CONSTRUCTION OF A SEEPAGE CUT-OFF WALL USING DEEP SOIL MIXING AND JET GROUTING

Transcription

1 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

2 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.

3 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.

4 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 10 6 cm/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

5 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

6 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 Segment and 7 Barrier Wall Type And Length Sheet Pile 120m Jet Grouting 100m Deep Soil Mixing with Reinforcement 35m Jet Grouting 35m Deep Soil Mixing with Reinforcement 130m Deep Soil Mixing except jet grouting at west end of Segment 7 400m Reasons Rapid installation to meet tight schedule possible due to local availability of necessary equipment Ability to support adjacent excavation Later excavation would allow sealing Ability to install wall between and beneath buried services Low headroom requirement Low waste soil generation Adequate headroom Low waste soil generation Adjacent future excavation Low cost compared to jet-grouting Low headroom requirement Service crossings Low waste soil generation Adequate headroom Adjacent future excavation Low waste soil generation Low cost compared to jet-grouting Adequate headroom Low cost compared to jet-grouting Low waste soil generation 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 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

7 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 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) (b) (c) Grout mix design Installation of a jet grout test wall Installation of deep soil mixing test wall (d) Installation and field and laboratory performance testing of barrier wall Segment 2 (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) (b) unconfined compressive strength of solidified groutsoil mix >0.35 MPa permeability of solidified grout-soil mix < 1 x 10-6 cm/s

8 (c) bentonite content in grout-soil mix > 2% (d) (e) grout-soil mix to be compatible with ground water and contaminants 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 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.

9 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 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 10-6 cm/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.

10 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/m 2 and for Deep Soil Mixing $200/m 2. The

11 total cost of the project including design and supervision and related construction activities and their costs resulted in a composite $800/m 2 cost for the wall. The barrier wall construction contract was started on March 09, 1992 and completed on August 26, 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.

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