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The piezocone test (CPTu) is a cone penetrometer deployed during a CPT which simultaneously measures cone tip and friction sleeve resistance, inclination and excess pore water pressure. The addition of pore water pressure measurements allows for a more reliable determination of soil stratification and soil type than standard CPT (cone tip and sleeve resistance only). This provides a better basis for interpretation in terms of mechanical soil properties.
The piezocone test is performed by fitting a porous filter to the cone at one of three positions on the cone; the most common filter locations include the cone tip (u1) or the shoulder (u2), and the less common position is located behind the sleeve (u3). Lankelma uses disc-shaped filters made of a durable bronze material, which has been saturated in silicone fluid to remove any air. A filter is fitted to the saturated pore pressure measurement system (usually in the u2 position). After each CPT sounding the filter is replaced and the pore pressure measurement system saturated once again, this has proven to enhance the quality of the pore pressure readings.
CPTu tests are a standard requirement in marine work, the results have direct applications in pile design, and correlation with specialist CPT results such as environmental testing with Lankelma’s membrane interface probe (MIP).
Dissipation tests measure the decay of pore water pressure over a specified amount of time. With the advancing CPT push excess pore water pressure builds up around the cone tip and this is measured via the filter element. During a dissipation test the CPT is paused and the pore water pressure decay readings are recorded in-situ. The test is usually run until 50% of the pore water has dissipated (t50); this allows for the calculation of geotechnical parameters to assess the hydraulic conductivity for use in understanding of the material’s settlement properties. Dissipations are useful in helping understand how the soil will behave with heavy surface loads.
A dissipation test can be carried out during any CPTu sounding and does not require any additional set up. Furthermore the results are instantaneous allowing for quick decisions on site.