deep foundations - The Portland Cement Association

DEEP FOUNDATIONS
Summer 2008
The Magazine of the Deep Foundations Institute
The Vancouver Island Conference Centre
An OPA Special Recognition Award
The CSM was able to carry out work in close proximity to on-site utilities without any damage
Vancouver Island Conference Centre
Cutter Soil Mixer Technology Transforms Site
I
n November 2004, the residents of the City of Nanaimo, B.C.,
Canada voted to approve a new conference centre as the centrepiece of a proposed revitalization project for the city’s historic
downtown core. The proposal goal was to replace a cluster of derelict buildings with a dynamic and modern complex that would
connect existing city facilities to a new conference facility, the
Vancouver Island Conference
Centre (VICC). The conference
centre is expected to create 150
full-time jobs, attract visitors/
Brian Wilson, P.Eng., John
delegates from across North
Scholte, P.Eng., and Megan
America and generate new ecoAtkinson, E.I.T.
nomic opportunities for local
Golder Associates Innovative
businesses.
Applications (GAIA) Inc.
However, the proposed site
British Columbia, Canada
posed a number of geotechni-
AUTHORS:
8 • DEEP FOUNDATIONS • SUMMER 2008
cal and geoenvironmental challenges. As is often the case with
coastal cities, historic growth of the Nanaimo downtown and harbour area was achieved through reclamation of former waterfront
areas and inlets. These areas were often infilled with random fills
and even wastes, leaving a legacy of variable density heterogeneous fills contaminated with a variety of pollutants. Preliminary
assessment of the site indicated a high likelihood of a requirement
for deep foundations, a high risk of liquefaction in the event of an
earthquake and the presence of both soil and groundwater contamination. Construction would be further complicated by the
requirement for two levels of basement parking and the proximity
of adjacent buildings and major roadways to the work area.
With a limited budget (fixed by the referendum) and timeframe, the city needed a cost-effective solution to proceed with
construction. A number of options were considered, but an innovative design using leading-edge Cutter Soil Mixer (CSM) tech-
nology best met the geotechnical and budgetary requirements of
the site. The CSM approach also addressed the environmental
concerns of contaminated soils and groundwater. Golder Associates Innovative Applications (GAIA) Inc., in association with
the designers of their parent company Golder Associates Ltd.
(Golder), proposed the CSM technology — at the time the first
North American application.
Geotechnical Challenges
10 to 30%, however, lateral displacement of up to 4 m was considered possible without appropriate mitigation measures.
The engineers determined that construction of the new conference centre would require transfer of the structural loads to more
competent soil or rock strata at depth as well as the provision of
some form of confinement, or densification of the liquefiable soils,
both within the footprint of, and possibly beyond the footprint of,
the proposed conference centre.
Environmental Challenges
Nanaimo’s downtown area was home to heavy industrial businesses decades before transforming into the retail-residential
hub it is today. Soil samples collected during an environmental
site investigation indicated that in some locations the concentration of inorganic contaminants and hydrocarbons in the site soils
exceeded the acceptable standards set out by B.C. regulation. At
one location, the concentration of inorganics was high enough to
be considered hazardous waste. The contaminants complicated
the choices for the geotechnical construction solutions by introducing additional costs associated with both groundwater and soil
management and/or disposal. The presence of random debris also
increased the difficulties. Based on the results of the analytical
testing, any material excavated at the site would need to be disposed of at a landfill facility appropriate for the type of contaminant, at significant additional costs.
Downtown Nanaimo in 1900s, showing the future site of the
VICC as the original inlet harbour
Sections of historic downtown Nanaimo, including the VICC site,
were built on an area that was originally a narrow inlet once used
as a harbour. Starting as early as the late 1800s, the inlet was filled
with heterogeneous natural and man-made fills, including loose
sandy soils, large boulders, wood and metal debris, broken concrete and brick, blast rock and coal waste. Geotechnical investigation revealed a complex mixture of materials of varying relative
density/consistency with significant lateral and vertical variability.
In general, however, subsurface soils were observed to be very
loose to loose silty sands overlying dense to very-dense till and
sandstone bedrock. The bedrock profile appeared to be consistent
with a U or V-shaped channel over most of the site, with inferred
depths ranging between 1.5 to 14 m and measured bedrock slopes
ranging between 10 and 30%.
