Shear Transfer Behavior of a 30-in. Pile in Silty Clay

ABSTRACT Pile to soil shear transfer and load distribution behavior for a 30-inch diameter test pile driven from 190 to 263 feet of penetration in stiff to hard, moderately over consolidated (OCR = 2–3) silty clay are presented. Measured shear transfer vs. displacement (t-z) curves are compared to those predicted using the methods of Vijayvergiya (1977)1 and Kraft, Ray and Kagawa (1981)2. Effects of degradation during cyclic one-way tension and two-way tension-compression on both load distribution and shear transfer behavior are presented. Degradation effects were minimal for the one-way loading and substantial for the two-way loading, Rate of loading effects on shear transfer are also presented. The measured static and cyclic pile behavior was modeled using the PSAS pile3soil interaction algorithm in INTRA (8ea et al, 1984). The pile was modeled as a series of beam elements. Non-linear hysteretic springs and dashpots were used to model the soil. When elasto - plastic soil support curves were used in conjunction with the Unconsolidated- Undrained Triaxial (UU) Strengths from driven sample and the recommended design method of API-F!P2A (1986), INTRA under predicted the long term static pile capacity. Using the measured t-z curves, the measured pile-top load deflection behavior under both static and cyclic loading was successfully modeled. INTRODUCTION A series of static and cyclic testing was carried out over a period of 90 days at an onshore site in Long Beach, California, to determine pile response in soils similar to those found at the site of the 700-foot water depth Eureka platform in the Beta field offshore Southern California. Load tests on the 30-inch diameter, open-ended driven pipe pile were conducted in a moderately over consolidated clay. The pile was lowered through a 42-inch diameter casing which had been drivel to a penetration of 190 feet and subsequently cleaned out. The test pile was driven from 190 to 263 feet of penetration. Some of the results of the load test program have been presented previously. Three test series were performed. Test Series 1 consisted of static tension tests performed immediately after driving. Test Series 2 and 3 consisted of static, cyclic one-way and cyclic two-way tests conducted 60 and 90 days after driving, respectively. Pile top load-deformation behavior for a portion of Test Series 2 is shown in Figure 1. Shear transfer response was measured in order to obtain correlations between insitu pile behavior and insitu and laboratory tests. Such correlations could then be used in conjunction with similar tests performed at offshore platform locations to predict overall response of prototype piles. Measured shear transfer (t-z) response from Test Series 2, performed when at least 95% of the excess pore pressure generated by driving had dissipated, are presented and discussed in this report. SOIL PROFILE The soil at the Long Beach site is a low plasticity silty clay from 190 to 264 feet below ground surface, Natural water contents ranged from 22 to 31%. Liquid limits ranged from 30–47% and plastic limits ranged from 20 to 28%.