# Study on Strength of Slip Soil-Tuff Contact Surfaces in Tuff Landslide Based on Model Test
## 1 Introduction  
As a major mountain geohazard worldwide[PERSON] landslide research focuses on the strength parameter of the slip surface. The study of the strength parameters of slip soils has produced rich results by many scholars worldwide[PERSON] In recent years, with the deepening of research, more and more scholars have shifted the focus of research from the strength of the slip soil itself to the strength of the contact surface between the slip soil and its neighboring rock and soil layers [1; 2; 3; 4; 5; 6; 7; 8; 9; 10][PERSON]  
[PERSON] [11] and [PERSON] [12] first studied the mechanical properties of the contact surface between various structural materials and the geotechnical body with the help of strain-controlled straight shear instrumentation. They summarized through experimental research that the water content of the geotechnical body and the structure indicates that the roughness and the vertical stress will affect the contact surface shear strength. [PERSON] and [PERSON] [13] introduced an X-ray photographic technique to a sand-steel contact surface shear test. They found that the steel surface roughness significantly affected the shear strength. [PERSON] and [PERSON] [3] applied a digital camera to a sand-steel contact surface straight shear test. They pointed out that a critical level of roughness classified contact surfaces as low roughness surface and high roughness surface. These two types of surfaces determined two modes of shear failure on the contact surface. [PERSON] et al. [14], based on straight shear tests on calcareous sand-steel contact surfaces with different interfacial roughness levels and grain sizes, pointed out that roughness and average grain size significantly affected the shear behavior of calcareous sand-steel contact surfaces. [PERSON] and [PERSON] [15]conducted an experimental study on the mechanical properties of sand-concrete contact surfaces on a box-type straight shear apparatus. Based on the test results, they proposed a hyperbolic relationship between the shear stress and the relative tangential displacement at the contact surface between sand and concrete. [PERSON] and [PERSON] [16] proposed the existence of critical roughness by using a direct simple shear test (DSS-test) for two types of cohesive soils in contact with steel plates. When the roughness of the steel plate was less than the critical roughness, the strength of the contact surface was lower than that of the clay, and when it was more significant than the critical roughness, the failure occurred in the clay, and the strength was equal to that of the clay. [PERSON] et al. [17] concluded that the shear strength of sandy soils in contact with structures conforms to the Moore-Cullen law. [PERSON] et al. [18] selected different types of soils and structures to determine the shear mechanical properties of contact surfaces. The results indicated that soil type, saturation, and structural surface material control the shear performance of soil-structure contact surfaces. Until now, only a few researchers have conducted the strength testing of contact surfaces using undisturbed slip soils. Most of them collect slip soils through drilling or surface excavation, and some scholars even simulate the mechanical characteristics of slip soils through indoor proportioning. Although these results have a robust discussion and reference value, they ultimately need to establish a direct link with the actual situation of landslide engineering. In this paper, with the help of the existing investigation results of the Xiageliao landslide located on the east coast of China as a research base, undisturbed slip soil was observed and taken from the exploration tunnel for testing.