# Performance Study of Black Shale Modified Soil for Road Use Based on Eshelby-Mori-Tanaka Theory
## 2 Experimental Materials and Their Physical Properties  
This experiment primarily investigates the physical and mechanical properties of black shale aggregate as roadbed fill material. Due to the susceptibility of black shale to disintegration in alternating wet and dry environments, it is necessary to incorporate a suitable amount of low-permeability clay to prevent further disintegration of the shale aggregate, as clay can envelop the aggregate and provide a more stable environment. However, clay has poor water stability and low strength when wet, so further incorporation of cement is required to enhance the water stability of the clay. Under the combined action of clay and cement, the mechanical properties of the black shale aggregate remain stable. Therefore, this paper explores the feasibility of using black shale aggregate as a highway subbase material through a strength test of the clay, cement, and black shale mixture. The experimental materials mainly include black shale aggregate, clay, and cement. The strength of the roadbed material depends on the comprehensive strength of the aggregate and binding materials (cement and clay, among others). This paper begins by conducting experimental research on the ratio and strength characteristics of cement-soil (a mixture of cement and clay). Combining the strength patterns of cement-soil, the paper further explores the feasibility of using cement-soil for the improvement of the performance of black shale aggregate and determines the optimal ratio for the black shale mixture. The experimental black shale used in this study was extracted from the newly excavated bedrock on a slope along the Zhangjiajie-to-Sangzhi Expressway. The surface black shale aggregate was collected after being exposed in the open for 15 to 30 days. The apparent density of the shale is \(\rho_{\text{shale}}=2.73\) g/cm\({}^{3}\). X-ray diffraction (XRD) analysis revealed that the main mineral components of the black shale include mica, chlorite, quartz, sodium feldspar, as well as small amounts of gypsum and pyrite (Figure 1). In its slightly weakened or unweathered state, the overall integrity of the black shale bedrock is good, Long-max black shale outcrop in Shizhu Country, Chongqing city, China. The peak stress of samples ranges from 100 to 195 MPa, whereas Young's modulus changes from 22.22 to 42.31 GPa [25]. The unconfined compressive strength of the exposed black shale from the Longmaxi Formation in Changing, Sichuan is between 80 MPa and 200 MPa [26]. However, upon exposure to the environment, it is prone to weathering and disintegration. The strength of pre-disintegrating carbonaceous shale can meet the requirement of roadbed filling, but it is not suitable to be used as roadbed filling directly [27]. The rock block in Figure 2b disintegrated into the state shown in Figure 2c after undergoing five cycles of wet and dry conditions. The strength of the shale aggregate decreases as the degree of disintegration deepens. Figure 1: Mineral content of black shale. To understand the particle size distribution of the shale aggregate, sieving was conducted using the screening method, and the particle size distribution curve of the shale aggregate was obtained, as illustrated in Figure 3. The unevenness coefficient of the particle size distribution curve is 11.6, and the curvature coefficient is 1.12. This indicates that the particle size distribution of the shale aggregate under natural disintegration is relatively uniform, and the gradation is good, meeting the recommended gradation upper and lower limits specified in the \"Technical Guidelines for Road Surface Base Construction\" [28] (JTG/T F20-2015). The clay used in the experiment was sourced from an excavated hillside at a construction site in Changsha, Hunan province. The soil samples were collected, air-dried, pulverized, and sieved to obtain samples with particle sizes not exceeding 3 mm for experimental purposes.