Thermosyphon foundations for buildings in permafrost regions

Transcription

CAN/CSA-S500-14
National Standard of Canada
(approved August 2014)
Thermosyphon foundations for buildings
in permafrost regions
REVISED AUGUST 2014
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Revision History
S500–14, Thermosyphon foundations for buildings in permafrost regions
National Standard of Canada — AUGUST 2014
Outside front cover, National Standard of Canada text, title page, and preface.
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CAN/CSA-S500-14
June 2014
Title: Thermosyphon foundations for buildings in permafrost regions
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National Standard of Canada
(approved August 2014)
CAN/CSA-S500-14
Thermosyphon foundations for
buildings in permafrost regions
Prepared by
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oper ating as “CSA Group”
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ICS 91.120.99
ISBN 978-1-77139-604-2
© 2014 CSA Group
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the prior permission of the publisher.
CAN/CSA-S500-14
Contents
Technical Committee on Northern Built Infrastructure
4
Working Group on Thermosyphon Foundations for New Buildings in Permafrost Regions
Preface
6
7
1 Scope, objectives, and application
1.1
Scope 8
1.2
Objectives 8
1.3
Application 8
1.4
Exclusions 8
1.5
Terminology 9
2 Reference publications
3 Definitions
8
9
10
4 Performance and service life requirements 12
4.1
General 12
4.2
Basic performance requirements 12
4.2.1 Maintaining the integrity of the permafrost foundation 12
4.2.2 Setting the design maximum subgrade temperature 12
4.3
Fundamental service life requirements 12
4.3.1 Extending the service life of the thermosyphon system 12
4.3.2 Ensuring consistency in the service life of all thermosyphon system components
5 Geotechnical site characterization 12
5.1
General 12
5.2
Requirement for a geotechnical site characterization
5.3
Requirements for a subsurface investigation 13
5.4
Phased approach to geotechnical investigation 13
5.4.1 General 13
5.4.2 Information review 13
5.4.3 Site specific geotechnical investigation 14
5.4.4 Data analysis and reporting 14
12
13
6 System design 15
6.1
General 15
6.2
Assessing the applicability of a thermosyphon foundation for a certain site and building
6.2.1 Presence of permafrost 15
6.2.2 Presence of surface water 16
6.2.3 Stability of native subgrade soils 16
6.2.4 Ensuring suitability of sub-grade before construction 16
6.2.5 Staging of construction 16
6.2.6 Buildings with ventilated air spaces 16
6.2.7 Unheated structures 16
6.2.8 Constructability of the foundation system 17
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CAN/CSA-S500-14
6.3
6.3.1
6.3.2
6.3.3
6.3.4
6.4
6.4.1
6.4.2
6.4.3
6.4.4
6.4.5
6.4.6
6.5
6.5.1
6.5.2
6.5.3
6.5.4
6.5.5
6.6
6.6.1
6.6.2
6.6.3
6.6.4
6.6.5
6.6.6
6.6.7
6.6.8
6.6.9
6.6.10
6.6.11
6.6.12
6.6.13
6.6.14
6.6.15
6.6.17
Thermal analysis and modelling 17
Requirement to undertake thermal analysis 17
Thermal analyses responsibility and documentation 17
Design criteria 17
Hybrid thermosyphon systems 18
Available analytical tools 19
Thermal analysis models 19
One- and two-dimensional heat conduction simulation 19
Necessary input variables 19
Soil condition 19
Initial conditions 20
Boundary conditions 20
Derivation of climate parameters needed for thermal analysis 20
Key climatic parameters 20
Setting up a site-calibrated model and conducting analysis 20
Process to develop climate inputs 21
Calibration of ground thermal models to existing ground temperature data
Testing for extreme weather events 22
Design considerations for thermosyphon systems 22
General 22
Planning 22
Design documents 23
Project documentation 23
Evaporator layout 23
Pipes 23
Radiators 24
Granular pads 24
Excavation of a site 24
Use of gravel 25
Insulation 25
Grading 26
Reducing the potential for seepage 26
Ancillary design considerations 26
Use of sumps and underground utilities 26
Coordination with other ancillary elements of the building design 27
7 Construction, installation, and commissioning 27
7.1
Site preparation 27
7.2
System construction 27
7.2.1 Development of standard operating procedures 27
7.2.2 Welding recommendations and requirements 29
7.2.3 Record drawings recommendations and requirements
29
8 Monitoring 29
8.1
Monitoring plan 29
8.2
Implementation and operation of the monitoring program
8.2.1 General 29
8.2.2 Monitoring program 30
8.2.3 Thermosyphon operation verification test 30
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CAN/CSA-S500-14
8.2.4
8.2.5
8.2.6
Instrumentation for ground temperature monitoring 31
Data review and documentation 31
Evidence of building distress as a result of thermosyphon underperformance or failure
Annex A (informative) — Background on thermosyphons 32
Annex B (informative) — Two-dimensional numerical models for thermal analysis
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37
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CAN/CSA-S500-14
Technical Committee on Northern Built
Infrastructure
S. Brown
Northwest Territories Association of Communities
(NWTAC),
Yellowknife, Northwest Territories
Category: Owner/Operator/Producer
Co-Chair
J. Streicker
City of Whitehorse,
Whitehorse, Yukon
Category: General Interest
Co-Chair
A. Applejohn
Government of the Northwest Territories,
Category: Regulatory/Policy/Underwriter Interest
H. Auld
Risk Sciences International,
Ottawa, Ontario
Category: Supplier/Contractor/Consultant Interest
S. Dueck
Yukon Government Community Services,
Whitehorse, Yukon
D.W. Hayley
Hayley Arctic Geoconsulting,
Kelowna, British Columbia
K.R. Johnson
Stantec Consulting Ltd.,
Edmonton, Alberta
C. Larrivée
OURANOS Impacts & Adaptation,
Montréal, Québec
N. Pisco
Government of Nunavut Dept of Community Services,
Iqaluit, Nunavut
Category: Owner/Operator/Producer
B. Roy
Dept of Community & Government Services,
Government of Nunavut,
Pond Inlet, Nunavut
June 2014
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Associate
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CAN/CSA-S500-14
T. Sheldon
Nunatsiavut Department of Lands and Natural
Resources,
Nain, Newfoundland and Labrador
E. Sparling
Risk Sciences International,
Ottawa, Ontario
G. Strong
Dillon Consulting Limited,
R. Trimble
Tetra Tech EBA,
Whitehorse, Yukon
R. Van Dijken
Council of Yukon First Nations,
Whitehorse, Yukon
Category: General Interest
M. Westlake
Aboriginal Affairs and Northern Development Canada
(AADNC),
Gatineau, Québec
Associate
M. Braiter
CSA Group,
Mississauga, Ontario
Project Manager
June 2014
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Associate
Associate
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CAN/CSA-S500-14
Working Group on Thermosyphon
Foundations for New Buildings in
Permafrost Regions
D.W. Hayley
Hayley Arctic Geoconsulting,
Kelowna, British Columbia
R.G. Campbell
Town of Inuvik,
Inuvik, Northwest Territories
E. Cormier
Government of the Northwest Territories Public
Works & Services,
E. Hoeve
Tetra Tech EBA,
D. Malcolm
Malcolm & Associates,
J. Oswell
Naviq Consulting Inc.,
Calgary, Alberta
R. Trimble
Tetra Tech EBA,
Whitehorse, Yukon
B. Wall
Achieve Engineering Inc.,
Winnipeg, Manitoba
M. Braiter
CSA Group,
Project Manager
P. Steenhof
CSA Group,
Project Manager
June 2014
© 2014 CSA Group
Chair
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CAN/CSA-S500-14
Preface
This is the first edition of CAN/CSA-S500, Thermosyphon foundations for buildings in permafrost regions.
Previous to this Standard, there have been no guidelines or standards for the design, construction, and
maintenance for thermosyphon supported foundations.
CSA Group received funding for the development of this Standard from Standards Council of Canada, as
part of the Northern Infrastructure Standardization Initiative, supported by the Government of Canada’s
Clean Air Agenda.
This Standard was developed by the Working Group on Thermosyphon Foundations for New Buildings in
Permafrost Regions, under the jurisdiction of the Technical Committee on Northern Built Infrastructure
and the Strategic Steering Committee on Construction and Civil Infrastructure, and has been formally
approved by the Technical Committee.
This Standard has been approved as a National Standard of Canada by the Standards Council of Canada.
Notes:
1) Use of the singular does not exclude the plural (and vice versa) when the sense allows.
2) Although the intended primary application of this Standard is stated in its Scope, it is important to note that it
remains the responsibility of the users of the Standard to judge its suitability for their particular purpose.
3) This Standard was developed by consensus, which is defined by CSA Policy governing standardization — Code
of good practice for standardization as “substantial agreement. Consensus implies much more than a simple
majority, but not necessarily unanimity”. It is consistent with this definition that a member may be included in
the Technical Committee list and yet not be in full agreement with all clauses of this Standard.
4) To submit a request for interpretation of this Standard, please send the following information to
[email protected] and include “Request for interpretation” in the subject line:
a) define the problem, making reference to the specific clause, and, where appropriate, include an
illustrative sketch;
b) provide an explanation of circumstances surrounding the actual field condition; and
c) where possible, phrase the request in such a way that a specific “yes” or “no” answer will address the
issue.
