What can you learn from pore pressure data?
The most basic CPT tests classify soil based on tip resistance and sleeve friction measurements. In coarse soils and shallow testing depths, this data may be sufficient to accurately characterize the soil behavior.
However, most modern CPT cones incorporate a third measurement: pore water pressure. What does this measurement mean and how can it add to our understanding of soil behavior?
Pore pressure is simply a measure of the in-situ groundwater pressure, i.e. the water pressure in the “pores” between soil grains. This data is used to determine the compressibility and permeability of the soil, as well as indicating groundwater conditions. It is used to correct or “normalize” the sleeve friction and tip resistance readings in the presence of in-situ moisture and overburden stress. This is especially important in soft, fine-grained soils where in-situ moisture takes longest to dissipate, and in tests at depths greater than 100 feet.
A CPT cone that is equipped with one or more pore pressure sensors is called a piezocone, and a CPT test using a piezocone is often indicated with the abbreviation CPTu. Piezocones may have between one and three pore pressure sensors, located on the cone (denoted u1), directly behind the cone (u2), or at the top of the friction sleeve (u3). Most piezocones for everyday applications use one sensor located at u2 (see image below).
The pore pressure sensor consists of a porous filter (usually made of plastic resin), a small cavity of incompressible, low-viscosity fluid, and a pressure transducer. The filter and tubing between the filter and transducer must be fully saturated with fluid, usually glycerin or silicon oil, to ensure fast and accurate readings. The filter must be replaced frequently so that it does not become clogged with soil.
The procedure for the CPTu test is slightly different than the basic CPT test, where tip resistance and sleeve friction data are taken continuously as the cone is pushed into the ground at a constant rate. To collect pore pressure data, the pushing of the cone must be paused long enough to take an initial reading, then allow pore pressure around the cone to dissipate as subsequent readings are taken.
When the cone is pushed into the ground, pore pressure builds up around it until the in-situ moisture dissipates into the surrounding soil. The rate of dissipation depends upon the soil’s coefficient of consolidation, which indicates the compressibility and permeability of the soil.
A fixed period of dissipation may be used, or in other cases dissipation is allowed to continue until reaching a specific value of U (usually 50%), calculated by the following ratio:
uo = equilibrium in-situ pore pressure (the pressure at which no further dissipation is observed)
ut = pore pressure measured at time t
In addition to correcting against the effects of in-situ moisture and overburden stress, pore pressure data helps to ensure consistent measurements and soil classifications among different types and sizes of CPT cones.
The process of pore pressure data analysis involves a handful of new terms and formulas, so we’ll discuss it in depth in our next blog. In the meantime, you can get the specifics on Vertek’s full line of CPTu cones on our website, or by downloading our Geotechnical and Environmental Products Catalog.