Potential biological and chemical clogging of piezometer filters in acid sulphate soil
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Instrumentation for performance monitoring of an embankment built on soft soils is vital for assessing the progress of consolidation and confirming (or refuting) soil parameters used in design when there are significant design uncertainties and the monitoring data can be used to calibrate the design soil parameters. A suite of instruments including settlement plates, extensometers, piezometers, inclinometers is often employed for this purpose. In the first Author's experience, erroneous readings interpretations of pore water pressure (PWP) readings have been reported in various case studies involving transport infrastructure development and reclamations works both in Australia and South East Asia, especially in low-lying acid sulphate soil floodplains. It has been observed that in spite of the presence of vertical drains (PVDs), excess pore water pressure readings from vibrating wire piezometers (VWPs) do not always dissipate as fast as expected especially after a certain period of time, typically a year. The article discusses the potential factors affecting the reliability of VWPs including filter tip clogging, extreme smearing of soil adjoining the filter, gas generation or cavitation, chemical alteration or corrosion of the filter, electro-osmotic effects and cavitation due to bacterial activity. Based on this, the response of VWPs may be divided into a distinct trilinear trend, observed for much of the Australian northern and eastern coastal belt that is predominantly affected by Acid sulphate soil (ASS) conditions where oxidisable pyrite layers are present within relatively shallow depths of the upper Holocene clay.