# 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 M\({}_{\text{\text{\text{\text{\text{\text{\text{\text{\text{\text{\text{\text{\text{\with black shale. Therefore, the rational disposal of black shale in road construction holds significant economic and ecological value. Many scholars have explored the feasibility of using black shale as a road material in road construction to avoid the aforementioned problems and reduce construction costs. Therefore, numerous scholars have conducted feasibility studies on its use as a road material. [PERSON] et al. [11], through indoor disintegration resistance tests, found that the disintegration index of carbonaceous rocks in the Guangxi region decreased with the increase in clay mineral content, showing a power law relationship. [PERSON], [PERSON], and others [12; 13] suggested that compacted shale exhibits acceptable engineering properties and performance ranges. However, over time, under humid conditions, compacted shale may disintegrate and expand, leading to a reduction in durability, resulting in decreased stability and loss of bearing capacity. [PERSON] et al., through dynamic triaxial tests, investigated the influence of grading, confining pressure, moisture content, and compaction degree on the dynamic properties of carbonaceous shale coarse-grained soil. They confirmed that carbonaceous mudstone coarse-grained soil can be used for filling highway subgrades with light to medium traffic loads [14]. [PERSON] et al. [15] used a triaxial test to study the changes in strength of disintegrated carbonaceous mudstone from the Long Lang highway in Loudi, Hunan Province, by adding fly ash, clay, and cement. They found that cement had the most significant effect on improving the cohesion of the mixed soil, followed by clay. [PERSON] [16], through cement addition to modify weathered soft rocks along the Shiyan-Tianmen route, found that the California Bearing Ratio (CBR) and compressive strength could meet the requirements of highway engineering. The above studies fully demonstrate the feasibility of using carbonaceous shale gravel as highway subgrade fill material. However, modification is needed, and determining the type of modifier and controlling its content pose a challenging problem. The strength and performance of expansive soil cannot be improved through simple mechanical stabilization (compaction). Generally, chemical additives are used to achieve the desired strength. Calcium-based additives (such as cement and lime) are widely used for the stabilization of clay and some expansive roadbeds [17]. Compared to lime, cement has a relatively significant impact on the mechanical properties of mountain soils. Using cement to solidify samples for 28 days can increase the compressive strength by four to six times compared to untreated samples [18].