Pore pressures in the sediment were measured at two locations in the East Bay area of the Mississippi delta. These measurements showed the sediment to be highly under-consolidated with the effective stress approaching zero even at significant depths below the mudline. During storms the pore pressures exhibited cyclic response to the wave-induced bottom pressures. In at least two instances the pore pressures increased significantly during storms, which is probably indicative of major sub-bottom movements.
Several different piezometers systems were utilized to make the pore pressure measurements. Each system is described in detail and the advantages and shortcomings of the systems with respect to measurements of pore pressures in the marine environment are presented.
Analysis of the data leads to the conclusion that pore gas, rather than pore water, pressure was measured in most instances even though in one set of experiments high air entry stones were utilized to exclude gas from-the measuring system.
The two projects known as SEASWAB I and SEASWAB II were aimed at studying sediment behavior in the Mississippi delta under the action of storm waves. Details of the projects have been discussed elsewhere1,2 and will not be repeated here. During both SEASWAB projects, pore pressures in the sediment were measured. These measurements were made to help determine the degree of consolidation in the sediments, and to detect gross subsurface movements. The rationale for the latter was that any significant subsurface slides or sediment collapse would be accompanied by a marked change in pore pressures. Further, it was anticipated that the pore pressures would exhibit a cyclic response to the wave-induced bottom pressures which have been hypothesized as a cause for sub-bottom movements in soft sediments3,4.
The primary purpose of this paper is to describe the pore pressure measuring systems that have been used to accomplish these functions, to discuss the suitability of the various systems, and to present a preliminary analysis of the pore pressure results. Detailed analysis of the pore pressure data is still underway.
SEASWAB I was initiated in September 1975 when several sensors were implanted in South Pass Block 28 in East Bay. Among the sensors installed were an accelerometer to measure mudline motions, a wave staff, and a bottom pressure transducer. However, the two systems of primary interest to this paper were probes for measuring pore pressures which were developed at Lehigh University and at the Atlantic Oceanographic and Marine Laboratory of NOAA in Miami, Florida.
Devices used to measure pore pressures in soils are termed piezometers. They may range from a simple open stand pipe with a well point at the tip to the more sophisticated closed system utilizing an electronic pressure transducer. Closed system piezometers can be remotely read and if a hard system is utilized the response times to changes in pore pressures are very rapid.
