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57 400 Model, without Excavation 350 Model, with Excavation Model, Passive Force 300 Lateral Force (kips) Test, Passive Force 250 200 150 100 50 0 0.0 0.5 1.0 1.5 2.0 Displacement (in) Figure 5-6. Simulated and tested pile cap passive pressure. about 140 psi after 63 days with a standard deviation of about 8 psi. Assuming that the soil-cement mixture cured at the same Figure 5-5. Finite element mesh for model rate as concrete alone, the compressive strength of the mixture of pile group with excavation adjacent at the time of testing would be approximately 126 psi. to cap. Preliminary analyses suggested that the shear strength of the mass mixed wall was sufficient to allow the wall to behave steps with pile head lateral displacement of 0.03 in. per load- essentially as a rigid block. Based on this conclusion and the ing step. fact that the mass mix has much higher strength than the clay, The mesh of the FEM pile group model in virgin soil with the mass mix is modeled as elastic material. This assump- excavation adjacent to the cap is shown in Figure 5-5. A total tion will introduce little error to the pile load-displacement of 49,977 nodes and 44,666 elements were in the FEM mesh. response against a more complicated material model for the The load-displacement curves computed using the FEM model mass mix. The Young's modulus of the mass mix zone is with and without excavation adjacent to the cap were used to estimated as 6.4 × 105 psi from the compressive strength of produce the passive force-displacement curve. The passive the mixture at the time of testing (126 psi) with the assump- force on the pile cap was obtained by subtracting the load with tion that the Young's modulus can be estimated with the same excavation from that without excavation. The simulated pas- formula as that for concrete [E = 57000(f c )0.5]. The Poisson's sive force-displacement curve is compared with the measured ratio of the mass mix is assumed to be 0.2. curve in Figure 5-6. The simulated force is greater than the A comparison of load-displacement curves from the test measured force for displacements less than about 0.3 in., which data and the FEM model is provided in Figure 5-8. The FEM is as expected for small displacement due to the residual dis- model provides very good agreement with the measured placement. However, the agreement is satisfactory for displace- results. These results indicate that the linear elastic material ments more than 0.3 in. model with the material properties described previously can reasonably represent the lateral resistance of the pile group after treatment with mass mixing. Furthermore, this result 5.4 FEM Model of Pile Group provides confidence that additional parametric studies can be with Mass Mixing used as "virtual load tests" for the purpose of developing a The mass mixing treatment zone in the FEM model is shown simplified model. in Figure 5-7. The modeled zone is 10 ft deep, 4 ft long in the direction of the lateral loading direction, and 11 ft wide trans- 5.5 Pile Group Model verse to the loading direction. These dimensions are the same with Jet Grouting as in the field test. Six 3-in. diameter core samples were extracted and tested Soil improvement using jet grouting was undertaken in after 38 and 63 days of curing. The test results indicated an aver- Test 8. For Cap 1, improvement was limited to zones in the age strength of 131 psi after 38 days and an average strength of front and back sides of the pile cap, while for Cap 2 the improve-
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58 Figure 5-7. FEM model profile with mass mix treatment zone beside the cap--mass mix zone is 10 ft deep, 4 ft long, and 11 ft wide transverse to loading. 600 ment extended underneath the entire pile cap as shown in Fig- ure 5-9. 500 Figure 5-10 shows a schematic profile diagram of the FEM model with the jet grout treatment zone. The model 400 shown in Figure 5-2 was used to model the pile with jet grout Load (kips) treatment after appropriate elements in the model were 300 assigned the properties corresponding to the soilcrete. Analy- ses suggested that the shear strength of jet grouting soilcrete 200 was sufficient to allow the soilcrete zone to behave as a rigid Model block. Based on this conclusion and the fact that the soilcrete 100 Test has a much higher strength than the clay, the soilcrete was modeled as an elastic material. This assumption will introduce 0 little error into the pile load-displacement response relative to 0.0 0.5 1.0 1.5 2.0 2.5 a more complicated material model for jet grouting soilcrete. Displacement (in) The Young's modulus of the soilcrete is estimated as 1.4 × 106 Figure 5-8. Comparison of load-displacement psi based on the compressive strength of the mixture at the curves from FEM model and field test with mass time of testing, which would be approximately 600 psi. This mix zone adjacent to the pile cap. modulus value is based on the assumption that the Young's
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59 Figure 5-9. Layout of test using jet grouting under Cap 2. Figure 5-10. Schematic profile drawing of FEM model with jet grouting beside and/or underneath the cap.
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60 1000 modulus can be reasonably well estimated based on the same 900 formula as that for concrete, E = 57000(f c )0.5. The Poisson's ratio of the mass mix is assumed to be 0.2. 800 A comparison of the measured load-displacement curve 700 Lateral Force (kips) with that computed with the FEM model is provided in Fig- 600 ure 5-11. Once again, the agreement between measured and 500 computed results is generally satisfactory especially for pile cap Test 400 displacement more than 0.5 in. The researchers noted that Model 300 for small displacements of the cap less than 0.5 in., the load- displacement curve from the test is somewhat stiffer than 200 the curve obtained by the FEM model. This could be due to the 100 linkage between the pile cap and jet grouting soilcrete. Once 0 again, the relatively good agreement suggests that using the 0.0 0.5 1.0 1.5 2.0 2.5 3.0 linear elastic material model with the above-mentioned prop- Displacement (in) erties can produce reliable estimates of the measured load- Figure 5-11. Comparison of FEM computed and deflection curve obtained from the field tests with jet grout measured load-displacement curve with jet treatment around the pile group. Therefore, the same model grouting below and beside the cap. The jet can be used for parametric studies using the variations on grouting soilcrete has a depth of 10 ft from the geometries to expand our understanding of the increased lat- cap base, a length of 15 ft, and a width of 9 ft. eral resistance provided by jet grouting.