Design Guidelines for Increasing the Lateral Resistance of Highway-Bridge Pile Foundations by Improving Weak Soils (2011) / Chapter Skim
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From page 14... ...
and sand (SM) layers as will be highlighted by the subsequent plots of CPT cone tip resistance.
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The interpreted soil profile and cone tip resistance are also provided in Figure 3-6 for reference. The shear wave velocity in the upper 10 ft of the profile is between 300 and 400 ft/sec, which is relatively low, and suggests a low shear strength.
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Virgin Mass Mixing5ft x 9ft x 10ft Jet Grouting 5ft x 9ft x 10ft Flowable Fill 9ft x 9ft x 5ftVirgin Soil Flowable Fill 5ft x 9ft x 5ft Jet Grouting 9ft x 9ft x 10ft VirginVirgin Compacted Fill Compacted Fill5ft x 9ft x 5ft Geopiers 5ft x 9ft x 10ft Pile Cap 1Pile Cap 2Pile Cap 3Pile Cap 4 32 ft9 ft 9 ft 9 ft 9 ft32 ft 32 ft N
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with Silty Sand and Lean Clay Layers SANDY LEAN CLAY (CL) w/ Sand Lenses LEAN CLAY (CL)
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Plots of cone tip resistance, friction ratio and pore pressure vs depth curves from cone penetration test (CPT) Sounding 2 near the center of the site along with soil profile.
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Plots of cone tip resistance, friction ratio, and pore pressure vs depth curves from all CPT soundings at the site along with soil profile.
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Plots of cone tip resistance and shear wave velocity versus depth from seismic cone testing along with soil profile. Direction of Loading 12.75 inch OD pipe pile with 0.375 in wall thickness (fy=58.6 ksi)
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diameter conduit was installed to a depth of 30 ft. A shape accelerometer array was inserted into this conduit at the beginning of the load test so that deflection versus depth profiles could be determined at various load increments.
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During reloading, the load-defection curve was stiffer than that observed during virgin loading at the same deflection. The virgin pile head load versus deflection curve is plotted in Figure 3-10.
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The lateral load tests were carried out with a displacement control approach with target pile cap displacement increments of 0.25, 0.5, 0.75, 1.0, and 1.5 in. During this process, the actuator extended or contracted at a rate of about 40 mm/min.
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Finally, vertical pile cap displacement was measured at two points along the length of each pile cap to evaluate pile cap rotation. On both caps, string potentiometers were located 2 in.
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A shape accelerometer array could be inserted into this pipe at the beginning of the load tests so that deflection versus depth profiles could be determined at various load increments. Using triaxial accelerometers embedded into a flexible cable at 1-ft intervals, the shape arrays provided real-time displacement versus depth profiles throughout the process of testing.
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(During Test 2, the soil adjacent to the pile cap was excavated to the base of the cap and the pile caps were pushed apart by the actuator.)
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Rotation versus Load Pile cap rotation versus load curves based on the string potentiometer and shape arrays for Cap 1 are provided in 27 -200 -150 -100 -50 0 50 100 150 200 250 300 350 -0.5 0 0.5 1 1.5 2 Displacement (in)
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Although pile cap rotation is clearly observed, it is considerably lower than the rotation of the single pile under free-head conditions. Displacement versus Depth Curves Displacement versus depth curves obtained from the shape accelerometer arrays in the piles within Pile Cap 1 are provided in Figure 3-18.
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It should be noted, however, that the inclinometer profiles, which extend deeper into the pile, indicate that some negative displacement is occurring below the base of the shape arrays. Maximum Moment versus Load Curves Figures 3-20 and 3-21 provide plots of the maximum negative and positive bending moments versus applied pile cap load, respectively, for Cap 2 during Test 1.
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Maximum negative moment vs total pile cap load for Piles (a)
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Maximum positive moment vs total pile cap load for piles (a)
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. 3.7 Pile Group Load Tests Involving Jet Grouting Plan and profile views of the jet grout columns around Pile Caps 1 and 2 are shown in Figure 3-24.
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NFigure 3-24. Plan and profile views of Pile Groups 1 and 2 after treatment with jet grouting.
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Although the compressive strength of the untreated soil prior to treatment was approximately 4 psi, the average compressive strength after jet grout treatment reached about 680 psi with mean ±1 standard deviation bounds ranging from about 34 Column Length 10 ft Estimated Column Diameter 5 ft Grout Pressure 6000 lbs/in.2 Grout Flow Rate 90 gallons/min Rotation Speed 7 revolutions/min Pull Rate 0.79 in./min Column Length 12 ft Estimated Column Diameter 4 ft Grout Pressure 6000 psi Grout Flow Rate 90 gallons/min Rotation Speed 8 revolutions/min Pull Rate 1 in./min Table 3-2. Jet grouting installation parameters for columns created beneath Pile Cap 2.
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Therefore, lateral movement of the piles engaged the soilcrete mass and produced the same lateral resistance. Test Results for Pile Cap 2 (Jet Grouting below the Cap)
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. 3.8 Pile Group Load Tests Involving Soil Mixing Construction Details Plan and profile drawings of the pile group with a soil mix wall on one side of Pile Cap 1 are provided in Figure 3-24.
