By definition, cohesion is the stress (act) of sticking together. Yet, in engineering mechanics, particularly in soil mechanics, cohesion refers to shear strength under zero normal stress, or the intercept of a material's failure envelope with shear stress axis in the shear stress-normal stress space. Cohesion under saturated drained conditions is called drained cohesion and under variably saturated conditions is called apparent cohesion. This paper summarizes and extends recent work showing that there exists a unique functional relation between drained cohesion and apparent cohesion as they share common sources of physical origin, namely, van der Waals attraction, double layer repulsion, cementation, and capillary attraction. All of these forces depend on saturation and material (soil) type. Cohesion and apparent cohesion can be unified by a unified effective stress called suction stress under variably saturated conditions. Cohesion can be unambiguously de-composed into the product of tangent frictional angle and suction stress or tensile stress, and thus can be physically interpreted as the mobilized suction stress to internal frictional stress. Under such conceptualization, suction stress is the true cohesion as it is the stress of sticking granular particles together. Together with the internal friction angle, it can be used to mathematically describe cohesion of all granular materials under variably saturated conditions.
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Lu et al. (2013) studied this question.