# Experimental Study on Shear Creep and Long-Term Strength of Clay-Type Muddy Interlayer
## 4 Conclusions  
Investigations on clay-type muddy interlayer were conducted through laboratory direct shear-creep tests using the convenient staged loading method. Experimental data were recorded, and the complete creep curve was obtained. The 'Chen method' was subsequently employed to transform it into separate loading creep curves. Analysis of the shear-displacement curve was carried out to study the creep characteristics of the muddy interlayer. Based on the analysis of the creep-test curve and the use of empirical formulas to fit several creep stages, the study then explored the relationships between average and stable creep rates and shear stress. A method for determining the long-term strength limit was proposed. The main results obtained are as follows:  
1. The shear creep deformation of the clay-type muddy interlayer is obvious. At the same normal stress level, the specimen has instantaneous deformation, initial creep and stable creep at the lower shear stress level. There are two cases at the level of rupture shear stress: the creep curve includes three stages of typical initial creep, stable creep and accelerated creep failure or directly enters the accelerated creep stage until the specimen failure. The empirical formula of shear creep proposed, \(u=u_{0}+A\left[1-\mathrm{e}^{(-Bt)}\right]+Ct^{n}\), can well reflect the instantaneous deformation, initial creep and stable creep of shearing interlayer, but the fitting effect of accelerated creep is not good. The coefficient of determination (\(R^{2}\)) for the fitting of the shear creep empirical formulas is consistently above 0.9. 2. Under the constant normal stress level, the shear creep increases with the increase in the shear stress level, while at the same shear stress level, the shear creep decreases with the increase in the normal stress level. At all levels of stress, the initial attenuated creep stage of the specimen has smaller deformation, shorter duration, longer stable creep duration and the steady creep rate at lower shear stress level is a constant greater than 0, which shows that the creep of the clay-type muddy interlayer is very strong. 3. Under the same normal stress level, the average and steady creep rate of the clay-type muddy interlayer can be characterized by the exponential function of \(\dot{u}=me^{n\tau}\), and the average creep rate of the clay-type muddy interlayer specimens is generally one- to three-times the stable creep rate. 4. In comparison to previous studies on no-clay-type muddy interlayer, the long-term strength of clay-type muddy interlayer is notably low. A method was proposed to determine long-term shear strength indicators based on the long-term strength limit. By curve fitting, \(\varphi_{\infty}\) was determined to be \(20.4^{\circ}\) and \(c_{\infty}\) was \(4.75\) kPa. The relationship between the shear strength limit (\(\tau_{\infty}\)) and normal stress (\(\sigma\)) was formulated as \(\tau_{\infty}=4.75+\sigma\) tan\(20.4^{\circ}\). Conceptualization, P.G. and J.Z. ; methodology, W.L., Z.W. and J.Z. ; software, P.G. and J.Z. ; validation, P.G. and F.L. ; formal analysis, P.G. ; investigation, P.G., W.L., Z.W., J.C. and J.Z. ; resources, W.L. and Z.W. ; writing--original draft preparation, P.G. and J.Z. ; writing--review and editing, P.G., W.L., Z.W. and J.Z. ; visualization, P.G. and J.Z. ; supervision, J.C. and J.Z. ; project administration, W.L., Z.W. and J.Z. ; funding acquisition, J.Z. All authors have read and agreed to the published version of the manuscript. This research was funded by the National Nature Science Foundation of China, grant number 41807258, Application Code: D0705, Project Category: Youth Science Fund Project, Leader: [PERSON]. Not applicable. Data Availability Statement: Data are containedwithin the article. Author [PERSON] and [PERSON] were employed by the company China Nonferrous Metals Industry Kunming Survey and Design Institute Co.