This paper discusses the planning, execution and performance of a geotechnical site investigation program conducted at four deepwater sites in the Santa Ynez Unit, offshore California. Due to the seismic activity of the region, extreme water depths and wide range of subsurface soil conditions, the geotechnical program used state-of-the-art field technology. The field program recovered push samples and conducted in-situ cone penetrometer, remote vane and gamma log tests. For calibration purposes, conventional percussion samples were al so obtained. Push sampling resulted in high quality samples of adequate lengths for laboratory tests, even in silty, fine sands, and the samp1e quality far surpassed the quality of the percussion samples. With the combined use of high quality push samples and in-situ test data, the uncertainties in characterizing the soil conditions at the sites were significantly reduced.
During the last quarter of 1981, McClelland Engineers, Inc. (MEl) conducted geotechnical site investigations at four deepwater sites in the Santa Ynez Unit (SYU), Santa Barbara Basin, offshore California. These sites are potential future locations of pile founded structures to be installed by Exxon Company, U.S.A. All the locations are in a seismically active region. Three of the sites are located on a submarine fan, which is underlain by an extensive deposit of interbedded sands. The fourth site is on the smooth slope and is underlain by a series of clay deposits.
The overall purpose of the geotechnical program was to assess the soil conditions in order to develop the foundation design parameters and account for local soil effects in the earthquake design criteria for future platforms. This purpose was accompli shed by performing extensive in-situ testing and conducting 1aboratory tests on high quality samp1es recovered from soil borings.
An important focus of the field program was to obtain high quality soil samples; i.e., relatively "undisturbed" samples of adequate soil recovery. A high priority was placed on this objective because of the following considerations:
Due to the seismic activity of the SYU area, relatively undisturbed samples were required for dynamic laboratory tests to obtain soil parameters needed for the dynamic analysis of the platform foundation and to account for soil effects in earthquake design criteria. This was particularly important for fine sands which in even a mildly "disturbed" state can give an inaccurate indication of material's dynamic response.
Due to extreme water depths (up to 1200 ft) and the associated sampling difficulties, some sample disturbance due to stress relief (reduction of in-situ stress and handling) was expected. Minimizing mechanical disturbance was thus essential.
Previous investigations in the SYU area which used the conventional percussion sampling technique had resulted, especially in sandy regions, in less than desired sample quality and recovery.
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