Experimental study of the shear strength of carbonate gravel

Current studies on carbonate soils are mostly directed at carbonate sands rather than widely distributed carbonate gravels. In this study, a series of large-scale direct shear tests were conducted to evaluate the parameters that affect the mechanical behaviours of carbonate gravels, including the particle size, gradation, relative density and normal stress. Simultaneously, several tests on quartz gravels with similar gradations were conducted for a comparison. The experimental results showed that a large cohesion, which is defined as the interlocking cohesion in this paper, appears in carbonate gravels and it increases as the average particle size and relative density increase. The strength envelope of the carbonate gravel with an average particle size greater than or equal to 3.9 mm could be expressed as a broken-line with two straight lines in the stress range of 0–1600 kPa, but for quartz gravel, the broken-line mode appears when the average particle size reaches 7.7 mm. This result could be related to the breakage of the particles. Therefore, a certain number of gravel particles with different sizes and different shapes were coloured to trace the breakage mode of the particles. At the same time, it could be found that the strength of the carbonate gravel is not always greater than that of the quartz gravel with a similar gradation.