Charlie C. Li

Influences of Loading Condition and Rock Strength to the Performance of Rock Bolts

The influences of displacing angle, joint gap, and rock strength to the performance of rebar bolts and D-Bolts are evaluated in this study. A new method was developed to apply pull and shear loads to the bolt specimen in any combination so that a displacing angle can be established in the range from 0 (pure pull) to 90° (pure shear). In the tests, five displacing angles, two joint gaps, and three “rock” materials were used. The tests showed that the ultimate loads of both the D-Bolt and the rebar bolt remained constant for all the five displacing angles. The deformation capacity of D-Bolt is approximately 3.5 times the rebar under pure pull and 50 % higher than the rebar under pure shear. Both 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 energy absorption capacity of the D-Bolt is 3.7 to 1.5 times that of the rebar bolt, depending on the displacing angle. The bolts installed in weak concrete blocks absorbed more energy than those installed in hard rock and high-strength concrete blocks. The loading angle is increased by the displacing angle. These two angles can be calculated with each other in an analytical solution.

Analysis of Inflatable Rock Bolts

An inflatable bolt is integrated in the rock mass through the friction and mechanical interlock at the bolt–rock interface. The pullout resistance of the inflatable bolt is determined by the contact stress at the interface. The contact stress is composed of two parts, termed the primary and secondary contact stresses. The former refers to the stress established during bolt installation and the latter is mobilized when the bolt tends to slip in the borehole owing to the roughness of the borehole surface. The existing analysis of the inflatable rock bolt does not appropriately describe the interaction between the bolt and the rock since the influence of the folded tongue of the bolt on the stiffness of the bolt and the elastic rebound of the bolt tube in the end of bolt installation are ignored. The interaction of the inflatable bolt with the rock is thoroughly analysed by taking into account the elastic displacements of the rock mass and the bolt tube during and after bolt installation in this article. The study aims to reveal the influence of the bolt tongue on the contact stress and the different anchoring mechanisms of the bolt in hard and soft rocks. A new solution to the primary contact stress is derived, which is more realistic than the existing one in describing the interaction between the bolt and the rock. The mechanism of the secondary contact stress is also discussed from the point of view of the mechanical behaviour of the asperities on the borehole surface. The analytical solutions are in agreement with both the laboratory and field pullout test results. The analysis reveals that the primary contact stress decreases with the Young’s modulus of the rock mass and increases with the borehole diameter and installation pump pressure. The primary contact stress can be easily established in soft and weak rock but is low or zero in hard and strong rock. In soft and weak rock, the primary contact stress is crucially important for the anchorage of the bolt, while in hard and strong rock it is the secondary contact stress that plays a vital role.

Measurement of the basic friction angle of planar rock discontinuities with three rock cores

The paper presents the results of tilt tests using a large number of three-core samples, aiming to establish a method for determining the basic friction angle of planar rock discontinuities. The three-core tilt test is easy to operate, and the results are comparable with the saw-cut method after modification. Saw-cut and two-core samples were also tested for the purpose of comparison. The test results show that the cylindrical surface of the core is slightly rougher than the surface of saw-cut samples so that the friction angle measured on the cylindrical surface of the core is slightly larger than that measured on saw-cut samples. The basic friction angle of the planar rock discontinuity is obtained by decreasing 2° from the friction angle measured on three-core samples. The appropriate number for test repetitions is 10∼20 in order to obtain a reliable mean friction angle. Grinding or polishing of the cylindrical surface of cores is not recommended, since it will lead to an increase in the friction angle in most of rock types.

A Benchmark Experiment to Assess Factors Affecting Tilt Test Results for Sawcut Rock Surfaces

From the earliest studies on the topic, plane sliding techniques, usually known as tilt tests, have shown contradictory features. On the one hand, they reflect on a small-scale basic principles regarding definition of the friction angle and can reproduce conditions very similar to those of the sliding of blocks on rock slopes; furthermore, tilt angle results agree with friction angles derived from shear and pull tests (Hencher 1977; Muralha 1996). On the other hand, the literature includes several reports of erratic results and it is recognized that even apparently smooth surfaces are actually rough at the microscopic level (Hencher and Richards 2015). Thus, adhesion and textural interlocking may contribute to the variability and the nonreproducibility of tilt test results. A number of authors have carried out simple tilt tests in the past, and several examples illustrate the above-mentioned arguments. Thus, Hencher (2012) observed extreme variability in the results of tilt tests; Nicholson (1994) found that friction angles for sawcut Berea sandstone in direct shear tests varied by 12.5 , despite great attention paid to sample preparation and reproducibility; Coulson (1972) demonstrated that the friction angle of planar surfaces of rock varied with surface finish; Krahn and Morgenstern (1979) reported similar variation for surfaces prepared in different ways and with different surface finishes; and Kveldsvik et al. (2008) found that the basic friction angle for a rock slope, derived from tilt testing of core, varied between 21 and 36.4 . These great differences in measured tilt angles are mainly attributed to different surface finishing and to wear of the rock contacts (Pérez-Rey et al. 2015, 2016), although other reasons, such as testing and ambient conditions, cannot be ruled out. Mehrishall et al. (2016) revealed that the residual friction coefficients of grinded joint surfaces and of rough rock surfaces were almost identical. This manuscript contains interesting findings in rock mechanics practice, derived from large efforts to test the same rocks under different conditions and different laboratories. At the same time, this manuscript presents part of the work which will be used to prepare a Suggested Method on Tilt Testing. It is published in the form of a Technical Note, although its length exceeds what is generally accepted for this type of publication.