ASCE Geotechnical Division Specialty Conference Grouting, Soil Improvement and Geosynthetics New Orleans, Louisiana. February 25-28, 1992

Transcription

1 ASCE Geotechnical Division Specialty Conference Grouting, Soil Improvement and Geosynthetics New Orleans, Louisiana February 25-28, 1992 Shallow Soil Mixing A Case History David Broomhead 1, P. Eng., and Brian H. Jasperse, P.E. 2, M. ASCE Abstract Shallow Soil Mixing (SSM) is a sister technology to Deep Soil Mixing (DSM) developed to more economically improve soils within ten meters of the surface. SSM was recently utilized to provide a foundation for large effluent storage tanks as well as to contain foundation soils in the event of liquefaction from an earthquake. Two large storage tanks were required to be built in a pulp mill. The only area available which suited the overall building program was land reclaimed from the Strait of Georgia by end dumping silt and fine sand out into water. The fill was not compacted enough to adequately support the tank and was subject to liquefaction. The geotechnical engineers considered a number of ground improvement and foundation support options. SSM was selected to provide optimal tank support under both static and seismic loading in relation to its cost. The project yielded successful results. The columns achieved the proper strength, the schedule was shortened, and the solution cost less. This was the first use ever of large diameter soil mixed columns for foundations. 1 Senior Geotechnical Engineer, Klohn Leonoff Ltd., Shellbridge Way, Richmond, BC, Canada V6X2W7 2 Vice President of Construction, Geo-Con, Inc. 1

2 Introduction The process of soil mixing originated in the United States in the 1950 s, but is major development occurred over the last twenty years in Japan. Several Japanese companies have developed different types of soil mixing methods and built large geotechnical construction divisions of their businesses based on the use of these processes. To date, there have been thousands of projects performed in Japan using some form of soil mixing. Deep Soil Mixing Five years ago, the process was re-introduced to the United States as Deep Soil Mixing (DSM). DSM is a relatively simple process involving standard construction equipment rearranged for the process. The equipment is a cranesupported set of leads that guide a series of one to four hydraulically driven mixing augers 450 to 900 mm in diameter. As penetration occurs, a bentonite, cement, lime or other slurry is injected into the soil through the tip of the hollowstemmed augers. The auger flights penetrate and break loose the soil, sand lift it to the mixing paddles, which blend the slurry and soil. As the auger continues to advance, the soil and slurry are re-mixed by additional paddles attached to the shaft. DSM can be used to treat soils more than 30m deep. A zone of contaminated soil or a complete block of contaminated soil can be treated. Water table elevation has no effect on the process. If the work is performed under the water table, the groundwater is mixed into the treated soil mass. If the work is performed above the water table, then the slurry waster-solids ratio can be adjusted to allow for the lack of water in the final soil-mixed product. The ability to perform under the water table is a key advantage to using a soil mixing system, because dewatering is not required. This saves on cost, particularly when groundwater is contaminated and would have to be treated or could not be lowered. DSM has many excellent applications such as structural cut-off walls, nonstructural cut-off walls, bloc treatment for foundations and low strength piles, However, for large, shallow applications, DSM is not economical. 2

3 Shallow Soil Mixing Because economics are important when treating a large mass of soil, Shallow Soil Mixing (SSM) was developed. This process uses a single auger 1 m to 3.7 m in diameter. A large diameter auger treats nearly 10 cubic meters per meter of penetration, compared to 1 cubic meter per meter of penetration with DSM augers. SSM can be used in soft soils and sludges up to 9m deep. Beyond those depths, torque limitations would require the DSM method. SSM uses a crane-mounted mixing system (Figure 1). The mixing auger is driven by a high-torque turntable via a kelly bar. In the case of the application of a slurry to the soil, the kelly bar is hollow. At the top of the kelly bars is a swivel which allows the connection of a 76 mm diameter hose which carries the slurry from the grout plant. The slurry grout exits the bottom of the mixing augers through three ports, one in each of the three auger flights. The pitch on the auger flights and the centrifugal force causes the grout to reach all parts of the column. Although the linear velocity of the mixing auger increases towards the edge of the auger, the discontinuous flighting lifts the soil-grout mixture and redeposits it at the end of the flight yielding a homogenous mix. In the case of mixing sludges or using wet soils, reagents can be added pneumatically. SSM has both geotechnical and environmental applications. It can be used for foundation elements, block stabilization, gravity walls, and fixation/solidification of contaminated soils. The first geotechnical construction application ever was completed in 1990 in Canada. Project Background In a coastal pulp and paper mill, located at Crofton on the southeast coast of Vancouver Island, British Columbia, installed secondary effluent treatment facilities including two large spill tanks. Site restrictions and operating considerations resulted in the spill tanks being located on an area of land reclaimed from the sea. This site was used as a laydown area and had been reclaimed by uncontrolled dumping of sand and silt spoil, during the original mill site development in A site plan is shown on Figure 2. 3

