A history of understanding crack propagation and the tensile strength of soil

Despite the highly dynamic nature of cracks in soil and their significance for a wide range of processes including soil structure dynamics, few studies have attempted to quantify the physical processes involved in their genesis. In this chapter the development of fracture mechanics theories from materials science to describe crack propagation in soil will be discussed. It starts by considering some of the pioneering research on capillary bonding by Haines and Fisher that was developed for soil but has been extended to a wide range of particulate materials. By simplifying soil structure, this research showed how basic concepts can be developed and then extended to more natural systems. Soil mechanics research investigating the strength of unsaturated soil is then reviewed. In soil containing cracks, however, traditional unsaturated soil mechanics approaches have been found to provide poor predictions of soil strength. To incorporate the influence of cracks, various researchers have adopted linear elastic fracture mechanics. This approach assumes that all energy imparted to a soil system is recoverable on unloading, with only limited dissipation of plastic energy occurring. Later research applied the traditional J integral, which defines the elastic-plastic energy criterion for crack initiation. Simplified testing approaches and analysis have been developed recently that may allow for more widespread use of elasto-plastic fracture mechanics to describe crack initiation in soil. The need for future research on a wider range of soils, the coupling of fracture mechanics with soil hydrology, and advanced modeling approaches to predict crack depth are highlighted in this chapter. Such an understanding is essential for predicting the dynamic nature of soil structure.