Evaluation of the shear strength parameters of a compacted S-RM fill using improved 2-D and 3-D limit equilibrium methods

It is a common construction practice to use mixtures of soils and rock fragments for engineered fills in slopes and embankments, making the assessment of their shear strength properties a fundamental step for determining the stability of such structures. In situ bimtests were performed in an artificial soil-rock mixture (S-RM) slope constructed for an electronic converter station in the Funing County, Yunnan Province, China. Improved 2-D and 3-D limit equilibrium methods, accounting for the difference between the peak pushing horizontal forces applied during the first and the second loading cycle of the test, neglected in previous researches, are proposed for correctly assessing the shear strength parameters of the S-RM. Results show that neglecting the difference between the two peak pushing horizontal forces can produce relatively large errors in the back-calculated strength properties of S-RMs, especially when the cohesive term is low. These errors are particularly significant when the analysis is performed in 3-D condition. The number of sections required in the simplified 2-D approach is also determined based on the relative errors of estimated strength parameters in comparison to those deriving from a 2-D analysis considering a very large number of sections (i.e. 133 sections). The shear mechanical properties of S-RM samples with different volumetric block proportion (VBP) and block strength are subsequently described. S-RMs with higher VBP have more tortuous shear surface, especially when stiff rock blocks are employed. The cohesion of stiff S-RMs decreases with the increment of VBP, while the friction angle increases. Conversely, both the strength parameters increase with VBP for the S-RMs containing soft rock blocks (i.e. soft S-RMs), even if the cohesion assumes negligible values. The shear behavior of soft S-RMs is different from that of stiff SRMs when the VBP is the same, indicating that the block strength also may affect the strength of this class of geomaterials. Finally, by adopting a fractal geometry approach, it is demonstrated that there is a large correlation between the degrees of tortuosity of shear surfaces and the friction angles of S-RMs.