# Experimental Study on Shear Creep and Long-Term Strength of Clay-Type Muddy Interlayer
## 3 Results and Discussion
### Study on Long-Term Strength of Muddy Interlayer  
The long-term strength of the muddy interlayer is the boundary value that distinguishes the attenuation creep from the nonattenuation creep, and researching the long-term strength of the muddy interlayer can provide valuable references for the stability analysis and evolution prediction of landslides. Rock and soil, in a given period of time, produce damage to the impedance ability that is known as its long-term strength \(\tau(t)\), and its long-term strength limit \(\tau_{\infty}\), that is, the rock and soil in the long-term load under the action of the impedance ability of the critical strength value, that is, the rock and soil strength with the extension of the action of the time and the lowest value of the reduction. At the same moment, shear creep deformation increases with the increase in shear load; when the shear load is below the long-term strength, the sample undergoes attenuation creep, and the specimen is not damaged; when the shear load surpasses the long-term strength, the sample exhibits steady-state creep and accelerated creep, which finally leads to the destruction of the specimen with the development of time. Usually, the long-term strength of soil creep can be determined by the following two methods [40,41]: (1) To draw the stress-deformation curve based on the test results of cascade loading in logarithmic coordinates. It is found that the stress with obvious inflection point on the curve in logarithmic coordinates is taken as the limit value of long-term strength \(\tau_{\infty}\). (2) The long-term strength limit is the boundary value that distinguishes attenuated creep from nonattenuated creep, so the curve between stress and strain rate under this stress can be drawn, and the intersection of the curve and stress axis is the long-term strength limit value \(\tau_{\infty}\). The isochronous cluster curve of the muddy interlayer under different normal stress is shown in Figure 11, and the inflection point of the cluster curve is not obvious. Therefore, according to the method of [PERSON] et al. and [PERSON] et al. [42, 43], it is challenging to precisely determine the long-term strength of muddy interlayer. In view of this, according to the definition of long-term ultimate strength, the long-term strength of muddy interlayer is determined by the following methods: take normal stress 100 kPa as an example, as shown in Figure 7a, the specimen shows attenuated creep at a shear stress of 35 kPa, and nonattenuated creep when shear stress is 42 kPa, indicating that long-term strength limit \(\tau_{\infty}\) is between 35-42 kPa, and their average value is taken as long-term strength limit, namely 38.5 kPa. As shown in Figure 12, the long-term cohesion \(c_{\infty}\) is 4.75 kPa and the long-term internal friction angle \(\varphi_{\infty}\) is 20.4\({}^{\circ}\). Previous researchers have classified weak interlayers based on their material composition, microstructure and origins, generally categorizing them into four types: primary soft rock, interbedded shear zones (fault zones), muddy interlayer and sliding zones [44]. [PERSON] et al. [45] conducted a study on the long-term strength evolution of weak interlayers in Permian carbonaceous shales in the southwestern Karsat mountain area. They investigated three evolutionary stages, including primary soft rock, interbedded shear zones and sliding zones. During the evolutionary process, long-term strength decreases, with the long-term cohesion (\(c_{\infty}\)) of the final evolutionary stage being 0.096 MPa and the angle of internal friction (\(\varphi_{\infty}\)) being 29.6\({}^{\circ}\). [PERSON] et al. [46] performed indoor direct shear-creep tests on weak interlayers in red sedimentary rocks. For sample 1, they determined a long-term cohesion (\(c_{\infty}\)) of 18 kPa and an angle of internal friction (\(\varphi_{\infty}\)) of 17.8\({}^{\circ}\).