Brazilian Tensile Strength Test Conducted on Ductile Unsaturated Soil Samples

The tensile strength of soil is often neglected. Because of the presence of matric suction, unsaturated soils may have substantial tensile strengths with implications for a range of geotechnical problems, such as slope stability, bearing capacity, and the integrity of clay liners. Direct measurement of the tensile strength of soils is complex to carry out, while the simplicity of the Brazilian tensile strength (BTS) test offers an attractive alternative. However, this method was developed to measure the tensile strength of brittle materials. While suction-bound unsaturated soils may behave in a brittle fashion at low moisture contents, such materials become more ductile as the moisture content increases. This study investigated the application of the BTS test to measure the tensile strength of unsaturated soil samples over a range of moisture contents. Because of the low sample strength of these soils, the use of curved loading platens is recommended. The required load angle depends on the ratio between the compressive and tensile strength of the material tested. When brittle behavior is obtained during testing, conventional interpretation may be used, i.e., the maximum mobilized load during load application is used for tensile strength calculation. This is not appropriate when testing ductile materials. During load application on ductile samples, the initial mobilized load-deformation behavior is approximately linear and becomes nonlinear when tensile yielding starts to occur, originating from the sample center where the full tensile strength is first mobilized. It is recommended that, to obtain a conservative estimate of tensile strength, the inflection point where behavior becomes nonlinear should be taken as representative of soil tensile strength.