construction of a seepage cut-off wall - Geo-Con

CONSTRUCTION OF A SEEPAGE CUT-OFF WALL
USING DEEP SOIL MIXING AND JET GROUTING
Pacific Place Soils Remediation Project
Vancouver, B.C.
Submitted to
Consulting Engineers of British Columbia
for
1993 Awards of Engineering Excellence Competition
by
Golder Associates Ltd., Vancouver, B.C.
Sandwell Inc., Vancouver, B.C.
Geo-Con Inc., Pittsburgh, PA
for
B.C. Environment, Lands and Parks
FEBRUARY 1993
EXECUTIVE SUMMARY
As part of the overall environmental controls required for the Pacific Place site
(former EXPO ’86 site), B.C. Environment Lands and Parks constructed a
groundwater control system using deep cut-off walls as part of their commitment
to protection of the environment and their contractual responsibility as former
owners of the lands. The construction of the groundwater cut-off barrier walls
through deep, highly contaminated and heterogeneous fill material in the Pacific
Place site was completed using deep soil mixing and jet-grouting construction
techniques. The work was completed during the summer of 1992 within the
construction window to meet project commitments. The frequent inclusions of
large objects and obstructions in the fill materials inhibited construction and were
dealt with in a pragmatic and cost effective manner. The highly variable nature of
the fill materials and their chemical composition and site constraints imposed
significant challenges for the construction of the barrier wall and is believed to be
a somewhat unique application in Canada. These environmental control
measures have substantially reduced the discharge of dissolved contaminants in
groundwater entering False Creek and fulfill the objectives of the approved
remedial plan for site.
1.0 INTRODUCTION
The former Expo ’86 property in downtown Vancouver comprises 83 hectares of
contagious lots bordering the north shore of False Creek, as illustrated in Figure
1. After Expo ’86 this provincially owned property was tendered for sale on the
international market and purchased by Concord Pacific Developments Ltd. in
May 1988 for development as mixed residential and commercial land uses. The
property – now referred to as Pacific Place – had a long history of industrial
activity dating aback to the late 1800’s when Vancouver was first developed.
Much of the site is reclaimed land formed by filling into False Creek. The origin
and quality of the fill material was highly variable. Former industrial activities on
the site included coal gasification plants, hydrocarbon-handling facilities, sawmills
and wood preservative operations, and blacksmith shops/ironworks plants.
These industrial operations left a variety of chemicals and refuse materials in the
ground at locations now being exposed during site development. An area of
particular concern, located at the east end of the Pacific Place site, is an area
referred to in the development as Parcel 6, 7, 8 and 9, (shown on Figure 1). The
area in the southern part of Parcel 9 included a coal gasification plant which
produced coal tar and other products from the coal gasification industry. The
extensive discharge of coal gasification products to the ground surface ahs
resulted in extensive contamination to depths exceeding 10 meters over parts of
the Parcel 9 site. It is this eastern portion of the Pacific Place site which is the
subject of the remedial actions discussed in this paper.
As part of the sale agreement for the property, the province accepted the
responsibility to remediate the site to meet the objectives of protecting human
health and the environment. Shortly after the property transfer, the British
Columbia Ministry of Environment, Lands and Parks adopted standards for the
clean-up of contaminated lands which are based on two approaches; a) a
numerical criteria approach which sets a concentration limit for a variety of
chemicals that are considered to be protective of human health, and b) a risk
assessment/risk management (RA/RM) approach which involves the evaluation
of the sources of contamination, pathways or the exposure of human and animal
receptors and an evaluation of the toxilogical effects of these exposures on
health and the environment.
An evaluation of remedial alternatives for Parcel 9 led to the conclusion that inplace management of wastes using control measures was the most cost effective
and suitable for the project. Thus, the RA/RM approach was applied and a
baseline risk assessment was conducted to assess the control measures
required. Since groundwater discharging into False Creek was a major pathway
of concern, a surface liner was designed to limit infiltration into part of the site
and a groundwater collection and groundwater control system using vertical cutoff walls, or barrier walls, was selected to meet the required environmental
controls.
The results of initial groundwater modeling at the site indicated that over 90% of
groundwater discharging to False Creek could be controlled by the construction
of low permeability cut-off walls (< 1 x 106cm/s) and with internal groundwater
controls. The engineering of the barrier wall, which was completed within the
year of 1992, is the subject of this paper.
