Describing soil crack formation using elastic-plastic fracture mechanics

Crack development is predominant in soil structure formation. A number of fracture mechanics models have been applied to soil to describe cracking, but most are not applicable for soil in a wet, plastic state. We address this weakness by applying a new elastic-plastic fracture mechanics approach to describe crack formation in plastic soil. Samples are fractured using a deep-notch (modified four-point) bend test, with data on load transmission, sample bending, crack growth, and crack-mouth opening collected to assess the crack-tip opening angle (CTOA). CTOA provides a powerful parameter for describing soil cracking since it can be induced by soil shrinkage (an easily measured parameter) and can be used to describe elastic-plastic fracture in numerical approximations, such as finite element modelling. The test variables we studied were the direction of the applied consolidation stress, clay content, and pore water salinity. All samples were formed by consolidating soil slurry one-dimensionally with a 120-kPa vertical effective stress. Tests on pure kaolinite showed that the direction of the consolidation stress did not affect CTOA, which was 0.23 +/- 0.02 m m(-1) for specimens cut both in a horizontal and in a vertical direction to the applied stress. Soil clay content had a marked influence, however, with silica sand:kaolinite mixtures by weight of 20:80 and 40:60 reducing CTOA to 0.14 +/- 0.02 m m(-1) and 0.12 +/- 0.01 m m(-1), respectively. These smaller values of CTOA indicate that less strain is required to induce fracture when the amount of clay is less. Salinity (0.5 m NaCl) caused a reduction in the CTOA of pure kaolinite from 0.23 +/- 0.02 m m(-1) to 0.17 +/- 0.03 m m(-1).