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 76... ...
was used to develop and calibrate the model. Test 2 was selected such that the unknown passive resistance generated by an embedded pile cap was not included.
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Test 2 was the horizontal test conducted without passive resistance of the pile caps (e.g., soil adjacent to the pile cap was excavated prior to the test so the cap was not embedded)
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The passive resistance is equal to the difference between the load applied during the test and the load applied in GROUP Figure 7-3. Comparison of measured and computed load-deflection curves for Test 1 (virgin soil, pile caps embedded)
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From page 79... ...
7.4 Development of Simplified Method The simplified method proposed for design is based on estimating the contribution of the treated ground around the pile group using a limit equilibrium analysis of the treated soil mass. This analysis includes passive resistance acting against the face of the treated soil mass and adhesion acting on the sides of the treated soil mass as lateral displacement mobilizes the passive soil resistance.
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Passive Resistance Acting on the Face of the Treated Soil Mass Based upon the computed magnitude of passive resistance acting against the leading face of the pile cap (50 kips at 1-in.
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This additional resistance is likely to have been developed by adhesion along the two sides and the base of the treatment block, assuming the unit passive resistance acting on the leading face of the block is equal to that of the pile cap with identical dimensions. Because the dimensions of the treated block are known and the magnitude of additional resistance provided by adhesion 81 Figure 7-8.
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0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 0.00 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00 2.25 2.50 2.75 D ep th b el ow G ro u n d Su rf ac e (ft ) ksf Undrained Shear Strength Effective Vertical Stress Rankine Unit Passive Resistance Bottom of Pile Cap
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Figure 7-11. Results from FE parametric length study for mass mixing.
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Pile Cap Pile Mass Mix Treatment Projected Area Contributing to Additional Lateral Resistance (Typical) 52° Passive Resistance Adhesion (2 Sides)
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7.5 Evaluation for Jet Grouting Cases Similar to the mass mix tests, jet grout ground improvement techniques were performed in or around the pile groups and tested to evaluate the benefit on performance of the group under lateral loading. Jet grout ground improvement was constructed adjacent to Pile Cap 1 and the leading row of piles in the group, as shown in Figure 7-16.
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From page 86... ...
0 20 40 60 80 100 120 140 160 180 0 1 2 3 4 5 6 7 8 9 10 11 12 13 Pa ss iv e + Ad he si on (ki ps ) Mass Mix Treatment Depth (ft)
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This simplified procedure is shown graphically in Figures 7-17 and 7-18 for the actual and simplified cases, respectively. Considering the jet grout ground improvement geometry beneath Cap 2 and the unit passive resistance and shear strength versus depth data presented in Figure 7-9, the summation of passive and adhesive resistance can be estimated as follows for the truncated depth: • Passive Resistance: 10 ft × 7.5 ft × 1.85ksf = 139k – where 10 ft is the treatment dimension perpendicular to loading, – where 7.5 ft is the truncated depth, and 87 Figure 7-17.
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From page 88... ...
: 2 sides × 5 ft × 15 ft × 0.9 × 0.58ksf = 78k – where 5 ft is the truncated treatment depth beneath the cap (note the sides of the cap are not in contact with the soil) , – where 15 ft is the treatment dimension parallel to loading, – where 0.9 is αpass, and – where 0.58 ksf is the average undrained shear strength over this depth.
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From page 89... ...
Recall that the simplified procedure truncates the treated soil block at a depth equal to the outside-to-outside dimension of the piles in the group perpendicular to the direction of loading. 7.6 Design Recommendations Pile Group Improved with Cemented Soils or Flowable Fill Assuming that the treated soil has a compressive strength of at least 75 psi, the following simplified design approach is recommended for the purpose of estimating the additional lateral resistance provided by the ground improvement: 1.
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Reduce the external horizontal loading force magnitude applied to the pile cap by the lateral resistance contributed by the treated block.
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The ground improvement involves introducing and mixing Portland cement into the virgin soil such that a mass of treated soil is created that substantially exceeds the in-situ strength of the virgin soil. To model the geotechnical benefit provided by the ground improvement, the simplified method described herein is used to compute the additional resistance provided by the ground improvement.
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From page 92... ...
Compute the reduced horizontal load to be used in foundation analyses, Preduced, by subtracting the total lateral resistance contributed by the treated block, F, from the external horizontal loading force magnitude applied to the foundation, P: 5. Use Preduced to analyze the foundation using commercially available software such as LPILE, GROUP, FBPier, etc.
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To model the geotechnical benefit provided by the ground improvement, the simplified method described herein is used to compute the additional resistance provided by the ground improvement (see Figure 7-25)
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From page 94... ...
Compute the total lateral resistance contributed by the treated block, F, as the sum of the passive resistance, Fp, and cumulative adhesion, Fα, acting on the projected area of the treatment block: 4. Compute the reduced horizontal load to be used in foundation analyses, Preduced, by subtracting the total lateral resistance contributed by the treated block, F, from the external horizontal loading force magnitude applied to the foundation, P: 5.
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From page 95... ...
In addition, the granular zone should be compacted to a minimum of 95% of the modified Proctor maximum unit weight. The passive resistance for the pile cap can then be computed using the following procedure: 1.
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