Continuous Measurement of Matrix Suction and Degree of Saturation of Unsaturated Soils with a New Soil-Water Retention Curve Device

The relationship between matrix suction and degree of saturation within the soil is a fundamental parameter in studying many behavioral aspects of unsaturated soils and is referred to as the soil-water retention curve (SWRC). Due to difficulties associated with negative pore water pressure measurements in unsaturated soils, most SWRC measuring techniques benefit from suction controlling methods such as axis translation or osmosis techniques. These methods provide only a few data points on the matrix suction-degree of saturation relationship and cause a discrete measured soil-water characteristic curve. However, many SWRC elements like drying and wetting curve slopes, air entry, and air expulsion values are fundamental parameters in describing the hydro-mechanical behavior of unsaturated soils. Therefore, a realistic understanding of these parameters requires continuous measurement of the degree of saturation-matrix suction relationship at more points. To this end, this paper examines the performance of a new SWRC device developed for the continuous measurement of the soil-water retention curve of unsaturated deformable soils along drying paths. The new apparatus is equipped with new miniature tensiometers enabling direct measurement of soil suction without the need for an artificial increase in pore air pressure. The variation of the degree of saturation is calculated by contiguous weighing of soil samples along drying paths. The credibility of the new SWRC apparatus is examined to investigate the influence of initial compaction on the soil-water retention response of sandy soil along with drying. This was experimentally achieved by SWRC tests on compacted soil samples with a range of void ratios between maximum and minimum void ratios to examine the influence of compaction on the slope of SWRC along with drying and variation of air entry value. The results are thoroughly discussed and compared against other available data in the literature. Also, the results suggest the fast performance of newly developed tensiometers for direct measurement of soil suction with a minute without the need for the application of elevated pore air pressure, which leads to the continuous SWRC measurement of the soil samples within 3 to 5 days along drying paths. The credibility of the new SWRC device is also examined with additional suction measurement tests using conventional jet-fill tensiometers, showing consistent results.