Summer 2008 The Magazine of the Deep Foundations InstituteDEEP FOUNDATIONSDEEP FOUNDATIONS

The Vancouver Island Conference Centre An OPA Special Recognition Award

8 • DEEP FOUNDATIONS • SUMMER 2008

cal and geoenvironmental challenges. As is often the case with coastal cities, historic growth of the Nanaimo downtown and har-bour area was achieved through reclamation of former waterfront areas and inlets. These areas were often infi lled with random fi lls and even wastes, leaving a legacy of variable density heteroge-neous fi lls 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 con-tamination. 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 (fi xed by the referendum) and time-frame, the city needed a cost-effective solution to proceed with construction. A number of options were considered, but an inno-vative design using leading-edge Cutter Soil Mixer (CSM) tech-

AUTHORS:Brian Wilson, P.Eng., John Scholte, P.Eng., and Megan Atkinson, E.I.T.

Golder Associates Innovative Applications (GAIA) Inc.

British Columbia, Canada

In November 2004, the residents of the City of Nanaimo, B.C., Canada voted to approve a new conference centre as the cen-

trepiece of a proposed revitalization project for the city’s historic downtown core. The proposal goal was to replace a cluster of der-elict 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/delegates from across North America and generate new eco-nomic opportunities for local businesses.

However, the proposed site posed a number of geotechni-

Vancouver Island Conference CentreCutter Soil Mixer Technology Transforms Site

The CSM was able to carry out work in close proximity to on-site utilities without any damage

DEEP FOUNDATIONS • SUMMER 2008 • 9

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 Asso-ciates 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 fi rst North American application.

Geotechnical Challenges

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 fi lled with heterogeneous natural and man-made fi lls, including loose sandy soils, large boulders, wood and metal debris, broken con-crete and brick, blast rock and coal waste. Geotechnical investiga-tion revealed a complex mixture of materials of varying relative density/consistency with signifi cant 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 profi le 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 fi rm ground horizontal accel-eration of 0.23 g, Golder engineers determined that the upper silty sands could potentially lose strength during the design event, and that if laterally confi ned, anticipated post seismic settlements could range from 100 mm to 250 mm. Given the bedrock slope of

10 to 30%, however, lateral displacement of up to 4 m was con-sidered possible without appropriate mitigation measures.

The engineers determined that construction of the new confer-ence 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 confi nement, or densifi cation of the liquefi able 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 busi-nesses 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 concentra-tion 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 intro-ducing additional costs associated with both groundwater and soil management and/or disposal. The presence of random debris also increased the diffi culties. Based on the results of the analytical testing, any material excavated at the site would need to be dis-posed of at a landfi ll facility appropriate for the type of contami-nant, at signifi cant additional costs.

Downtown Nanaimo in 1900s, showing the future site of the VICC as the original inlet harbour

Preparation for pouring the building foundations exposed an intersection of CSM wall

Finished foundation

10 • DEEP FOUNDATIONS • SUMMER 2008

nomic importance of the project to the commu-nity, it was vital that the project be carried out within the planned time frame and within bud-get. The adoption of a GAIA’s innovative deep soil mixing seemed the best construction approach.

Design Build Golder determined that the foundation solution would have to exhibit a number of properties including: 1) suffi cient fl exural strength (and tensile capacity) to withstand the lateral forces imposed by the liquefi ed soil; 2) suffi cient fl exibility to move with the soil as the vibrations propagate during an earthquake; 3) suffi cient connection to the under-lying dense soils and rock to mitigate the potential for ratcheting downslope during an earthquake; 4) a limited footprint beyond the plan area of the building; 5) suffi cient compressive strength to take

the applied vertical loads and limit settlement; 6) construction using a low vibration installation technique; and, 7) minimization of the need for excavation of existing soils or the removal and treatment of groundwater.

In assessing the construction options available, GAIA deter-mined that Cutter Soil Mixer technology provided the best pos-sible alternative, meeting most of the major criterion set by the design team at Golder. CSM, a relatively new approach to the more conventional Deep Soil Mixing (DSM), is an in-situ ground modifi cation technology which incorporates cutter technology, historically used for cut-off wall construction, with soil mix-ing to develop rectangular columns of soil-cement that can be interlinked and even reinforced to create a series of subterranean walls. CSM makes use of two sets of cutting wheels that rotate about a horizontal axis cutting the soil and mixing at the same time. By cutting about a horizontal axis, the CSM technology allows for greater control of the position of the cutting head and hence improved QA/QC. The location of the gearbox at the cut-ting head further allows the inclusion of instrumentation that allows the operator to assess cutting wheel speed, torque, slurry fl ow etc., giving the operator improved information on both the variation in stratigraphy with depth, and also the quality of the injection and mixing process required to produce the desired rectangular panels of soil-cement.

