# 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  
Scanning electron microscopy revealed detailed structural features of the sample, in which \"P\" denotes particles, \"O\" denotes pores, and \"C\" denotes cracks. As can be seen from Figure 4, at the microscopic level, red mudstone exhibits numerous cracks and impurity particles, while yellow mudstone contains fine particles in an independent state. At the microscopic level, clay blocks demonstrate abundant fine particles with poor interparticle bonding, inhibiting the formation a stable cohesive whole. The presence of hydrophilic substances in clay, such as kaolinite, contributes to its strong plasticity and sensitivity to moisture changes. When exposed to water, these hydrophilic minerals can absorb moisture, leading to swelling, softening, and a reduction in the overall strength of the clay layer. The mudstone's microstructure, characterized by numerous fine particles with relatively weak particle bonding, also plays a crucial role in its mechanical properties. The  
Figure 4: Microstructure of the sample. Figure 3: Sample XRD test results. weak bonding between particles suggests that the mudstone has a lower cohesive strength compared to rocks with stronger inter-particle bonds. This weak bonding can make the mudstone more susceptible to deformation and failure under stress, especially when combined with the presence of a moisture-sensitive clay layer. Therefore, it is necessary to analyze the mechanical properties of the influencing factors. -clay layers, the initial loading stage is dominated by clay compaction and overflow effects, exhibiting significant plastic behavior. As the clay layer is compacted, the overall deformation is distributed between mudstone and clay. Compared to conventional composites, the stress-strain curve under uniaxial compression displays distinct abrupt and fluctuating characteristics. From Figure 6b, in the pre-peak stage, the variation in clay layers shows limited impact on the stress-strain behavior of mudstone-clay composites, manifested by highly overlapping stress-strain curves. Compared to composites T2, the stress-strain curves of T4 and T5 under uniaxial compression exhibit distinct bimodal characteristics. Specifically, the mudstone thicknesses in T4 and T5 samples are 20 mm and 70 mm, respectively. The bimodal phenomenon in their stress-strain curves reflects the macroscopic failure of the mudstone layer. During compression, uneven deformation of saturated clay creates a non-uniform state on the clay layer's upper surface, inducing significant shear effects on the upper mudstone. Notably, due to its thin upper mudstone layer (20 mm), the T5 sample exhibits weaker shear resistance. In the post-peak stage, the stress response of the T5 sample deviated from our expectations, and severe fracture occurred due to strong shear from uneven clay deformation. Additionally, when the thick mudstone layer experiences macroscopic damage, the entire mudstone-clay composite struggles to form an effective load-bearing structure, leading to more severe damage. This highlights the significant impact of shale layer thickness and shear performance on the composite's mechanical behavior. As shown in Figure 6c, with relative moisture contents of 0%, 50%, and 100%, the uniaxial compressive strengths of the mudstone-clay composites are 3.61 MPa, 1.00 MPa, and 1.59 MPa, respectively. Water exposure significantly reduces the uniaxial compressive strength, indicating increased deformation of the mudstone-clay composite roof during heavy rainfall. Compared to the dry clay combination, those with 50% and 100% moisture contents showed strength decreases of 72.30% and 55.96%, respectively. Notably, saturated clay composites exhibit slightly higher strength values than those with 50% moisture  
Figure 6: Influence of different factors on stress–strain curves of double soft composites. content, primarily due to the differences in clay mechanical properties under different moisture conditions.