Based on a design earthquake with a 425-year return period of
Magnitude 7 and an associated peak firm ground horizontal acceleration of 0.23 g, Golder engineers determined that the upper
silty sands could potentially lose strength during the design event,
and that if laterally confined, anticipated post seismic settlements
could range from 100 mm to 250 mm. Given the bedrock slope of
Finished foundation
Preparation for pouring the building foundations exposed an
intersection of CSM wall
DEEP FOUNDATIONS • SUMMER 2008 • 9
1. CUTTING PHASE
2. SOCKETING INTO BASE LAYER
3. WITHDRAWAL AND MIXING PHASE
Water or Grout
Injection to
Fluidify Soils
DISTURBED SOIL
DISTURBED SOIL
Direction of
Cutting
nomic importance of the project to the community, it was vital that the project be carried out
within the planned time frame and within budget. The adoption of a GAIA’s innovative deep soil
mixing seemed the best construction approach.
Design Build
Direction of
Mixing
Golder determined that the foundation solution
would have to exhibit a number of properties
including: 1) sufficient flexural strength (and tensile
capacity) to withstand the lateral forces imposed by
the liquefied soil; 2) sufficient flexibility to move
with the soil as the vibrations propagate during an
earthquake; 3) sufficient connection to the underlying dense soils and rock to mitigate the potential
Schematic diagram of the cutting wheels at work, on the down and up stroke
for ratcheting downslope during an earthquake;
4) a limited footprint beyond the plan area of the
building; 5) sufficient compressive strength to take
Options Analysis
the applied vertical loads and limit settlement; 6) construction
using a low vibration installation technique; and, 7) minimization
The design-build team of GAIA and Golder worked to provide a
of the need for excavation of existing soils or the removal and
solution that would address both the geotechnical and environtreatment of groundwater.
mental issues at the site and meet the project’s budgetary requireIn assessing the construction options available, GAIA determents. A number of foundation support options were considered,
mined
that Cutter Soil Mixer technology provided the best posincluding stone columns, dynamic compaction, piles, and even
sible alternative, meeting most of the major criterion set by the
complete excavation and replacement of the unsuitable soils.
design team at Golder. CSM, a relatively new approach to the
After studying the options, only the deep soil mixing option could
more conventional Deep Soil Mixing (DSM), is an in-situ ground
address the site constraints effectively, particularly with respect to
modifi
cation technology which incorporates cutter technology,
seismic performance.
historically used for cut-off wall construction, with soil mixIn the case of stone columns, analysis indicated that due to the
ing to develop rectangular columns of soil-cement that can be
silty nature of the soils and the sloping bedrock foundation, that
interlinked and even reinforced to create a series of subterranean
it would be necessary to install columns in very close proximity to
walls. CSM makes use of two sets of cutting wheels that rotate
each other and well beyond the actual building footprint to mitiabout a horizontal axis cutting the soil and mixing at the same
gate large lateral movements. Similarly, the level of effort required
time. By cutting about a horizontal axis, the CSM technology
by dynamic compaction appeared to be excessive, and the resultallows for greater control of the position of the cutting head and
ing vibrations likely unacceptable to the surrounding structures
hence improved QA/QC. The location of the gearbox at the cutand businesses.
ting head further allows the inclusion of instrumentation that
Excavating down to competent ground and replacing all
allows the operator to assess cutting wheel speed, torque, slurry
unsuitable soils with structurally sound and uncontaminated
flow etc., giving the operator improved information on both the
granular fill was discounted largely on the basis of cost. This
variation
in stratigraphy with depth, and also the quality of the
option would result in approximately 50,000 tonnes of material
injection and mixing process required to produce the desired
being relocated to a remediation landfill with expensive transrectangular panels of soil-cement.
portation and disposal fees, not to mention the cost of importaThe design-build team was confident that the CSM could key
tion of clean fill at a significant premium. Similar to the stone
into
bedrock or dense strata at depth, something conventional
column option, this approach would require the excavation to
auger arrangements often struggle with. Based on this confiextend beyond the building footprint and would require extendence, the team developed an innovative design that resists large
sive dewatering and shoring.
lateral ground movements with a cellular structure (grillage) of
A steel pile foundation appeared to offer the best potential
strengthened soil, while still providing vertical support to the
from a design perspective but required the installation of large
building loads. The in-situ structure provides shear resistance to
and costly steel piles socketed sufficiently deep into bedrock to
the lateral forces exerted by the liquefied soils, confining most
withstand the significant lateral force resulting from a liquefied
of those soils within its cells. The soil-cement structure also prosoil mass moving around the piles under the design earthquake
vides vertical foundation support by transferring the load of the
conditions.
building foundations to the underlying competent ground (till
Preliminary pricing indicated that these options would each
or bedrock) without the need for deep excavations. In addition
cost between $5 and $10 million to complete, well beyond the
to the geotechnical solutions, strengthening the existing soils
budgetary constraints of the project. Given the social and ecoGrout
Injection
MIXED CSM COLUMN
BASE LAYER
BASE LAYER
Grout Injection
10 • DEEP FOUNDATIONS • SUMMER 2008
BASE LAYER
allowed them to remain on the site, negating the need for the
expense of removal and disposal at a remediation site. Further,
the reduced permeability of the soil-cement structure after CSM
treatment also offered the advantage of containing existing contaminated groundwater.
