Mechanical Behavior of BFRP Cable Rock Bolts: Experimental and Analytical Study

Abstract

In this study, the mechanical behavior of novel fully grouted cable rock bolts made of basalt fiber–reinforced polymer (BFRP) was investigated, which included pullout and double-shear joint behavior. Three BFRP cables were prepared: (1) single-tendon; (2) untwisted multiple-tendon; and (3) twisted multiple-tendon. In the pullout tests, the effects of cable type, borehole diameter, and encapsulated length were studied. However, the influence of the pretension load level was focused on in the double-shear joint tests. The results suggested that an overly thick grout annulus significantly reduced the pullout strength and stiffness. For the BFRP cable with multiple tendons, the initial pullout stiffness was enhanced compared with the BFRP cable with a single tendon due to better interfacial bonding with the grout. In the double-shear joint tests, the pretension of the cable significantly improved the shear capacity and stiffness. With an enhancement in the confinement from pretension, the partial failure of the grout at an early stage could be prevented. The parallel and twisted-tendon BFRP cable rock bolts had larger failure displacements than the single-tendon cable. This analytical study found that a pullout analytical model available in the literature was accurate when back-calculating the stress conditions along the rock bolt. The predicted shear capacity of the rock joint showed good agreement with the experimental values. A parametric study indicated that the joint shear capacity was sensitive to the tensile and shear strengths of the cable but was irrelevant to the cable moduli. Considering the effectiveness of pretension and creep safety for BFRP, a suitable range for the pretension load was suggested. This study attempts to promote the use of novel, fully grouted cable rock bolts made of basalt fiber–reinforced polymer (BFRP) materials in geotechnical engineering applications. Different types of cable rock bolts are produced, which include single-tendon, untwisted multiple-tendon, and twisted multiple-tendon cables. In the pullout tests, the untwisted multiple-tendon cable exhibited the highest pullout capacity, because of its better interfacial bonding with the grout. From the double-shear joint tests, the pretension of the cable could significantly improve the shear capacity and stiffness. In general, the BFRP cable rock bolts could have a competitive mechanical behavior compared with the steel rock bolts. The respective analytical models for pullout and double-shear joint specimens have proved to be accurate for predicting the mechanical responses of the cable rock bolts. These concise but reliable analytical models could allow researchers and designers to better utilize these novel cable rock bolts in potential applications.