EURIPIDES, Load Tests on Large Driven Piles in Dense Silica Sands

Abstract

Abstract In 1995 an extensive load testing program was conducted on 30' pipe piles in dense silica sands similar to those found in the southern North Sea. A highly instrumented pile was driven at one location, extracted and redriven at a second location. A total of twelve static compression and tension load tests were performed at three penetration depths between 30 and 47 meters. Test objectives included acquiring reliable data on capacity of offshore type piles in sands which hitherto were hardly available and to understand the failure mechanisms in order to improve current foundation design. The paper describes the design of the testing programme and project organisation and funding. Details of the lest pile and instrumentation to measure the loads in the pile and the pile-sand interaction are highlighted with an emphasis on quality of the measurements. The field-work and innovative testing facilities are also described. Introduction Estimates of axial capacity of open-ended pipe piles in sand are an area of great uncertainty. Various authors (e.g. Pelletier et al., 1993) have argued that the world-wide used API recommended practice (RP2A, 1982-1991) does not properly account for fundamental parameters (pile length, plug behaviour, horizontal stresses, driving history, etc.) and underestimates the capacity of driven piles in very dense sands. On the other hand no other widely recognised design method is currently available. The base for a design method should be an understanding of the pile-sand load transfer in parallel with representative full scale pile load tests to confirm this (Hobbs, 1992). Some new promising design methods have been published in recent years (Randolph et nf., 1992, Lehane et al., 1992), but the existing data base on pile tests in sand is limited to piles of relatively small size and/or low capacity. Moreover few tests provide instrumented data of enough quality and resolution to confirm the load transfer in detail. While this unsatisfactory situation has been recognised by most parties involved in foundation design it appeared very difficult to resolve because high capacity pile tests in sand are relatively costly. In 1985 Heerema and Fugro developed a plan to test three 42' piles in very dense sands nearshore Eemshaven in The Netherlands (Fig. 1). On each pile compression and tension tests were foreseen at four penetration depths with a potential ultimate compression load of 90 MegaNewton (MN). The programme costs were estimated at US$ 12 million. Even though a 40% European subsidy was available not enough support from industry could be generated. This contrasts with the fact that about US$ 50 million is spent annually on pile foundations in the southern North Sea and a 30% increase in allowable load could lead to a saving of about US$ 60,000 per pile installed (van Zandwijk, 1986).