A novel multifunctional suction-controlled apparatus for investigating tensile strength of unsaturated soils

Tensile failure is a critical mechanism in the cracking and destabilization of unsaturated soil structures such as slopes, landfill covers, and compacted embankments. Accurate characterization of tensile strength under controlled suction remains experimentally challenging due to limitations in existing apparatuses. This study presents a novel multifunctional apparatus designed and manufactured for unsaturated soil testing. The apparatus integrates suction control, tensile loading and vertical loading systems, enabling it to conduct tensile strength testing under controlled suction and constant normal stress, as well as pressure plate tests and unsaturated oedometer tests. A series of tests were conducted to assess its efficiency and applicability. The apparatus produced repeatable soil–water retention curves (SWRCs), demonstrating its effectiveness in suction control. Tensile strength characteristic curves (TSCCs) obtained from compacted clay samples revealed strong dependence on suction and compaction level, including the observation of double-peak tensile strength behaviour at high compaction. In addition, the application of cover loading (normal stress) was found to reduce the tensile strength relative to uniaxial conditions, particularly under low-suction states. Unsaturated oedometer tests under constant suction yielded compression and recompression indices. The developed apparatus offers a reliable, space-efficient, and cost-effective solution for unsaturated soil testing. Its multifunctionality and control precision enable in-depth investigations into suction-dependent tensile strength and compressibility behaviours, contributing to improved understanding and prediction of cracking and settlement in unsaturated soil structures.