# Instability Characteristics of and Control Techniques for Mudstone-Clay Composite Roof Roadways
## 3 Microscopic Experimental Study on Mudstone-Clay
### Analysis of Mudstone-Clay Experimental Results
#### 3.2.2 SEM Scanning Experimental Analysis
Additionally, as clay moisture content increases, the plastic-elastic properties of mudstone-clay composites become more pronounced in the pre-peak stage. The stress-strain curve of dry clay composites shows linear decline post-peak, while composites with 50% and 100% moisture contents display stepped decline. This suggests that water-exposed composites are more likely to form a load-bearing structure after With constant clay layer thickness and varying layer numbers, the failure behavior changes distinctly. In two-layer composites, the clay layer fails first, and cracks propagate through it before propagating into the mudstone. In this case, the middle mudstone experiences tensile failure, while the rocks at both ends undergo shear failure. In three-layer composites, tensile cracks increase in the central mudstone section, while rock cracks at the ends are less pronounced. Increased clay layer numbers promote tensile failure in the center mudstone and shear failure at the ends. More clay layers result in more macroscopic cracks and a more complex fracture morphology, as shown in Figure 7b,h,i. During the uniaxial loading experiment, failure typically begins with cracking in the clay segment. As the clay layer is compacted, cracks gradually widen. Once tension cracks penetrate the clay, the mudstone ruptures as the clay overflows outward. With increasing axial load, tensile cracks in the mudstone grow in number, length, and width. When axial stress peaks, macroscopic damage occurs in the mudstone, with cracks extending toward the top and bottom of the composite. The clay's bonding effect creates a stable triangular structure between rocks adjacent to the tensile crack. As pressure continues to rise, axial deformation increases, disrupting the balance of this triangular structure and further expanding the tensile cracks in the mudstone. The clay restricts lateral movement of the fractured rock mass, gradually increasing interfacial friction, which halts the expansion of tensile cracks after a certain distance. This results in the formation of a new triangular  
Figure 7: Failure characteristics under combined uniaxial loading experiment (**a**–**k**). stable structure. This process repeats as tensile cracks advance toward the composite ends until the fractured rock mass collapses. This periodic disruption of triangular stability ultimately leads to composite failure. The failure mode of mudstone-clay composites with water-bearing clay is primarily tensile failure, often characterized by penetrating cracks. During uniaxial loading, cracks initiate in clay layer first due to the lower strength and deformation resistance of water-bearing clay compared to mudstone. The cracks begin in the middle of the clay and propagate outward to the mudstone-clay interface. This indicates that clay's outward extrusion after compaction, which creates a tearing effect on the mudstone, is the main mechanism causing tensile failure in the composite material. Shear cracks in mudstone-clay composites are relatively sparse, with smaller crack lengths and openings compared to tensile cracks. Higher relative moisture content leads to more shear cracks. This occurs primarily because compacted clay cracks overflow outward under dry conditions. The central part of the mudstone-clay interface plane is elevated, causing uneven stress during uniaxial loading and resulting in shear cracks. These cracks are primarily located at the edges and corners of the mudstone, affect a limited area, and are closely linked to the horizontality of the composite interface. Therefore, tensile cracks caused by tensile failure significantly influence the stability of uniaxial loading experiments, while shear cracks have minimal impact on the bearing characteristics.