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 weights of the working machinery. The objective of this research study was to assess the shear stress behaviour of rice growing vertisols at Mwea Irrigation Scheme with a view to manipulating resultant shear stress parameters at empirically established magnitudes for optimal tillage and traction operations. Collected samples 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 model of resultant induced deviatoric stresses within the three deformation zones at various strain levels predicted the shear stress behaviour of the study soil. Cohesive strength that is believed to represent maximum cohesive force holding soil particles together was observed at 70 kPa at a corresponding 100 frictional angle. Thus, maximum traction accompanied by optimal tillage operation by working machinery on vertisols is likely to be achieved at shear strength parameters of 70 kPa and 100 for cohesion and angle of internal friction respectively.
Published Version
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