One capability of a typical CPT probe that is often overlooked is the ability to perform dissipation tests – pore pressure versus time. This test involves recording the dissipation of pore pressure as it approaches static equilibrium. Dissipation tests can be performed at any depth by pausing the advancements of the probe. Dissipation test data determines the groundwater table.
The CPT probe has the capability to acquire pore pressure data versus time. At anytime during a CPT test, the push can be paused to record pore pressure dissipation as at approaches static equilibrium, thus allowing real time decisions regarding the depth(s) of the test(s), compressibility and permeability within the surface strata. No time is required to initiate a test. The time required for a test – from seconds to hours – is fully dependent on the soil.
The addition of pore pressure measurements during static cone penetration testing has added a new dimension to the interpretation of geotechnical perimeters. The continuous measurement of pore pressures along with bearing and friction has enhanced the electric penetrometer as the premier tool for stratigraphic logging of soil deposits.
The excess pore pressure ( u) measured during penetration is a useful indication of the soil type and provides an excellent means for detection details in stratigraphy. The differential pore pressure ratio ( u/qc) also appears to be a good index of soil type and relative consistency and a rough indicator of stress-history.
In addition, when the steady penetration is stopped, the excess pore pressure decay with time can be used as an indicator of the coefficient of consolidation. Finally, the equilibrium pore pressure ( uo), after complete dissipation is reached, provides important data on the ground water conditions.
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