Potential biological and chemical clogging of piezometer filters in acid sulphate soil

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\u27s 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

Potential Biological and Geochemical Clogging of Vibrating Wire Piezometers in Low-lying Acid Sulphate Soil

Installing a suite of appropriate instruments such as piezometers, settlement plates, extensometers, and inclinometers etc., in strategic locations to monitor the performance of an embankment built on soft soils is vital when there are major design uncertainties; the monitoring data can also be used to calibrate the design parameters. Questionable readings of pore water pressure (PWP) have been reported in various case studies involving the development of dams, embankment foundations and reclamation work in Australia and in South East Asia, especially in low-lying acid sulphate soil (ASS) floodplains. Despite having 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. This paper describes the biological and geo-chemical factors affecting reliability of Vibrating Wire (VW) piezometers, filter-tip clogging, smearing of soil adjoining the filter, gas generation, chemical alteration or corrosion of the filter, as well as electro-osmotic effects and cavitation. To that end, several VW piezometers installed in ASS terrain were extracted after being in place for 1.5 years and the soil surrounding the tips was tested for iron related and sulphate reducing bacteria. It is found that sulphate reducing bacteria has medium to high aggressivity whereas iron related bacteria has very high aggressivity with the bacteria count exceeding 20,000. VWPs with ceramic/stainless steel filter tips installed in acidic ground with organic contents exceeding say 4-5% have shown impeded dissipation of excess pore water pressure after a year or so. Accordingly, it appears that this issue is likely in other types of piezometers fitted with such ceramic or stainless filters when installed in ASS soils. Further Scanning Electron Microscopy (SEM) analysis of the piezometer filter is also ongoing at the University of Wollongong (UOW) laboratory to determine how ionic precipitation causes a VW piezometer to clog. In addition, several samples were collected from Victorian Dams and are being tested in University of Wollongong (UOW) laboratory to quantify the clogging effect in Dam practice when installed in ASS terrain