Tensile strength of clayey soil and the strain analysis based on image processing techniques

Tensile strength is one of the most important mechanical parameters controlling the development of cracks in soil. However, it is frequently neglected in conventional geotechnical practice, because its magnitude is small and difficult to measure relative to other soil strength parameters. In this paper, a newly designed direct tensile test apparatus was employed to measure the tensile strength of an unsaturated clayey soil. A digital image acquisition and analysis system was developed for tensile strain analysis with the help of Particle Image Velocimetry (PIV) and Digital Image Correlation (DIC) techniques. Six groups of samples were compacted at a dry density of 1.7 Mg/m(3) and different water contents (6.5%, 8.5%, 10.5%, 12.5%, 16.5% and 20.5%). Test results show that the tensile strength characteristic curve (tensile strength versus water content) of the compacted unsaturated soil exhibits mono-peak feature. When water content is relatively low, the tensile strength increases with increasing water content and reaches the maximum value at a critical water content of about 9.3%. Then, it declines with further increase in water content. The evolution of tensile strength with water content depends on both suction and microstructure. Based on plotted tensile load-displacement curves, the tensile failure process can be divided into three typical stages which are: stress increasing stage (I), failure developing stage (II) and post-failure stage (III). It is found that the overall tensile failure process presents different patterns controlled by water content. Generally, the failure developing stage (II) lasts longer and the failure ductility is more pronounced when the sample is compacted at higher water content. Using PIV and DIC techniques, the development of displacement direction and strain concentration during tension can be well captured for appreciation of the soil failure mechanism. Based on the strain concentration information, the tensile fracture location and direction can be pre-determined for soil samples.