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14 CHAPTER 3 Field Load Testing Based on preliminary lateral pile load analyses, improvement A generalized soil boring log at the test site is provided in of soft clay was considered to provide the greatest potential for Figure 3-3. The depth is referenced to the top of the excavation, increasing the lateral resistance of pile foundations. Therefore, which was 2.5 ft above the base of the pile cap as shown in the a test site was selected where a variety of soil improvement figure. The soil profile consists predominantly of cohesive methods appropriate for treating soft clay around a pile group soils; however, some thin sand layers are located throughout could be investigated. The field load tests provided basic per- the profile. The cohesive soils typically classify as CL or CH formance data that also could be used to calibrate and verify materials with plasticity indices of about 20 as shown in Fig- computer models. ure 3-3(b). In contrast, the soil layer from a depth of 15 to 25 ft consists of interbedded silt (ML) and sand (SM) layers as will be highlighted by the subsequent plots of CPT cone tip 3.1 Test Site Location resistance. The test site was located north of Salt Lake City, Utah, at the The liquid limit, plastic limit, and natural moisture content interchange of Redwood Road and I-215 on a Utah Depart- are plotted in Figure 3-3(b) at each depth where Atterberg limit ment of Transportation right of way. An aerial view of the site testing was performed. The water table is at a depth of 1.5 ft. is provided in Figure 3-1. The site offered several advantages The natural water content is less than the liquid limit near the including the following: ground surface, suggesting that the soil is overconsolidated, but the water content is greater than the liquid limit for soil speci- 1. Presence of a consistent layer of relatively soft saturated mens from a depth of 5 to 27 ft suggesting that these materials cohesive soil near the ground surface, may be sensitive. Below a depth of 30 ft the water content is 2. Fill over the soft clay to allow easy access for construction approximately equal to the liquid limit suggesting that the equipment, soils are close to normally consolidated. 3. Access to water, and The undrained shear strength is plotted as a function of 4. Permission to drive piles and use ground improvement depth in Figure 3-3(c). Undrained shear strength was mea- methods. sured using a miniature vane shear test or Torvane test on undisturbed samples immediately after they were obtained in Four pile groups were constructed at this site according to the the field. In addition, unconfined compression tests were per- basic layout shown in Figure 3-2 and discussed subsequently. formed on most of the undisturbed samples. Both the Torvane and unconfined compression tests indicate that the undrained shear strength decreases rapidly from the ground surface to a 3.2 Geotechnical Site depth of about 6 ft but then tends to increase with depth. This Characterization profile is typical of a soil profile with a surface crust that has Geotechnical site conditions were evaluated using field and been overconsolidated by desiccation. However, the undrained laboratory testing. Field testing included one drilled hole shear strength from the unconfined compression tests is typi- with undisturbed sampling, four cone penetration test (CPT) cally about 30% lower than that from the Torvane tests. The soundings, and shear wave velocity testing. Laboratory testing unconfined compression tests at a depth of 27 and 48 ft appear included unit weight and moisture content determination, to have been conducted on soil with sand lenses because the Atterberg limits testing, and undrained shear testing. measured strength is substantially lower than that from the