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Suggested Citation:"Chapter 6 - Parametric Studies." National Academies of Sciences, Engineering, and Medicine. 2011. Design Guidelines for Increasing the Lateral Resistance of Highway-Bridge Pile Foundations by Improving Weak Soils. Washington, DC: The National Academies Press. doi: 10.17226/14574.
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Suggested Citation:"Chapter 6 - Parametric Studies." National Academies of Sciences, Engineering, and Medicine. 2011. Design Guidelines for Increasing the Lateral Resistance of Highway-Bridge Pile Foundations by Improving Weak Soils. Washington, DC: The National Academies Press. doi: 10.17226/14574.
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Suggested Citation:"Chapter 6 - Parametric Studies." National Academies of Sciences, Engineering, and Medicine. 2011. Design Guidelines for Increasing the Lateral Resistance of Highway-Bridge Pile Foundations by Improving Weak Soils. Washington, DC: The National Academies Press. doi: 10.17226/14574.
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Suggested Citation:"Chapter 6 - Parametric Studies." National Academies of Sciences, Engineering, and Medicine. 2011. Design Guidelines for Increasing the Lateral Resistance of Highway-Bridge Pile Foundations by Improving Weak Soils. Washington, DC: The National Academies Press. doi: 10.17226/14574.
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Suggested Citation:"Chapter 6 - Parametric Studies." National Academies of Sciences, Engineering, and Medicine. 2011. Design Guidelines for Increasing the Lateral Resistance of Highway-Bridge Pile Foundations by Improving Weak Soils. Washington, DC: The National Academies Press. doi: 10.17226/14574.
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Suggested Citation:"Chapter 6 - Parametric Studies." National Academies of Sciences, Engineering, and Medicine. 2011. Design Guidelines for Increasing the Lateral Resistance of Highway-Bridge Pile Foundations by Improving Weak Soils. Washington, DC: The National Academies Press. doi: 10.17226/14574.
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Suggested Citation:"Chapter 6 - Parametric Studies." National Academies of Sciences, Engineering, and Medicine. 2011. Design Guidelines for Increasing the Lateral Resistance of Highway-Bridge Pile Foundations by Improving Weak Soils. Washington, DC: The National Academies Press. doi: 10.17226/14574.
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Suggested Citation:"Chapter 6 - Parametric Studies." National Academies of Sciences, Engineering, and Medicine. 2011. Design Guidelines for Increasing the Lateral Resistance of Highway-Bridge Pile Foundations by Improving Weak Soils. Washington, DC: The National Academies Press. doi: 10.17226/14574.
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Suggested Citation:"Chapter 6 - Parametric Studies." National Academies of Sciences, Engineering, and Medicine. 2011. Design Guidelines for Increasing the Lateral Resistance of Highway-Bridge Pile Foundations by Improving Weak Soils. Washington, DC: The National Academies Press. doi: 10.17226/14574.
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Suggested Citation:"Chapter 6 - Parametric Studies." National Academies of Sciences, Engineering, and Medicine. 2011. Design Guidelines for Increasing the Lateral Resistance of Highway-Bridge Pile Foundations by Improving Weak Soils. Washington, DC: The National Academies Press. doi: 10.17226/14574.
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Suggested Citation:"Chapter 6 - Parametric Studies." National Academies of Sciences, Engineering, and Medicine. 2011. Design Guidelines for Increasing the Lateral Resistance of Highway-Bridge Pile Foundations by Improving Weak Soils. Washington, DC: The National Academies Press. doi: 10.17226/14574.
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Suggested Citation:"Chapter 6 - Parametric Studies." National Academies of Sciences, Engineering, and Medicine. 2011. Design Guidelines for Increasing the Lateral Resistance of Highway-Bridge Pile Foundations by Improving Weak Soils. Washington, DC: The National Academies Press. doi: 10.17226/14574.
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Suggested Citation:"Chapter 6 - Parametric Studies." National Academies of Sciences, Engineering, and Medicine. 2011. Design Guidelines for Increasing the Lateral Resistance of Highway-Bridge Pile Foundations by Improving Weak Soils. Washington, DC: The National Academies Press. doi: 10.17226/14574.
