A Practical Method for Extrapolating Ambient Pore Pressures from Incomplete Pore Pressure Dissipation Tests Conducted in Fine Grained Soils

A Practical Method for Extrapolating Ambient Pore Pressures from Incomplete Pore Pressure Dissipation Tests Conducted in Fine Grained Soils

Author: J. Scheremeta
Conference: Tailings and Mine Waste 2014
Date: October 5-8, 2014

The estimation of ambient pore water pressures in an existing soil stratum for use in developing appropriate pore pressure distributions for input to various geotechnical engineering analyses often presents the engineer with challenges in locations devoid of sufficient piezometer coverage. One method of obtaining pore pressure data is by utilizing Pore Pressure Dissipation (PPD) testing conducted during cone penetration tests (CPTs). If the CPT advancement is paused for a sufficiently long time, the excess pore pressures generated during probe advancement decay to ambient levels. This stabilization of pore pressures can be exceedingly lengthy in actual practice if the existing stratum consists of a very low permeability soil. Often, PPD tests are terminated prior to the test reaching equilibrium which may result in misleading inputs to subsequent geotechnical analyses if the data is used without appropriate corrections. Based on equations presented by Mayne (2002) describing the decay of excess pore pressure during a pore pressure dissipation test, a technique for predicting ambient pore pressures from incomplete PPD data has been developed utilizing the solver function in Microsoft ® Excel, and is presented herein.

The majority of analyses performed by geotechnical engineers are sensitive to the characterization of effective stresses within the modelled soil stratum. Among other parameters and properties, an accurate characterization of effective stresses is dependent on an accurate characterization of pore water pressures within the soil.

The estimation of existing pore water pressure can be conducted using a variety of methods. Finite element seepage modelling can be used; however, the results are dependent on accurate input parameters and boundary conditions, which are often estimated and may be inexact. If sufficient piezometer coverage exists, a geotechnical engineer will often interpolate between piezometers in order to estimate a pore pressure distribution for input to various analyses.

In soils sufficiently loose or soft enough to allow for advancement of a cone penetrometer, Cone Penetration Testing (CPT) is often conducted to assist the engineer in characterizing insitu material properties and site conditions. During a CPT, an electronic cone penetrometer is hydraulically pushed into the soil strata at a constant rate. Measurements are made of the resistance at the tip of the cone, the resistance along the shaft, and the pore pressure is monitored with a transducer as the cone is advanced. These measurements are used with appropriate correlations to estimate various engineering characteristics and in-situ conditions.

During CPT probe advancement in saturated soils, penetration pore pressures are recorded continuously. However, pore pressures observed as the probe is advancing are normally higher or lower than the ambient values due to excess pore pressures (positive or negative) due to contraction or dilation of the material. During the course of CPT testing, probe advancement can be periodically interrupted to conduct Pore Pressure Dissipation (PPD) tests. PPD tests allow for the measured pore pressures to equilibrate to their ambient values. These values can then be used to develop pore pressure profiles within the material.

The amount of time required for the excess pore pressures to dissipate to equilibrium in a PPD test is dependent on the hydraulic conductivity and consolidation characteristics of the soil. The excess pore pressures generated during CPT in a highly permeable soil will dissipate almost immediately. On the other end of the spectrum, the excess pore pressures generated during a CPT in a low permeability silt or clay could take several hours or even days to dissipate. This could make the characterization of ambient pore pressure conditions in these types of materials using PPD testing extremely time consuming, expensive and impractical. If the PPD test is terminated prior to reaching equilibrium, this could lead to an incorrect characterization of ambient pore pressures if the final pore pressure measurements are incorporated into geotechnical interpretations without appropriately correcting for the fact that the ambient pore pressure has not been reached, as some excess pore pressures are still being measured at the incomplete conclusion of the PPD test.


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