Influence of soil water on stress–strain behaviour of a compacting soil in semi-arid Kenya A.N. Gitau * , L.O. Gumbe, E.K. Biamah Department of Environmental and Biosystems Engineering, University of Nairobi, P.O. Box 30197, Nairobi, Kenya Received 23 December 2004; received in revised form 31 May 2005; accepted 7 July 2005 Abstract Depending on the top and subsoil textures, semi-arid soils exhibit cohesive and frictional properties that are associated with the relatively high soil strength, bulk density and penetration resistance. The objective of this study was to gain the knowledge of mechanical properties of the compacting chromic luvisols in order to improve the design of tillage tools. Therefore, we applied critical state soil mechanics to study the stress–strain behaviour of the luvisols using triaxial tests under laboratory conditions. Field investigations involved random collection of undisturbed soil samples which were subjected to triaxial testing first by isotropic consolidation and compression and then triaxial shearing. Plots of deviatoric stress against axial strain were made to determine the soil shear strengths at the critical states over different soil water levels and the two soil depths of 0–20 cm for the plough and 20–40 cm for the hard pan layers, respectively. An exponential model used to fit the deviatoric stress–axial strain test data accurately predicted the trends. Soil water significantly influenced the shear strength, cohesion (c 0 ) and internal angle of friction (f 0 ) and hence the mechanical behaviour of the luvisols. The regression equations developed showed that c 0 and f 0 have quadratic relationships with soil water. The very high clay bonding strength in the subsoil (hard pan) layer resulted in high shear strength, bulk density and penetration resistance values for this soil layer. The increase in shear strength with decreasing water content affected the deviatoric stress–axial strain relationships between the upper and lower plastic limits of the sandy soil. Thus, as the soil dried, the soil ceased to behave in the plastic (ductile flow) manner and thus began to break apart and crumble. The crumbling was indicative of brittle failure. The transition stage from an increase to a decrease in c 0 and f 0 values with soil water occurred in the soil water content range of 6–10%. Knowledge of stress–strain behaviour of compacting soils is of practical significance in the design of appropriate tillage tools for the specific soil type. # 2005 Published by Elsevier B.V. Keywords: Soil water; Stress–strain; Plough layer; Hard pan layer; Tool design; Critical state; Soil mechanics 1. Introduction In semi-arid Kenya, rainfall is bimodal and characterized as low, erratic and poorly distributed. The short and long rainy seasons receive about 55 and 45% of the total annual rainfall, respectively (Biamah www.elsevier.com/locate/still Soil & Tillage Research 89 (2006) 144–154 * Corresponding author. Tel.: +254 722878029. E-mail address: gitauan@yahoo.co.uk (A.N. Gitau). 0167-1987/$ – see front matter # 2005 Published by Elsevier B.V. doi:10.1016/j.still.2005.07.008