The local shaft capacity of displacement piles is governed by effective stress friction laws. Laboratory tests carried out on clay soils in the ring-shear apparatus described by Bishop et al. (1971) predict reasonably accurately the soil-steel Interface friction angles (d) measured directly m the field with specially instrumented piles. The laboratory test values of d depend on: the soil grading and mineral composition; interface material and roughness; and in some cases previous shearing history. Recognising these factors, Jardine and Chow (1996) recommended in their pile design procedure that in clay sods site-specific soil-steel interface shear tests should be performed that model the above factors. This paper explores (i) the possible sensitivity of dto apparatus type and test procedure, and (ii) differences that can arise between test data and the "default" guidance curves given by Jardine and Chow. Results are presented of a series of comparative soil-steel interface ring shear tests, involving the relatively simple "Bromhead equipment and the more complicated "Bishop" apparatus, where clay soils of varying plasticity were sheared against a standard steel pile interface with the roughness of a typical pile. It is shown that d is not unduly sensitive to the choice of apparatus. Recommendations are made on how the commercially available "Bromhead" apparatus should be used to ad pile design in clays. In addition, a summary of recently performed interface ring-shear test results is used to comment on the "default curves" offered by Jardine and Chow for clays. Site-specific tests are strongly recommended for all new site investigations; they are simple and inexpensive to perform, and provide the most secure means of ensuring a reliable balance between safety and economy in pile design
An extended programme1 of research carried out at Impend College (IC) has led to a new approach for designing displacement piles m sands and clays The provisional proposals of Bond et al. (1992), Lehane and Jardine (1994), and Lehane et al. (1994) have been developed into the fully fledged design procedures described by Jardine and Chow (1996), which are now being used extensively for the design of offshore driven piles.
A key finding of the research was that the simple Coulomb failure criterion applies to the skin friction developed by displacement piles, with the local vertical shear stress, ¿being limited by:
Full scale piles fad progressively from the top downwards. In brittle clays, the local operational values of 6f applying when the pile reaches its peak total capacity will fall between maximum and minimum values of df, denoted as dpeak and dult depending on the depth of the point of interest and the length and compressibility of the pile. With short rigid piles, such as micro-piles, dpeak often dominates, while most points on a long compressible offshore pile will have reached then ultimate conditions before the peak overall capacity is developed.
Field research with specially instrumented piles showed that the rate of pile installation could have a strong effect on the magnitude of dpeak Bond and Jardine (1991) noted that