# Research on Shear Behavior of Sand-Structure Interface Based on Monotonic and Cyclic Tests
## 1 Introduction  
The problem of the interaction between soil and structure is widespread in geotechnical engineering [1; 2; 3; 4; 5; 6; 7; 8; 9], such as the interaction between piles and surrounding soil, underground diaphragm wall and soil behind the wall, etc. Due to the great difference between the stiffness of soil and structure, under the action of external load, discontinuous phenomena such as shear slippage, dislocation and disengagement may occur at the contact surface. Therefore, the shear stiffness, strength, deformation and other mechanical properties of the contact surface have become one of the core topics in the study of the interaction between soil and structure [10; 11; 12; 13; 14; 15; 16]. To date, scholars have conducted research on the contact surface between soil and structure through static tests. [PERSON] and [PERSON] [17] employed the direct simple shear test to investigate the shear behavior of the soil-structure interface. [PERSON] [18] was the first to use an improved shear box to carry out interfacial shear tests to study the problem of interfacial friction between sand and structural materials. [PERSON] and [PERSON] [19] used a direct shear box to demonstrate the shear strength of the interface between lean clay and steel and concluded that normal stress may influence shearing resistance for both smooth and rough interfaces. Considering that there may be cyclic loading in actual engineering, such as seismic load, wave load, etc., [PERSON] et al. [20] introduced a two-degree-of-freedom cyclic direct shear instrument for cyclic testing and modeling research on the sand-concreteinterface. The experimental results showed that the peak or mobilized shear stress increased with the number of cycles. However, the increase for higher density was not as rapid as that for the lower density. This implied that, in general, for cohesionless soils, the interface response hardened with an increasing number of cycles, and the rate of such hardening decreased with an increasing number of cycles. [PERSON] et al. [21] constructed a modified multi-purpose interface apparatus to replace the field test of the pile-soil interface shear test. Additionally, he found a consistent conclusion with [PERSON], which was that the increase in the number of test cycles caused the shear stiffness of the interface to change. However, the effect of initial shear stress was ignored in their experiments. In many engineering applications, the shear stress already existed before the dynamic load was applied. For example, under cyclic loads such as seismic loads, wave loads, and traffic loads, the shear stress of the contact surface between the pile and the soil around the pile might fluctuate around the initial shear stress. However, the difference of loading and unloading stiffness and the accumulated residual deformation are important factors to be considered in engineering design. According to [PERSON]'s law [22], if unloading occurs under the initial shear stress, the unloading stiffness is consistent with the loading stiffness. Loading and unloading under the condition of constant amplitude forms a closed hysteresis loop. If, according to [PERSON]'s loading and unloading criteria [23], the stiffness of each load is only related to the current turning point (Figure 1), this behavior will produce the obvious accumulation of residual plastic deformation, which is similar to the ratchet effect [24; 25; 26; 27; 28]. Additionally, the applicability of the two models to the experimental results needs to be verified. Therefore, it is of great engineering significance to study the cyclic shear behavior of the interface between sand and structure considering the initial shear stress. This paper aimed to study the cyclic shear behavior of the sand-structure interface considering the initial shear stress. The structure of this paper was mainly divided into two parts of static and dynamic tests, as well as the analysis and discussion of the results.