Design of temperature-controlled static and dynamic direct shear experimental system and its application in experimental teaching.

[Objective] To deepen the connection between theory and practice in the teaching of direct shear experiments in soil mechanics, a temperature-controlled static and dynamic direct shear experimental system was designed in this study. [Methods] The designed experimental system was based on the principle of direct shear. Normal stress and horizontal shear displacement were applied through an air cylinder and a servo motor, respectively. The condition of constant normal stiffness was achieved by installing a normal spring. The sample temperature was controlled through the flow of liquid at constant temperatures. The data was collected through a data acquisition system. The experimental process was also controlled through this system. Combined with practical issues in the application of energy geostructures, wherein the surrounding soil and soil-structure interface was subjected to thermal, static, and dynamic loads. Non-isothermal shear tests were conducted using this new system. To verify the reliability of the designed experimental system, monotonic direct shear tests were conducted on sand under constant normal stress and monotonic direct shear tests on sand-steel interfaces under constant normal stiffness conditions. To study the influence of the temperature on the static and dynamic shear characteristics of the soil and soil-structure interfaces, monotonic and cyclic shear tests on the sand, monotonic shear tests at the clay-steel interface under constant normal stress conditions, and monotonic shear tests at the sand-steel interface under constant normal stiffness conditions were performed at 22 °C and 60 °C, respectively. [Results] The results showed that the correlations between the shear strengths and the corresponding normal stresses for the tested sand and sand-steel interface under different boundary conditions could be fitted with straight lines, called the Coulomb strength lines. The results also showed that the monotonic shear stress curves at 22 C and 60 C almost overlapped for both sand and sand-steel interface samples. Irreversible displacements were observed during the cyclic shear process of sand samples at 22 C and 60 C, and no significant discrepancy was observed between the shear stress-shear displacement curves with continual shear. However, the monotonic shear at the interface between the clay and the steel exhibited strain hardening characteristics, and the interface shear strength slightly increased with temperature. [Conclusions] The experimental results verified the applicability and accuracy of the designed temperature-controlled static and dynamic direct shear experimental system. This experimental system can research thermal coupling characteristics of soils and soil-structure interfaces, evaluate the bearing characteristics of energy geostructures, enrich the direct shear experimental teaching content, cultivate awareness regarding experiments serving engineering problems, and improve their ability to solve practical problems. Based on the preliminary experimental results, temperature showed no significant effect on the shear mechanical properties of the sand and sand-steel interface but significantly affected the shear properties of the clay-steel interface. In the design and application of energy geostructures, the effect of temperatures should be considered reasonably based on the characteristics of the onsite soil and soil-structure interfaces. [ABSTRACT FROM AUTHOR]