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. 3.9 Pile Group Load Tests Involving Flowable Fill Several sets of lateral load tests were performed after excavating and replacing the soil around Pile Cap 3 with flowable fill.
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Figure 3-30. Plan and profile views of Cap 3 (right)
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Because of the lower than expected compressive strength of the original flowable fill zone, a second set of lateral load tests was subsequently performed after constructing a flowable fill wall adjacent to the pile cap. This technique would represent an approach for improving lateral pile group resistance after construction.
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Test 10 performed with flowable fill adjacent to pile cap and Test 12 performed after excavation of flowable fill adjacent to cap.
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Lo ad (k ip s) Weaker flowable fill beneath the cap with passive T3 Cap 3 untreated clay with passive T1 Cap 2 0 50 100 150 200 250 300 350 -0.25 0.25 0.75 1.25 1.75 2.25 Displacement (in)
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When the piles were installed, the ground heaved and, in order to maintain the correct pile cap thickness, approximately 0.75 ft of backfill had to be removed, leaving approximately 3 ft of sand under the cap. The sand fill extended 5 ft beyond the cap face on one side to evaluate the increased pile-soil resistance from extending the 42 0 50 100 150 200 250 300 350 400 450 0 0.5 1 1.5 2 Displacement (in)
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Figure 3-36. Plan and profile views of Pile Caps 3 and 4 after excavation and replacement with compacted fill around Pile Cap 4 and placement of flowable fill under Pile Cap 3.
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Test 1 involves the pile cap in untreated clay; compacted sand was placed directly below the pile cap for Test 5. The comparison shows an increase in lateral resistance of about 23 kips at a displacement of 1.5 in.
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When testing was complete on the compacted fill, 30-in. diameter geopiers were installed in a grid pattern south of Pile Cap 4.
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Figure 3-40. Plan and profile view of Pile Cap 4 showing the locations of the rammed aggregate piers and location of excavated zone for subsequent test.
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Comparison of load-displacement curves for Pile Cap 4 with RAP extending to the top of the cap relative to tests on Cap 4 without RAP columns and Cap 1 in untreated native clay. Figure 3-42.
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(kips) in Resistance Jet Grouting Below Cap 15'x10.5'x10' 58.3 282 500 160 Jet Grouting Adjacent to Cap 6.6'x13'x12' 38.1 214 398 185 Soil Mixing Adjacent to Cap 4'x11'x10' 16.3 282 170 60 Weak Flowable Fill Below Cap 13.5'x8.8'x6' 26.4 232 24 10 Flowable Fill Adjacent to Cap 6'x12'x6' 16.0 265 145 55 Compacted Fill to Edge of Cap 9.6'x8.75'x3.5' 10.9 232 23 10 Compacted Fill 5 ft beyond Edge of Cap 14.6'x8.75'x3.5' 16.6 232 40 18 Rammed Aggregate Piers Adjacent to Cap Top 13-2.5' dia x 13' deep 29.5 285 40 14 Rammed Aggregate Piers Adjacent to Cap Top 13-2.5' dia x 10.5' deep 23.6 50 35 70 1 Note 1: Increase in resistance is for passive resistance only Table 3-4.
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Increase in Equivalent Add'l Ground Savings Ground Treatment Method Resistance Number of Pile/Cap Improvement Relative to Percent Improvement Comments (kips) Pipe Piles Cost Cost Piles Savings Cost/kip Jet Grouting Below Cap 500 20 $84,200 $28,500 $55,700 66 $57 Jet Grouting Adjacent to Cap 398 16 $69,360 $38,000 $31,360 45 $95 1 Soil Mixing Adjacent to Cap 170 7 $30,345 $10,000 $20,345 67 $59 1 Weak Flowable Fill Below Cap 24 1 $4,335 $3,180 $1,155 27 $133 Flowable Fill Adjacent to Cap 145 6 1 $26,010 $3,600 $22,410 86 $25 1 Compacted Fill to Edge of Cap 23 $4,335 $544 $3,791 87 $24 Compacted Fill 5 ft beyond Edge of Cap 40 2 $8,670 $828 $7,842 90 $21 Rammed Aggregate Piers Adjacent to Cap Top 40 2 $8,670 $4,225 $4,445 51 $106 Rammed Aggregate Piers Adjacent to Cap Top 35 2 $8,670 $4,225 $4,445 51 $121 2 Note 1: Cost of soil improvement doubled to account for increased resistance in opposite direction Note 2: Increase in resistance is for passive resistance only Note 3: Cost/kip for pile/pile cap = $182/kip
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From page 50... ...
For the treatments adjacent to the pile cap, the cost was doubled assuming that a similar improvement zone would be required on the opposite side to account for load in the opposite direction. The excavation and replacement cost was assumed to be $50 per cubic yard for the small volumes involved and the cost of RAPs was assumed to be $50 per ft of length.
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