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5 CHAPTER 2 Available Ground Improvement Case Histories and Approaches Publications on ground improvement methods and their arrangement with a 0.75-m center-to-center (3D) spacing in applications to foundation design are extensive. However, engi- both directions. The original clay profile consisted of stiff, over- neering practice typically has not used soil improvement tech- consolidated clay with an undrained shear strength of about niques in combination with deep foundations to increase the 1150 psf (57 kPa) at the ground surface, which increased to lateral resistance of bridge foundations. If soil improvement about 3000 psf (150 kPa) at a depth of 18 ft (5.5 m) below has been undertaken, it has generally been with the goal of pre- the ground. Approximately 9 ft of the clay was excavated and venting liquefaction or increasing soil resistance so that deep replaced with a relatively uniform clean sand compacted to a foundations were not required. Three excellent publications dry unit weight (d) of 98 pcf (15.4 kN/m3), which is a relative that summarize the state of the art in soil improvement are density of about 50%. A direct shear test indicated a friction provided by Mitchell (1981), Terashi and Juran (2000), and angle of 38.5°, but back-calculated friction angles using LPILE ASCE (1997). Based on a review of methods documented in (Reese, Wang, Isenhower et al., 2004a) suggest a friction angle these and other publications, specific improvement methods of around 50°. A plot showing the total load versus deflection or technologies appropriate for applications to improve the lat- curves for the pile group in both clay and sand is presented in eral resistance of soils (stiffness and strength) associated with Figure 2-1. At deflections less than about 20 mm, the lateral pile foundations are summarized in Tables 2-1 and 2-2. resistance of the pile group in clay is about the same as that in A review of the literature indicates that there are also a sand. However, at greater deflections, the lateral resistance of number of case histories that provide some insight regarding the pile group in sand eventually exceeds that for the pile the degree of increased lateral pile resistance that could be group in clay by over 28%, despite the fact that the clay was obtained by employing soil improvement in concert with deep relatively stiff. foundations. In connection with research studies, field load The pile group tested by Rollins et al. (2005) and Rollins, tests have been performed on two pile groups where the native Snyder et al. (2010) consisted of 15 12.75-in. diameter steel clay soil was excavated and replaced with compacted granu- pipe piles driven closed-ended to a depth of about 40 ft. The lar soil. These studies were primarily undertaken to evaluate piles were driven in a 3 × 5 grouping with a center-to-center group interaction factors under lateral loading. Brown et al. spacing of 4.17 ft (3.92D) in the direction of loading and 3.5 ft (1987) conducted lateral load tests on a nine-pile group in sat- (3.29D) transverse to loading. The upper 2.5 m of clay in the urated stiff clay. Later, Brown et al. (1988) excavated the clay, original soil profile had an undrained shear strength of about compacted sand around the pile group, and repeated the lat- 900 psf. The pile group reacted against two 4-ft diameter drilled eral load test. Rollins et al. (2005) performed cyclic lateral load shafts. Prior to the second set of tests, the upper 1 m of clay was tests on a 15-pile group in medium-consistency clay. Rollins, excavated and replaced with compacted clean sand. In addi- Snyder et al. (2010) excavated the clay, replaced it with clean tion, an extra 1.5 m of sand was compacted above the original sand, and performed additional lateral pile group load tests. ground elevation so that the upper 2.5 of the profile consisted Although these tests were not designed to evaluate the effect of clean sand compacted to 93% of the modified Proctor max- of excavation and replacement on lateral pile group resistance, imum density. The load versus deflection curves for the pile the test results can be compared to provide this information. group in clay and sand are compared in Figure 2-2. The pile group tested by Brown et al. (1987, 1988) was a Because the clay strength was relatively soft, the lateral resist- nine-pile group consisting of 0.25-m (10-in.) diameter steel ance of the pile group in sand was considerably higher than pipe piles filled with grout. The piles were driven in a 3 × 3 that for the pile group in clay. Analyses using the computer
