West Delta 58A Site Selection And Characterization

Abstract

Abstract Prior to final design, installation and testing of Conoco's test pile, it was necessary to complete a comprehensive geotechnical site characterization. The initial site selection was made using existing geotechnical borings near the West Delta 58A (WD58A) platform, which was to be the test site. These early borings, performed in 1956 and 1979, were supplemented in 1981 with three additional borings, a cone penetrometer boring, an in situ vane boring and, in 1983, a cyclic vane test program. To augment the laboratory tests from 1956 and 1979, a full suite of index and strength tests was performed on the samples from the 1981 program. In addition to conventional testing, isotropically and anisotropically (CIU and CKoU) consolidated triaxial tests, direct simple shear tests and cyclic triaxial and cyclic simple shear tests were performed. The results of all the site investigation work was synthesized to produce an interpreted shear strength profile which was used as the geotechnical basis for the pile test program. Cyclic in situ vane and cyclic laboratory tests were used to help understand the soil degradation observed during the cyclic testing portion of the pile test. Introduction An in-depth site characterization program was required in advance of the proposed pile testing program at the West Delta 58A platform. The purposes were (I) to understand the detailed stratigraphy at the site to assure the proper pile length and location of instruments, and (2) to obtain all soil property and strength information which would later be incorporated and compared to load test results. The West Delta 58A platform was located about 15 to 20 miles southwest of Venice, Louisiana, on the west side of the Mississippi River delta. Water depth at the location was approximately 49 ft. General Site Geology The area's geologic history is relatively well documented in the open literature (Coleman et al., 1980). The geology at the site was formed by the rapidly changing face of the Mississippi River delta and its rapid discharge of sediments into the Gulf of Mexico. Soils at the site are characterized as medium to high plasticity clays of recent geologic deposition with typically normal to "under" consolidated stress histories. Site Investigation Programs' Overview The WD58A location was the site of a number of geotechnical borings, both prior to and for the purpose of the pile testing program. The location of each of the test programs, relative to the WD58A platform, is shown in Figure I. The original site investigation of Boring I (MEI, 1956) was performed in June, 1956, following installation of the WD58A platform, to verify the foundation information assumed for design of the piling for the platform. Boring 2, performed in January, 1979, and approximately 2500 ft south of WD58A (Fig. I), was drilled to provide foundation information at the site of a proposed adjacent structure (MEI, 1979). A large portion of the site characterization effort performed at WD58A was conducted as part of Conoco's driven pile test program to study the axial behavior of piles under static and cyclic tension loading.