Committee interpretations are processed in accordance with the CSA Directives and guidelines governing
standardization and are available on the Current Standards Activities page at standardsactivities.csa.ca.
5) This Standard is subject to review five years from the date of publication and suggestions for its improvement
will be referred to the appropriate committee. To submit a proposal for change, please send the following
information to [email protected] and include “Proposal for change” in the subject line:
a) Standard designation (number);
b) relevant clause, table, and/or figure number;
c) wording of the proposed change; and
d) rationale for the change.
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CAN/CSA-S500-14
CAN/CSA-S500-14
Thermosyphon foundations for buildings in
permafrost regions
1 Scope, objectives, and application
1.1 Scope
This Standard provides requirements for all life-cycle phases of thermosyphon foundations for new
buildings on permafrost, including site characterization, design, installation, and commissioning phases as
well as for monitoring and maintenance phases. This Standard is meant to ensure the long-term
performance of thermosyphon-supported foundation systems under changing environmental conditions.
1.2 Objectives
The objectives of this Standard are to
a) describe performance expectations for thermosyphon foundations together with monitoring
requirements necessary to support an appropriate maintenance program;
b) specify the materials to be used in thermosyphon foundations;
c) foster an awareness and understanding of application technology, with a focus on factors that could
compromise the functionality of foundation systems reliant on thermosyphons;
d) describe the typical phases of the life cycle of thermosyphon foundations for buildings on
permafrost, including design, installation, commissioning, monitoring, and maintenance;
e) provide guidance to maximize the long-term viability of thermosyphon-supported foundation
systems under changing environmental conditions; and
f) describe performance expectations for thermosyphon foundations together with monitoring
requirements necessary to support a maintenance program.
1.3 Application
This Standard is intended to be used by designers, contractors, building owners, and operators. For
owners, it provides an understanding of the design and construction processes required to permit
verification that adequate measures are taken during these phases. The Standard also sets out
monitoring and maintenance expectations for building operators.
This Standard is applicable to new buildings on permafrost sites. It is not intended to provide guidance
for initial selection of the most appropriate foundation type for any particular structure on a permafrost
site. It is assumed that a thorough review of alternative foundation systems has been undertaken and
that that the site has been categorized as potentially thaw-unstable. Preservation of the permafrost for
support of the structure has been identified as a design objective before this Standard is implemented.
1.4 Exclusions
This Standard does not cover
a) abandonment/demolition of buildings with thermosyphon foundations;
b) thermosyphons in areas of non-permafrost or retrofitting thermosyphons to existing buildings; and
c) thermosyphons used for infrastructure other than buildings.
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1.5 Terminology
In this Standard, “shall” is used to express a requirement, i.e., a provision that the user is obliged to
satisfy in order to comply with the standard; “should” is used to express a recommendation or that
which is advised but not required; and “may” is used to express an option or that which is permissible
within the limits of the standard.
Notes accompanying clauses do not include requirements or alternative requirements; the purpose of a
note accompanying a clause is to separate normative clauses from explanatory or informative material.
Notes to tables and figures are considered part of the table or figure and may be written as
requirements.
Annexes are designated normative (mandatory) or informative (non-mandatory) to define their
application.
2 Reference publications
This Standard refers to the following publications, and where such reference is made, it shall be to the
edition listed below.
CSA Group
B51-09
Boiler, pressure vessel, and pressure piping code
Plus 4011-10
Technical Guide – Infrastructure in permafrost: A guideline for climate change adaptation
ASME
ASME Boiler and Pressure Vessel Code (2013)
ASTM International
A106/A106M-13
Standard Specification for Seamless Carbon Steel Pipe for High-Temperature Service
C518-10
Standard Test Method for Steady-State Thermal Transmission Properties by Means of the Heat Flow
Meter Apparatus
D698-12
Standard Test Methods for Laboratory Compaction Characteristics of Soil Using Standard Effort (12 400 ftlbf/ft3 (600 kN-m/m3))
D2842-12
Standard Test Method for Water Absorption of Rigid Cellular Plastics
D4083-89(2007)
Standard Practice for Description of Frozen Soils (Visual-Manual Procedure)
Other publications
Farouki, O.T., 1986. Thermal Properties of Soils. TransTech Publications, Germany, 136 p.
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CAN/CSA-S500-14
Hayley, D.W., 1982. “Application of heat pipes to design of shallow foundations on permafrost.”
Proceedings of the 4th Canadian Permafrost Conference, Calgary, AB, March 1981. NRC Press, pp. 535–
544.
Hayley, D.W. and Horne, B. 2008. “Rationalizing climate change for design of structures on permafrost: A
Canadian perspective.” Proceedings Ninth International Conference on Permafrost, University of Alaska,
Fairbanks, pp. 681–686.
Haynes, F.D. and Zarling, J.P., 1988. “Thermosyphons and foundation design in cold regions.” Cold
Regions Science and Technology, Vol. 15, pp. 251–259.
Holubec, I., 2008. Flat Loop Thermosyphon Foundations in Warm Permafrost. Report submitted to
Government of the Northwest Territories, Asset Management Division, Department of Public Works and
Services, and Public Infrastructure Engineering Vulnerability Committee, March 2008.
Holubec, I., 2010. Geotechnical Site Investigation Guidelines for Building Foundations in Permafrost.
Report submitted to the Government of the Northwest Territories, Department of Public Works and
Services, January 2010.
Hwang, C.T., 1976. “Prediction and observations on the behaviour of a warm gas pipeline in permafrost.”
Canadian Geotechnical Journal, 13(4), pp. 452–480.
Johnston G.H. (Editor), 1981. Permafrost Engineering Design and Construction. Wiley & Sons, Toronto,
540 p.
Long, E.L. 1963. “The Long Thermopile.” Proceedings of the First International Conference on Permafrost,
Purdue University, US National Academy of Sciences, pp. 487–490.
Yarmak, Jr., E. and Long, E.L., 2002. “Recent developments in thermosyphon technology.” Proceedings of
the 11th International Conference on Cold Regions Engineering, Anchorage, Alaska, May 2002, pp. 656–
662.
3 Definitions
The following definitions apply in this Standard:
Active layer — the top layer of ground subject to annual thawing and freezing in areas underlain by
permafrost.
Competent individual — a person who through training, qualification, and experience has acquired the
knowledge and skills necessary for undertaking tasks assigned to him or her.
Construction — includes the original installation, as well as any subsequent repair, modification, or
replacement of utility infrastructure.
Construction drawings — design drawings indicating the scope of the proposed work activity that have
been approved and issued for construction.
Contractor — a person who undertakes a project for an owner.
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Designer — a professional engineer responsible for the design, and prepares or issues a drawing for a
construction project.
Excess ice — the volume of ice in the ground that exceeds the total pore volume that the ground would
have under natural unfrozen ground conditions.
Engineer — a professional engineer experienced with design of building foundations on permafrost sites
and registered with the Association of Professional Engineers in the jurisdiction where the project is
located.
Freezing-point depression — the number of degrees by which the freezing point of an earth material is
depressed below 0 °C.
Ice-rich permafrost — permafrost containing ice in excess of 20% by volume.
Life cycle — the phases of planning, design, construction, operations, maintenance/monitoring, and
decommissioning of the thermosyphon.
Mean annual air temperature (MAAT) — the annual air temperature as an average of mean monthly
temperatures for a site taken over a thirty-year period.
Note: MAAT is available from Environment Canada as published Canadian Climate Normals for specific locations.
Non-frost-susceptible soil (NFS) — sand and gravel containing less than 10% by mass passing the
0.08 mm sieve.
Note: For example, silt and clay-sized particles, fines.
Owner — the proprietor of the thermosyphon, including contractors, agents, or other persons acting on
behalf of the owner.
Permafrost — ground (soil or rock and included ice and organic material) that remains at or below 0 °C
for at least two consecutive years.
Notes:
1) Permafrost occurring everywhere beneath the exposed land surface throughout a geographic region is
considered “continuous” whereas it is considered “discontinuous” if some areas within the geographic region
are free of permafrost.
2) Cold permafrost is generally considered to have a ground temperature at or below –5 °C. Warm temperature is
generally considered to have a ground temperature at or above –2 °C. The ground temperature refers to that
measured at a depth where it is constant year around.
Record — information created or received that is retained in a tangible or reproducible format, as might
be required by law or used directly or indirectly in the management or transaction of business.
Subgrade — the natural soil below a building or foundation, generally below any engineered fill that can
comprise part of the foundation structure.
Talik — a layer of year-round unfrozen ground that lies in permafrost areas.
Note: In regions of continuous permafrost, taliks often occur underneath shallow thermokarst lakes and rivers,
where the deep water does not freeze in winter, and thus the soil underneath will not freeze either.
Thaw stable soil — soil with an ice content such that no significant volume change occurs on thawing.
Thermosyphon — a two-phase passive refrigeration device charged with a working fluid that transfers
heat from the ground to the air when appropriate temperature differentials prevail.
Note: See Annex A.
