Mechanical Behavior of a Hard-Setting Luvisol Soil as Influenced by Soil Water and Effective Confining Stress

A. N. Gitau, L. O. Gumbe , S. K. Mwea


A series of experiments were carried out using a triaxial system modified for unsaturated soil
testing to investigate the mechanical behavior of a luvisol soil under varying soil water and
effective confining stress levels. Mechanical properties and deviatoric stress-strain
relationships of the soil were also established. The specimens were prepared under
laboratory conditions where the inter-particle cementation bonds were allowed to form to
their natural state. An unsaturated soil mechanics approach was used to define critical state
relations for the soil. All specimens attained a critical-state under continuous shearing as a
function of the level of effective confining stress. The results demonstrate that brittle / ductile
behavior of unsaturated soils and their tendency to dilate / compact at failure are both
controlled by soil water and effective confining stress levels. An exponential model used to
fit the deviatoric stress - axial strain test data accurately predicted the trends. Soil water
significantly influenced the shear strength and internal angle of friction (φ′) and hence the
mechanical behavior of the luvisol soil. The regression equation developed showed that φ′
have quadratic relationships with soil water with an asymptotic surface (transitional stage).
Hence, in soil tillage the transitional range (w.c. 5 - 9% d.b.) could be taken as a useful guide
(soil friable state) towards understanding soil behavior upon loading when machinery and
equipment traverses agricultural fields. Numerous researchers have placed great emphasis in
performing tillage operations when soils are at the friable states hence minimizing

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