ABSTRACT

The Geotechnically Instrumented Seafloor Probe (GISP) can make long-term, in-situ measurements of pore water pressure in marine sediments in remote, undeveloped areas. The seafloor package is completely self-contained and includes data processing electronics, solid-state memory, acoustic telemetry, batteries, and pressure transducers. Acoustic telemetry provides the communication link with a surface command and recording system.

Two GISP units operated successfully for five months in a 1981 field test on the Mississippi Delta. Data obtained during this test, when combined with the results of SEASWAB I, indicate that from the mudline to a depth of 6 meters, excess pore pressure is near zero. From 6 to 15 meters (the maximum depth studied), excess pore pressure increases at about the same rate as total stress.

INTRODUCTION

Over the last several years, effective stress models of marine soil behavior have continued to improve. As these models are applied to a wider range of foundation and slope stability problems the information required from geotechnical investigations is changing. Effective stress itself is not a measurable quantity. Rather, it is defined as the difference between total stress and the pressure in the pore water in the soil. Some information about effective stress can be inferred from consolidation and shear strength tests; however, the most desirable source of data is direct measurement of total stress and pore water pressure.

Piezometers have long been used to measure pore water pressure in onshore geotechnical applications, especially to monitor the performance of large embankments and fills or heavy foundations on highly compressible, saturated soils. Also, the use of various types of piezometers to measure hydraulic head is an integral part of groundwater studies. But until recently, the technology needed to make these same measurements on the seafloor was not available.

The development of a fast-responding piezometer probe using small-displacement pressure transducers was a major advance for pore pressure measurements. Participants in the U. S. Geological Survey's SEASWAB experiments applied this new pore pressure measurement technology to the study of the stability of marine sediments1,2 .The pore pressure instruments fielded during the SEASWAB tests were specifically designed for shallow water. The data gathering and recording techniques took advantage of the shallow water depth and of the availability of nearby oil-production platforms to support the equipment.

The U. S. Department of Energy and the U. S. Geological Survey perceived a need to be able to make similar in-situ measurements in deep water and in remote, undeveloped areas. Sandia National Laboratories was contracted to develop the necessary technology beginning with the basic components of a self-contained seafloor data gathering system. 3. Sandia then applied this technology to the Geotechnically Instrumented Seafloor Probe (GISP) and the Seafloor Earthquake Measurement System (SEMS}. 4

GISP was designed to make long-term, in-situ measurements of pore-water pressure in marine sediments in remote areas. This paper describes the GISP instrumentation and its operation and compares GISP with the technology of the SEASWAB experiments.

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