Experimental Research on Mechanical Properties of Tension-Compression Composite Bolts

Bolt reinforcement technology has been widely used in supporting structures for slope engineering. However， traditional bolts are difficult to balance high strength and high ductility under complex conditions， such as those encountered in high and steep slopes. This study employed Twinning-Induced Plasticity （TWIP） steel as the material for the compression-bearing anchorage section， introduced a tension-compression composite bolt system， and developed a new type of anchorage structure with both high bearing capacity and large deformation capacity. Through pull-out tests， the mechanical properties of three types of bolts were compared and analyzed： TWIP steel tension-type bolts， traditional HRB400 tension-type bolts， and TWIP/HRB400 tension-compression composite bolts. The results showed that all tension-compression composite bolts experienced fracture of the TWIP steel rod， with an ultimate load ranging from 260.89 to 265.82 kN， which was twice that of HRB400 tension-type bolts. In terms of deformation capacity， the displacement at the ultimate load was approximately 207 mm， 3.25 times that of HRB400 tension-type bolts and 51 times that of TWIP steel tension-type bolts. The strain rate in the parallel section of the TWIP steel reached 53.7%， demonstrating significant high-ductility characteristics. The shear stress distribution in tension-compression composite bolts was more reasonable. The shear stress at the bearing plate did not reach the ultimate bond strength， and no slippage occurred. In the initial loading stage， the load was primarily borne by the compression-bearing anchorage section. After the displacement reached 80 mm， it transferred to the tension-bearing anchorage section， with the two sections working together in good synergy. These research findings provide theoretical support for the application of tension-compression composite bolts in complex geotechnical engineering.