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4 Performance and service life requirements
4.1 General
Clause 4 provides basic system design requirements that are considered essential to satisfactory
performance over the service life of the structure.
4.2 Basic performance requirements
4.2.1 Maintaining the integrity of the permafrost foundation
The permafrost condition within the foundation soil or rock shall be preserved. The thermal and
mechanical properties of the frozen ground shall be sustained throughout the designated life of the
structure.
Note: The fundamental purpose of thermosyphons in a foundation application is to maintain the thermal stability of
permafrost soils supporting a structure. The objective of the system design is that the combination of thermosyphon
and insulation placement limits seasonal freezing and thawing to an engineered layer of non-frost-susceptible soil
and thaw stable granular soil that separates the structure from the underlying natural permafrost soil.
4.2.2 Setting the design maximum subgrade temperature
The maximum sub-grade temperature predicted throughout the life of the structure should be
established as a design criteria in accordance with the properties of the soils and deformation risktolerance associated with the type of structure.
Note: Fine-grained soils exhibit freezing-point depression, specifically related to the clay content and mineralogy.
The presence of ions in the porewater/ice, such as salinity, will bring about freezing-point depression. Maintaining
the temperature of soil below a structure colder than 0 °C might therefore not be sufficient to prevent thaw. A
maximum subgrade design temperature of –2 °C is typical, but the selected value should reflect project specific
considerations.
4.3 Fundamental service life requirements
4.3.1 Extending the service life of the thermosyphon system
An appropriate monitoring and maintenance program shall be used to extend the service life of the
thermosyphon system to align with the service life of the building (see Clause 8 for details of the
monitoring and maintenance).
4.3.2 Ensuring consistency in the service life of all thermosyphon system components
Components of the thermosyphons shall be selected to be consistent with the service life of the
thermosyphon system. The service life of the thermosyphon system shall be at least as long as the service
life of the building.
Note: The service life of thermosyphon systems described in this Standard are generally in the range of 20 to
30 years based on experience to-date. There is a tendency to increase service life expectations to 50 years based on
limited Alaska experience, but verification by precedent for the horizontal loop system in common use in northern
Canada is currently lacking.
5 Geotechnical site characterization
5.1 General
Clause 5 provides recommendations and requirements for geotechnical site characterization.
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Note: There are no specific codes or standards for the geotechnical investigation. Holubec (2010) provides one
perspective on the types and extent of geotechnical investigation for any structure founded on permafrost.
5.2 Requirement for a geotechnical site characterization
Geotechnical site characterization shall be undertaken before the final design process begins. This shall
involve sourcing and assessing all available information on site surface and subsurface conditions as a
component of project planning. The following parameters should be assessed as part of the geotechnical
site characterization:
a) confirmation that the site is underlain by permafrost and that thermosyphons are an appropriate
design option;
b) determination of ground temperature at or near the depth of zero annual amplitude, natural water
or ice content, and pore water salinity;
c) identification of deep seasonal thawing or potential presence of a talik; and
d) potential surface and groundwater flow within the active layer.
An extensive site investigation should be undertaken where the permafrost is thin in order to
characterize the initial and boundary conditions necessary for thermal modelling (see Clause 6.3).
Note: This Standard is not intended to provide guidance for initial selection of the most appropriate foundation type
for any particular structure on a permafrost site. It is assumed that a thorough review of alternative foundation
systems has already been undertaken and that the site has been categorized as potentially thaw-unstable.
Preservation of the permafrost for support of the structure has been identified as a design objective before this
Standard is implemented.
5.3 Requirements for a subsurface investigation
A subsurface investigation shall be undertaken to determine site-specific conditions appropriate for
thermosyphon foundation design. In some cases, where there is significant appropriate geotechnical and
geothermal data for sites in close proximity, the scope of the investigation can be reduced.
Note: Planning and executing the geotechnical investigation is more important for the design of a thermosyphon
foundation than other foundation systems because the foundation system performance is contingent on several
important factors, including
a) confirmation that the site is underlain by thaw-unstable permafrost and that thermosyphons are an
appropriate design option;
b) confirmation of the depth and variability of the active layer soils;
c) determination of ground temperature and ice or water content;
d) identification of deep seasonal thawing or potential presence of a talik; and
e) identification of surface and groundwater flow within the active layer across the site.
5.4 Phased approach to geotechnical investigation
5.4.1 General
The geotechnical investigation should comprise several phases, even if some are undertaken in a cursory
manner.
5.4.2 Information review
A desktop study should be performed to review the available general project area data, including
a) geotechnical/environmental reports from the community;
b) site observations, including local/traditional knowledge;
c) interpretation of surficial geology from aerial or satellite imagery;
d) identification from historic records the mean annual air temperature (MAAT) representative of the
site; and
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e)
technical literature.
5.4.3 Site specific geotechnical investigation
A site specific geotechnical investigation should be performed, comprising
a) borehole drilling with appropriate sampling;
b) installation of ground temperature monitoring instruments; and
c) associated laboratory testing.
5.4.4 Data analysis and reporting
5.4.4.1 Use of testpits
The use of testpits to provide geotechnical data for foundation design should be discouraged.
Note: Testpits will, at best, only reach the top of the permafrost and thus will provide very little meaningful
subsurface data. The use of testpits for foundation design does not represent prudent or sufficient engineering
practice.
5.4.4.2 Number of boreholes
One borehole should be drilled for every approximately 150 m2 to 250 m2 of building footprint area with
a minimum of three boreholes per site. Additional boreholes should be drilled if subsurface conditions
are highly heterogeneous. Borehole locations should be chosen based on observed surface features and
topography.
Note: As the project site area increases, the number of boreholes per unit area necessary to adequately characterize
the site generally decreases, unless the subsurface conditions are found to be complex.
5.4.4.3 Depth of boreholes
The recommendations and requirements for the depth of boreholes are as follows:
a) the depth of boreholes should be 10 m or the narrowest dimension of the proposed structure;
b) at least one borehole should be drilled to a minimum depth of 15 m;
c) in the deepest borehole, a multi-bead ground temperature cable should be installed following good
practice that will ensure its survival for several years;
d) where bedrock is encountered, boreholes may be terminated at shallower depths provided at least
one borehole is advanced into bedrock deep enough to prove its existence and to assess its
competency in both a frozen condition and a thawed condition; and
e) boreholes that remain dry and open should be inspected by looking down the hole with a strong
light or mirror.
In addition to Items a) to e), there shall be flexibility to adjust borehole depth and spacing in the field
based on stratigraphy that is encountered.
Note: Ground temperatures typically experience seasonal changes to depths up to 15 m, and therefore one borehole
to this depth with ground temperature measurements will provide a more complete understanding of the
geothermal character of the ground.
5.4.4.4 Collection of cores and other samples
The recommendations and requirements for the collection of cores and other samples are as follows:
a) if there is only a rotary percussion (air-track) drill available, bulk samples of cuttings shall be
collected at minimum 1 m intervals or every stratigraphic unit, whichever is less;
b) core and other samples shall be logged and photographed as they are received; and
c) permafrost and ground ice shall be described using the terminology in ASTM D4083.
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In addition to the requirements specified in Items a) to c), undisturbed cores or samples from a split
spoon sampler, if practical, should be collected every one metre of hole depth or every stratigraphic unit,
whichever is less;
Note: This provides the best data for soil and permafrost classification.
5.4.4.5 Length of ground temperature data record
A year of ground temperature data or a minimum of four sets of readings representing the late winter
(coldest ground conditions) and the late summer or fall (warmest ground conditions) should be collected.
The ground temperature data shall be representative of the variability of the ground temperature with
depth over an annual cycle. Data loggers are useful to ensure continuity of information and should be
installed where manual readings are expected to be difficult to obtain.
5.4.4.6 Geotechnical laboratory testing
Representative samples should be preserved and transported to a geotechnical laboratory for testing and
analyses. Samples should be protected against moisture loss, and thawing if frozen strength or creep
testing is required. At a minimum, the laboratory tests should include
a) natural moisture content;
b) particle size distribution;
c) porewater salinity; and
d) Atterberg limits (where applicable).
6 System design
6.1 General
Clause 6 outlines the thermosyphon design process sequentially, including providing requirements and
recommendations on
a) assessing the applicability of a thermosyphon foundation for a certain site and building;
b) data collection to support the design analysis;
c) analysis methodology; and
d) design and construction requirements of the individual components of the system.
6.2 Assessing the applicability of a thermosyphon foundation for a certain site and
building
6.2.1 Presence of permafrost
A thermosyphon stabilized foundation shall only be considered for a site that is underlain by permafrost.
The presence and characteristics of permafrost at the development site should be verified.
Notes:
1) The primary objective of a thermosyphon foundation is to stabilize a permafrost subgrade against heat loss
from an overlying building. A common misconception is that insulation alone will serve this purpose. Insulation
only slows heat transfer, but does not prevent it. Insulation alone is not an effective measure to maintain an
ice-rich permafrost subgrade below a heated building.
2) The most robust thermosyphon designs are often the simplest, with one thermosyphon configuration at a
single level beneath the entire structure. Thermosyphons are best suited and most commonly used for
buildings that require at-grade floors. These would typically be buildings with heavily loaded floors, such as
garages, warehouses, fire halls, aircraft hangars, and certain recreational facilities.
