Critical evaluation of some suction measurement techniques

Suction is an important stress-state variable of unsaturated soils. The magnitude of suction affects the shear strength, the hydraulic conductivity, and the volume change behaviour of unsaturated soils. The measurement of soil suction is a prerequisite for the characterisation of unsaturated soils. Soil suction can be determined either by adopting direct or indirect measurement techniques. Despite several techniques available currently for measuring and controlling matric and total suctions of soils in the laboratory, several aspects related to various suction measurement techniques, such as the water phase continuity in null- type tests and compatibility of test results from various measuring techniques are yet to be explored in detail. Similarly, studies concerning determination of air-entry values (AEVs) and residual suctions of soils that exhibit volume change during the drying process are limited. Suctions of two soils from Libya (a silty sand and an inorganic clay with intermediate plasticity) were experimentally measured using null- type axis-translation, filter paper, and chilled-mirror dew-point techniques. Axis-translation and vapour equilibrium techniques were used for establishing the drying and wetting suction-water content soil-water characteristic curves (SWCCs) of the soils. Compacted soil specimens were prepared by varying moulding water content, dry density, compaction type, and compaction effort in order to investigate the influence of initial compaction conditions on measured suctions and SWCCs of the soils. The water content-void ratio relationships (shrinkage curves) of the soils from Clod tests were used in conjunction with the drying suction-water content SWCCs to establish the suction-degree of saturation SWCCs that enabled determination of the air-entry values (AEVs) and residual suctions of the soils. Initially saturated slurried specimens of the soils were also considered for comparing with the test results of compacted soil specimens. The test results from the investigation showed that the influence of compaction conditions on SWCCs of the soils was distinct only at a low suction range, whereas their impact was insignificant at higher suctions. The volume change of the soils during the drying process had significant impact on the AEVs and residual suctions. For initially saturated slurried specimens, the AEVs and the residual suctions of the soils determined form the suction-water content SWCCs were found to be distinctly lower than their counterparts determine from the suction-degree of saturation SWCCs. Suctions corresponding to the plastic limits of the soils agreed well with those determined from suction-degree of saturation SWCCs, whereas suctions corresponding the shrinkage limits overestimated the AEVs. An increase in the chamber air pressure soon after the null-type tests were completed clearly indicated that the water phase continuity between the water in the soil specimens, the water in the ceramic disk, and the water in the compartment below the ceramic disk was lacking for all specimens tested. Soil specimens with higher water contents created better continuity in the water phase. At high suction range, the test results from the techniques based on vapour equilibrium (i.e., non contact filter paper, salt solution and chilled-mirror dew-point tests) showed very good compatibility, whereas differences were noted between the test results at low suction range from the techniques that are based on liquid phase equilibrium (i.e., pressure plate and null-type tests).