Effect of sample size on shear strength of basaltic residual soils

Abstract

This paper first provides a brief review of the very limited data available on the size effect on strength of soils. Then it presents the results of an investigation of this effect on the drained strength of two residual soils derived from basalt. The dense basaltic soil, derived from weathering of columnar basalt, is fissured, whereas the vesicular basaltic soil, product of weathering of amygdaloidal basalt, is remarkably free of discontinuities. The results of tests on 500 mm square, 100 mm square, and 63.5 mm diameter direct shear tests, as well as on 36 mm diameter triaxial samples were obtained. The data clearly indicate the significant effect of fissures on the strength of dense basaltic soil, whereas the effect is absent in the vesicular soil. The reduction in strength with size in the former can be attributed almost totally to a loss of the cohesive component of shear strength. In the absence of tests on large-sized samples, a method is suggested to estimate the mass strength of such soils from results of tests on small-sized samples. Key words: fissures, residual soil, size effect, shear, strength, testing.