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3)
If the permafrost is very warm (warmer than –1 °C) or there is a deep active layer or talik at the site, these
characteristics of the permafrost introduces complexities that need to be carefully considered in the selection
of a foundation type and as design proceeds.
6.2.2 Presence of surface water
Thermosyphons should not be used in areas where there is significant surface water in the summer or
significant subsurface flow at any time of year.
Note: Thermosyphons can preserve permafrost, but it is difficult for them to create it, especially when there is
excess water in the foundation soils.
6.2.3 Stability of native subgrade soils
Thermosyphons may only be considered if the native subgrade soils are judged to be unstable when
thawed.
Note: If the development site is underlain by thaw stable permafrost, such as competent bedrock or gravel or sand
without excess ground ice, consideration should be given to constructing the building with no measures to protect
the permafrost. Identifying sites underlain by suitable bedrock is common, however finding thaw stable soil in
permafrost areas is not.
6.2.4 Ensuring suitability of sub-grade before construction
The sub-grade of the building should be in a thermally stable, permafrost condition prior to constructing
the superstructure.
6.2.5 Staging of construction
One of the following two approaches should be considered for staging of construction:
a) If equipment and supplies can be on-site in the late spring or early summer, site preparation and
placement of the evaporator, gravel, and insulation may occur early in the summer in an effort to
preserve cold ground temperatures from the previous winter.
b) If equipment and supplies arrive later in the summer, as would typically be the case if delivered by
sea-lift, then the site preparation and the placement of thermosyphon, gravel, and insulation may
occur in the late summer or fall. Construction should be delayed until freeze-back occurs through
the following winter.
6.2.6 Buildings with ventilated air spaces
Ventilated air spaces should be kept above outside ground level. In some cases, this is not practical and,
if the ventilated air space is designed to be below ground level, all risks associated with the design should
be carefully evaluated. Air temperature within ventilated air spaces shall be controlled and monitored to
provide the coldest conditions practical to prevent winter freeze up.
Notes:
1) This is also applicable for the main floor of buildings (i.e., the main floor should always be above ground).
2) Installations requiring a portion of the building to extend below grade have proven to be problematic in past
applications and are not recommended. The risks associated with crawl spaces below ground level generally
are associated with the potential for seepage and the accumulation of water or ice in the crawl space.
6.2.7 Unheated structures
Thermosyphons should not be used for unheated structures unless these structures are on warm
permafrost near the southern fringe and there is no realistic option to prevent or mitigate thawsettlement due to ground disturbance associated with the building construction.
Note: Thermosyphons might have a future application for unheated structures in order to provide resiliency to
climate warming over the service life of the structure.
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6.2.8 Constructability of the foundation system
The constructability of the foundation system at the project location should take into consideration the
following:
a) contractor experience and available manpower;
b) availability of supplies and equipment, or the anticipated timing of their arrival; and
c) anticipated ground conditions at the time of foundation construction.
6.3 Thermal analysis and modelling
6.3.1 Requirement to undertake thermal analysis
The design of thermosyphon foundation systems shall be supported by thermal analyses to predict the
interaction of the proposed building with permafrost at the site. The steps in the modelling process
should include the following:
a) determining the thermal properties of the constituents of the thermosyphon system and site,
including the foundation soils, fill materials, and insulation;
b) determining the current and future climate parameters applicable to the site;
c) calibrating the model to known ground temperatures, usually with a one-dimensional analysis; and
d) undertaking design analysis of the foundation configuration, usually with a two-dimensional
analysis, but possibly with an axisymmetric or a three-dimensional analysis.
Note: The reference to buildings includes the foundation as well as crawlspace.
6.3.2 Thermal analyses responsibility and documentation
The analyses conducted to support the design of the thermosyphon system shall be performed under the
direction of an engineer and documented in a design report. The engineer selected for this task should
be able to document past successful delivery of similar projects. If the engineer cannot document past
successful delivery of similar projects, an independent review should be undertaken.
6.3.3 Design criteria
6.3.3.1 General
The designer shall consider the following factors (design criteria) that impact the thermal performance of
the system:
a) interior space temperatures as they reflect interior floor temperatures and the crawl space, for both
the summer and the winter;
b) insulation system design for the building, including insulation around the perimeter and directly
below the floor;
c) the thickness of engineered non-frost susceptible gravel fill;
d) climate data with appropriate projections over the life of the building;
e) natural snow accumulation at the site and how it will change as a result of construction. Identify
what snow management procedures should be implemented;
f) the heat extraction capacity of the individual thermosyphons; and
g) location of heating lines within the floor slab. This design feature shall be coordinated if the
thermosyphon designer and the foundation designer are different engineers.
The designer should balance the design in order to stabilize the permafrost and protect the building
foundation over the life of the project when considering these conditions.
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6.3.3.2 Interior space temperatures
The summer and winter interior space temperatures shall be estimated based on climate data,
ventilation system design, and space usage.
Note: Changing the interior building temperatures, in both the summer and the winter, will increase or decrease the
heat flow through the floor.
6.3.3.3 Insulation system design
The following conditions should be applied for insulation system design:
a) the lower the insulation thermal resistance (R-value), the greater number of thermosyphons that
should be used;
Note: Insulation thickness directly impacts the rate of heat transfer from the interior of the building to the
thermosyphons.
b)
more thermosyphons (closer spaced evaporators) should be used for warmer buildings versus cooler
buildings; and
Note: For any given insulation thickness, the higher the space temperatures, the greater the heat load to the
thermosyphons and the potential thaw depth in the fill during the summer.
c)
more thermosyphons should be used for buildings with in-floor heating, as in-floor heating
significantly increases the rate of heat transfer due to warmer than normal floor temperatures.
6.3.3.4 Engineered pad thickness
The thickness of engineered non-frost-susceptible fill should be selected to contain the summer period
thaw depth predicted by geothermal modelling based on the interior space temperatures, insulation
thickness, and climate data.
6.3.3.5 Consideration of regional climate change
Consideration shall be given to regional climate change projected to occur during the life of the building.
Clause 6.5 provides requirements and recommendations on related analytical tools.
Notes:
1) Climate data is used to specify the exterior boundary conditions for the geothermal analysis. It also determines
the time period where the thermosyphons will be active and not active. As the climate warms, the number of
freeze and thaw days available to the thermosyphons will be affected. This could have a significant effect on
the system design.
2) For a given set of climatic data and interior space/slab temperatures, the designer should optimize the
thickness of engineered fill, the insulation system design, and the spacing of thermosyphons.
6.3.4 Hybrid thermosyphon systems
A hybrid thermosyphon system should be considered if summer heat load becomes excessive or during
the early startup years if construction scheduling does not allow the permafrost temperature to stabilize
before building erection.
Note: A hybrid system incorporates a heat exchanger into the thermosyphon condenser. This allows the system to
connect to a refrigeration system to supplement the effective operating time period. Hybrid systems can also be
used to increase the rate of ground freezing immediately after the installation of thermosyphons, particularly in
summer. Hybrid heat exchangers can also be used to provide a backup cooling source for situations where the thaw
depth exceeds expectations and the permafrost is in jeopardy. They should also be considered where rapid cooling is
required for construction scheduling, logistical reasons, or where there is a risk that regional temperatures could be
warmer than predicted.
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6.4 Available analytical tools
6.4.1 Thermal analysis models
Numerical models are commonly used for the thermal analyses as they have the capability to accurately
represent boundary conditions and soil profile irregularities. The applicability of any model should be
confirmed by reference to past successful performance in a similar application.
Notes:
1) Analytical tools for predicting future ground temperatures below the structure, taking into account the various
design variables, are based on numerical models that solve equations for heat flow in the ground. These
analyses are readily available for desktop use by engineers who specialize in design for permafrost conditions.
2) Examples of models that are commercially available and that have been used successfully are listed in
Annex B.
6.4.2 One- and two-dimensional heat conduction simulation
The geothermal model used shall have the ability to simulate one- and two-dimensional heat conduction
with change of phase (solid-to-liquid and vice versa) within the elements for a variety of boundary
conditions.
Note: Two-dimensional analyses are satisfactory for modelling buildings as the thermal regime can be effectively
represented by a cross-section of width and depth beneath the structure. The algorithm that deals with freezing or
thawing is critical to any ground thermal analysis, as it captures the exchange of latent heat as a thaw or freeze
boundary migrates through the model’s domain. Another important consideration is the simulation of ground
surface heat flux. Some models represent the ground surface temperature as a function of air temperature or
freeze-thaw indices. More refined models consider the energy balance at the ground surface, which includes energy
flux components of solar radiation, long wave radiation, convective heat transfer between the air and the ground
surface, and the insulating effects of snow cover at the ground surface. Hwang (1976) provides information in this
regard.
6.4.3 Necessary input variables
The input parameters for any model should represent the soil conditions, boundary conditions, and initial
temperature conditions.
6.4.4 Soil condition
6.4.4.1 Soil parameters
The soil parameters for each element in the site stratigraphy shall include moisture content, frozen and
unfrozen thermal conductivity, frozen and unfrozen heat capacity, and latent heat.
