Use of a chilled-mirror dew-point potentiometer to determine suction in a railway formation material

Compacted soils are used in many civil engineering works, such as railway formations and highway pavements, earth dams, backfills, and soil covers. Compacted soils are invariably unsaturated and possess negative pore-water pressure or suction that can dominate their mechanical behaviour. A growing interest in unsaturated soil mechanics is increasing the need to routinely measure soil suction. The soil water retention curve, SWRC, is often used to relate the soil suction to water content or degree of saturation. A number of methods of measuring suction have become well established, including the filter paper technique, pressure plate apparatus, and tensiometers. In this study, the chilled-mirror dew-point technique has been used to measure suctions of a compacted material typical of South African railway foundations. The test allows for an evaluation of total suction greater than about 200 kPa but is of most value for measuring suctions greater than 500 kPa. The ability of the chilled-mirror dew-point potentiometer to measure high suctions, in the low water content range, means that it can be used to complement results from pressure plate tests at lower suctions and add to the results of filter paper tests at higher suctions. The paper compares suctions measured using the chilled-mirror dew-point technique with those obtained using more established techniques.