The importance of accurate pore water pressure measurements when conducting CPTu as exemplified using data collected in Christchurch following the Canterbury earthquake sequence

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Published 16 November 2013
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The importance of accurate pore water pressure measurements when conducting CPTu as exemplified using data collected in Christchurch following the Canterbury earthquake sequence

Piezocone penetration testing (CPTu) is a widely used investigation method for geotechnical analysis and liquefaction assessments. In addition to recording tip resistance (qc) and sleeve friction (fs), pore water pressure (u) is measured by a pressure transducer via a porous filter commonly located behind the cone tip. In this paper, data collected in Christchurch following the 2010-2011 Canterbury earthquake sequence is used to exemplify the variable quality of CPTu data, focusing on pore pressure response. It has been found that if the probe is not properly maintained or prepared prior to testing, it can have undesirable effects on the measured CPTu data. A skilled operator could improve the quality of the CPTu sounding by monitoring the data and pausing the test to verify the pore pressure response. With accurately measured pore pressure response, dissipation tests can be conducted during CPTu. Dissipation tests can provide important geotechnical parameters such as ground water level and soil permeability. Accurate pore water pressure data can improve liquefaction analysis results by providing a representative groundwater level and providing a mean for a more accurate estimation of liquefiable layer thickness. Inaccurate pore pressure response during CPTu sounding can lead to incorrect interpretation of the data when describing soil behavior, especially for fine grained soils such as those found in Christchurch. Quality control is important when conducting CPTu to assure accurate pore pressure response recordings and effective dissipation testing.

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Issue 1
Volume 38
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ISSN 0111-9532