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The purpose of sampling and geotechnical testing of marine sediments is to ascertain and predict their in-situ properties. For many applications, limitations in accuracy imposed by the sampling and testing procedures may not be critical. However, for the safe disposal of some toxic wastes, there is a requirement for higher degrees of precision and reliability in the determination of some pertinent parameter. For waste products that are to be buried in marine sediments, and where the sediments themselves provide a geologic barrier to the release of waste products into the oceans, it is essential to determine accurately the flow regime of the natural pore waters. Furthermore, it may be desirable to monitor any changes that occur in the flow regime after emplacement. In-situ measurement of pore-water pressure and permeability is possibly the simplest, most reliable, and perhaps the most accurate technique for determining the rate of pore-water advection in marine sediments.
Experience of developing the techniques required to make these measurements and in interpreting the data has been gained in one area of the North Atlantic Ocean (Madeira abyssal plain) for the feasibility study into the disposal of high-level radioactive wastes and in other areas for different applications. Over 40 deployments have been made using pop-up pore-pressure instruments (PUPPIs) over the past few years to depths of up to 6 m below the seabed in water depths greater than 5000 m. PUPPIs have a resolution as low as 15 Pa, are calibrated in-situ at the end of each deployment, and can be deployed in water depths of up to 6000 m for periods of up to a year. Ambient differential pore pressures are measured at the end of each deployment after the insertion pressure has fully decayed (this may take several days depending on the sediment type). Permeability values are obtained using the hydrostatic tidal cycle. Natural pore-water advection rates are determined using these in-situ values assuming Darcian flow. Depending on the permeability, the resolution can be as low as 1 mm/year.
In addition to determining pore-water flow rates, the PUPPIs provide other pertinent geotechnical information. For example, the ambient pore-pressure measurements enable the effective stress profile to be calculated from sediment density core data; hence the consolidation state of the sediments can be determined. The maximum insertion pressure is indicative of the shear strength, and the horizontal coefficient of consolidation can be obtained from the pressure decay curve.
Apart from the geotechnical site assessment aspects of pore-pressure measurements, the PUPPIs can be used for short and long-term monitoring of changes that may occur either naturally or artificially as a direct result of the waste emplacement.
pore préssure, geotechnical aspects, in-situ measurements, sediment properties, pore-water flow, permeability, site assessment
Research scientist, Haslemere, Surrey