Development of Selective Pore Pressure Transducers to Measure In Situ Pc-S Curves During Model Tests

The relation between capillary pressure and saturation, the so-called Pc-S relation describes the transport of a Non-Aqueous Phase Liquid NAPL in saturated sand. An on-site measurement of this curve is of interest to follow in-situ the extension of this type of contamination. As a preliminary study to possible application in the field of environmental on-site measurements, model tests have been performed in the GeoCentrifuge of GeoDelft. The use of a geo-centrifuge is very relevant to increase the driving force on fluids and therefore to accelerate the transport process at equal stresses in prototype and model. The purpose of this study is to measure ‘dynamic’ Pc-S curves related to the transport of a Dense Non-Aqueous Phase Liquid DNAPL in a saturated sand. To do so, one of the tasks is to design pressure transducers able to measure separately pressures of the DNAPL phase and of the water phase. Time Domain Reflectometry (TDR) sensors have been used to measure on-line change in water saturation in the sand. Direct capillary pressures can be measured by using a tensiometer. However, the use of these sensors is limited by the time response, which is too long to characterize mobile DNAPL, especially under the conditions of the centrifuge tests. In our tests, infiltration through a sand sample of 18 cm took place within ~10 minutes. As the capillary pressure is the pressure in the NAPL phase substracted by the one of the water phase, the capillary pressure can then be deduced from the separate measurements of these two types of pressures. Hence in that case, reasonable time response should be reached.