This study is to investigate shear properties and rock block breakage characteristics of soil-rock mixtures (SRMs) by lab large-scale direct shear tests and particle discrete element numerical simulation using PFC2D. A particle discrete element numerical method for SRM

accurately describing the shape characteristics and the breakage effect of rock block

is presented with comparisons of the results of large-scale direct shear tests and sieving analysis tests. The shear properties and rock block breakage characteristics of SRMs of six different rock block contents under four different normal stresses were studied. The result basically complies with Mohr-Coulomb strength criterion

showing the higher the rock block content

the higher the shear strength of SRMs. With the increase of rock block content

the internal friction angle of SRMs shows slow-fast-slow growth trend

but the cohesion presents a trend of first increase

then decrease and then increase. The breakage forms of rock block can be classified into 4 categories: surface grinding

partial breakage

complete rupture and complete breakage. A new particle breakage index is presented to accurately reflect the breakage degree of rock block with particle size greater than 5 mm. The index increases as the rock block content and normal stress increasing. The analysis of shear surface of particle discrete element numerical model shows that the shear surface appears ‘shear oblique’ phenomenon. The breakage degree is more obvious and the number of shear cracks near shear surface increase as rock block content increasing. The failure of S-RMs during shearing is tension-shear mixing failure.