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Suggested Citation:"Chapter 6 - Parametric Studies." National Academies of Sciences, Engineering, and Medicine. 2011. Design Guidelines for Increasing the Lateral Resistance of Highway-Bridge Pile Foundations by Improving Weak Soils. Washington, DC: The National Academies Press. doi: 10.17226/14574.
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Suggested Citation:"Chapter 6 - Parametric Studies." National Academies of Sciences, Engineering, and Medicine. 2011. Design Guidelines for Increasing the Lateral Resistance of Highway-Bridge Pile Foundations by Improving Weak Soils. Washington, DC: The National Academies Press. doi: 10.17226/14574.
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61 The parametric studies provide numerical simulation of the behavior of the pile group subjected to lateral loading and eliminate the cost of additional field tests. The basic approach for the parametric studies is to develop calibrated soil and pile parameters based on the results of the field testing. Then, using these calibrated parameters, the depth, width, and strength of the improved ground can be systematically varied to determine the effect on computed pile group response such as load-deflection curves, maximum moment-load curves, etc. This chapter describes parametric studies associated with the various soil improvement methods. 6.1 Mass Mix Depth Effect (Beside the Cap) on Lateral Resistance The first parametric study involved an investigation of the depth of the mass mix block produced by soil improvement on the pile group capacity. The mass mix block is assumed to be immediately adjacent to the pile cap but does not contact the piles themselves. As shown in Figure 6-1, the mass mix block length was held constant at 4 ft in the direction of loading with a width of 9 ft (cap width) perpendicular to the lateral load direction while the depth was increased. The top of the mass mix was assumed to be at the same depth as the cap top, which is at a depth of zero ft. Analyses were performed for mass mix blocks with bottom depths of 2.5, 5.0, 7.5, 10.0, and 12.5 ft (about 2.4D, 4.7D, 7.0D, 9.4D, and 11.8D, where D is the pile diameter). Other soil and pile parameters were kept the same as described in Section 5.4. The load-displacement curves calculated by the FEM analy- sis for each soil mix depth are plotted in Figure 6-2. The per- cent increase in lateral resistance for improved soil relative to the virgin soil at a reference lateral displacement of 1.5 in. is 14%, 27%, 33%, 36%, and 41% for the mass mix bottom depth of 2.5, 5.0, 7.5, 10.0, and 12.5 ft, respectively. It is readily appar- ent that the lateral load increases as the depth of the block increases; however, the increase is not uniform, indicating that a greater proportion of the lateral resistance is carried by the upper part of the mass mix block. The improvement ratio is defined as the lateral load in the improved soil over the lateral load in the virgin soil at a refer- ence cap lateral displacement of 1.5 in. Figure 6-3 provides a plot the improvement ratio versus the depth of the mass mix zone adjacent to the cap. An equation for the trend line computed using the least square method is also shown in Figure 6-3. The slope of the trend line becomes flatter as the depth of the mass mix depth increases, which suggests increasing the depth is becoming less effective in increasing the lateral resistance. Beyond a certain limit, increasing the depth of mass mix treat- ment provides only a relatively limited increase in lateral capacity. These results suggest that the optimal soil improve- ment depth of mass mixing beside the pile cap would be about 10 pile diameters for a similar soil and pile profile. 6.2 Mass Mix Depth Effect (Below the Cap) on Lateral Resistance The second parametric study investigated the effect of the depth of a mass mix layer below the pile cap on the lateral pile group resistance. The mass mix block is assumed to have the same cross section as the cap (length of 9 ft in the direc- tion of loading and a width of 9 ft perpendicular to loading), and has variable depths along the depth direction. The top of the mass mix block is at the base of the pile cap (a depth of 2.5 ft) and the bottom of the mass mix block is at depths of 5.0, 7.5, 10.0, and 12.5 ft (see Figure 6-4). Other soil and pile parameters remain the same as described in Section 5.4. There is no linkage between base of the cap and the top of the mass mix block. The load-displacement curves computed with the FEM model are presented in Figure 6-5. The depth of the mass mix block below the cap has a significant effect on the lateral capac- ity of the pile group. The increased lateral capacity at the refer- ence cap lateral displacement of 1.5 in. is 41%, 60%, 75%, and C H A P T E R 6 Parametric Studies

