Shear Strength Behaviour of Vertisols : Case of Rice Growing in Mwea Irrigation Scheme

Abstract

Rice growing vertisol soils at Mwea Irrigation Scheme in Kenya are routinely subjected to dry ploughing prior to establishment of a new crop. During ploughing, the soil experiences varying levels of induced shear stress emanating from the weight of the working machinery which are mostly self-propelled tractors as well as soil engaging plough components. Characteristics response/behaviour of the vertisols under these loading variabilities is not only significant in influencing resultant traction developed but also tillage operation performances. The objective of this research study was to assess the shear stress behaviour of rice growing vertisols at Mwea Rice Irrigation Scheme with a view to manipulating resultant shear stress parameters at empirically established magnitudes for optimal tillage and traction operations. Collected samples (disturbed and undisturbed) from the study field were triaxially tested through successive three phases of saturation, isotropic consolidation and shearing based on critical state soil mechanics principles. Deviatoric stress – strain plots to establish shear strengths of samples from three profile depths ( 0 - 20, 20 – 40 and 40 – 60 cm) over varied water contents were developed. The study established that three patterns deformation characteristic occurred in vertisols when subjected to loading from working tillage machinery. A predictive exponential model satisfactorily correlated deviatoric stress – strain response pattern of the the study soil. Cohesive strength that is believed to represent maximum cohesive force holding soil particles together was established at 70 kPa with a corresponding frictional angle of 30 . Thus, maximum traction with optimal tillage performances on vertisols at Mwea Irrigation Scheme are likely to be achieved by operating tillage machinery at a cohesive force and frictional angle just above 70 kPa and 30 respectively. Consequently, any tillage tractor with a tractive effort above 70 kPa is optimal for use by farmers and tractor hire service providers on rice growing vertisols in Mwea Irrigation Scheme. Key Words: Vertisol; Stress – strain; Traction; Tillage; Working machinery; Isotropic consolidation; Critical state; Deviatoric stress; Profile depth