A study of mixed-mode fracture and size effects of weakly cemented sand

The mechanical behavior of stiff soils, such as overconsolidated clays and cemented sand is characterized by brittleness, which means that once the strength reaches the peak level, it decreases rapidly to its residual state. Although there is no single reason for this behavior, many researchers have brought attention to the fact that the presence of flaws i.e. fissures, cracks, joints, have a great effect on the strength and overall stress-strain behavior of such materials. These defects results in stress concentrations that lead to local failure and reduction in the overall strength of the material as the failure propagates through the intact area or volume. This phenomenon indicates that the failure mechanisms of structures such as slopes in cemented sand are different from shear failure. In other words, conventional analysis techniques that are based on classical strength criteria might not be adequate. Fracture mechanics can appropriately be adopted as a good tool for analysis of these materials. However, the use of fracture mechanics concept especially for cemented sands, is faced with difficulties in obtaining relevant parameters, because fracture parameters and predictions are highly dependent on the material constituents and the size of specimens, as well as the size of particles.; Four-point tests with three different specimens and grain sizes were conducted in the investigation and a numerical finite element analysis (MERLIN) was performed as well. Through these techniques we found that the fracture properties of cemented sands are greatly dependent on grain size of the constituent material as well as the size of specimens. Using FEM coupled with nonlinear fracture mechanics concepts, in which the fracture energy was used as the dominant parameter, we obtained good predictions of the load vs. CMSD (Crack Mouth Sliding Displacement). The 4-point beam experiments did not provide pure Mode II conditions but rather a mixture of both Modes I and II. Using fracture mechanics concept, steep slopes in cemented sand were assessed. The results showed that FEM coupled with fracture mechanics concepts provides an excellent alternative in the design and safety assessment of earth structures in cemented soils.