62 0 50 100 150 200 250 300 350 400 450 0.0 0.5 1.0 1.5 2.0 Displacement (in) Lo ad (k ip s ) Depth to 2.5 ft Depth to 5.0 ft Depth to 7.5 ft Depth to 10.0 ft Depth to 12.5 ft Virgin Soil Figure 6-1. Mass mixing depth intervals adjacent to cap for parametric study. Figure 6-2. Parametric study of mass mix depth adjacent to the cap on the computed load-displacement curve.

63 y = 1.0204x0.1289 R2 = 0.9956 1.0 1.2 1.4 1.6 1.8 2.0 0.0 5.0 10.0 15.0 Depth (ft) Im pr ov em e n t R a tio Figure 6-3. Improvement ratio as a function of mass mix depth adjacent to cap. Figure 6-4. Mass mixing depth intervals below cap for parametric study.

tens somewhat as the depth of the mass mix zone increases, which suggests that the upper mass mix zone provides more lateral resistance, as expected. It should also be noted that the improvement ratio pro- duced by mass mixing below the cap is much higher than that produced by mass mixing beside the cap for the same depth of treatment. This may be due to the larger cross section (9 × 9 ft for mass mix below the cap in this section, 9 × 4 ft for mass beside the cap as in Section 6.1) as well as the constraint of piles by the mass mix. However, in practical design, since the external load (e.g., wind or earthquake load) direction is very random, the soil improvement beside the cap should be pre- formed on all four sides of the cap. In contrast, the mass mix below the cap will resist external load in any direction. 64 0 100 200 300 400 500 600 0.0 0.5 1.0 1.5 2.0 Displacement (in) Lo ad (k ip s ) Depth to 5.0 ft Depth to 7.5 ft Depth to 10.0 ft Depth to 12.5 ft Virgin Soil y = 0.8691x0.3021 R2 = 0.9986 1.0 1.2 1.4 1.6 1.8 2.0 0.0 5.0 10.0 15.0 Depth (ft) Im pr o v e m e n t R a tio Figure 6-5. Results of parametric study of the effect of the mass mix depth below the cap on the computed load-displacement curve. Figure 6-6. Improvement ratio as a function of mass mix depth below the cap. 86% for the mass mix depths of 5.0, 7.5, 10.0, and 12.5 ft, respectively. Figure 6-6 provides a plot of the improvement ratio versus the depth of the mass mix below the cap using a best-fit trend line. These results suggest that mass mixing below the cap will dramatically increase the lateral capacity of the pile group rel- ative to that in virgin clay. For mass mix zones with thick- nesses ranging from 2.5 ft to 10.0 ft (2.4D to 9.4D) underneath the cap, the lateral resistance will increase by approximately 40% to 86% for similar soil and pile profiles. The increased resistance from the soil treatment is partially due to the increased passive area and partially due to constraint of piles by the mass mix (so that the mass mix and the piles can be consid- ered as an integrated block). The slope of the trend line flat-

6.3 Mass Mix Length Effect (Beside the Cap) on Lateral Resistance The third parametric study involved an investigation of the length of the mass mix block in the direction of the loading on the increase in lateral pile group resistance. In this case, the mass mix block also was assumed to be beside the pile cap but not in contact with the piles. The depth of the block was fixed at 12.5 ft and the width was taken as 9.0 ft perpendicular to the lateral load direction (4.5 ft in the FEM model due to sym- metry), which is equal to the pile cap width. Other soil and pile parameters were kept the same as described in Section 5.4. Finite element analyses were then performed for mass mix block lengths ranging from 3 ft to 7 ft in the direction of lateral loading as illustrated in Figure 6-7. The load-displacement curves calculated with the FEM model are plotted in Figure 6-8. As the length of the mass mix zone increases, the lateral resistance gradually increases in a rather consistent fashion. Figure 6-9 provides a plot of the improvement ratio as a function of the length of the mass mix block in the direction of the lateral loading relative to the vir- gin clay resistance at a lateral displacement of 1.5 in. The improvement ratio is 1.36, 1.42, 1.47, 1.54, and 1.60 for the mass mix lengths of 3.0, 4.0, 5.0, 6.0, and 7.0 ft, respectively. As observed in Figure 6-9, the improvement ratio increases almost linearly with the mass mix length. This is considerably different from the nonlinear trend lines obtained from the parametric studies relative to mass mix depth presented in Sections 6.1 and 6.2. The correlation equation shows that each additional foot of length leads to an additional increase in the improvement ratio of about 0.06. Since the passive pressure area is the same for various lengths of the mass mixes, the increase in the lateral capacity is considered to be a result of the increase in the shear area of the mass mix block. However, 65 Figure 6-7. Mass mix length intervals beside cap for parametric study.

