Pore pressure penetrometers

PENETRATION-INDUCED PORE PRESSURE MAGNITUDES - METHODS TO DETERMINE TRANSPORT PARAMETERS FROM TERRESTRIAL AND MARINE PENETROMETER TESTING... 

Notably, this solution is similar to that recovered for a moving point dislocation (Equation 1), but includes three important improvements the effect of failure in a mechanical process zone is incorporated, as are the influence of both dilation in the process zone and effect of a finite size penetrometer tip. Despite these additions, the approximate solution is not able to discriminate between pore pressures measured on tbe tip, shoulder, or shaft. Therefore although unable to differentiate between the response resulting from different tip geometries and locations, the most crucial components regulating the response are included. 

With magnitudes of penetration induced pore pressure, p-p available in Equation 1, substituting this relation directly into Equation 5 enables plots of Qi versus 6, to be contoured for permeability, for the case of an infinitesimal radius penetrometer. The resulting relationship is... 

Figure 5 Schematic of a lance falling at terminal velocity, Uo, and impacting the seabed. For the evolving partially-drained analysis of embedmentgenerated pore pressures, the coordinate system is fixed to the penetrometer tip.

Solutions are developed for the pore pressure fields that develop around penetrometers either advanced at constant or decelerating rate and under partially-drained flow conditions. The maximum pore pressure, recorded at the penetrometer is suggested as indicative of the permeability of the surrounding soil, since the peak-pressure is conditioned by the ability for fluids to escape. This full suite of solutions promotes a consistent framework to understand the processes operating during penetrometer penetration and arrest, and to enable the conventional sounding indices inclusive of tip resistance, Q friction factor, and pore pressure ratio, to be related to penetration-induced pore pressures, and therefore to permeability. 

Zuidberg, H.M., Schaap, L.H.J., and Beringen, F.L. 1982. A penetrometer for simultaneously measuring of cone resistance, sleeve friction and d5mamic pore pressure. In Penetration Testing, Verruijt, A., Beringen, F.L., and de Leeuw, E.H., eds., A.A. Balkema, Rotterdam, the Netherlands, pp. 963-970. 

A value of 140 °C was used for the contact angle of mercury on the solid (0), and the surface tension of mercury (7) was taken as 0.485 N/m. These values correspond to an effective working range for the instrument of 150 pm to 1.7 nm in pore radius. The samples were outgassed at room temperature to a pressure of 50 mtorr (7 Pa) immediately prior to analysis to facilitate filling the penetrometers with mercury. All data were fully corrected for mercury compression with calibrated penetrometers. 

Relations may be developed to represent the steady pore fluid pressures that develop around a penetrometer under steady penetration, at penetration rate, U. These rtKxlels necessarily employ simple linearized constitutive relations, but incorporate the important influence of a porous medium migrating past the penetrometer tip, albeit in a simplified form. Dislocation models (Elsworth, 1991 1993) may be applied to represent a penetrometer of infinitesimal-radius, but suffer the disadvantage that penetration-induced pressures become singular at the assumed penetrometer tip. The approximate solution for a finite radius penetrometer avoids this shortcoming, as explored in the following. 

A solution is developed for the build-up, steady and post-arrest dissipative pore fluid pressure fields that develop around a penetrometer that self-embeds from freefall into the seabed. Arrest from freefall considers deceleration under undrained conditions in a purely cohesive soil, with constant shear strength with depth. Consider a lance falling through the water column that has reached terminal velocity, Uo, and subsequently impacts the soft sediments of the seabed, as illustrated in Figure 5. The non-dimensional pressure, Pd, may be used to define the build-up of pressure following the impact of the penetrometer with the surface of the seabed. The penetrometer impacts the seabed at velocity Uo, represented in dimensionless magnitude as Ud, and decelerates to arrest. Non-dimensional pressures are plotted as the product PdXd, since it is known that the peak pressures, sh-... 

In the piezocone, a cone penetrometer is combined with a piezometer, the latter being located between the cone and the friction sleeve. The pore water pressure is measured by the piezometer at the same time as the cone resistance, and sleeve friction is recorded. Because of the limited thickness of the piezometer (the filter is around 5 mm), much thinner layers can be determined with greater accuracy than with a conventional cone penetrometer. If the piezocone is kept at a given depth so that the pore water pressure can dissipate with time, then this allows assessment of the in situ permeability and consolidation characteristics of the soil to be made (Sills and Hird, 2005). 

Most of the commercially available porosimeters include certain common features. First, the sample is evacuated and then the penetrometer is backfilled with Hg in the low-pressure port. The seeond step of the low-pressure analysis is the eolleetion of the data at pressures up to the last low pressure point specified. When the low-pressure analysis is complete, the high-pressure measurement is carried out up to the maximum pressure. Pore volume data are calculated by determining the volume of Hg remaining in the penetrometer stem. When the maximum pressure is aehieved, the extrusion curve starts by reducing slowly the applied pressure. Commercial instruments ean work in one or both modes ineremental and continuous. In the former the pressure, or amount of Hg introdueed, is inereased step by step and the system is allowed to stabilize before the next step. In the continuous mode the pressure is increased continually at a predetermined rate [106]. 

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