4 Figure 1. Crane Mounted Mixing System for SSM Figure 2. Site Plan 4

5 The Project The two concrete spill tanks have diameters of 82 m and 55 m and a height of 10.7 m. the operating condition involves 1.2m of fluid, but several times a year they will fill rapidly to within half a meter of their rim for a period unlikely to exceed one week. The tanks are set partially in ground and therefore no net load increase is experienced by the ground due to tank contents during normal operating conditions. The tanks will be connected to each other, and to other structures, by 1.2 m diameter glass fiber reinforced pipe work. Ground Conditions The ground conditions were established from a geotechnical investigation program consisting of 12 test pits, 6 samples drill holes, 3 dynamic cone penetration tests (DCPT) and 3 penetration vane shear strength profiles. The DCPT and 3 penetration vane shear strength profiles. The DCPT is a continuous penetration test, using a 60 mm diameter sleeved cone driven with standard penetration test (SPT) equipment, with no correction for rod friction or tip diameter. The site generally consisted of pit-run sand and gravel surfacing, over desiccated, very stiff, becoming loose and soft sand and silt fill. The average thickness of the fill crust was 1.8 m overlying typically 3.7 m of loose deposits. The fill was underlain by about 1.2 m of medium dense beach sand overlying very dense, over consolidated silt, which is an inter-till deposit. The groundwater was typically 3 m below the site surface. A typical section through the site is shown on Figure 3. The fill materials are characterized by the following gradation limits: % Passing U.S. Standard Sieve Size #40 #200 Range Average The variation of the fill strength is represented by the penetration profile envelope shown on Figure 4. 5

6 Figure 3. Typical Section Through Site Figure 4. Penetration Resistance Profile 6

7 Design Considerations The mill is located in an area of potentially high seismic activity (Puget Sound Seismic Zone) which has a maximum magnitude of 7.5 (Richter scalezz0. the design seismic horizontal peak firm ground acceleration (PGA) is 0.26 g at the National Building code of Canada (NBCC) minimum risk level of 10% probability of exceedance in 50 years (475 yr return period). The new facilities are designed to survive the NBCC event but not the lower risk, higher magnitude events considered possible in the region. The character of the poor site soils and the seismicity of the region combines to make it expensive to design fully against earthquake damage; hence, an optimization process of balancing risk against foundation costs was carried out. The principal design aspects are summarized below, with estimates of displacements with no ground improvement, where appropriate: static design - high compressibility, 150+ (tank full) 75 mm of settlement - edge stability, Su=70kPa (St=2) aseismic design - liquefaction (tank almost empty) - high settlements mm - high lateral displacements mm - large translation of foreshore zone construction - tidal range (4.9 m) - high water table - possible uplift pressures from the inter-till - foreshore stability - adjacent site facilities The liquefaction potential was evaluated by comparing the cyclic shear stress ratio values, based on the design PGA with the values required to cause liquefaction, for a Magnitude 7.5 earthquake, as proposed by Seed et al. (1985). The analyses indicated a zone of loose fill between 1.5 m and 3 m thick could liquefy under the design event. The range of liquefaction-induced settlement was estimated based on the cyclic shear stress ratios at the design PGA and normalized penetration resistance values to obtain the volumetric strain, as proposed by Tokimatsu and Seed (1987). 7

8 Several estimates of horizontal ground displacement were made for the liquefied ground based on yield accelerations from Newmark (1965) and shear strains from Franklin and Chang ( 1977). Values of displacement varied with assumptions and the areas of ground considered but relatively large displacements were generally predicted. The proximity of the foreshore to the tanks, i.e. 15+ m combined with the predicted large lateral displacements created a major risk to the tanks. One early concept was an in-situ dyke of stabilized soil around the seaward perimeter of the site to try to restrain the site soils. A fundamental element of the design approach was the involvement of the owner and his civil consultant in reviewing the foundation alternatives with respect to their hazard reduction potential, relative to their cost. This involvement led, ultimately, to the perimeter dyke being abandoned due to serviceability concerns following the design, or greater, seismic activity. The interaction between the project group also produced the concept of partial foundation treatment, instead of the original total footprint treatment. The partial treatment approach required positive support to the tank walls but allowed the tank floor to settle. Foundation Alternatives Several foundation alternatives were initially reviewed for their static condition benefits and included the following groups: preload excavate and replace ground improvement piles Consideration of ground improvement techniques included the following methods: dynamic compaction columns jet grouting soil mixing vibro-mortar columns Consideration of seismic loading was a major determinant in the alternative selection and helped to eliminate preloading and piling. Excavation and replacement would have been extremely beneficial but material disposal was a problem and overall costs were high. The final selection was made from the ground improvement group of options. 8