2.0
SOIL CONDITIONS AND EVALUATION OF BARRIER WALL TYPES
2.1 Soil Conditions
Based on subsurface investigations, the general site stratigraphy was found to
comprise four layers of soil: upper random fill, marine clayey silt/silty clay, silty
sand (beach deposits), and glacial till as show in the schematic cross section
given in Figure 2. The glacial till was encountered at relatively shallow depths of
about 3 m below the ground surface at the northern part of the site at depths of
up to 17 m along the barrier wall alignment. Both the fill and the clayey silt/silty
clay layer generally thicken towards the False Creek.
The glacial till was found to be generally the least permeable soil layer with
hydraulic conductivity ranging from 2 x 10-7 to 2 x 10-6 cm/s. The thin silty sand
layer above the till has hydraulic conductivity values ranging from 7 x 10-5 to 3 x
10-4 cm/s. Overlying this layer is the marine clayey silt/silty clay layer covering
almost two-thirds of the site and having hydraulic conductivity values between
3 x 10-6 and 4 x 10-5 cm/s. The uppermost fill layer is heterogeneous and its
hydraulic conductivity range between 2 x 10-6 and 1 cm/s. the zones of higher
hydraulic conductivity reflect fill consisting of predominantly wood waste.
The ground surface at the site is generally flat and varies between Geodetic
Elevation +3.0 m to +4.0 m. The ‘average” pre-development water table
contours for the site is presented in Figure 3. Ground water originating form
northeast flows towards the southwest through Parcel 7 and then flows directly
south through Parcel 6 to False Creek. Over the normal year, the changes in the
water table have been found to be small with maximum fluctuations of about
0.5m.
2.2 Evaluation of Alternative Barrier Wall Types
The barrier wall design consisted of seven segments in the alignment as shown
in Figure 4. Alternative barrier wall types for each segment were evaluated
relative to the performance requirements, subsurface conditions, site constraints,
and relative cost. The types of barrier wall considered were:
•
sheet pile walls;
•
soil-bentonite, cement-bentonite and plastic concrete walls installed by the
slurry trench method;
•
soil-cement-bentonite walls installed by the deep soil mixing method;
•
soil-cement-bentonite walls installed by the jet grouting method; and
•
soil-cement-bentonite and asphaltic barrier walls installed by the vibrated
beam technique.
The performance requirements were a barrier wall permeability of 1 x 10-6 cm/sec
or less and a design life of 50 years. In addition, several site constraints were
considered. The barrier wall Segments 1, 3, and 5 were required to accommodate
future adjacent excavations. In Segment 1, the adjacent excavations of up to 3m
below underlying glacial drift was required. The selection of wall types for
Segments 2 and 4 was influenced by the low headroom requirements of Dunsmuir
and Georgia traffic viaducts, and BC Transit ALRT guideway. In addition, the
required alignments of these segments crossed a significant number of active
utilities such as City of Vancouver sewers, BC Hydro high voltage transmission
ducts, and BC Tel communication lines. Segments 6 and 7, being parallel and in
close proximity to the False Creek shoreline, demanded the installation of the wall
while maintaining adequate shoreline slope stability.
With the exception of the sheet pile wall, all of the above barrier wall types could
be used to meet the permeability requirement. The sheet pile wall could be made
to meet the permeability requirement by excavation of the wall and sealing of its
joints. Slurry barrier walls, although able to meet the permeability requirement and
suitable for installation through the random site fills, have the disadvantage of
requiring disposal of large amounts of contaminated soil which would add
considerably to use the barrier wall cost. Also along the shoreline slope
conventional slurry trenches would have to be installed in short panels and a
cement-bentonite or plastic concrete wall material would be required in order to
limit lateral thrust due to the slurry and semi-fluid backfill and maintain shore
stability. Barrier walls installed by the vibrated beam method were ruled out due to
the presence of debris in the site fills which would make formation of the wall by
this method difficult and unreliable at best. Deep soil mixing and jet grouting were
judged to be practical and would involve less waste soil generation than slurry
trench barrier walls. The jet grout barrier wall, although more expensive than the
deep soil mixing wall, has the advantage of low headroom requirement for
installation and the ability to install barrier walls below shallow services by inclined
drilling.
