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OCR for page 37
37 5 to 10 ft below the top of the cap was progressively excavated 400 with the excavator bucket and mixed with the jet grout spoils. Mixing was accomplished by repeatedly stirring the native 300 soil and grout spoil until the consistency of the mixture Load (kips) became relatively homogeneous and no large blocks were obvious in the mixture. This process required approximately 200 10 to 15 minutes of mixing and provided a 1:1 ratio of soil to grout spoil mixture. 100 Virgin Excavated T2 Cap 1 The grout used in the jet grouting procedure was designed to have a specific gravity of approximately 1.52, which is the Soil Mix Excavated T15 Cap 1 equivalent of a 1:1 water to cement ratio by weight using nor- 0 mal Type I cement. The cement content per volume of jet -0.5 0 0.5 1 1.5 2 2.5 3 3.5 grout slurry was computed to be about 24 lbs/ft3. Mixing the Displacement (in) jet grout slurry with the underlying clay at a 1:1 ratio by vol- Figure 3-29. Comparison of measured load- ume reduced the cement content of the resulting soilcrete wall displacement curves for Cap 1 in virgin soil and after to approximately 12 lbs/ft3. This corresponds to about 10% construction of a soil mix wall on one side of the cap cement by weight. Six core samples obtained from the soil- followed by excavation of soil adjacent to the cap. crete wall indicate that the mean compressive strengths were 130 and 140 psi after 30 and 60 days of curing, respectively. 0.1 in. was 800 kips/in.; after soil mixing the stiffness increased This strength gain is consistent with past experience for soil to 1300 kips/inch. This represents an increase in stiffness of mixed walls (Terashi, 2003). about 62%. Figure 3-29 provides plots of the load-displacement curves for Cap 1 during Test 2 in virgin clay and Test 4 after the mass Test Results for Pile Cap 1 with Soil Mixing mix wall construction. In contrast to Tests 1 and 3, in these two Figure 3-28 presents plots of the load-displacement curves tests the soil adjacent to the cap was excavated to the base of the for Cap 1 in untreated virgin clay and Test 3 after the mass mix cap. Because the soil had been previously loaded, the load- soil improvement. With the soil mix wall, the pile cap resisted displacement curve for the pile cap with the mass mix wall is 452 kips compared to the 282 kips resisted by the pile cap in actually lower than that for the pile cap in virgin clay. However, the virgin clay at a displacement of 1.5 in. This represents an as displacement increases to the maximum previous displace- increase of 60% in the lateral resistance provided by the pile ment, the load-displacement curve appears to follow the load- cap. It also is interesting to evaluate the increase in initial stiff- displacement curve for the pile cap in virgin clay with little ness due to the mass mixing. Prior to treatment, the secant apparent increase. Because the soil adjacent to the pile cap had stiffness of the load-displacement curve at a displacement of been excavated, the pile cap no longer pushed the soil-mixed wall laterally; hence, no increase in lateral resistance was pro- duced. This behavior is in contrast with that for Pile Cap 2 500 where jet grouting allowed the soilcrete to extend underneath the pile cap and contact the piles themselves. 400 To produce a more readable report, additional plots, simi- lar to those presented for the pile group in virgin clay are not Load (kips) 300 presented here but are available in Herbst (2008, Appendix 3). 200 3.9 Pile Group Load Tests Soil Mixing-T9 Cap 1 Involving Flowable Fill 100 Virgin Soil-T1 Cap1 Several sets of lateral load tests were performed after exca- 0 vating and replacing the soil around Pile Cap 3 with flowable 0 0.5 1 1.5 2 fill. One set of tests was performed for the case where the soil Displacement (in) below the pile cap was excavated and replaced with flowable Figure 3-28. Comparison of measured load- fill prior to driving the test piles. The technique would repre- displacement curves for Cap 1 in virgin soil and sent an approach for improving lateral resistance for new after construction of a soil mix wall on one side of construction. Plan and profile drawings for this case are the cap. shown in Figure 3-30. In this case the flowable fill extended

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Figure 3-30. Plan and profile views of Cap 3 (right) and Cap 4 (left) during Tests 3 and 5.