Design and Installation of Steel Open End Piles in Weathered Basalt

The Dabhol Power Company (DPC) is constructing India's first Liquefied Natural Gas (LNG) Terminal on a remote strip of the western coast along the Arabian Sea about 160-km south of Mumbai. Designed to handle the largest LNG carriers, the Terminal's marine facilities includes a Jetty that extends 1750-m into open sea to reach adequate depth, a jetty head supporting the unloading arms and the control tower, four berthing dolphins, four mooring dolphins, walkway support dolphins, three navigation dolphins and one tug berth. 473 steel open-end piles (dia.762-mm) support all these structures and are driven into weathered basalt to support working compression loads up to 4000 kN. Tensile loads up to 2000 kN are supported by 610-mm diameter sockets that are bored beneath the pile tip in the underlying basalt. The subsoil consist of three subsequent layers: 1) Soft clay layer with a thickness varying between 0 and 6-m, 2) Weathered Basalt with a thickness of 1 to 5-m's, and a RQD value varying between 0 to 90%, and 3) Sound Basalt with unconfined compression strength varying between 29 and 115 MPa. A significant problem consists of defining a installation procedure for the piles, which reconcile the requirement to guarantee an adequate bearing capacity with a risk of damaging the pile tip, and the requirement of limiting the deformation of the pile tip in such way that the installation of the socket through the steel open end pile remains possible without damaging the bore hammer. An onshore test program was performed to solve this problem. To confirm the findings of the test program, Dynamic Pile Tests were carried out on offshore piles. In this way the bearing capacity of the installed piles was controlled. This paper discusses the test program which was carried out to define the installation conditions (Hammer Energy and Penetration per blow) in order to guarantee the bearing capacity in compression and tensile together. For compression piles, a penetration of 1 mm per blow is required and for tension piles 5 mm per blow for 16 m piles and 2,5 m per blow for 19 m piles is not to exceed.