The selection of barrier wall types for each of the above segments are given in
Table 1.
TABLE 1
Barrier Wall Type
Segment
Reasons
And Length
• Rapid installation to meet tight
schedule possible due to local
Sheet Pile
1
availability of necessary equipment
120m
• Ability to support adjacent excavation
• Later excavation would allow sealing
• Ability to install wall between and
Jet Grouting
beneath buried services
2
• Low headroom requirement
100m
• Low waste soil generation
• Adequate headroom
Deep Soil Mixing with
• Low waste soil generation
3
Reinforcement
• Adjacent future excavation
35m
• Low cost compared to jet-grouting
• Low headroom requirement
Jet Grouting
4
• Service crossings
35m
• Low waste soil generation
• Adequate headroom
Deep Soil Mixing with
• Adjacent future excavation
5
Reinforcement
• Low waste soil generation
130m
• Low cost compared to jet-grouting
Deep Soil Mixing
• Adequate headroom
except jet grouting at
• Low cost compared to jet-grouting
6 and 7
west end of Segment 7 • Low waste soil generation
400m
• Low impact on shoreline slope stability
3.0 BARRIER WALL SEGMENTS 2, 6, AND 7
The results of groundwater modeling indicated that the target cut-off levels in
groundwater discharge to False Creek could be effectively achieved by the
installation of barrier wall Segments 1, 2, 6, and 7 combines with a groundwater
collection system and a low permeability surface cap over Parcel 9. The
installation of the Segment 1 sheet pile barrier wall was completed during the
latter part of 1990. The work related to the surface cap (HDPE liner) in the
northern portion of Parcel 9, which began in 1991, is still ongoing. The
construction of barrier wall Segments 2, 6, and 7, which was accomplished during
the summer of 1992, involved the use of modern and highly specialized jet
grouting and deep soil mixing techniques. The following text focuses on the
construction work of the above Segments 2, 6, and 7 of the barrier wall which
could be considered as the most challenging construction phases of the Pacific
Place soils remediation project during 1992.
The design and construction inspection of the barrier wall project was carried out
by Golder Associates Ltd., Vancouver, B.C. as members of the Soils Remediation
Group. Sandwell Inc. of Vancouver, B.C. acted as the Construction Manage for
the work and was responsible for the contract management aspects of the project.
The prime contract for the construction of wall Segments 2, 6, and 7 was awarded
to Geo-Con Inc., Pittsburgh, Pennsylvania, U.S.A., a specialist contractor in jet
grouting and deep soil mixing. Wherever possible, work was awarded to local
sub-contractors.
3.1 Construction
Segment 2 of the barrier wall was completed using jet grout construction
techniques; whereas, Segments 6 and 7 were constructed using Deep Soil Mixing
(DSM) techniques. In zones where obstructions or services prevented DSM
augering, jet grouting was used. The scope of the construction work for
Segments 2, 6, and 7 consisted of the following main phases:
(a)
Grout mix design
(b)
Installation of a jet grout test wall
(c)
Installation of deep soil mixing test wall
(d)
Installation and field and laboratory performance testing of
Segment 2
barrier wall
(e)
Installations and field and laboratory performance testing of barrier wall
Segment 6 and 7
The contract specifications required that the grout-soil mix used in the
barrier wall construction meet the following criteria:
(a)
unconfined compressive strength of solidified groutsoil mix >0.35 MPa
(b)
permeability of solidified grout-soil mix < 1 x 10-6cm/s
(c)
bentonite content in grout-soil mix > 2%
(d)
grout-soil mix to be compatible with ground water and contaminants
(e)
grout to be workable and placeable
Initially, a laboratory test program was carried out on test mixes of in-situ
soils and grout compounded in different proportions to determine the most
suitable construction grout mix for the Pacific Place site. The test program
included the compressive strength, hydraulic conductivity, and immersion
testing of solidified soil-grout mix. Based on the results of the test
program, a mixture of Portland Cement, fly ash, gypsum, bentonite, in-situ
soil, and water, making up 4.6, 1.6, 3.3, 2.0, 72.5, and 16 percent by
weight respectively, was selected as the design mix for the construction.