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6 Table 2-1. Summary of soil improvement methods in loose cohesionless soils. Method Advantages Disadvantages Strength Cost Gain Excavate and Replace Compacted ·Simple, widely used method ·May require excavation support or Low to mod. Low to mod. Fill ·Equipment widely available dewatering ·Compaction can be measured easily Flowable Fill ·No special equipment or ·Must dispose of excavated material Mod. to high Mod. to high personnel required ·No compaction necessary In-Situ Densification Dynamic Densification to 25-ft depth from ·Decrease in improvement with depth Low Low Compaction surface impact ·Produces vibration and noise ·Could produce downdrag in existing piles ·Requires overhead clearance for dropping weight Vibro ·Relatively uniform improvement ·Could produce downdrag in existing Low Moderate Compaction with depth piles Stone ·Increased densification relative to ·Must install stone through entire layer to Low Moderate Columns vibrocompaction treat loose sand at depth ·Increased shear resistance of ·Could produce downdrag in existing reinforced soil mass piles ·Drainage provided by columns in the event of liquefaction concerns Stone ·Improved effectiveness with high ·Increased cost and logistical effort of Low Moderate Columns fines content soils installing wick drains prior to stone with Wick ·Does not produce spoils column treatment Drains Compaction ·Can be used for retrofit below a ·Less effective at shallow depths where Moderate Moderate Grouting pile cap pressure is restricted ·Can treat zones of interest ·More difficult to evaluate improvement without treating all soil above the for retrofit conditions below pile zone ·Does not produce spoils Soil Mixing Deep Soil ·Mixing can occur to 60- to 80-ft ·Can decrease the strength of sensitive Mod. to high Mod. to high Mixing depths clays ·Significant strength gain can be ·Produces spoils achieved · Can produce columns (3-ft dia.) or wall panels at desired depths Grouting Permeation ·Can be used for retrofit below a ·Limited to very coarse sands Moderate Moderate Grouting pile cap ·More difficult to evaluate improvement ·Can treat zones of interest for retrofit conditions below pile than in without treating all soil above the areas around periphery of pile group zone ·Uniformity of treatment is often difficult ·Does not produce spoils Jet Grouting ·Low noise and vibration ·Creates spoil material High High ·Can treat soil under pile cap after ·Requires mobilization of specialized construction equipment and personnel ·Can transform pile group into equivalent pier for scour resistance
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7 Table 2-2. Summary of soil improvement techniques in soft cohesive soils. Method Advantages Disadvantages Strength Gain Cost Excavate and Replace Compacted ·Simple, widely used method ·Must dispose of excavated material Low to mod. Low to mod. Fill ·Equipment widely available ·May require excavation support or ·Compaction can be easily measured dewatering Flowable ·No special equipment or personnel May need to dispose of excavated Mod. to high Mod. to high Fill required material ·No compaction necessary Cement ·Strength can be increased by 3 to 5 ·Requires special care in mixing Mod. to high Mod. to high Treated times the in-situ strength operations Excavated ·Significant laboratory testing Clay necessary to develop treatment plan ·Field strength gain usually much less than laboratory because of less efficient mixing Lime ·Strength can be increased 3 to 5 times ·Requires special care in mixing Mod. to High Mod. to High Treated the in-situ strength operations Compacted ·Requires less cement to achieve same ·Significant laboratory testing Clay strength as using cement alone necessary to develop treatment plan ·Field strength gain usually much less than laboratory because of less efficient mixing ·Difficult to treat soils with high sulfate content Rammed ·Can be constructed above or below Creates relatively little increase in Low Low Aggregate water table density of surrounding soil Piers ·Creates extremely dense gravel ·Columns provide no flexural columns with high-friction angle resistance. ·Increases lateral pressure in ·Creates extremely dense high- surrounding soil friction angle columns ·Increases shear resistance of the reinforced soil mass In-Situ Soil Mixing Vibro ·Increases shear resistance of the ·Can cause heave of surrounding Low Low Replacement reinforced soil mass ground Compaction ·Can be used for retrofit below a pile ·Less effective at shallow depths Moderate Low Grouting cap where pressure is restricted ·Can treat zones of interest without ·Can decrease the strength of treating all soil above the zone sensitive clays ·Does not produce spoils ·More difficult to evaluate improvement for retrofit conditions below pile Deep Soil ·Mixing can occur to 60- to 80-ft depths ·Can decrease the strength of Mod. to high Mod. to high Mixing ·Significant strength gain can be sensitive clays achieved ·Produces spoils ·Can produce columns (3 ft dia.) or wall panels at desired depths Mass ·Mixing can occur in-situ to 10- to 15-ft ·Significant laboratory testing Mod. to high Mod. to high Mixing depths necessary to develop treatment plan ·No need for excavation or ·Field strength gain usually much recompaction less than laboratory because of less ·Strength increase of 3 to 5 times efficient mixing original shear strength Grouting Jet Grouting ·In-situ treatment with flexibility to ·Creates spoil material High High produce variety of geometries (columns ·Requires mobilization of and panels) specialized equipment and personnel ·Flexibility to treat only zones of interest ·Can treat soil under pile cap after construction ·Can transform pile group into equivalent pier for resistance during scour events
