Relationship between pre-compaction stress and shear strength under confined and semi-confined loadings for a sandy loam soil

Abstract

Mechanical soil strength is typically measured in terms of the maximum normal pressure which can be applied before a substantial increase in soil compression occurs. This soil strength is termed pre-compaction stress (σpc) and is usually regarded as the stress at which the soil deformation changes from “elastic” behaviour to the “plastic and irreversible” case. It was hypothesised that the σpc is the point at which the shear stress generated by the applied mechanical forces is equal to the soil shear strength (τ). Therefore, the objectives of this research were (i) to determine the σpc values by both plate sinkage test (PST) and confined compression test (CCT); and the τ from shear box test on pre-compressed soil specimens and (ii) to obtain the relations between values of the σpc and τ of the same soil. Tests were conducted on the remoulded specimens of a sandy loam soil collected from the topsoil. Experiments were conducted at two gravimetric water contents: 17 and 19%. Preload stress ranged from 0 to 200 kPa. The centre section of the preloaded soil specimen was firstly submitted to a 49.6 mm PST; then immediately one cylindrical sample was cored for CCT and three separate samples were taken for the shear tests. The σpc prediction by PST was accurate, whereas the values obtained with CCT were 4.5 (for water content of 17%) and 8.5 (for water content of 19%) times higher than the nominal preload stresses. The results showed that there was a highly significant (R2 = 0.95**) linear relationship between the σpc values and the corresponding τ for both PST and CCT. Although there was a good relationship between the σpc and τ, the former was greater than the latter, indicating that shear deformation may occur in the zone that is theoretically believed to be reserved for elastic deformation. Therefore, it might be the case that σpc is not at a transition point between the small elastic deformations and the large plastic deformations, but it is located in a transition area where both types of deformation occur at the same time.