Measuring fracture toughness of clayey soils in a wide suction range

Fracture toughness is a fundamental variable in describing cracking behaviour, yet it remains challenging to measure for clayey soils partly due to its extremely high dependence on water content or suction. Herein, a novel expansion ring method is proposed for measuring the fracture toughness of clayey soils in a wide suction range from below the air-entry value to several hundred megapascals. This method utilises an expanding ring mechanism to generate outward pressure on the inner boundary of thin concentric clay samples. This set-up exhibits two features: approximately zero gravitational stress in samples; and no stress concentration around loading points. These features enable mode I crack nucleation in very soft, even saturated clay samples. Thereafter, the strain variations around the crack tip are captured using digital image correlation, facilitating the calculation of the critical J-integral at the crack initiation point (JIc), which is a measure of fracture toughness. The applicability of the proposed method is tested using three types of clayey soils representing a wide range from non-expansive to highly expansive. The validity of the method is assessed through the existence of the J-dominated zone and the path independence of the J-integral. Moreover, the repeatability of the method is validated through independent trials. Results reveal that JIc can vary non-monotonically with suction, reaching maximum values of 1·56 J/m2, 3·55 J/m2 and 6·26 J/m2 for bentonite, HN clay and kaolinite, respectively.