3.1.1 Methods of Wall Construction
Deep soil mixing (DSM)
The Deep /soil Mixing (DSM) system of Geo-Con Inc. uses a set of four
hydraulically driven augers with mixing paddles on crane supported set of leads.
Each auger is 0.91 m in diameter and the augers are spaced horizontally at 0.69
m to provide overlap between adjacent augers. As the augers are advanced
through the soil, the grout is pumped through the auger shafts. The augering
breaks the soil at the auger shaft locations and lifts it to the mixing paddles which
blend the soil with the grout. Each stroke of the DSM equipment produces four
overlapping columns of soil-grout mix forming a 2.06 m long panel of the barrier
wall between the outer augers. The DSM strokes along the wall alignment are
arranged such that one full column overlap is provided between adjacent strokes
in order to ensure formation of a continuous soil-grout wall.
Jet Grouting
Jet grouting on this project was carried out by initially drilling, using a tricone bit
and water, to the required wall depth and then introducing the grout mix under
high pressure (about 400 bars) at a flow rate of about 500 litres/min. through small
orifices near the tip of the jet grout nozzle as the drilling rods are withdrawn. The
jet grout nozzle has a total of 14 orifices arranged such that a 0.91 m length of a
column is formed when grouting is carried out by rotating the nozzle while holding
it at a specific depth. The nozzle was rotated at a speed of
about 2 rpm as the grout jets cut and mix the native soil with the grout. In order to
install one 0.91 m lift of jet grout column in areas of normal soil conditions at
Pacific Place site, jet grouting was carried out over a period of 1 minute at the
level of that lift.
3.1.2 Segment 2
Initially, a jet grout test wall along the Segment 2 wall alignment was constructed
in order to verify the applicability of the jet grouting method to the Pacific Place
site. laboratory strength and permeability testing on core samples obtained from
the solidified wall, in-situ recovery testing, and finally, visual inspection of the test
wall (after exposing by excavation) was carried out as a part of this program. It
was found that the jet grout test wall generally met the specified criteria except in
heavy debris zones such as tin sheet and brick zones that were encountered in
the upper fills of Segment 2. It was decided that in those zones where debris is
encountered or inferred by higher resistance to drilling, jet grouting over a double
the normal period of time (“double-cutting”) should be carried out.
Upon completion of the test wall program, the main construction work of Segment
2 jet grout wall was carried out. During installation of each jet grout columns the
drilling process was closely, monitored and possible zones of minor and major
zones of obstructions were identified for “double-cutting”. All underground
services crossing the wall alignment were first exposed by careful subexcavation
and the construction at these locations was completed by jet grout construction
from either side of the services to the required depth. Adjacent Dunsmuir and
Georgia viaduct and ALRT foundations, pile caps were periodically surveyed to
monitor potential movements during jet grout construction in vicinity. In order to
achieve a proper seal and continuity of the seepage cut-off between the Segment
1 sheet pile wall and the Segment 2 jet grout wall, a 1.5 m long jet grout overlap
was installed at the junction of these two segment.
3.2.3 Segments 6 and7
Following a similar approach to that of Segment 2, a test section, rectangular in
plan, was first installed using the deep soil mixing (DSM) method. During the
initial construction stages of the DSM test wall, a larger number of obstructions
than anticipated were encountered. Therefore, the test wall alignment had to be
pre-excavated in order to remove potential obstructions. Obstructions including
concrete pieces, steel bars, woodpiles, old foundations etc. were removed during
this operation. The DSM test installation was then checked by in-situ permeability
testing in a test well located within the rectangular area enclosed by the test wall.
Test results indicated that the construction of the DSM test wall was not
successful where the obstructions prevented the penetration of the augers.
These zones were then repaired by drilling through the existing DSM columns and
jet grouting the underlying ungrouted zones with a 1m overlap into the DSM
columns above. In-situ tests conducted after jet grout repair indicated that test
wall construction conformed to the specifications with estimated permeability
values less than 1 x 10-6cm/s. Moreover, this test program also indicated that the
repair of an ungrouted zone below DSM construction could be effectively
achieved by subsequent jet grouting.
