Performance Study of Black Shale Modified Soil for Road Use Based on Eshelby–Mori–Tanaka Theory

Abstract

Black shale, as a type of soft rock, exhibits high strength when freshly exposed. However, it easily disintegrates upon contact with water, making it unsuitable for direct use in roadbed construction. Using it as discarded material not only increases construction costs but also pollutes the environment. Therefore, the reuse and modification of black shale have become particularly important. Based on the theory of composite material equivalent inclusions, this study investigates the strength and water stability characteristics of black shale gravel after being mixed with cement and compacted with clay. The results show that the strength of cemented soil increases linearly with the cement content. The water absorption properties of the modified soils with different amounts of black shale added are similar, with an average water absorption rate of about 2.53%. The strength of black shale modified soil is generally positively correlated with the cement content, although the linear correlation is not significant. The modified black shale soil used in the experiment is suitable for the subgrade of medium- and light-grade secondary roads and below. The recommended mass ratio is Mshale:Mclay:Mcement = 70:21:9. The unconfined compressive strength of the material under 7-day curing is 1.36 MPa. The relationship between the strength of modified soil, clay strength, cement content, and gravel addition has been established, clarifying the physical significance of each parameter. The “drying and soaking” cycle can accelerate the strength degradation of modified soil. It is recommended to strengthen the construction of roadbed drainage facilities during construction to maintain a stable and dry environment for the modified soil as a roadbed filling material. The research results not only provide clear technical indicators for the reuse of discarded black shale in engineering but also serve as a basis for proportion of crushed stone discarded material as roadbed fill.