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8 900 800 700 600 Total Load (kN) 500 400 300 Sand 200 Clay 100 0 0 10 20 30 40 50 60 70 Displacement (mm) Figure 2-1. Load vs deflection curves for nine-pile groups in stiff clay and dense sand based on Brown et al. (1987, 1988). program GROUP (Reese, Wang, and Arrellaga et al., 2004b) 1.9-m (6.3-ft) wide anchor block. The block was originally were successful in matching the measured response of the pile poured flush against an excavation into partially saturated stiff groups in clay. However, for the pile groups in sand, successful clay and a lateral load test was performed. The clay was then agreement with measured response generally required the excavated and replaced with a compacted sandy gravel backfill use of friction angles that are somewhat higher than would and the test was repeated. Rollins, Gerber, and Kwon (2010) normally be used in engineering practice. The potential evaluated the passive force provided by various soils against a for improved resistance using excavation and replacement pile cap that was 1.1 m (3.67 ft) deep and 5.2 m (17 ft) wide. increases as the clay becomes softer and compacted granular Tests were conducted on a silty sand at two densities and on soil becomes denser. Increases of 60% are possible (Rollins, loose silty sand with a 1- to 2-m wide zone of dense compacted Snyder et al., 2010). gravel immediately adjacent to the pile cap. Two field test studies have evaluated the passive force on a The native clay in the tests performed by Mokwa and Dun- pile cap as a function of soil type and density. Mokwa and can (2001) was partially saturated. Triaxial shear tests on the Duncan (2001) performed tests on a 1.1-m (3.5-ft) deep and clay at the natural moisture content indicate that the cohesion 2000 1800 1600 Total Group Load (kN) 1400 1200 1000 800 Sand 600 Clay 400 200 0 0 20 40 60 80 100 Average Group Deflection (mm) Figure 2-2. Load vs deflection curves for 15-pile group in medium stiff clay and dense sand based on Rollins et al. (2005) and Rollins, Snyder et al. (2010).
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9 700 600 500 Total Load (kN) 400 300 200 Clay Gravel 100 0 0 10 20 30 40 50 Displacement (mm) Figure 2-3. Comparison of passive force provided by stiff, partially saturated clay and compacted sandy gravel against 1.1-m deep 1.9-m wide cap block. was 1 ksf and that the friction angle ranged from 32° to 38°. Tests also were performed using the loose silty sand backfill The clay was excavated to the base of the cap block and along with a well-compacted zone of sandy gravel adjacent replaced by a compacted, sandy gravel. The sandy gravel to the pile cap. The compacted zones were 0.9 m (3 ft) and (GW-GM in the Unified Soil Classification System) was com- 1.83 m (6 ft) thick. These tests indicate that compacting rela- pacted to a relative density of approximately 80%. Triaxial tively thin zones (3 to 6 ft wide) of sandy gravel around a pile shear tests indicate that the friction angle could range from a cap can significantly increase the passive resistance as illus- low of 48° to a high of 52°. A comparison between the passive trated in Figure 2-5. In this case, replacing a 0.9 m (3 ft) zone force-deflection curves for the clay and gravel is provided in of loose silty sand around the pile cap with compacted gravel Figure 2-3. In this case, the lateral resistance provided by the increased the lateral resistance on the pile cap from an initial stiff, partially saturated clay was considerably higher than that value of 70 kips to more than 180 kips, which is an increase of for the gravel at the shallow depths involved. As a result, the lat- over 200%. Crack patterns from the tests, shown in Figure 2-6 eral resistance actually decreased substantially when the com- indicate that the compacted gravel zones increase the effective pacted gravel was used in place of the clay. Duncan and Mokwa (2001) concluded that the log-spiral method provided the best 400 estimate of the ultimate capacity and that the passive force- Dense Silty Sand 350 (98%) deflection relationship could be estimated reasonably using a Loose Silty Sand hyperbolic curve. This conclusion was supported by analyses 300 (88%) Passive Force (kips) of additional large-scale tests by Rollins and Cole (2006) and 250 Cole and Rollins (2006). Rollins, Gerber, and Kwon (2010) performed lateral load 200 tests on a pile cap supported by 12 0.324-m diameter pipe 150 piles. The piles provided sufficient vertical resistance so that 100 the full wall friction force could develop. Basic passive force- deflection relationships were developed for two tests involv- 50 ing silty sand compacted at 88% and 98% of the modified 0 Proctor maximum unit weight as shown in Figure 2-4. The 0 0.5 1 1.5 2 2.5 3 increased compactive effort produced a considerable increase Deflection (inches) in passive resistance. Preliminary analyses indicate that this Figure 2-4. Measured passive force vs deflection behavior is predicted quite well using the Duncan and Mokwa relationships for two full-scale tests with silty (2001) approach along with the soil properties measured in sand compacted to 88% and 98% of the modified the field. Proctor maximum unit weight.
