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OCR for page 85
85 estimating the undrained shear strength acting along the sides olation beyond the results investigated herein is not recom- (assuming the shear strength profile varies with depth). If the mended. With any simplified approach, some discrepancy is treatment block projected area contains a horizontal base, anticipated, and 15% would appear to be within the realm of the adhesion acting on the base of the block also should be sufficient accuracy and precision for general design purposes. included. If the treatment depth is deeper, a tension crack is The graph in Figure 7-15 provides a comparison of results expected to develop behind the trapezoidal block beneath the including the FE parametric studies involving changes in the bottom of the pile cap, thereby nullifying any base adhesion. treatment depth. The FE results and projected area dimensions were presented in Figures 7-11 and 7-13, respectively. For all cases, excellent agreement is obtained and further substanti- Incorporation of the Lateral Resistance ates truncating the projected area depth at three times the from the Treated Soil Mass into embedded pile cap depth. the Analysis of the Pile Group The proposed method decouples the limit equilibrium 7.5 Evaluation for Jet analysis of the treated soil mass from the analysis of the pile Grouting Cases group. The magnitude of lateral resistance provided by the projected area is computed in accordance with the methodol- Similar to the mass mix tests, jet grout ground improvement ogy outlined previously. In order to incorporate this contri- techniques were performed in or around the pile groups and bution into the analysis of the foundation response using tested to evaluate the benefit on performance of the group available software (such as GROUP or similar), the magnitude under lateral loading. Jet grout ground improvement was con- of the applied horizontal force acting on the pile group is structed adjacent to Pile Cap 1 and the leading row of piles in reduced by the contribution of the treated soil mass. the group, as shown in Figure 7-16. It also was constructed To illustrate the applicability of this simplified approach, beneath Pile Cap 2 around the piles in the group. Figures 7-14 and 7-15 have been developed for comparison The plan dimensions provided in Figure 7-16 indicate a purposes. The plots in Figure 7-14 provide a comparison of treatment area of 10 ft perpendicular to the direction of load- results including the FE parametric studies involving changes ing and 15 ft parallel to the loading. The profile dimensions in the treatment dimension parallel to the direction of loading. indicate ground improvement from the bottom of the pile cap The FE results and projected area dimensions were presented to a depth of 12.5 ft below ground surface. All dimensions in Figures 7-10 and 7-12, respectively. For all cases, the simpli- defining jet grout ground improvement must be considered as fied approach is within 15% of the FE model. Less agreement estimates only. Neat lines do not exist considering the nature is apparent for increasing dimensions of treatment, so extrap- of the jet grout process. 275 250 225 Passive + Adhesion (kips) 200 175 150 125 100 75 Simplified (Rankine + Adhesion) 50 Modeled (FE & GROUP) Modeled + 15% 25 Modeled -15% 0 0 1 2 3 4 5 6 7 8 Mass Mix Treatment Dimension in Direction of Loading (ft) Figure 7-14. Simplified and FE results with varying treatment width.

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86 180 160 140 Passive + Adhesion (kips) 120 100 80 Simplified (Rankine + Adhesion) Modeled (FE & GROUP) 60 40 20 0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 Mass Mix Treatment Depth (ft) Figure 7-15. Simplified and FE results with varying treatment depth. Schematic plan view of Test 4. Figure 7-16. Layout of test for pile group with jet grout treatment.

