# The Shear Strength of Granite Weather Soil Under Different Hydraulic Paths
## 1 Introduction  
Shear strength is the ultimate ability of soil to resist shear force, which is one of the most important indexes of soil mechanical properties. The pressure of retaining wall, bearing capacity of foundation, and stability of landslide and collapse in a natural environment are all related to the shear strength of the soil. Soils in unsaturated states are much more commonly seen in nature [1; 2; 3]. The shear strength of unsaturated soils depends on several factors such as soil type, particle distribution, density, hydraulic path, and stress state [3; 4; 5]. Over the past few decades, a number of laboratory tests have been conducted to study the hydraulic behavior of unsaturated soils in the low suction range using axial translation techniques [6; 7; 8; 9]. The suction was less than 500 kPa in most direct shear or triaxial tests, and the test soil was mainly sand or silt. However, in geotechnical engineering, unsaturated soils with high or wide suction range are widespread, and soil types contain a lot of silty clay or clay. At the same time, it is challenging to control suction in mechanical tests in high suction range accurately, and the vapor flow technique or the osmotic suction techniques have been adopted to control the high suction of specimens [10; 11; 12; 13]. However, the test results of unsaturated soil mechanical behavior under high suction in the literature are still intensely limited. In addition, most existing shear strength models are based on analysis of test results obtained by direct shear or triaxial shear tests at low suction ranges (typically below 500 kPa). As a result, the shear strength of unsaturated soils in the medium suction range is significantly overestimated,especially for clays [14; 15]. Through the verification and comparison of various existing strength models, it is found that most of them are based on an empirical and phenomenological basis, and there are few explanations and simulations for the strength in the medium and high suction range [14]. On the other hand, different soil types, such as sand and clay, exhibit differences in microscopic states, which often significantly affect the strength of the soil. Soils in nature are complex and vary widely in their microscopic structure and mineral composition [16]. Although there are numerous models, there is no widely accepted method for strength influence mechanism and model calculation. Therefore, it is necessary to study the influence mechanism of different soil types, particle distribution, hydraulic path, and stress state on soil strength under high suction or wide suction range and establish a corresponding strength model. The widely distributed granite weathered soil is affected by its soil-forming environment and geotechnical composition [17; 18]. Its physical and mechanical properties are unique, especially the microstructure of its profile variation [19; 20], which also causes difficulties in numerical simulation of the mechanical behavior and engineering characteristics of weathered granite soil. Therefore, the granite weathered soil can be used as a typical research object in the study of unsaturated strength. There is an urgent need to clarify the mechanism of action of various factors that affect the strength of granite weathered soil, and establish a model directly used to simulate or explain its physical and mechanical characteristics under a wide suction range. Therefore, in this paper, the direct shear strength test of two different weathered granite soils under different hydraulic paths is carried out, and the evolution law of the soil strength on the effect of mineral composition and particle size gradation is analyzed under a wide suction range. The factors affecting the strength change of granite weathered soil are explained by the contact modes between soil particles established, and further verified and explained by scanning electron microscope (SEM) from the perspective of microscopic mechanism.