A PORE WATER PRESSURE PROBE FOR THE IN SITU MEASUREMENT OF A WIDE RANGE OF SOIL SUCTIONS

During the last 30 years, theories for the behaviour of unsaturated soil have been developed. Their key parameter is the soil moisture suction, i.e. the negative pore water pressure. Hitherto, they have been difficult to apply, because of the inability to measure the negative pore water pressure in situ. A new design of tensiometer is described, which now makes such measurement possible. Like any other tensiometer, it uses a saturated porous ceramic to transmit the soil suction from the soil to the water reservoir. The new tensiometer has been adapted from a device for measuring absolute pore water pressure in the range 0 to -1500kPa. It has a rapid response, so that representative measurements of the in situ negative pore water pressure can be obtained within a few minutes. As part of a long-term research programme on the measurement of soil suction, a 1.5m diameter reinforced polyvinylchloride (PVC) bin was partly filled with London clay, compacted at a moisture content of 26%, and its surface was covered with polythene. Three years afterwards, the pore water pressure was measured in situ, and this measurement was validated by measuring the suction of a London clay sample with the same moisture content, which was compacted in the laboratory. For the covering abstract see IRRD 883023.