OCR for page 85
87 The unconfined compressive strength of the jet grout The lateral resistance provided by the jet grout ground improved soil at the time of the tests is estimated to be greater improvement is estimated using a limit equilibrium approach, than 600 psi. Further discussion of the strength is provided and the contribution of the improved soil incorporated into the in Chapter 4. However, the results of these analyses, as with analysis by reducing the magnitude of the applied force by the the FE analyses, are not sensitive to the strength of the jet contribution to lateral resistance from the block of improved grout improved soil since it is significantly stronger than ground. This reduced force is then used in GROUP (or the in-situ soil. equivalent) to compute the response of the pile group separately The first attempt at testing the two groups was not success- from the contribution of the improved ground. ful. Because only one actuator was initially used, the load to The passive resistance is estimated using Rankine earth pres- produce significant deflection was not available. Accordingly, sure theory and should be truncated at a depth equal to the an additional actuator was used in series with the first to dou- width of the group from outside pile edge to outside pile edge ble the horizontal load applied to Cap 2. However, additional perpendicular to the direction of loading. The adhesive resis- loading capacity was not available for Cap 1. The measured tance along the sides of the treatment block, also truncated load-displacement curves for Caps 1 and 2 are provided in with depth, is estimated using pass=0.9 times the undrained Figures 3-26 and 3-27, respectively. The maximum loading shear strength. The undrained shear strength may be com- magnitude applied to Caps 1 is about 450k, slightly more than puted using a weighted average over the surface of the block. half the 800k maximum load applied to Cap 2. The 800k The adhesive resistance along the base should also be included maximum load applied to Cap 2 appears to initiate an ulti- in a similar manner to that along the sides. This simplified pro- mate, or limit state, condition with excessive deflection under cedure is shown graphically in Figures 7-17 and 7-18 for the constant load. actual and simplified cases, respectively. The simplified procedure described previously is compared Considering the jet grout ground improvement geometry with the results of the jet grout ground improvement, based on beneath Cap 2 and the unit passive resistance and shear strength the results measured at Cap 2 (jet grout beneath cap and versus depth data presented in Figure 7-9, the summation of around piles). Insufficient load versus deflection data exist for passive and adhesive resistance can be estimated as follows for Cap 1 to draw definitive conclusions. However, based on the the truncated depth: similar load-deflection behavior observed at both caps at rela- tively small displacements, and the applicability of the simpli- Passive Resistance: 10 ft 7.5 ft 1.85ksf = 139k fied procedure to mass mix or jet grout improved soils, it is where 10 ft is the treatment dimension perpendicular to considered reasonable that the simplified procedure is appro- loading, priate for conditions tested at Cap 1. where 7.5 ft is the truncated depth, and 1 0 Ground Surface -1 -2 -3 -4 -5 Feet -6 -7 -8 -9 -10 Jet Grout -11 -12 -13 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 Figure 7-17. Jet grout test as performed (Cap 2).

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88 2 Reduced Load 1 0 -1 -2 -3 Passive Adhesion (2 Sides) -4 Resistance -5 Feet -6 -7 -8 Adhesion (Base) -9 Truncated Depth -10 -11 -12 -13 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 Figure 7-18. Forces on pile cap for simplified procedure for jet grout test. where 1.85 ksf is the average unit passive resistance over where 1.84ksf is the average unit passive resistance over the truncated depth. the treatment depth. Adhesive Resistance (sides): 2 sides 5 ft 15 ft 0.9 Adhesive Resistance (sides): 2 sides 10 ft 15 ft 0.9 0.58ksf = 78k 0.56ksf = 151k where 5 ft is the truncated treatment depth beneath the where 10 ft is the truncated treatment depth beneath cap (note the sides of the cap are not in contact with the the cap (note the sides of the cap are not in contact with soil), the soil), where 15 ft is the treatment dimension parallel to loading, where 15 ft is the treatment dimension parallel to loading, where 0.9 is pass, and where 0.9 is pass, and where 0.58 ksf is the average undrained shear strength where 0.56 ksf is the average undrained shear strength over this depth. over this depth. Adhesive Resistance (base): 10ft 15ft 0.9 0.58ksf = 78k Adhesive Resistance (base): 10 ft 15 ft 0.9 0.50ksf = 68k where 10 ft is the treatment dimension perpendicular to where 10 ft is the treatment dimension perpendicular to loading, loading, where 15 ft is the treatment dimension parallel to where 15 ft is the treatment dimension parallel to loading, loading, where 0.9 is pass, and where 0.9 is pass, and where 0.58 ksf is the undrained shear strength at this where 0.50 ksf is the undrained shear strength at this depth. depth. Therefore, reduce the applied load by: 230k + 151k + 78k = Therefore, reduce the applied load by: 139k + 78k + 68k = 459k. 285k. If the magnitude of the applied load is reduced by the Alternatively, if the depth were not truncated, the summa- amounts listed above (285k for truncated depth and 459k for tion of passive and adhesive resistance would be estimated as full treatment depth), and applied considering the appropri- follows for the entire treatment depth: ate effective cap depth (7.5 ft and 12.5 ft beneath ground sur- face, respectively) using the GROUP model discussed above, Passive Resistance: 10 ft 12.5 ft 1.84ksf = 230k good agreement with measured performance is obtained. A where 10 ft is the treatment dimension perpendicular to comparison of the results predicted using the aforementioned loading, simplified procedure versus the measured data is provided in where 12.5 ft is the treatment depth, and Figure 7-19.