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10 300 Direction of Lateral Load Bf Loose Silty Sand/6 ft 250 Gravel Backfill Dense Sand Pile Cap or Loose Silty Sand/3 ft H (D) 200 abutment Gravel Backfill Load (kips) Loose Silty Sand 150 2 ft min. 100 Loose Sand (L) 50 0 0 1 2 Deflection (inches) Figure 2-7. Layout of geometry for limited width Figure 2-5. Passive force vs deflection curves for dense backfill zone adjacent to pile cap or abutment. loose silty sand against a 17-ft wide by 3.67-ft high pilecap after excavation and replacement with 3-ft and 6-ft zones of compacted gravel backfill against the friction angle of the looser surrounding sand (L ). Fig- the cap. ure 2-7 shows the layout of the improved zone relative to the pile cap or abutment and defines the basic parameters involved. width of the pile cap and reduce the pressure on the loose silty To facilitate generalization, the width of the dense zone is nor- sand behind it, thereby increasing passive resistance. Rollins malized by the height of the pile cap. In addition, the ultimate and Nasr (2010) completed a follow-up study to develop a gen- passive resistance provided by the limited-width dense granu- eralized equation to predict the increased passive resistance lar backfill (PLW) is normalized by the ultimate passive resist- that could be obtained by constructing a narrow dense granu- ance for a homogeneous dense backfill (PFW) that fully encloses lar zone adjacent to a pile cap or abutment surrounded by loose the passive failure surface. This ratio is defined as the passive sand. This investigation followed the same pattern employed force ratio (PFR). PFR is plotted as a function of normalized in this NCHRP study. The full-scale load tests reported by width (Bf /H) for a number of cases in Figure 2-8 where the Rollins, Gerber et al. (2010) and Gerber et al. (2010) were used dense zone consisted of gravel. As the normalized width to calibrate soil parameters within the finite element computer increases, the PFR increases. When Bf/H exceeds 1.0, the PFR program PLAXIS. In addition, the computer model was veri- is normally greater than 60%, and the PFR increases as the fric- fied against analytical solutions for computing passive force tion angle of the surrounding sand increases. Typically, the (e.g., Duncan and Mokwa, 2001). Once the FEM analysis Bf/H ratio would need to be around 4.0 to fully enclose the fail- model was calibrated, parametric studies were performed to ure surface; therefore, the narrow dense zone is relatively effec- evaluate the influence of changes in wall height (H), the dense tive in mobilizing the majority of the total passive resistance. zone width (Bf), the friction angle of the dense zone (D) and Based on the result from the FEM parametric study, Equation 1 2 ft x 2 ft Grid 2 ft x 2 ft Grid 10 ft x 17 ft x 3.67 ft Gravel Backfill 10 ft x 17 ft x 3.67 ft Pile Cap Loose Silty Sand Pile Cap (a) 3 ft sandy gravel zone plus loose silty sand backfill (b) 6 ft sandy gravel zone plus loose silty sand backfill Figure 2-6. Plan view of crack patterns behind a pile cap after excavation and replacement of loose silty sand with (a) a 3-ft and (b) a 6-ft zone of compacted sandy gravel behind the pile cap.