increasing mass mix length will increase the cost. The optimal length requires a balance analysis between engineering capacity and economic efficiency. 6.4 Jet Grout Depth Effect (Beside the Cap) on Lateral Resistance This parametric study is very similar to that conducted for the mass mix treatment except that the improved soil is now soilcrete produced by jet grouting, which has a higher strength than that from the soil mixing. The jet grout soilcrete is assumed to be at the side of the pile cap and has a length of 4 ft in the direction of loading with a width of 9 ft transverse to the lateral load direction (see Figure 6-10). FEM analyses are conducted for variable soilcrete depths. The top of the soilcrete is assumed to be at the ground surface and the bottom of the soilcrete is at depths of 2.5, 5.0, 7.5, 10.0, and 12.5 ft as shown in Figure 6-10. Other soil and pile parameters are the same as those described in Section 5.5. The load-displacement curves computed by the FEM model are presented in Figure 6-11. The depth of the jet grout beside the cap has a significant effect on the lateral resistance of the pile group. The increased lateral resistance (or improve- ment ratio minus 1) at the cap lateral displacement of 1.5 in. is 13%, 25%, 31%, 35%, and 40% for the mass mix depth of 2.5, 5.0, 7.5, 10.0, and 12.5 ft, respectively. 66 0 100 200 300 400 500 600 0.0 0.5 1.0 1.5 2.0 Displacement (in) Lo a d (ki ps ) Virgin Soil Length = 3.0 ft Length = 4.0 ft Length = 5.0 ft Length = 6.0 ft Length = 7.0 ft y = 0.0607x + 1.1746 R2 = 0.9976 1 1.2 1.4 1.6 1.8 2 0 2 4 61 3 5 7 8 Length of Mass Mix (ft) Im pr o v e m en t R a tio Figure 6-8. Results of parametric study on the length of the mass mix zone beside the pile cap on the computed load-displacement curve. Figure 6-9. Improvement ratio as a function of mass mix length adjacent to the cap.

67 0 50 100 150 200 250 300 350 400 450 500 0.0 0.5 1.0 1.5 2.0 Displacement (in) La te ra l F or ce (ki ps ) Virgin Soil Depth to 2.5 ft Depth to 5.0 ft Depth to 7.5 ft Depth to 10.0 ft Depth to 12.5 ft Figure 6-10. Jet grout soilcrete depth intervals beside cap for parametric study. Figure 6-11. Parametric study of jet grout depth adjacent to the cap on the computed load-displacement curve.

