Anchorage performance of fully-encapsulated rock bolts under pull-and-shear loading

Abstract

This thesis aims to study the anchorage performance of rock bolts under pull-andshear loading conditions. Laboratory tests, an analytical solution and a numerical simulation are included. The research results can be applied to mining engineering and civil engineering, in which rock bolts have been widely used as a conventional support element. A new laboratory test method was developed to apply pull and shear loads at the same time to the bolt specimen so that a displacing angle could be established in a range from 0° (pure pull) to 90° (pure shear). The influences of the displacing angle, the joint gap, and the rock strength on the performance of rebar bolts and D-Bolts are evaluated in this study. In the tests, five displacing angles (0°, 20°, 40°, 60°, and 90°), two joint gaps (0 and 30 mm), and three “rock” materials (weak concrete, strong concrete, and concrete-granite) were used. The test results show that the linear elastic stiffness of both the D-Bolt and the rebar bolt is mobilised quickly after a small displacement. When the displacing angle is larger than 40°, grout crushing may occur underneath the bolt shank, resulting in a reduction in the stiffness of the bolt. The ultimate load of the bolts remains approximately constant no matter what the displacing angle is for both the D-Bolt and the rebar bolt. The displacement capacity of the D-Bolt, however, is dependent on the displacing angle. The ultimate displacement of a 1-m-long D-Bolt section varies from 140 mm under pure pull (0°) to approximately 70 mm when the displacing angle is larger than 40°. The ultimate displacement of the rebar slightly increases from 29 mm under pure pull to 53 mm under pure shear. In general, the displacement capacity of the D-Bolt is larger than that of the rebar bolt. It is approximately 3.5 times greater than the rebar under pure pull and 50% higher than the rebar under pure shear. Both the D-Bolt and rebar displaced more in the weak “rock” (concrete) than in the hard rock. The ultimate load of the bolts slightly decreased in the hard rock at pure shear. The deformation capacity of the bolts increased with the joint gap. The rebar bolts absorbed about 6 kJ of energy regardless of the displacing angle, while the DBolts absorbed about 25 kJ under pure pull. The energy absorption capacity of the DBolt is 3.7 to 1.5 times that of the rebar bolt, depending on the displacing angle. The bolts installed in the weak concrete blocks absorbed more energy than those installed in the hard rock and high-strength concrete blocks. The compressive stress exists at 50 mm from the loading point, and the maximum bending moment value rises as the displacing angle increases. The rebar bolt mobilises greater applied load than the DBolt when they are subjected to the maximum bending condition. The D-Bolt yielded in a longer section than the rebar bolt under pure shear. A bolt overriding an open joint gap becomes yielded more quickly and has a greater bending moment than those overriding a tightly closed rock joint. The displacing angle, α, of the bolt is greater than the loading angle, θ. Loading angle θ can be determined from displacing angle α for a rock bolt subjected to pull-andshear loading via an analytical solution, or vice versa. The analytical solution of the loading angle is in good agreement with the loading angle obtained in the laboratory tests. The loading angle is greater than the displacing angle in hard rock but smaller in weak rock. In terms of the pure shearing, the loading angle is in the range of 40° to 50°. The anchorage performance of two types of rock bolts, the D-Bolt and the rebar bolt, are studied numerically under the loading condition of combined pull and shear. In the FLAC3D modelling, a tri-linear material model was used for the bolt steels in order to simulate the strain hardening behaviour of the materials. Different interfaces were defined for the D-Bolt and the rebar bolt to simulate the different bonding mechanisms. The modelling results are in good agreement with the experimental results in the aspects of the axial load in the bolt and the shear stress along the bolt– grout interface. The numerical simulations show that the two types of bolts have similar load capacities, while their deformation capacities are different. The deformation capacity of the D-Bolt is influenced by the displacing angle, while the rebar bolt is not. The axial load and axial displacement in the bolt and the shear stress on the bolt are quite different for the D-Bolt and the rebar bolt. They are attributed to the different bonding conditions and the stretching lengths of the bolts. The axial load decreases exponentially as the distance from the loading position for the rebar bolt increases, although it remains constant in the bolt sections between the adjacent anchor positions for the D-Bolt. When the bolt is subjected to a lateral load, shear stress is created on the bolt surface within a short distance from the loading position. The axial stress in the D-Bolt is greater than that in the rebar bolt, except at the loading position. The shear stress on the D-Bolt is in general lower than that which is on the rebar bolt and limited in a small area close to the loading position.