The formation of desiccation cracks in soils is often interpreted in terms of tensile strength. However, this mechanistic model disregards the cohesionless, effectivestress-dependent frictional behaviour of fine-grained soils. An alternative theory is explored using analyses, numerical
simulations based on an effective-stress formulation, and experiments monitored using high-resolution timelapsed photography. Results show that desiccation cracks in fine-grained sediments initiate as the air–water interface invades the saturated medium, driven by the increase in suction. Thereafter, the interfacial membrane causes an increase in the local void ratio at the tip, the air-entry value decreases, the air–water interface advances into the tip and the crack grows. The effective stress remains in compression everywhere in the soil mass, including at the tip of the desiccation crack. This crack-growing mechanism can explain various observations related to desiccation crack formation in finegrained soils, including the effects of pore fluid salt concentration, slower crack propagation velocity and right angle realignment while approaching a pre- existing crack, and the apparent strength and failure mode observed in fine-grained soils subjected to tension. Additional research is required to develop a complementary phenomenological model for desiccation crack formation in coarse-grained sediments.