Figure 6-12 plots the improvement ratio versus the depth of the jet grout beside the cap. The equation for the best-fit trend is a power function. As was the case for the mass mix treatment, the slope of the trend line flattens as the depth of the jet grout soilcrete increases. This result suggests that the upper soilcrete zone carries more lateral resistance. Increasing the depth of the jet grout zone beside the cap will increase the lateral capacity of the pile group; however, when the depth reaches a certain value (roughly 10 times that of the pile diameter), increasing addi- tional depth will only provide limited increases in lateral pile group resistance relative to virgin clay. 6.5 Jet Grout Depth Effect (Below the Cap) on Soil Improvement For the numerical tests of the jet grout below the cap, the jet grout soilcrete has the same cross section as the cap (length of 9 ft in direction of loading and width of 9 ft perpendicular to the direction of loading). The bottom of the soilcrete block is at depths of 5.0, 7.5, 10.0, and 12.5 ft below the ground surface as shown in Figure 6-13. There is no linkage between the base of the cap and the top of the jet grouting block. Other soil and pile parameters remain the same as described in Section 5.5. Figure 6-14 provides load-displacement curves computed by the FEM model. Again, the depth of the jet grout treatment below the cap is found to have a significant impact on the lat- eral resistance of the pile group. At a lateral pile cap displace- ment of 1.5 in. the lateral capacity is increased by 40%, 59%, 74%, and 85% for the mass mix depths of 5.0, 7.5, 10.0, and 12.5 ft, respectively. Figure 6-15 provides a plot of the improvement ratio versus the depth of the soilcrete below the cap. A best-fit curve and the accompanying equation also are provided. These results sug- gest that the soil improvement method of jet grouting soilcrete below the cap will dramatically increase the lateral capacity of the pile group in virgin clay. A jet grouting soilcrete with the height of 2.5 ft to 10 ft (2.4D to 9.4D) will increase the lateral capacity by approximately 40% to 85% for similar soil and pile profiles. The slope of the trend line flattens somewhat as the soilcrete depth increases, which suggests that the upper layers in the improved zone provide more lateral resistance than do the lower layers. It should be noted that the improvement ratio produced by jet grout below the cap (1.40 to 1.85) is much higher than that produced by jet grout beside the cap for the soilcrete with the same depth. This result is likely a result of the larger cross sec- tion (9 × 9 ft for the jet grouting soilcrete below the cap in this section, 9 × 4 ft for jet grout beside the cap as in section 6.4) as well as the constraint of piles by the jet grouting. Again, the improved soil below the cap will resist external load in any direction. It also is important to note that the increase in lateral resis- tance for the soilcrete produced by jet grouting was very sim- ilar to that obtained for the soilcrete produced by mass mixing despite the lower compressive strength. This result suggests that lower strengths that can be produced by less expensive treatment approaches might still be effective in improving the lateral resistance. This issue will be investigated further in a subsequent parametric study. 6.6 Jet Grout Length Effect (Beside the Cap) on Soil Improvement This parametric investigation focuses on the block length effect on the pile group lateral capacity for the jet grout mix beside the cap. The jet grouting soilcrete has a fixed depth of 12.5 ft and a width of 9 ft (4.5 ft in the FEM model due to symmetry) perpendicular to the lateral load direction, and has variable length from 3 ft to 7 ft along the lateral loading direction (see Figure 6-16). Other soil and pile parameters are kept the same as described in Section 5.5. The load-displacement curves calculated with the FEM model are presented in Figure 6-17 and the improvement ratio versus soilcrete length is provided in Figure 6-18. As the length increases, the load-displacement curves increase relatively con- sistently. The improvement ratios are 1.34, 1.40, 1.45, 1.51, and 1.58 for the jet grouting soilcrete lengths of 3.0, 4.0 5.0, 6.0, and 7.0 ft, respectively. The trend line in Figure 6-18 shows that the improvement ratio versus the jet grout soilcrete length is roughly linear, which is different from the nonlinear trend lines associated with the jet grout depth as presented previously. The correlation equation shows that each additional foot of jet grouting soilcrete length will cause the improvement ratio to increase by 0.06. This result is identical to that found for the parametric study of soilcrete length with soil mixing. Since the passive pressure area is the same for various lengths of the soil- 68 y = 1.0059x0.1291 R2 = 0.9953 1.0 1.2 1.4 1.6 1.8 2.0 0 5 10 15 Depth (ft) Im pr o v e m e n t R a tio Figure 6-12. Effect of jet grout depth beside the cap on the improvement ratio for a lateral displacement of 1.5 in.

69 0 100 200 300 400 500 600 0.0 0.5 1.0 1.5 2.0 Displacement (in) La te ra l F o rc e (ki ps ) Virgin Soil Depth to 5.0 ft Depth to 7.5 ft Depth to 10.0 ft Depth to 12.5 ft Figure 6-13. Jet grout depth intervals below cap for parametric study. Figure 6-14. Results of parametric study of the effect of the mass mix depth below the cap on the computed load-displacement curve.

