Search

Quasi-NPR (negative Poisson’s ratio) steel is a new type of super bolt material with high strength, high ductility, and a micro-negative Poisson’s effect. This material overcomes the contrasting characteristics of the high strength and high ductility of steel and it has significant energy-absorbing characteristics, which is of high value in deep rock and soil support engineering. However, research on the shear resistance of quasi-NPR steel has not been carried out. To study the shear performance of quasi-NPR steel bolted rock joints, indoor shear tests of bolted rock joints under different normal stress conditions were carried out. Q235 steel and #45 steel, two representative ordinary bolt steels, were set up as a control group for comparative tests to compare and analyze the shear strength, deformation and instability mode, shear energy absorption characteristics, and bolting contribution of different types of bolts. The results show that the jointed rock masses without bolt reinforcement undergo brittle failure under shear load, while the bolted jointed rock masses show obvious ductile failure characteristics. The shear deformation capacity of quasi-NPR steel is more than 3.5 times that of Q235 steel and #45 steel. No fracture occurs in the quasi-NPR steel during large shear deformation and it can provide stable shear resistance. However, the other two types of control bolts become fractured under the same conditions. Quasi-NPR steel has significant energy-absorbing characteristics under shear load and has obvious advantages in terms of absorbing the energy released by shear deformation of jointed rock masses as compared with ordinary steel. In particular, the shear force plays a major role in resisting the shear deformation of Q235 steel and #45 steel, therefore, fracture failure occurs under small bolt deformation. However, the axial force of quasi-NPR steel can be fully exerted when resisting joint shear deformation; the steel itself does not break when large shear deformation occurs, and the supporting effect of the jointed rock mass is effectively guaranteed.

This study presents a visualized approach for tracking joint surface morphology. Three-dimensional laser scanning (3DLS) and 3D printing (3DP) techniques are adopted to record progressive failure during rock joint shearing. The 3DP resin is used to create transparent specimens to reproduce the surface morphology of a natural joint precisely. The freezing method is employed to enhance the mechanical properties of the 3DP specimens to reproduce the properties of hard rock more accurately. A video camera containing a charge-coupled device (CCD) camera is utilized to record the evolution of damaged area of joint surface during the direct shear test. The optimal shooting distance and shooting angle are recommended to be 800 mm and 40°, respectively. The images captured by the CCD camera are corrected to quantitatively describe the damaged area on the joint surface. Verification indicates that this method can accurately describe the total sheared areas at different shear stages. These findings may contribute to elucidating the shear behavior of rock joints.

Rock bolts have been widely used for stabilizing rock mass in geotechnical engineering. It is acknowledged that the bolt profiles have a sound influence on the support effect of the rock bolting system. Previous studies have proposed some optimal rib parameters (e.g. rib spacing); unfortunately, the interface shear behaviors are generally ignored. Therefore, determination of radial stress and radial displacement on the bolt-grout interface using traditional pull-out tests is not possible. The load-bearing capacity and deformation capacity vary as bolt profiles differ, suggesting that the support effect of the bolting system can be enhanced by optimizing bolt profiles. The aim of this study is to investigate the effects of bolt profiles (with/without ribs, rib spacing, and rib height) on the shear behaviors between the rock bolt and grout material using direct shear tests. Thereby, systematic interfacial shear tests with different bolt profiles were performed under both constant normal load (CNL) and constant normal stiffness (CNS) boundary conditions. The results suggested that rib spacing has a more marked influence on the interface shear behavior than rib height does, in particular at the post-yield stage. The results could facilitate our understanding of bolt-grout interface shear behavior under CNS conditions, and optimize selection of rock bolts under in situ rock conditions.

This paper investigates the explicit use of rock reinforcement in a discontinuous stress analysis model. A series of numerical experiments was undertaken to evaluate the performance of local and global reinforcement models implemented in universal distinct element code (UDEC). This was made possible by calibrating the reinforcement models to the laboratory behavior of a fully-grouted rebar bolt tested under pure pull and pure shear loading conditions. The model calibration focuses on matching different loading stages of the force–displacement curve including the initial elastic response, the hardening behavior and the bolt rupture. The paper concludes with a discussion on the suitability of the different reinforcement models in UDEC including their advantages and limitations. Finally, it addresses the choice of input parameters required for a realistic simulation of fully-grouted rebar bolts.

This is a review paper on the performances of both conventional and energy-absorbing rockbolts manifested in laboratory tests. Characteristic parameters such as ultimate load, displacement and energy absorption are reported, in addition to load–displacement graphs for every type of rockbolt. Conventional rockbolts refer to mechanical rockbolts, fully-grouted rebars and frictional rockbolts. According to the test results, under static pull loading a mechanical rockbolt usually fails at the plate; a fully-grouted rebar bolt fails in the bolt shank at an ultimate load equal to the strength of the steel after a small amount of displacement; and a frictional rockbolt is subjected to large displacement at a low yield load. Under shear loading, all types of bolts fail in the shank. Energy-absorbing rockbolts are developed aiming to combat instability problems in burst-prone and squeezing rock conditions. They absorb deformation energy either through ploughing/slippage at predefined load levels or through stretching of the steel bolt. An energy-absorbing rockbolt can carry a high load and also accommodate significant rock displacement, and thus its energy-absorbing capacity is high. The test results show that the energy absorption of the energy-absorbing bolts is much larger than that of all conventional bolts. The dynamic load capacity is smaller than the static load capacity for the energy-absorbing bolts displacing based on ploughing/slippage while they are approximately the same for the D-Bolt that displaces based on steel stretching.