Shear strength of repacked remoulded samples of a calcareous soil as affected by long-term incorporation of three organic manures in central Iran

Improvement of soil organic matter content and its effect on soil mechanical properties is a key goal for sustainable agriculture. This study examines the effects of seven-year incorporation of organic manures (municipal solid waste compost; air-dry sewage sludge; cattle farmyard manure) on shear strength parameters (cohesion, c; internal friction angle, φ) of a silty clay loam soil. Disturbed samples collected from the topsoil of experimental plots to which the organic manures had been added at three rates (25, 50 and 100 t ha−1) were remoulded at two gravimetric water contents (WC) of 17.1 and 20.9%. Three intact samples were removed from the large soil specimen and the shear tests were conducted using direct shear box. To study the effect of WC on the shear strength of the soils with different bulk densities (BD) arising from the organic manure management, an index of consistency (IC) was introduced, defined as the ratio of WC of a soil to its plastic limit (PL). By increasing WC from 17.1 to 20.9%, c was almost doubled, whereas φ significantly decreased, but only by 13%. The changes in c due to changes in WC were explainable using effective stress theory. The relationships of c and φ with IC were linearly increasing and decreasing below the plastic limit, respectively, with some evidence of a plateau or non-linearity above this level. The results showed that for soils having different organic matter contents (or BD), the change in shear strength parameters with WC could be unified and explained using the IC concept.