70 Figure 6-16. Jet grout treatment length intervals beside cap for parametric study. y = 0.8553x0.3068 R2 = 0.9987 1.0 1.2 1.4 1.6 1.8 2.0 0 5 10 15 Depth (ft) Im pr ov em en t R a tio Figure 6-15. Improvement ratio as a function of jet grout treatment depth below the cap.

on the increase in lateral pile group resistance. For the first of these studies, the improved soil block is assumed to be at the side of the pile cap and have a length of 4 ft in the direction of loading, a width of 9 ft perpendicular to the loading direction, and a depth of 12.5 ft (see Figure 6-19). The compressive strength of the improved soil (f ′c) is assumed to vary over a wide range between 21 and 7700 psi. Young’s modulus is assumed to abide by the same relation as that of conventional concrete [E = 57000(f ′c)0.5] and is in the range of 260 to 5000 ksi, which covers the typical strengths of mass mixed and jet grouted soil (see Table 6-1). Since the improved soils with the Young’s moduli shown in Table 6-1 are much stiffer than the virgin clay, the improved soils have been modeled as linear elastic materials. The load-displacement curves computed using the FEM model are presented in Figure 6-20 and the improvement ratio is plotted versus compressive strength in Figure 6-21. As shown in Figure 6-20, the load-displacement curves all plot on top of each other for the range of compressive strengths investigated. As a result, the improvement ratio is essentially constant rela- tive to the compressive strength of the treated zone as shown in Figure 6-21. These results show that the lateral capacity of the pile group is not sensitive to the compressive strength of the treated zone, which is as expected since all of the improved soils are much stiffer than the virgin clay. Therefore, for practical purposes, the improved soils can be considered to act as a rigid block for the range of material properties in Table 6-1. The numerical model suggests that the lateral capacity of the pile group is sensitive to the geometry of the improved soil, but not to the material strength (or Young’s Modulus), provided the improved soil is much stiffer than the virgin clay. The typical mass mix and jet grouting soilcrete are much stiffer than the soft clay. 71 0 50 100 150 200 250 300 350 400 450 500 0.0 0.5 1.0 1.5 2.0 Displacement (in) La te ra l F or ce (ki ps ) Virgin Soil Length = 3 ft Length = 4 ft Length = 5 ft Length = 6 ft Length = 7 ft y = 0.0595x + 1.1592 R2 = 0.9976 1.0 1.2 1.4 1.6 1.8 2.0 0 1 2 3 4 5 6 7 8 Length (ft) Im pr o v e m e n t R a tio Figure 6-17. Results of parametric study on the length of the jet grout treatment zone beside the pile cap on the computed load-displacement curve. Figure 6-18. Improvement ratio as a function of jet grout treatment length adjacent to the cap. crete, the increase of the lateral capacity is mainly caused by the increase of the shear area of the jet grouting soilcrete. There- fore, these results indicate that a lower strength soilcrete, which could be produced with a lower cost treatment method, could produce the same adhesive resistance as that obtained with jet grouting. This finding increases the potential that soil improve- ment methods can be a cost-effective approach for increasing lateral pile group resistance. 6.7 Material Strength Effect on Lateral Pile Group Resistance Based on the results from the previous parametric studies, another set of parametric studies was performed to investi- gate the effect of the strength of the soil improvement zone

72 Strength (psi) Young’s Modulus (ksi) Note 21 261 63 452 126 640 Typical mass mix 252 905 300 987 600 1400 Typical jet grout 900 1710 1200 1980 4000 3600 7700 5000 Typical concrete The final set of parametric studies involved varying the compressive strength of the soilcrete zone directly below the pile cap as shown in Figure 6-22. As shown, the soilcrete zone was 9 ft square in plan view and extended 12.5 ft below the ground surface. As in the previous case, the computed load-displacement curves plot on top of each other as shown in Figure 6-23 and the improvement ratio shown in Figure 6-24 is nearly constant with strength. These results indicate that the lateral resistance is relatively insensitive to the soilcrete strength, provided the soilcrete is much stiffer than the vir- gin clay. 6.8 Conclusions Based on Parametric Studies Verification and validation procedure was conducted for the finite element model before the parametric studies. Mesh and boundary sensitivity analyses were performed and the soil material properties were carefully calibrated by comparison with the test data for the single pile and pile groups. The load- displacement curves obtained from the numerical models fit satisfactorily with the test data. Parameter studies were then performed to examine the sensitivity of the depth (beside and Table 6-1. Material strength of the improved soil. 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.

