abstract
Determining the stability of soft sediments requires knowledge of the in situ pore pressure state. In offshore soft sediments, where strengths are typically very low and the deposits are either still consolidating (due to high sedimentation rates) or normally consolidated, accurate measurement of the pore pressure profile is critical. Excess in situ pore pressures are believed to have been a major contributing factor to the Storegga slide (Solheim et al. 2005) and many other offshore regions of the world have excess pore water pressure caused by rapid sedimentation rates, mud volcano activity, and other mechanisms (e.g., Gulf of Mexico, Caspian Sea, Offshore West Africa). While measurement of the water pressure at the seabed floor (top of sediment) is relatively straightforward, determination of the pore pressure within soft sediments is much more complex. The complexity in measuring the in situ pore pressure arises from the current state-of-the-art: measurement requires installation of a sensor or probe that in turn displaces soil and creates excess pore pressures. Immediately after installation, the probe measures the excess pore pressure during installation, and it is only after prolonged durations (often hours and even days; which is costly because it ties up the drilling vessel) that the excess pore pressure dissipates and hydrostatic pore pressures can be measured. Efforts to reduce the time required for dissipation have primarily focused on development of a miniature tapered piezoprobe since the degree of excess pore pressure is proportional to the square of the probe diameter (Houlsby and Teh 1988). Constrained by practical implementation issues, the miniature piezoprobe begins to taper up to a larger diameter about 100 to 150 mm behind the tip (Whittle et. al. 2001). During dissipation the excess pore pressure generated by the miniature tip dissipates relatively quickly. However, before it reaches the in situ equilibrium pore pressure, the excess pore pressure from the upper tapered section propagates forward and produces an increase in the excess pore pressure at the tip. This results in the equilibrium pore pressure at the tip not being measured until the excess pore pressure from the tapered section is also dissipated, effectively negating the tapered section’s benefit. To overcome this issue Whittle et. al. (2001), using numerical modeling, proposed the use of a dual element piezoprobe and a more complex analysis where the correlations between two dissipation curves are analyzed. This enables a rigorous prediction of the equilibrium pore pressure but not a direct measurement. Determination of the equilibrium pore pressure from piezoprobe dissipation is essentially a time-dependent process during which an instantaneous pore pressure differential must diffuse and return to equilibrium. To date, all piezoprobe methods have required dissipation excess pore pressure via flow of water away from the probe into the surrounding soil. The dissipation time has been accelerated by using a smaller probe since a smaller probe produces a smaller differential pressure. This research proposes a novel approach that enables accelerated dissipation of excess pore pressure via the flow of water into the piezoprobe. Prior to penetration, an estimate of the in situ equilibrium pore pressure will be made. After piezoprobe penetration to the target depth, the pore pressure will be measured continuously. For a specified time (to be determined by this research) water flow in/out of the probe will be regulated so that the measured pore pressure remains at the estimated in situ equilibrium conditions. After the specified time, water flow in/out of the probe will cease and the pore pressure will come to equilibrium. This approach will rapidly reduce the initial pore pressure differential since water flow into the probe will occur at the location of highest excess pore pressure. Once most of the excess pore pressure has been relieved, the time required for equilibrium conditions to be restored will be minimal and less than the time required for the pore pressure front generated by the tapered section to reach the measurement location. The novel device and technique proposed above requires significant design, analysis, modeling, and testing components. The external dimensions of the miniature piezoprobe will be maintained as closely as possible to those by previous researchers, enabling use of prior results and analysis (e.g. Whittle et al. 2001). The control of water flow in and out of the piezoprobe (fluid chamber in which the pore pressure sensor is located) will be performed using a stack of piezoceramic disks. The piezoceramic disk stack expands with positive voltage and contracts with negative voltage. During penetration the piezoceramic will be in the expanded state with high positive voltage. By varying the voltage the volume of the piezoceramic stack will be varied, which will be equal to the volume of water flow into the probe. In addition to the design details, analytical and numerical analyses will be performed to determine the time duration during which the flow should be regulated, the time required to reach equilibrium conditions, whether this time is less than that at which the tapered pore pressure front reaches the measurement, and how these parameters vary with soil properties.