Note: These properties are sensitive to the soil moisture or ice content and the bulk density of the soil. It is seldom
necessary to measure soil thermal properties in the laboratory as there are published values available for the
experienced geotechnical engineer to use once common soil properties are known. For certain soils, such as
relatively dry sand and gravel, the thermal properties are strongly influenced by mineralogy of the constituents.
Gravels used for site grading and engineering pad construction that have high quartz content will have relatively
high thermal conductivity and low water content.
6.4.4.2 Determination of soil properties
The thermal properties of soil are determined indirectly from well-established correlations with soil index
properties, as described in Farouki (1986) and Johnston (1981). In some instances, soil properties might
need to be estimated from experience. The following properties might be required as input to a
geothermal model:
a) water content (%);
b) unfrozen water content (%);
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c)
d)
e)
f)
g)
porewater salinity (ppt);
bulk density (kg/m3);
frozen and unfrozen thermal conductivity (W/m-K);
frozen and unfrozen heat capacity (kJ/kg-K); and
latent heat (MJ/m3).
The design maximum temperature selected for the engineered fill should consider freezing point
depression when considering the grain size distribution of the soil and potential for soil porewater
salinity.
6.4.5 Initial conditions
The initial conditions for the simulation should be the current soil temperatures. This information should
come from the site investigation and be obtained from ground temperature sensors that are installed in
drill holes where stratigraphy has been characterized. Engineering judgment shall be used to establish
the baseline ground temperature regime.
6.4.6 Boundary conditions
Boundary conditions simulate the heat flow into and out of the soil.
Note: These boundary conditions can specify known or design-based temperature conditions, such as the
temperature of the top surface of a floor slab, or they can specify heat energy flux rates, such as the geothermal
gradient. The boundary condition at the natural ground surface surrounding the structure is often a critical driver
for the computed short and long term ground temperatures within the engineered pad below the structure. Ground
surface boundary conditions, where the ground surface temperature is implicitly derived are preferred to those
where the ground surface temperature is explicitly specified. Ground surface energy balance boundary conditions
are therefore preferred over the N-factor approach, where ground surface temperatures are prescribed based on a
multiplier of air temperature or freeze-thaw indices.
6.5 Derivation of climate parameters needed for thermal analysis
Note: Much of Clause 6.5 originates from CSA Plus 4011.
6.5.1 Key climatic parameters
Key climatic parameters required for thermal analyses parameters should be evaluated in order to
determine likely values throughout the life of the project. These include the following:
a) air temperatures, usually taken as mean monthly air temperatures, and from which annual freezing
and thawing indices are calculated;
b) monthly wind speed;
c) snow cover related to monthly average; and
d) daily solar radiation, usually taken in one-month increments and factors relating to the ground
surface heat exchange (albedo and evapotransportation) where models utilize these data.
Note: The climatic input can be developed using data available from Environment Canada's Weather Office historic
data (www.weatheroffice.gc.ca) and The Canadian Climate Change Scenarios Network (CCCSN) lead by Environment
Canada (www.cccsn.ec.gc.ca ).
6.5.2 Setting up a site-calibrated model and conducting analysis
A site-calibrated model should be set up in an appropriately sized grid with temperature-controlled
boundaries that include the contact of the building with the ground.
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The process of setting up the model and conducting analysis should include the following steps and
conditions:
a) microclimatic effects such as exposure of radiators to sunshine (north or south walls) and wind
together with snow accumulation on the ground should be taken into consideration;
b) the simulation should proceed in set time monthly intervals during which time heat flow between
points in the grid can be calculated and changes in ground temperature can be determined; and
c) the results should be presented as ground temperature isotherms at chosen times during the
analyses.
The temperature prediction shall be examined at the end of service life for fall conditions (September–
October) when the ground is the warmest. This condition should be the initial basis for judging adequacy
of design.
Note: The climate surface boundaries may be set to vary annually on a linear trend from the present climate to the
climate at the end of the service life.
6.5.3 Process to develop climate inputs
Climate inputs to a site-calibrated model should consider annual or seasonal climate warming. This
should involve the following steps when data is available:
a) Review and plot all available mean annual air temperature data for the closest meteorological
station(s).
b) Plot current published Canadian Climate Normals (temperature) for reference.
c) Develop a “dynamic normal” by averaging the most recent 30 years of air temperature data, for
comparison with the plotted Canadian Climate Normals.
d) Conduct trend analyses using a least squares fitting technique for the entire data set as well as for
the past thirty-year dynamic normal period.
e) Derive air temperature projections from an ensemble of global climate models (GCMs) evaluated
and verified for the project region (CCCSN, www.cccsn.ec.gc.ca). Tables 5.2 and 5.3 in CSA Plus 4011
provide a useful starting point and might be sufficient for most common projects. Where greater
certainty is required, a specialist who is conversant with regional GCMs or projections from model
ensembles should be consulted.
f) Extrapolate the linear trends to the end of building life, or 30 years, whichever is the shorter time
period. Take into consideration that since changes in air temperature are highly variable from yearto-year, they might in the future depart from linear trends of previous decades (see Note below).
When analyzing climate and the atmosphere, the present temperature trajectory should be
considered as one of various pieces of evidence, and one that, like most others, becomes less
precise as the time span for prediction increases.
g) Compare the end-of-life projections from the GCM ensemble with the extrapolated historical trends.
A comparison of this type is considered reasonable, where a comprehensive historical database
exists, over a limited service life (not exceeding 30 years, although climate model projections are
provided for consideration beyond this timeframe).
h) Using all available results from above, make a value judgment regarding an appropriate design
climate at the end of structure life.
i) Convert the mean annual air temperature at the end of life to projected design mean monthly air
temperatures, recognizing the seasonal variation in climate change and increasing uncertainty when
considering long design lives.
Note: Climate scientists have indicated that due to the natural variability of air temperature trends in the Arctic,
extrapolations in excess of 10 years will be subject to significant uncertainty even when based on long-term data
sets. However, most community infrastructure is expected to remain in service for periods of much greater than
10 years from the time of construction. Therefore, a range of evidence, including temperature Normals, modelled
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projections, as well as historical trends, need to be combined in order to predict mean air temperatures at the
anticipated end-of-life of a structure. While an important input, once derived, end-of-life air temperatures will
ultimately be combined with many other factors for the dynamic modelling of ground temperature over time. A
level of uncertainty will be associated with all temperature and all other geothermal inputs; however, a combination
of sources will allow for a sufficient amount of accuracy and is preferable to assuming no change in air
temperatures over time.
6.5.4 Calibration of ground thermal models to existing ground temperature data
The following are recommendations and requirements for calibrating ground thermal models to existing
ground temperature data:
a) The interpretation of calibration runs and minor adjustments to the input parameters shall be made
with considerable judgment by an experienced engineer.
b) Once the ground thermal model has been selected and appropriate input parameters determined,
the simulation shall be calibrated to current conditions.
c) The model should be set up and run for the present site conditions and the ground temperature at
the site should be simulated.
d) The predicted ground temperatures should be compared with available measured values. Deviations
between predicted and observed ground temperatures shall be rationalized and, in some cases,
minor adjustments to the input parameters made.
Note: In many cases, the baseline ground temperatures and surface boundary conditions may be calibrated using a
one dimensional model.
6.5.5 Testing for extreme weather events
The foundation configuration should be tested for extreme weather events. A foundation configuration
should be able to provide an acceptable result according to a climatic warming trend. In order to do so,
the historic database should be analyzed statistically to determine frequency of extreme annual mean
temperatures. Mean monthly temperatures representative of these conditions may be developed. The
design should then be checked for robustness against a combination of extreme conditions.
Note: The purpose of checking the design for an extreme year or a series of extreme years is to satisfy the designer
and owner that the thermosyphon system as designed is robust enough to withstand several “what-if” scenarios. An
example of the what-if scenario used for the Inuvik Hospital (Hayley and Horne, 2008) was the following:
a) five consecutive 1 in 5 warm years, followed by a 1 in 100 warm year; and
b) ten consecutive 1 in 5 warm years.
It is a matter of engineering judgment which scenario is adopted by the designer, but it should be clearly
documented in the design report.
6.6 Design considerations for thermosyphon systems
6.6.1 General
Clause 6.6 discusses how the elements of a thermosyphon stabilized foundation system are incorporated
into an overall design.
6.6.2 Planning
The process of planning should include the following steps and conditions:
a) contingency planning should be included in the design process;
b) at the outset, it should be recognized that adaptation of the design during construction may be a
field requirement. Any deviations from the design should be done in consultation with the designer
and should be reflected in the as-built documentation;
c) the design and construction plan should be reviewed for robustness;
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d)
e)
the review should examine the design within the context of the overall site development and
consider site drainage and snow management both during the construction and operational phases;
and
there should be an independent review by a competent individual who has not been part of the
design process.
Note: An independent review is particularly warranted when a new configuration of the technology or a warm
location without precedent are being considered.
