Pre-compression stress and its relation with the physical and mechanical properties of a structurally unstable soil in central Iran

Pre-compression stress approach as a mean of assessing compressive nature of soils has received more attention nowadays. A pot experiment of a silty clay loam soil (fine-loamy, mixed, thermic Typic Haplargids, USDA; Calcaric Cambisols, FAO) was conducted to examine the effect of internal forces upon drying on pre-compression stress. The pots were saturated by flood irrigation. Bulk density (BD), water content (θm), matric suction (−ψm), cone index (CI), unconfined compressive strength (UCS), indirect tensile strength (ITS) and pre-compression stress (σpc) were measured at different water contents/matric suctions during soil drying. It was found that σpc prediction using BD as strain-related property in functional relationship with logarithm of major principal stress (σpc(BD)) is higher compared with σpc prediction using e (void ratio) (σpc(e)) by a factor of 1.3. Because the non-linear function of BD vs. e can affect the curvature of the stress–strain relationship and therefore the σpc determination. Pre-compression stress (σpc(e)) was strongly affected by θm, but the packing state of the soil, i.e. BD had a weaker effect on σpc(e). Pre-compression stress (σpc(e)) increased linearly with effective stress for 0<σ′<100 kPa (wet moisture range), after which the relation deviated from the linear line. The internal forces are the major source of hardening in this structurally unstable soil. Pre-compression stress (σpc(e)) was predicted fairly well by using the UCS and the ITS data. However, a very close linear relationship was obtained between σpc(e) and CI. The results may be useful for rapidly predicting the σpc(e) of the soil. It is indicated that UCS and ITS can be used to predict one another.