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11 100% 90% 80% 70% PFR = PLW/P FW Gravel 60% (gravel)=39° (sand)=36° 50% (gravel)=39° 40% (sand)=32° 30% (gravel)=39° (sand)=27.7° 20% 10% 0% 0 0.5 1 1.5 2 2.5 BF/H Figure 2-8. Percentage of total passive resistance developed by a narrow dense gravel zone adjacent to an abutment relative to that for a homogenous dense gravel zone. was developed to predict the PFR for a limited-width dense to treatment, correlations with SPT and CPT test results granular backfill. (Kulhawy and Mayne, 1990) indicated that the sand had a relative density of about 50%; however, after treatment, the PFR = 3.1418 - 0.139 D - 0.033 D + 0.484 (B f H ) relative density was increased to between 85% and 100%. - 0.043(B f H ) + 0.003 2 - 0.007 L (B f H ) 2 L (1) A comparison of the load versus deflection curves for the CISS pile and the pile group before and after treatment but At a test site on Treasure Island in San Francisco Bay, stone prior to blast liquefaction are shown in Figure 2-10. The lateral column treatment was used to improve a 6-m-thick liquefi- resistance was increased by about 25% for the pile group and able sand layer around two test foundations (Ashford, Rollins, about 33% for the CISS pile. Back-analyses using the computer and Baez 2000a; Ashford, Rollins, Bradford et al., 2000b; and program LPILE indicate that the friction angle was increased Weaver et al., 2005). One test foundation was a 0.6-m dia- from about 39° prior to treatment to about 48° after treatment. meter cast-in-steel shell (CISS) shaft that reacted against a Prior to stone column treatment the sand layer liquefied group of four 0.324-m diameter driven steel pipe piles. Lateral (excess pore pressure ratios of 100%) following blasting and load tests were first performed on the test foundations prior to remained liquefied for at least 10 minutes. Settlement fol- treatment, then comparable lateral load tests were performed lowing dissipation of excess pore pressures amounted to after treatment for comparison. A high-speed hydraulic actu- about 12 in. A plot of the load-deflection curve for the CISS ator was used to apply load. In addition to the conventional pile following blast-induced liquefaction is provided in Fig- static lateral load tests, a pattern of small explosive charges was ure 2-11(a). The pre-blast load-deflection curve also is shown detonated sequentially to produce a liquefied volume of soil for comparison. Following liquefaction, about 11 times more within which the test foundations also could be laterally movement was required to develop the same lateral resist- loaded to large displacement levels (9 in.). The layout of the ance as that prior to liquefaction. After treatment, high excess test foundations is shown in Figure 2-9. The 0.9-m diameter pore pressure developed initially after the blasting but dissi- stone columns were installed in a square pattern with a spac- pated within a few seconds, presumably due to the increased ing of 2.5 m from center to center (see Figure 2-9) using the drainage provided by the stone columns. The load-deflection dry, bottom feed method. The sand had a mean grain size of curve after blasting is presented in Figure 2-11(b) for compar- about 0.2 mm with a fines content between 5% and 10% and ison and the stiffness of the curve is several hundred percent was initially placed using hydraulic filling techniques. Prior higher in comparison to that prior to treatment.
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12 Limit of Excavation o o o Limit of Densification x x o o x x x x o o x x x x o o x x x x o o x x o o o Scale: 1m = 2 x 2 Pile Group x = Charges for first and second blast @ 3.5 m o = Charges for second blast only @ 3.5 m = 0.6 m CISS Pile = Stone Column, 0.9 m Diameter, Installed to a depth of 6 m Figure 2-9. Layout of test foundations, stone columns, and explosive charges.
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13 (a) Pile Group (b) CISS Pile 400 400 Post-Treatment Post-Treatment 300 300 Pre-Treatment Pre-Treatment Load (kN) Load (kN) 200 200 100 100 0 0 -100 -100 -20 -10 0 10 20 30 40 50 -20 -10 0 10 20 30 40 50 Displacement (mm) Displacement (mm) Figure 2-10. Static load vs deflection curves before and after stone column treatment for (a) pile group and (b) 0.6-m CISS pile. 700 700 (a) Before Stone Column Treatment (b) After Stone Column Treatment 600 600 Pre-Blast 500 Non-Liquefied 500 Post-Blast Liquefied 400 400 Load (kN) Load (kN) 300 300 200 200 100 100 0 0 -100 -100 -200 -200 -25 0 25 50 75 100 125 150 175 200 225 250 -25 0 25 50 75 100 125 150 175 200 225 250 Displacement (mm) Displacement (mm) Figure 2-11. Pre- and post-blast cyclic load-deflection curves (a) before and (b) after stone column treatment for a 0.6-m CISS pile.