6.6.3 Design documents
The design documents shall show a complete design of the thermosyphons system. In addition to the
information required by the applicable building code, the design documents shall include, but not be
limited to, the following information:
a) the design standards used;
b) the material or product standards used;
c) a design basis document, which includes documentation of how climate change has been accounted
for over the service life of the project;
d) a thermal design report that describes the methodology, stamped by the geotechnical engineer of
record;
e) operating limitations knowingly accepted by the designer and the owner and their role in on-going
maintenance;
f) monitoring plan documentation with reporting procedures (see Clause 8); and
g) non-routine maintenance and operating procedures that arise from the design process.
6.6.4 Project documentation
Project documentation shall be thorough and easily interpreted by future owner-representatives who
were not involved in either design or construction. The design analysis should be reproducible in order to
test contingency plans, if required.
6.6.5 Evaporator layout
The following should be considered for evaporator layout when considering evaporator spacing, loop
length, and system configuration:
a) for a given radiator capacity, evaporator pipes should be spaced in the range of 1 to 2 m apart;
b) evaporator spacing should reflect the length of loops;
i) long loops should use closer pipe spacing;
ii) if evaporator pipe spacing is greater than 2 m, the rationale and justification should be well
documented in the design report;
c) the maximum loop length should be 150 m. Loop lengths longer than this should be given special
consideration and justification;
d) the system should be configured to be redundant, such that if one evaporator loop fails, the
adjacent loops would continue to provide sufficient cooling of the subgrade.
Note: Practical considerations can dictate closer spacing, particularly as the pipe converges towards the radiators;
6.6.6 Pipes
The following are recommendations and requirements for pipes that represent current practice in
Canada for thermosyphons components:
a) the outside diameter of evaporator pipe should be 20 mm for flat loop thermosyphons and 75 mm
for sloped thermosyphons;
b) the outside diameter of radiator pipes is typically 90 mm;
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c)
d)
e)
f)
the thermosyphons should be constructed with ASTM A106B Schedule 40 steel pipe;
seamless pipe should be used;
butt welding of pipe shall meet appropriate standards for pressure vessels; and
if the gravel pad in which the evaporator pipes are installed contains particles larger than 20 mm,
the pipe should be protected with 100 mm thick layers of 20 mm minus sand or gravel above and
below the pipe.
6.6.7 Radiators
The following recommendations, representing current practice in Canada for the placement of radiators,
should be considered:
a) Radiators should be fabricated in multiples of 6.5 m2. Typically available sizes of radiators are 13 m2
and 19.5 m2. If additional radiator capacity is needed, multiple radiators should be placed on a
single riser pipe to provide up to 39 m2 of radiator area.
Note: Such increases in radiator capacity provides one remedial strategy in cases where the initial radiator
configuration provides insufficient cooling.
b)
Radiators may be grouped on one side of the structure.
Note: For small structures with only a few thermosyphon loops, this approach might be adequate. However,
for larger buildings with many loops, the concentration of the evaporator lines and radiators in one location
can lead to large temperature gradients across the plan area of the structure, with the engineered fill in the
vicinity of the radiators being much colder than the area remote from the radiators.
c)
d)
Radiators should be grouped at two or more locations around the structure, taking into account the
architectural and building access requirements.
Radiators should preferably be placed on the windy side of the structure and in a location, such as
the north side, where solar radiation will be minimized, for optimal heat transfer.
Note: Sensitivity and parametric studies have shown that air temperatures are the primary factor in
determining thermosyphon efficiency. Wind speed is a secondary factor.
e)
f)
If architectural or operation considerations, such as vehicle access, dictate that radiators are situated
in a sheltered location, then the decreased efficiency should be accounted for in the design.
The radiator array should be supported on a reinforced concrete footing, buried at evaporator level.
They should be placed at least 1.5 m from the structure walls and extend well above the roof line of
the building.
Note: The size of the footing depends on the number and spacing of radiators. Typical footing thickness is
150 mm.
6.6.8 Granular pads
The design thickness of the non-frost-susceptible gravel pad thickness should consider a number of
factors that can impact the thermal characteristics of the gravel pad, including the following:
a) provision for mechanical installations, such as piping, conduits, and water piping, which should be
above the insulation;
b) subexcavation of poor subgrade soils; or
c) timing of construction (e.g., in the summer, it might become necessary to sub-excavate thawed
frost-susceptible soil to mitigate against seasonal thaw effects).
6.6.9 Excavation of a site
The need for excavation at a site should be carefully considered. Excavation should not be undertaken
simply to lower the grade of the building.
Note: There might be cases where subexcavation is warranted. However, excavation in permafrost areas can be
problematic in terms of permafrost disturbance and handling seepage.
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6.6.10 Use of gravel
The following should be considered for the use of gravel:
a) non-frost-susceptible gravel should be used;
b) a 1.5 m minimum thickness should be considered a practical minimum;
c) particles larger than 75 mm should be removed from fill;
Note: It is impractical to place and compact gravel with particles larger than 75 mm in the structural pad of a
thermosyphon foundation. It is common to use gravel with a 20 mm top-size in this application. If this is done,
the requirement for bedding against evaporator pipes and insulation is avoided.
d)
e)
fill supporting slab-on-grade construction should be compacted to at least 98% of standard proctor
maximum dry density. See ASTM D698; and
Fill supporting discrete foundation elements should be compacted to at least 100% of standard
proctor maximum dry density.
6.6.11 Insulation
6.6.11.1 Type of insulation
Rigid extruded polystyrene board insulation intended for subsurface applications should be used because
of its low water absorption characteristics.
Note: Water absorption will result in the degradation of the insulative properties.
6.6.11.2 Insulation properties
The insulation should have the following properties:
Property
Test method
Units
Value
Thermal resistance (typical 5-year
aged R value)
ASTM C518
m2 °C/W
0.87 minimum per
25 mm
Water absorption
ASTM D2842
%
0.7 maximum
6.6.11.3 Design thickness for foundation pad insulation
If the design thickness for insulation within the foundation pad of engineered fill is less than 150 mm,
justification should be provided in the design documentation. Insulation should extend beyond the
perimeter of building by at minimum of 1 m.
Note: Thicknesses ranging from 100 to 200 mm have been used, with 150 mm being a typical thickness.
6.6.11.4 Use of buffer zones
An external buffer zone should be used to limit impacts of edge effects on foundation performance. High
compressive strength non-water absorbent products such as extruded polystyrene should be used in
below grade applications and shall be used when under loaded foundation elements, such as strip and
spread footings. The design should include the insulation layer extending beyond the building perimeter.
Note: Edge effects associated with snow, wind, and solar radiation can be a significant driver on the design.
6.6.11.5 Use of liners
Insulation should be protected with a high density polyethylene (HDPE) liner if there is a possibility of
exposure to hydrocarbon liquid or vapour, as these will degrade insulation.
Note: This is a common consideration in garages or power stations.
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6.6.12 Grading
The following should be considered for grading:
a) final site grading should maintain positive drainage in the direction of natural drainage and should
direct water away from the building;
b) final grades within 3 m of the building should be at least 4%;
c) building embankment side-slopes should be no steeper than 4 horizontal to 1 vertical (4H:1V).
Gravel or landscaped areas beyond this should have a minimum grade of 2%; and
d) future and existing development should be taken into consideration when directing drainage so as
not to divert flow into adjacent developments.
Note: Improper drainage and ponding of water near or under the structure could initiate foundation distress.
6.6.13 Reducing the potential for seepage
The following measures should be implemented to reduce the potential for seepage:
a) runoff from the roof of the building should not be permitted to discharge adjacent to the building;
b) a collection system should be installed to direct the water away and to a discharge point not less
than 2 m away from the building;
c) the grade at the perimeter of the building should be constructed to protect against the infiltration of
runoff, either with relatively impervious soil or some form of membrane, for a width of at least 2 m
out from the building; and
d) there should be a seepage collection system installed at the inside perimeter of any below grade
portion of the building, directed into the storm water or sewer system.
6.6.14 Ancillary design considerations
Vertical undulations in flat loop systems shall be avoided to mitigate the risk of vapour-lock within the
evaporator segment.
6.6.15 Use of sumps and underground utilities
Sumps and underground utilities should be specifically analyzed in the design process. Where services
are installed below grade, they should be thermally isolated to protect the frozen state of the
surrounding fill.
Note: Sumps are often required in or below floors and these introduce a complexity into the design and should not
be added as an afterthought. In general, there are two purposes for sumps:
a) collecting seepage from around and below a buried portion of a building; and
b) collecting water accumulation within a building, such as snow melt from vehicles in garages. This type of sump
might need to be accounted for in design. It should be watertight to prevent the collected water from
migrating into the foundation and subgrade. While the sump might need to extend below the level of the
insulation, the walls and bottom of the sump should be adequately insulated to limit heat loss into the zone
below the insulation.
6.6.16 Water supply and waste water lines
Water supply and wastewater lines shall be installed in the granular layer above the insulation.
Notes:
1) Flat loop thermosyphon systems are most reliable when the entire evaporator system is installed on one plane.
2) Buildings such as garages often require sumps and it can be impractical to confine these to above the
evaporators. As these are typically of limited areal extent, they can be accommodated in design with special
attention to the evaporator and insulation configuration in that localized area.
Consideration should be given to the possibility of installing waterlines in raceways to facilitate access for
future inspection and maintenance.