9 As an aid in optimizing the benefits, risks and costs of the various alternatives, a cost-benefit parameter was developed during the early stages. The parameter was obtained from the product of the estimated cost, in millions of dollars, and the estimated static settlement, in inches. The approach is summarized in Table 1 below for full treatment of the tank footprints. TABLE 1 SUMMARY OF ESTIMATED SITE IMPROVEMENT COSTS Estimated Estimated Cost-Benefit Cost Settlement Parameter Methods $ Can. (mm) ($M x mm) Task Water Preload 1 175, Dynamic Compaction 755, Granular Preload 1 748, Soil Mixing 1,047, Vibro-Replacement 1,145, Timber Piles 1 1,032, Vibro-Mortar 1,454, Excavate & Replace 1,750, Excluding soil mixed containment dyke at $ 250,000. During the dyke concept, period specialist geotechnical contractors were requested to submit outline technical and cost proposals based on a data package and a set of simple criteria. Although the dyke concept was abandoned, the large diameter soil mixing technique (soilcrete columns) offered by Geo- Con, Inc. was reviewed and adopted for the tank wall support. This is believed to be the first time that this method of support has been used in North America. Soilcrete Column Design The 3.6 m diameter soilcrete columns were designed as a continuous tangential ring beneath the tank walls. The large and small tanks were supported on 71 and 45 columns, respectively. Eighteen additional columns were used to support some ancillary tanks and also to provide an arched dam connecting the two rings of tank columns, thereby providing ground restraint to facilities located between the two tanks. The column 28 day compressive strength of 860 kpa was specified to provide adequate strength for the tank wall foundation maximum bearing pressure of 428 kpa. The column strength was based on an empirical ratio between compressive 9

10 and shear strength of 0.3. As noted below, a substantial increase in strength occurs following the 28-day period. The column and foundation geometry was determined from stability analyses under static (full tank) and dynamic (almost empty tank) loadings. Normal bearing capacity, force and moment equilibrium analyses were carried out for the static case. In the seismic case similar analyses were carried out, but assuming the contained soil acted as a heavy fluid with an acceleration component (0.13g), which was also applied to the soilcrete column. Substantial loss of support was assumed for the external grade, together with the same acceleration component. Based on the results of the analyses, the soilcrete column was embedded 0.6 m into the dense till the column was located to create a stabilizing eccentric vertical load and surplus tensile capacity in the ring beam foundation was required. A schematic arrangement of the foundations and soilcrete columns is shown in Figure 5. the column layout is shown on Figure 2. The columns rings were intersected by cross-drains at regular intervals to allow water, generated from the dissipation of excess porewater pressure, to drain to the exterior of the tank. The cross-drains were linked by a continuous perimeter drain inside the wall footing which was, in turn, connected to a drain gravel blanket beneath the entire floor slab. Figure 5. Foundation Detail 10

11 Soilcrete Column Construction Construction commenced with preliminary in-situ mix trials to obtain the appropriate combinations of water-cement ratio, grouting rate and mixing rate. The water-cement-soil ratio initially was based on laboratory trials using site soils and was adjusted on site as the early test results became available. The basic mix design was 177 kg of cement with a water/cement ratio of 1.8 to 1 per cubic meter of soil. The procedure was also varied by the superintendent to reflect variations in material and groundwater conditions. Some initial problems with obstructions such as boulders and logs were experienced. Where the auger paddle could not displace the obstruction, it was displaced or removed by a backhoe, obtained from nearby construction activities. Samples were obtained from the soilcrete at various depths below the ground surface by a discrete sampler developed by Geo-Con, Inc. Samples were not always taken in the center of the column so as to verify that all areas of the column were of proper strength. Three cylinders were made from each sample and stored for testing. The results of the cylinder tests are summarized on Figure 6 below. Figure 6. Soilcrete Samples U.C.S. A swelling of the ground volume occurs in the soil mixing process due to the injection of cement grout. Once initial set had occurred, and usually the following day when of a stiff clay consistency, the excess mounded soilcrete was graded off to footing grade and the surplus material placed and compacted as structural fill beneath the tank floor. 11

12 No problems were experienced in keying into the very dense till stratum. On first filling, the tank floor is expected to experience several inches of settlement. Filling will occur under a controlled test fill procedure based on readings from pneumatic piezometers installed beneath the tanks at several locations and depths, together with center settlement gauges. The filling rate will be controlled to avoid an excessive build-up of porewater pressures and associated lateral loads. Acknowledgments The authors acknowledge, with thanks, the permission of Fletcher Challenge Canada Limited (FCC) to publish this paper. Klohn Leonoff also acknowledges the input of the engineering staff of FCC, together with their civil consultant, H.A. Simons Ltd. of Vancouver, B.C. in arriving at an optimum solution for the foundations on this site. Finally, we acknowledge the valuable comments and review provided by our colleagues in our respective organizations. Appendix. References (1) Franklin, A.G., Chang, K. (1977) Earthquake Resistance of Earth and Rockfill Dams: Permanent Displacements of Earth Embankments by Newmark Sliding Block Analysis, Misc. paper S-71-17(5), U.S. Army Eng. Waterways Exp. Sta. (2) Newmark, N.M. (1965) Effects of Earthquakes on Dams and Embankments, Geotechnique, 15(2), (3) Seed H.B,, Tokimatsu, K., Harder, L.F., Chung, R.M. (1985) Influence of SPT Procedures in soil Liquefaction Resistance Evaluations, J. Geot. Eng. (ASCE), 111 (12), (4). Tokimatsu, K., Seed, H.B., (1987). Evaluation of Settlements in Sands Due to Earthquake Shaking, J. Geot. Eng. (ASCE), 113 (8),