## 3 Test Results and Discussion
### Analysis of Strain Variation in the Intact Region of Rock Mass under the Influence of joints
This leads to significant oscillations in strain monitoring data, marking a notable departure from the previous stable changes. Consequently, this stage plays a critical role in identifying when the rock mass enters a state of complete instability. 3 Instability stage: In this stage, the test specimen has undergone complete failure. The strain in the jointed region relative to the intact region stabilizes at a specific value or reverts to zero, indicating the complete failure of the specimen. Sub-Instability Stage Strain Transient Characteristics in Jointed Rock Mass  
Sample 1-4, influenced by shear failure, did not exhibit a typical sub-instability stage, and its strain changes differed significantly from those of the other specimens. In Sample 1-1, Strain S1 displayed the following pattern: an accelerated increase during the NO stage (+2 \(\upmu\varepsilon\)), followed by relative stability during the OA stage, and a sudden sharp increase followed by a sharp decrease during the AB stage (+5 \(\upmu\varepsilon\) to \(-4\)\(\upmu\varepsilon\)). On the other hand, Strain S2 showed a sharp decrease during the NO stage (\(-7\)\(\upmu\varepsilon\)), followed by relative stability during the OA stage, and a rapid decrease to zero during the AB stage (\(-10\)\(\upmu\varepsilon\)), as depicted in Figure 11b. Consequently, during the strong departure from the linearity stage (NO), the jointed rock mass initially emits instability signals, such as a sudden increase or decrease in strain, although the magnitude of these signals is relatively low. In the static sub-instability stage (OA), the jointed rock mass maintains a phase of stability. In the dynamic sub-instability stage (AB), the jointed rock mass releases intense instability signals, with strain changes reaching as high as 50%. This stage serves as a precursor to the complete instability of the jointed rock mass. In Samples 1-2 and 1-3, Strains S1 and S2 exhibited a similar trend to that of Sample 1-1 during the N-B stage, characterized by significant strain changes in the NO stage and rapid changes in the AB stage. However, the extent of strain variation in Samples 1-2 and 1-3 was lower compared to Sample 1-1. This phenomenon can be attributed to differences in the sizes of the jointed portions. In fixed-sized red sandstone samples, as the proportion of the jointed portion increases, its influence on the intact portion of the red sandstone becomes more pronounced. Under the same external loading conditions, when the proportion of the jointed portion is relatively small (as in Sample 1-1), the strain energy released due to the jointed portion's failure is lower, resulting in a relatively higher accumulation of strain energy in the intact portion of the red sandstone. Consequently, the physical signals released during the instability process (with strain gauges located in the intact portion) become more pronounced. Conversely, when the joint proportion is larger (as shown in Figure 11b), the jointed portion concentrates stress to a greater extent, causing a higher release of strain energy, The specific areas of mutual influence are shown in Figure 12. Consequently, the physical signals released during the instability process in the intact portion become less prominent. Analysis of Acoustic Emission Characteristics during the Instability Process of Jointed Red Sandstone  
Figure 13 illustrates the acoustic emission signal characteristics during the sub-instability stage for the four test specimens. From the acoustic emission monitoring data, it is evident that Sample 1-4, which experienced sudden shear instability, did not generate a significant number of AE events during the instability stage. However, at 137 s in Sample 1-4, the sensor detected an amplitude of 97 dB and an energy of 29,000, which corresponds to the moment when an internal shear crack parallel to the joint orientation occurred within the jointed portion, as observed at the test site.