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Note: A raceway will again be of limited areal extent and should be able to be accommodated in the design with a
specific insulation and evaporator configuration. An additional consideration for raceways is to protect them
against the possibility of seepage entering the area.
6.6.17 Coordination with other ancillary elements of the building design
The design should be coordinated with other ancillary elements of the building design. In locations that
rely on on-site sewage disposal, the design and location of the septic tank and drainfield shall be
considered relative to their potential effects on the performance of the thermosyphons. Coordination
with the designers of the sewage disposal system shall be required.
Note: There are several instances of permafrost thaw below buildings, especially in warm permafrost (T > –2 °C),
caused by sewage disposal systems that are located too close to the building.
7 Construction, installation, and commissioning
7.1 Site preparation
The following should be considered for site preparation:
a) Construction of building elements that are sensitive to differential movements should not
commence until the subgrade is frozen.
Note: This is best accomplished by constructing the thermosyphon system one summer, allowing the
thermosyphons to freeze/cool the subgrade for one winter, or as indicated by analysis or monitoring, then
commence building construction the following year.
b)
In regions of cold permafrost, site preparation should be initiated as early as practical in the
summer. Site clearing and excavation, if required, should be done in late spring, with fill placed,
thermosyphons assembled, and followed by insulation being placed by early summer.
Note: This permits some of the previous winter’s cooling to be retained within the subgrade, reduces the
thermal impact of foundation construction, and permits foundation freeze-back to be expedited the following
fall and winter.
c)
In warm permafrost, excavation should be avoided where practical and construction should be
completed as late in the fall season to take advantage of the upcoming winter.
7.2 System construction
7.2.1 Development of standard operating procedures
System installers shall develop standard operating procedures appropriate for construction of the
thermosyphon system. This includes the following requirements and recommendations:
a) Requirements:
i) Material transportation and storage:
1) All material shall be properly protected, supported through the entire length, and secured
for transportation to the job site.
2) All piping ends shall be protected from damage, and ends of piping sealed.
3) All piping components shall be tagged with identifiers.
ii) Material receipt at jobsite:
1) Damaged material shall not be used. Any damage should be reported immediately and
replacement material of equal specifications/quality obtained.
iii) Jobsite review:
1) Drainage water shall be diverted away from the frozen zone.
iv) Installation of evaporator piping:
1) Maximum undulations shall meet specifications established by the engineer.
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v)
Installation of radiators:
1) Radiators shall be installed plumb.
2) The condensers shall be secured to minimize stresses caused by high winds, storm
conditions, and other outside forces.
Note: As thermosyphons become active, they will freeze the ground and potentially cause frost heave in
the vicinity of the risers. If the radiators become significantly out of plumb, it will stress the pipe and
could compromise pipe integrity. This warrants verification that the thermosyphons remain functional.
b)
Recommendations:
i) Material specifications and material tracking:
1) Material certificates should be kept on file and documented with each project.
2) When receiving material, confirmation should be made that the material received matches
the specifications, are accompanied with material certificates, tagged, logged, and
documented.
3) Material should be assigned to projects with associated documentation.
ii) Material transportation and storage:
1) Loose items should be shipped in secured closed, sealed containers.
2) All thermosyphon system materials should be stored and protected from weather and
installed in the condition described in their specification.
iii) Material receipt at jobsite:
1) Material received should be checked to ensure what was shipped and all material is in
good condition.
2) All components should be clear of debris.
3) Replacement material should be tracked and documented.
iv) Jobsite review:
1) The jobsite should be as described in tender documents.
2) Remedial action should be undertaken to eliminate excessive standing water, to improve
surrounding area drainage patterns to keep water away from the site, and to immediately
remove exposed water during excavation.
v) Installation of evaporator piping:
1) All evaporator piping should be level for a flat loop system or inclined at a specific angle
for a single pipe system.
2) The non-frost-susceptible fill should be compacted to the engineer’s specifications to
prevent settlement.
3) The elevation of the evaporator piping relative to a surveyed benchmark should be
documented and a survey plan prepared showing where all buried evaporators are located
relative to the building perimeter and other significant internal components of the
foundation system.
vi) Installation of ground temperature sensors (cables):
1) Ground temperature sensors should be installed as detailed on the design drawings.
2) All wiring should be connected to the control unit.
3) It should be confirmed that temperature transmitters are calibrated and reading properly
prior to completing the installation.
4) The elevation of the temperature sensors relative to a surveyed benchmark should be
documented.
vii) Initial preparation of the thermosyphon piping system:
1) The system should be evacuated to the level required.
2) All vacuum conditions should be documented for each loop including time at required
vacuum level.
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viii) The system should be charged with the specified refrigerant to the level (pressure) required by
the project’s operating procedure.
ix) The location of the evaporator piping and temperature transmitters relative to the radiator
sections should be recorded. Document these conditions on the drawings and update the files.
x) All electronics and data logging equipment should be installed. All temperature and other data
should be logged and recorded.
xi) The location of all evaporator piping should be staked out prior to installation of the remainder
of the non-frost-susceptible fill.
xii) The remainder of non-frost-susceptible fill should be installed. When installing fill, do not drive
over the evaporator pipes with heavy equipment unless appropriately covered by fill and
approved by the engineer. Compact fill as required by the design drawings. Confirm all
thermosyphons are operational after compaction of cover fill.
xiii) As-left condition should be documented for all installed systems and components.
7.2.2 Welding recommendations and requirements
The welding of pressure vessels shall conform to CSA B51.
Note: In Canada, the provincial and territorial authorities provide oversight for pressure vessel manufacturing. This
authority is normally granted by a pressure vessel act. These acts normally refer to CSA B51. CSA B51 generally
adopts the ASME Boiler and Pressure Vessel Code (BPVC) series of standards for specific construction requirements.
The local authorities may add their own additional requirements to CSA B51 or ASME BPVC.
7.2.3 Record drawings recommendations and requirements
Record drawings and reports shall be prepared and approved (stamped) by the designer. The record
drawings should highlight deviations from design. The report should document quality control activities
that were undertaken [such as pressure/leak testing and vacuum parameters (e.g., pressure and
duration)], charging pressure and verification.
8 Monitoring
8.1 Monitoring plan
A plan should be developed for the project specifying how the performance of the thermosyphon system
will be monitored. The plan should detail how to repair non-functioning thermosyphons by checking for
leaks, recharging, or initiating more complex remediation plans.
Note: A properly functioning thermosyphon system does not require maintenance. A thermosyphon is either
working or it is not. The monitoring plan can confirm that all thermosyphons are working as intended and the
design or predicted thermal conditions are being maintained. If one or more thermosyphons within a system does
not function, it is important to identify those units early. The monitoring program is indirectly linked to building
maintenance and is for the sole purpose of providing background information should a remediation program be
required.
8.2 Implementation and operation of the monitoring program
8.2.1 General
An instrumentation and monitoring program should be implemented during construction and should
operate over the service life of the thermosyphon system.
Notes:
1) If the thermosyphon system is not performing as intended, it is possible to determine this before the
performance issues manifest themselves as foundation distress. Early monitoring of foundation system
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2)
performance is the single most significant opportunity to mitigate risk. If it is not built into the project delivery
contract, it becomes likely that it will not be implemented as envisioned by the designer.
Periodic visual inspection of the radiators can provide clues about performance concerns. If the caps over the
valves have been damaged or removed, this damage could indicate that the seal has been compromised.
8.2.2 Monitoring program
Thermosyphons should be examined at least semi-annually prior to the onset of the freezing season for
evidence of damage or distress. Radiators should be inspected in the winter, either with a thermometer
or with an infrared surface temperature measuring device, to confirm that they are emitting heat relative
to ambient air temperature. A monitoring program should include
a) visual inspections for indications of system deterioration or tampering;
b) surface temperature measurements of the thermosyphon radiators during winter to ensure each
thermosyphon is functioning;
c) ground temperature monitoring prior to construction of the building superstructure to detect when
the subgrade freezes;
d) ground temperature recording at least monthly during construction until the superstructure is in
place;
e) ground temperature monitoring after construction at least semi-annually, with one set of readings
taken when the foundation is warmest (fall);
f) comparing the collected data over time to the values predicted in the design process;
g) verification of building deformations by setting reference points on structural elements for periodic
elevation survey; and
h) verification that radiators remain plumb and that the thermosyphons remain functional.
8.2.3 Thermosyphon operation verification test
The following simple field test steps should be performed at a time the air temperature is colder than
–15 °C to confirm individual thermosyphon system operation:
a) acquire a digital contact thermometer or an infrared surface temperature scanner with a sensitivity
of 0.1 oC;
b) measure the temperature of the radiator (the surface of the pipe) and compare this to the
surrounding air or a white painted metal plate (preferred method) kept adjacent to the riser for this
purpose;
c) take the temperature of the thermosyphon:
i) if the thermosystem is warmer than the air or plate, the thermosyphon is operating; or
ii) if the air temperature is the same, the thermosyphon is not operating;
d) if this test indicates that any of the thermosyphons are not operating, wait a few days and test
again, preferably at a colder outside air temperature;
e) an infrared thermometer or camera, if available, will show that the finned risers will appear warmer
than surrounding air;
f) where ground temperature sensors are installed, examine the available data for unexplained
temperature fluctuation. If temperatures fluctuate beyond the design limits, inform the persons that
have authority; and
g) if repairs are required, contact the design engineer or the supplier.