Based on the experience during test wall construction, it was evident that there
was a potential for encountering more obstructions, which would inhibit
construction of a significant portion of the Segment 6 and 7 wall by the DSM
method. In such occurrences, the uncompleted areas of the DSM wall in areas
where obstructions were too deep for removal by excavation would have to be
completed by jet grouting which is both expensive and time consuming.
Therefore, it was decided that removal of obstructions prior to deep soil mixing
would be desirable.
After considering several alternative options, the removal of obstructions was
carried out by pretrenching of the wall alignment under a bentonitic slurry using a
Caterpillar 245 excavator equipped with a long stick. One of the main concerns
that had to be addressed during pretrenching operations was the stability of the
adjacent False Creek shoreline. Design calculations indicated that in order to
maintain the factor of safety against shoreline failure at an acceptable level, the
hydrostatic thrust from the slurry trench on the shoreline had to be kept within
certain limiting values. This requirement warranted the careful control of the
maximum depth and panel length of slurry trench during construction. In addition,
due to the possible presence of highly permeable fill zones and/or unidentified
buried services close to the shoreline, there was a potential for an accidental loss
of slurry to the False Creek during pretrenching. Therefore, an Environmental
Contingency Plan including a monitoring program and a contingency action plan
was developed for rapid implementation in the event of such accidents.
In most of the zones, pretrenching for obstruction removal was carried out to a
depth of at least 1 m into the underlying clayey silt unit which is the design base
level of the wall. the trench was then backfilled with screened excavation spoil in
preparation for wall construction using deep soil mixing. Upon completion of
pretrenching, the installation of barrier wall Segments 6 and 7 mainly was first
carried out using the DSM method. In some areas, the wall could not be installed
to its full depth by the DSM method due to obstructions and the presence of
buried services. The wall within these zones was completed by jet grouting.
3.2 Budget Cost and Schedule
The initial engineers’ budget for Segments 2-6 of the barrier wall was $3,650,000.
Tenders for the work wee sought from across North America and three bids were
received ranging in value from $3,151,800 to $7,599,100. The reconfiguration of
the barrier wall to Segments 2, 2a, 6, 7, and 7a, produced a contract value of
$2,244,630. Obstructions in Segment 2, pretrenching for obstruction removal and
jet grout repair of DSM wall in Segments 6 and 7 with other extra work resulted in
final cost of $2,759,601. Costs for the jet grout wall production were in the range
of $400/m2 and for Deep Soil Mixing $200/m2. The
total cost of the project including design and supervision and related construction
activities and their costs resulted in a composite $800/m2 cost for the wall.
The barrier wall construction contract was started on March 09, 1992 and
completed on August 26, 1992.
Concurrent site activities such as the Dragon Boat Festival, the adjacent
construction of Carrall Street and Pacific Boulevard South, and the preparation for
the Molson Indy Race had to be accommodated during the construction period.
4.0
CLOSURE
The construction of a groundwater cut-of f
barrier
wall
through
deep
heterogeneous fill material in the Pacific Place site using deep soil mixing and jetgrouting construction techniques was completed within the construction schedule
to meet project commitments. The frequent inclusions of large objects
and obstructions in the fill materials which inhibited construction were dealt with in
a pragmatic and cost effective manner. The highly variable nature of the fill
materials and their chemical composition and site constraints imposed new
challenges for the construction of such a barrier wall and is believed to be unique
in application. The completion of the seepage control barrier forms part of the
overall environmental controls implemented on the Pacific Place site by B.C.
Environment as part of their commitment to protection of the Environment and
their contractual responsibility as former owners of the lands. The works have
now been completed and the groundwater collection system ahs been installed
with treatment of groundwater prior to discharge. A permanent groundwater
treatment facility is currently being designed and will be construction during the
1993 fiscal year. These environmental control measures have substantially
reduced the discharge of dissolved contaminants in groundwater entering False
Creek and fulfill the objectives of the approved remedial plan for site.
ACKNOWLEDGEMENTS
The authors wish to acknowledge the support of several members of B.C.
environment during the planning and implementation phases of this project,
including Mr. Barry Olson, Dr. John Weins, Dr. John Ward, Dr. John O’Riordon,
Mr. Roger Ord and Mr. Bob Ferguson. We would also like to acknowledge Mr. Bill
Mottershead, BCE Pacific Place Project Manager, for the opportunity to present
this paper.