73 0 50 100 150 200 250 300 350 400 450 0.0 0.5 1.0 1.5 2.0 Displacement (in) Lo a d (ki ps ) 21 psi 63 psi 126 psi 252 psi 300 psi 600 psi 900 psi 1200 psi 1.0 1.2 1.4 1.6 1.8 2.0 0 200 400 600 800 1000 1200 1400 Strength (psi) Im pr o v em en t R a tio 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. 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. below the cap) and the length of the improved soil, includ- ing mass mix and jet grout, as well as the sensitivity of the material strength of the improved soil. Based on the para- metric analyses some important conclusions have been devel- oped, as follows: 1. The lateral resistance of the pile group is not sensitive to the material strength of the improved soil (including mass mix and jet grouted soilcrete), provided that the improved soil is much stiffer than the virgin clay. When the stiffness is high relative to the surrounding clay, the soilcrete behaves more like a rigid block within the surrounding soil. This result suggests that soilcrete with lower strength, which can be produced by less expensive treatment approaches, can still produce the same increases in lateral pile group resis- tance as a higher strength soilcrete. 2. A relatively narrow zone of improved soil adjacent to the pile group increased the lateral resistance relative to untreated clay by 15% to 40% for improved soil block depths of 2.5 to 12.5 ft (2.4D to 11.8D), respectively. The trend line of the improvement ratio versus depth is a power function and flattens considerably for depths greater than 8D to 10D. As a result, the upper improved layer provides more lateral resistance than the deeper layers. 3. For the improved soil beside the cap to a depth of 12.5 ft below the ground, the lateral resistance increased 36% to

74 0 100 200 300 400 500 600 0.0 0.5 1.0 1.5 2.0 Displacement (in) Lo ad (k ip s ) 21 psi 63 psi 126 psi 252 psi 300 psi 600 psi 900 psi 1200 psi 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. 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.

60% for soil block length ranging from 3 to 7 ft. The improvement ratio versus the soil block length is roughly linear and increasing the length of the block 1 ft produced an increase in the improvement ratio of 0.06. This increase is likely associated with increased side and base shear since the passive resistance remains constant with a constant width. 4. Improving the soil directly below the pile cap increased lat- eral resistance by 40% to 85% relative to untreated clay for the improved soil depths of 5.0 to 12.5 ft (4.7D to 11.8D). The trend line of the improvement ratio versus depth is roughly a power function and flattens somewhat with depth. As a result, the upper improved soil layers provide more lat- eral resistance than do the deeper layers, but increasing the depth can still provide significant increases in resistance. Another advantage of the improved soil below the cap is that the soil improvement will increase the lateral capacity of the pile group in any direction and increase the constraint on the piles themselves. Generally, increasing the depth and length of the improved soil block will increase the lateral capability of the pile group. On the other hand, increasing the dimension of the improved soil also increases the volume of the improved soil and accord- ingly increases the economic cost. The optimal dimension of the improved soil is expected to be a balanced one that com- prehensively considers the engineering demand and the eco- nomic budget. It should be mentioned that the above conclusions were based on the conditions described in this report. Any extrap- olation of these conclusions should be carefully evaluated to take into account sensitivity of improved soil geometry and position, the properties of soil and pile, numerical method limitations, and other factors. 75 1.0 1.2 1.4 1.6 1.8 2.0 0 200 400 600 800 1000 1200 1400 strength (psi) Im pr o v e m en t r at io 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.

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Design Guidelines for Increasing the Lateral Resistance of Highway-Bridge Pile Foundations by Improving Weak Soils Get This Book
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TRB’s National Cooperative Highway Research Program (NCHRP) Report 697: Design Guidelines for Increasing the Lateral Resistance of Highway-Bridge Pile Foundations by Improving Weak Soils examines guidance for strengthening of soils to resist lateral forces on bridge pile foundations.

The report presents computational methods for assessing soil-strengthening options using finite-element analysis of single piles and pile groups and a simplified approach employing commercially available software.

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