The design-build team was confi dent that the CSM could key into bedrock or dense strata at depth, something conventional auger arrangements often struggle with. Based on this confi -dence, the team developed an innovative design that resists large lateral ground movements with a cellular structure (grillage) of strengthened soil, while still providing vertical support to the building loads. The in-situ structure provides shear resistance to the lateral forces exerted by the liquefi ed soils, confi ning most of those soils within its cells. The soil-cement structure also pro-vides vertical foundation support by transferring the load of the building foundations to the underlying competent ground (till or bedrock) without the need for deep excavations. In addition to the geotechnical solutions, strengthening the existing soils

Water or Grout Injection to

Fluidify Soils

BASE LAYER

Direction of Cutting

1. CUTTING PHASE 2. SOCKETING INTO BASE LAYER 3. WITHDRAWAL AND MIXING PHASE

DISTU

RB

ED SO

IL

BASE LAYER

Grout Injection

DISTU

RB

ED SO

IL

BASE LAYER

Grout Injection

MIX

ED C

SM C

OLU

MN

Direction of Mixing

Options Analysis The design-build team of GAIA and Golder worked to provide a solution that would address both the geotechnical and environ-mental issues at the site and meet the project’s budgetary require-ments. A number of foundation support options were considered, including stone columns, dynamic compaction, piles, and even complete excavation and replacement of the unsuitable soils. After studying the options, only the deep soil mixing option could address the site constraints effectively, particularly with respect to seismic performance.

In the case of stone columns, analysis indicated that due to the silty nature of the soils and the sloping bedrock foundation, that it would be necessary to install columns in very close proximity to each other and well beyond the actual building footprint to miti-gate large lateral movements. Similarly, the level of effort required by dynamic compaction appeared to be excessive, and the result-ing vibrations likely unacceptable to the surrounding structures and businesses.

Excavating down to competent ground and replacing all unsuitable soils with structurally sound and uncontaminated granular fi ll was discounted largely on the basis of cost. This option would result in approximately 50,000 tonnes of material being relocated to a remediation landfi ll with expensive trans-portation and disposal fees, not to mention the cost of importa-tion of clean fi ll at a signifi cant premium. Similar to the stone column option, this approach would require the excavation to extend beyond the building footprint and would require exten-sive dewatering and shoring.

A steel pile foundation appeared to offer the best potential from a design perspective but required the installation of large and costly steel piles socketed suffi ciently deep into bedrock to withstand the signifi cant lateral force resulting from a liquefi ed soil mass moving around the piles under the design earthquake conditions.

Preliminary pricing indicated that these options would each cost between $5 and $10 million to complete, well beyond the budgetary constraints of the project. Given the social and eco-

Schematic diagram of the cutting wheels at work, on the down and up stroke

DEEP FOUNDATIONS • SUMMER 2008 • 11

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 con-taminated groundwater.

Modeling, Monitoring and Performance Because this was the fi rst 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 qual-ity control testing on the fi nal product by GAIA. For design, Golder engineers carried out detailed analysis, including a soil-structure deformation analysis that used the fi nite difference program FLAC to estimate the anticipated ground movements under the design earthquake loadings before and after CSM treatment, and Sigma-W analysis to look at the anticipated stress distributions within the soil-cement grillage based on the anticipated loads.

In the fi eld, a calibrated and automated batching system con-tinuously 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. Work-ers collected wet samples at every intersection of the soil-cement walls, and these were cast into cylinders for subsequent Uncon-fi ned 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 inves-tigative measures were carried out to confi rm 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 struc-tures, particularly the library. The results showed levels within the adjacent buildings below the threshold for human perception.

SummaryThe 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 cel-lular structure that was approximately 150 m long, 40 m wide and of varying depths. The CSM option reduced the amount of con-taminated material that required relocation to a landfi ll 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, confi rming that the strength specifi cation 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.

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FOLCROFT, PAPERMIT NO. 100

Cutter Soil Mixer Technology Debuts at Vancouver Island Site