A free-fall piezometer (Pop Up Pore Pressure Instrument, PUPPI) has been developed to measure excess pore pressures in seabed sediments. The instrument is capable of operating in water depths of up to 6000 m for a period of one year. Excess pore pressure is measured with a resolution of 0.05 kPa at a port located near the tip of a 4 m long lance. The pressure caused by insertion, the insertion pressure decay curve and any residual (ambient) excess pressure can be used to assess some of the geotechnical properties of in-situ sediments. The acoustic, mechanical, and electronic systems of the PUPPI are described, and examples from deployments into soft sediments in oceanic depth are presented.
The impetus for the development of an instrument to measure pore pressures in sediments stemmed from the wide-ranging study into the feasibility of disposing of high-level radioactive waste in deep ocean sediments. In this study, accurate estimate of in-situ permeability and pore pressures are required to quantify any naturally occurring advection of pore waters at sites designated as study areas. Pore-water advection is one of many factors that could potentially accelerate or retard the release of dissolved radionuclides to the ocean waters from buried canisters.
Geotechnical characterization of the in-situ properties of a soil or sediment cannot be complete without some knowledge of the pore pressure. Soil behaviour depends to a large extent on the effective stresss' =s-u (where sis the total stress and u is the pore pressure).consequently, pore-pressure measurements also could be of significant interest to those concerned with the offshore construction industry.
Effective stresses will also influence many geological processes such as consolidation, lithification, sediment stability and liquefaction potential. Consequently there is a wide interest in measurements of pore pressure and the techniques used to determine in-situ values.
The primary requirement of the instrument is that it should accurately measure the ambient excess pore pressure, u (expressed as a head of water) at some depth, z, beneath the sea floor. (The excess pore pressure at any point in the sediment is defined as the difference between the actual pressure and the hydrostatic pressure at the depth of the measurement.) To achieve this, a pore-pressure port has to be inserted into the sediments on a lance which inevitably causes some disturbance. A pressure pulse will be generated which decays at a rate that depends on the geometry of the lance and on the physical properties of the sediments. For clays it may take many hours, or even days, for this insertion pressure to decay to a residual ambient value. Therefore, it was considered impractical to develop a tethered instrument and the free-fall Pop-Up-Pore-pressure-Instrument (PUPPI) was conceived. A two-part system was designed which consists of a recoverable buoyant section and a disposable section including the lance and pore-pressure port.
The initial design requirements dictated that a pore-water advection velocity as small as 1mm/year should be resolved. The advection velocity in permeable porous media can be calculated.