These tasks should be done in early winter, at least once a year, when the air temperature is 15 to 20 °C
colder than the expected ground temperature at the thermosyphon evaporator pipes.
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8.2.4 Instrumentation for ground temperature monitoring
Ground temperatures shall be monitored by horizontal and vertical ground temperature cables that are
connected to readily accessible data loggers. Instrumentation should target those areas of the foundation
system that are most vulnerable to temperature changes.
8.2.5 Data review and documentation
The monitoring system shall incorporate data review and documentation, initially on an annual basis.
Trigger points should be identified where action is needed. As the system demonstrates that it is robust,
the review period may be increased.
8.2.6 Evidence of building distress as a result of thermosyphon underperformance or
failure
Owners should request expert review from a competent individual of any sign of building distress as a
result of thermosyphon underperformance or failure.
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Annex A (informative)
Background on thermosyphons
Note: This Annex is an informative part of this Standard.
A.1 What is a thermosyphon and what is a thermosyphon foundation
system?
Thermosyphons are a two-phase passive refrigeration devices charged with a working fluid that transfers
heat from the ground to the air when appropriate temperature differentials prevail. They have been used
for foundation stabilization in continuous and discontinuous permafrost areas since the 1960s in Alaska
and since the mid-1970s in Canada. Thermosyphons have been applied to help stabilize permafrostdependent infrastructure ranging from rail lines and mine tailing facilities, to pipelines, tank farms,
roadways, and buildings. The technology has performed reliably, however there are notable cases where
inadequacies in site selection, design, construction or performance monitoring have resulted in
foundation settlement and building distress (Holubec, 2008). This is causing the reliability of the
technology to be questioned, both in present applications and in resiliency to climate change. The review
by Holubec (2008) raised concerns on the performance of thermosyphon stabilized building foundations
and provided recommendations for correcting past issues in design and construction. The development
of this Standard was another recommendation arising from this review.
A thermosyphon foundation system consists of more than the thermosyphon pipes alone. The
components of a thermosyphon system include
a) evaporator pipes below grade;
b) radiator section on top of vertical conductor pipe;
c) rigid insulation; and
d) a layer of non-frost-susceptible gravel, in which the evaporators and insulation are embedded.
The convective heat transfer characteristics of flat-loop thermosyphons are based on empirical
expressions established from laboratory experiments of full-scale horizontal thermosyphons (Haynes and
Zarling, 1988). Subsequently, thermosyphon technology has been described in some detail in Yarmak and
Long (2002).
A typical configuration of the components is shown in Figure A.3. The configuration of the various
components is inter-dependent with other components. For example, the designer may choose to
increase evaporator or radiator capacity in order to be able to reduce the required insulation thickness or
the designer may choose to increase the insulation thickness in order to be able to reduce the required
non-frost-susceptible gravel thickness.
A.2 How do thermosyphons work?
Thermosyphons extract heat from the ground and discharge it into the atmosphere whenever the air
temperature is colder than the ground temperature. A commonly-used gas/fluid medium is carbon
dioxide that functions in a closed “pipe”, pressure vessel, under a pressure varying from about 2100 to
4800 kPa. The thermosyphons enhance heat removal from below the building by liquid-gas phase
change. During the winter, the outside air is colder than the ground temperature causing the gas in the
pipe above the ground to condense and flow as a liquid to the base of the pipe. The warmer ground in
contact with the evaporator drops the pressure in the gas and thereby causes the fluid in the subsurface
pipe to evaporate. Heat is extracted from the ground supporting the building and dissipated to the
atmosphere throughout the winter, or as long as the air temperature is colder than the ground under the
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structure. The components are shown in Figure A.1. During the summer, the cold is preserved in a
permafrost condition by a layer of insulation located above the horizontal evaporator pipes.
Figure A.1
Thermosyphon components and their functions
(See Clause A.2.)
Consendsor (radiator)
sec on
Heat loss to
ambien t cold air
Ground surface
Condensate flow
Vapour flow
Crea on or maintenance
of frozen soil
Evaporator
secon
Heat gain
from ground
Note: This Figure is from CSA PLUS 4011.
A form of heat pipe or thermosyphon with the trade name Cryo Anchors was used for early building
foundations in Canada (Hayley, 1982). Cryo Anchors were manufactured by McDonnell Douglas and were
used extensively during construction for the Alyeska warm oil pipeline across Alaska in the 1970s. They
were manufactured in Canada in the 1970s and 1980s by Mobile Augers and Research Ltd. Cryo Anchors
used ammonia as the heat transfer medium and were manufactured in the shop to design requirements,
then shipped to site charged. The shipping and longevity were found to be problematic as pure ammonia
is an unstable gas that reacts with impurities in the containment system that can adversely affect
performance. This product is no longer available for permafrost foundation stabilization.
Arctic Foundations Canada Inc. is currently the only manufacturer of thermosyphon products in Canada.
Their thermosyphon system design was developed in Alaska by Arctic Foundations Inc. and was originally
known as the Long Thermopile (Long, 1963). They use carbon dioxide in steel tubing as the two-phase
heat transfer medium.
A.3 Types of thermosyphons available
There are four types of thermosyphons available, as shown in Figure A.2, and are described as follows:
a) Three variants of the trade name Thermoprobe are distinguished by having either a “vertical”,
“sloped”, or “flat looped” evaporator section. The vertical, then sloped Thermoprobes were the
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b)
earliest derivations and relied on gravity to circulate the condensate. While the sloped system is still
available, the flat looped system has gained prevalence for building foundation applications in
Canada over the last 20 years. The flat looped system relies on pressure differentials and slug flow
to circulate the vapour and condensate. Thermoprobes are installed for the sole purpose of
stabilizing permafrost by cooling/freezing the ground, but do not directly support building loads. The
structure is then supported on this stabilized permafrost by some other structural element, which
would typically be footings, but could be piles or a granular pad.
As the trade name implies Thermopiles are piles, which incorporate thermosyphon technology. The
radiators would typically be integral with the pipe, within the air space between the ground and the
structure, but could be welded to be external to the pipe. Condensate circulation occurs by gravity
in Thermopiles, with these technologies intended to directly support structural loads. Therefore, the
design process for Thermopiles requires direct consideration of foundation loads.
Figure A.2
Four thermosyphon foundation design options
(See Clause A.3.)
Structure
a) Thermopile
b) Ver!c al thermos yphon
Structure
Structure
c) Sloping thermo syphon
d) Flat looped evaporator thermosyphon
Note: This Figure is modified from CSA PLUS 4011.
This Standard focuses on flat loop and sloped thermosyphons. In Canada, these are currently exclusively
distributed under the trade name Thermoprobes. While Thermopiles were developed for conditions in
Alaska and have an experience base there, they are not commonly used in Canada.
The components of a thermosyphon foundation system are shown in Figures A.3 and A.4. It comprises
evaporator pipes below grade; radiators above ground, on top of the vertical conductor pipe; rigid
insulation; and a layer of non-frost-susceptible gravel, in which the evaporators and the insulation are
embedded.
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Figure A.3
Typical thermosyphon design sketch showing insulation placement and evaporator
pipes in an engineered foundation pad
(See Clauses A.1 and A.3.)
Thermosyphon
radiator
Floor structur e on
shallow founda on
Insulaon
Engineered fill
Natural permafrost
Maximum seasonal
tha w depth
Thermosyphon
evaporator
Note: This Figure is from CSA PLUS 4011.
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Figure A.4
Plan view of a flat loop thermosyphon system
(See Clause A.3.)
Structure
Engineered fill
Original ground
Thermosyphon
radiators
Thermosyphon
evaporators
Insula on
Plan view
Thermosyphons are generally used as a passive device, but they can be configured as a hybrid device,
where a cooling system is operated mechanically through the summer months. This may be used to
expedite the construction process, but is likely not an effective long-term solution.
Experience with use of thermosyphons in Canada has been achieved from other applications, such as
dams on a permafrost foundation and to rehabilitate thawing road and railway embankments. In dams,
the thermosyphons are stabilizing a foundation, but are also creating or maintaining permafrost to limit
seepage through the dam. While many of the principles discussed in this Standard apply, this is a
specialized application of thermosyphons that is outside the scope of this Standard.
June 2014
© 2014 CSA Group
36
CAN/CSA-S500-14
Annex B (informative)
Two-dimensional numerical models for thermal analysis
Note: This Annex is an informative (non-mandatory) part of this Standard.
Examples of models that are commercially available or currently utilized by Canadian engineers for
thermal analysis include, but are not limited to,
a) TEMP/W: a commercial finite element model marketed by Geo-Slope International;
b) SVHEAT: a finite element model marketed commercially by Soil Vision Systems;
c) GEOTHERM: a proprietary finite element model developed and used by EBA Engineering
Consultants; and
d) THERM2: a proprietary finite difference model developed by Nixon Geotech and used by
consultants.
June 2014
© 2014 CSA Group
37

Thermosyphon foundations for buildings in permafrost regions

Transcription

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