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72 Figure 6-19. Treatment zone relative to pile group for parametric study involving the effect of compressive strength of the improved soil beside the cap. The final set of parametric studies involved varying the Figure 6-23 and the improvement ratio shown in Figure 6-24 compressive strength of the soilcrete zone directly below is nearly constant with strength. These results indicate that the pile cap as shown in Figure 6-22. As shown, the soilcrete the lateral resistance is relatively insensitive to the soilcrete zone was 9 ft square in plan view and extended 12.5 ft below strength, provided the soilcrete is much stiffer than the vir- the ground surface. As in the previous case, the computed gin clay. load-displacement curves plot on top of each other as shown in 6.8 Conclusions Based Table 6-1. Material strength of the improved soil. on Parametric Studies Strength (psi) Young's Modulus (ksi) Note 21 261 Verification and validation procedure was conducted for 63 452 the finite element model before the parametric studies. Mesh 126 640 Typical mass mix and boundary sensitivity analyses were performed and the soil 252 905 300 987 material properties were carefully calibrated by comparison 600 1400 Typical jet grout with the test data for the single pile and pile groups. The load- 900 1710 1200 1980 displacement curves obtained from the numerical models fit 4000 3600 satisfactorily with the test data. Parameter studies were then 7700 5000 Typical concrete performed to examine the sensitivity of the depth (beside and
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73 450 400 350 300 21 psi Load (kips) 250 63 psi 126 psi 200 252 psi 150 300 psi 100 600 psi 900 psi 50 1200 psi 0 0.0 0.5 1.0 1.5 2.0 Displacement (in) Figure 6-20. Load-displacement curves computed using the FEM model assuming a variety of compressive strength values for the soil improvement zone beside the cap. below the cap) and the length of the improved soil, includ- be produced by less expensive treatment approaches, can ing mass mix and jet grout, as well as the sensitivity of the still produce the same increases in lateral pile group resis- material strength of the improved soil. Based on the para- tance as a higher strength soilcrete. metric analyses some important conclusions have been devel- 2. A relatively narrow zone of improved soil adjacent to the pile oped, as follows: group increased the lateral resistance relative to untreated clay by 15% to 40% for improved soil block depths of 2.5 to 1. The lateral resistance of the pile group is not sensitive to the 12.5 ft (2.4D to 11.8D), respectively. The trend line of the material strength of the improved soil (including mass mix improvement ratio versus depth is a power function and and jet grouted soilcrete), provided that the improved soil flattens considerably for depths greater than 8D to 10D. As is much stiffer than the virgin clay. When the stiffness is a result, the upper improved layer provides more lateral high relative to the surrounding clay, the soilcrete behaves resistance than the deeper layers. more like a rigid block within the surrounding soil. This 3. For the improved soil beside the cap to a depth of 12.5 ft result suggests that soilcrete with lower strength, which can below the ground, the lateral resistance increased 36% to 2.0 1.8 Improvement Ratio 1.6 1.4 1.2 1.0 0 200 400 600 800 1000 1200 1400 Strength (psi) Figure 6-21. Effect of material strength of improved soil beside the cap on improvement ratio relative to untreated clay at a lateral cap displacement of 1.5 in.
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74 Figure 6-22. Treatment zone relative to pile group for parametric study involving the effect of compressive strength of the improved soil below the cap. 600 500 400 21 psi Load (kips) 63 psi 300 126 psi 252 psi 200 300 psi 600 psi 100 900 psi 1200 psi 0 0.0 0.5 1.0 1.5 2.0 Displacement (in) Figure 6-23. Load-displacement curves computed using the FEM model assuming a variety of compressive strength values for the soil improvement zone beneath the cap.
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75 2.0 1.8 Improvement ratio 1.6 1.4 1.2 1.0 0 200 400 600 800 1000 1200 1400 strength (psi) Figure 6-24. Effect of material strength of improved soil below the cap on improvement ratio relative to untreated clay at a cap lateral displacement of 1.5 in. 60% for soil block length ranging from 3 to 7 ft. The of the pile group in any direction and increase the constraint improvement ratio versus the soil block length is roughly on the piles themselves. linear and increasing the length of the block 1 ft produced an increase in the improvement ratio of 0.06. This increase Generally, increasing the depth and length of the improved is likely associated with increased side and base shear since soil block will increase the lateral capability of the pile group. the passive resistance remains constant with a constant On the other hand, increasing the dimension of the improved width. soil also increases the volume of the improved soil and accord- 4. Improving the soil directly below the pile cap increased lat- ingly increases the economic cost. The optimal dimension of eral resistance by 40% to 85% relative to untreated clay for the improved soil is expected to be a balanced one that com- the improved soil depths of 5.0 to 12.5 ft (4.7D to 11.8D). prehensively considers the engineering demand and the eco- The trend line of the improvement ratio versus depth is nomic budget. roughly a power function and flattens somewhat with depth. It should be mentioned that the above conclusions were As a result, the upper improved soil layers provide more lat- based on the conditions described in this report. Any extrap- eral resistance than do the deeper layers, but increasing the olation of these conclusions should be carefully evaluated to depth can still provide significant increases in resistance. take into account sensitivity of improved soil geometry and Another advantage of the improved soil below the cap is position, the properties of soil and pile, numerical method that the soil improvement will increase the lateral capacity limitations, and other factors.