Modeling, Monitoring and Performance
Because this was the first North American application of this type
of deep soil mixing technology, the design-build team paid strict
attention to detail at every step of the project, with extensive quality control testing on the final product by GAIA. For design, Golder
engineers carried out detailed analysis, including a soil-structure
deformation analysis that used the finite difference program FLAC
to estimate the anticipated ground movements under the design
earthquake loadings before and after CSM treatment, and SigmaW analysis to look at the anticipated stress distributions within
the soil-cement grillage based on the anticipated loads.
In the field, a calibrated and automated batching system continuously supplied the CSM equipment with the appropriate
design mix cement. QA and QC of the cement was undertaken
on a per batch basis, and instrumentation was installed to ensure
the accurate dosing of cement for each soil-cement panel. Workers collected wet samples at every intersection of the soil-cement
walls, and these were cast into cylinders for subsequent Unconfined Compressive Strength (UCS) testing. The engineers then
cross-referenced wet sample strength results with UCS test results
on core samples taken from 14-day- and 28-day-old soil-cement
panels cored using a mud rotary drill rig. During the process, the
design-build team undertook detailed surveys to ensure accuracy
of the soil-cement wall placement. Extensive geotechnical investigative measures were carried out to confirm the depth of the
dense strata and allow recognition of the cutter wheel “signature”
(torque/wheel speed ratio) that would accurately and consistently
identify the foundation strata. Golder and GAIA also monitored
vibration as work progressed in close proximity to existing structures, particularly the library. The results showed levels within the
adjacent buildings below the threshold for human perception.
Summary
The CSM option presented a solution that addressed all of the
on-site issues at a total cost of $3 million dollars, substantially
less than the other options and within the project budget. The
CSM option selected for construction by the City of Nanaimo
was successfully completed on schedule and on budget in August
2006. Over 2,000 tonnes of cement were used to build the cellular structure that was approximately 150 m long, 40 m wide and
of varying depths. The CSM option reduced the amount of contaminated material that required relocation to a landfill by 85%
while containing the contaminated material that remained on site
and preventing off-site movement of contaminated groundwater.
Cast on-site wet samples and drilled core samples were tested in
independent concrete laboratories, confirming that the strength
specification of 1.5 MPa for the treated soil was met and indicating
that strength gain with time would likely result in ultimate in-situ
strengths in excess of 2 MPa.
SMALL
WONDER.
The little remote control drill rig that goes where others can’t.
FITS UNDER LOW CEILINGS.
GOES EVERYWHERE.
SAFE TO USE.
Only 4.5 ft. wide.
Remote control offers
operators a 360
range of motion
for safe and
accurate use.
o
HUSKY & POWERFUL.
65 HP Diesel engine.
Rig weighs 15,438 lbs.
Telescopic mast shrinks from
15 ft. (max) to 7 ft. (min).
INCREASE PROFIT PER METER.
20,000 ft. lbs. of powerful torque.
DRILL DEEP & FAST.
REMOVABLE
POWERPACK.
Removable power
pack for exhaust-free
use in enclosed areas.
RUBBER TRACKS.
Won’t damage
road services.
Complete impossible projects on hillsides, unstable terrain and tight
places with the versatile TesCar COMPACT. It’s a must-have addition
to your arsenal. In-stock now for sale or rent. Available only from
Hennessy International. Log on or call 800.656.6766 now.
45 ft. maximum drill depth.
LARGE DRILLING DIAMETER.
Max: 24” / Min: 14”
Very affordable and
available only from
Hennessy International
800.656.6766
hennessyinternational.com
DEEP FOUNDATIONS • SUMMER 2008 • 11
ITUTE
EP F O U
ST
DE
N
N
TIONS
DA
I
Deep Foundations
Institute
326 Lafayette Avenue
Hawthorne, NJ
07506 USA
973.423.4030
FAX 973.423.4031
Cutter Soil Mixer Technology
Debuts at Vancouver Island Site
PRESORTED STANDARD
U.S. POSTAGE PAID
FOLCROFT, PA
PERMIT NO. 100