Naj Aziz

The effect of sorbed gas on the strength of coal – an experimental study

This paper examines the influence of sorbed gas type and pressure on the strength of coal. Particular attention is focused on the influence of gas type and pressures on drillability characteristics of coal and particle size distribution of drill cuttings. Drillability of coal has a strong bearing on the strength properties of coal. Based on fracture mechanics and mechanical rock cutting, weaker rocks are easier to drill and the drill cuttings produced are generally coarse in size. Accordingly a specially designed precision drill was used to drill coal samples under both normal atmospheric and confined gas pressure conditions. Indications of changes to coal strength as a result of increased sorbed gas pressure were examined by analysing the particle size distribution of the drill flushing. A laser controlled Malvern Mastersizer S particle size analyser with measuring range between 0.05 μm and 900 μm was used to study the particle size distribution. This paper demonstrates that there is a definite variation in the range of particle size distribution with particles obtained from drilling coal samples not subjected to gas pressures in comparison with those obtained from samples drilled under confined gas pressure. Gas types also have an influence. Higher proportions of coarse particles were produced when the confined gas was changed from CH4 to CO2. A change in the rate of drilling under varying confining pressure and gas type was also evident. The rate of drilling in air (at normal atmospheric conditions) was slower than at higher confining pressures. The highest drill rates were obtained with CO2 confinement.

Laboratory modelling of shear behaviour of soft joints under constant normal stiffness conditions

Shear behaviour of regular sawtooth rock joints produced from casting plaster are investigated under constant normal stiffness (CNS) conditions. Test results obtained in this investigation are also compared with the constant normal load (CNL) tests. It is observed that the peak shear stress obtained under CNL conditions always underestimates the peak shear stress corresponding to the CNS condition. Plots of shear stress against normal stress show that a nonlinear (curved) strength envelope is acceptable for soft rock joints subjected to a CNS condition, in comparison with the linear or bilinear envelopes often proposed for a CNL condition. Models proposed by Patton (1966) and Barton (1973) have also been considered for the predictions of peak shear stress of soft joints under CNS conditions. Although Pattons model is appropriate for low asperity angles, it overestimates the shear strength in the low to medium normal stress range at higher asperity angles. In contrast, while Bartons model is realistic for the CNL condition, it seems to be inappropriate for modelling the shear behaviour of soft joints under CNS conditions. The effect of infill material on the shear behaviour of the model joints is also investigated, and it is found that a small thickness of bentonite infill reduces the peak stress significantly. The peak shear stress almost approached that of the shear strength of infill when the infill thickness to asperity height ratio (t/a) reached 1.40. This paper also introduces an original, empirical shear strength envelope to account for the change in normal stress and surface degradation during CNS shearing.

Behavior of fiber glass bolts, rock bolts and cable bolts in shear

This paper experimentally compares the shear behavior of fiber glass (FG) bolt, rock bolt (steel rebar bolt) and cable bolt for the bolt contribution to bolted concrete surface shear strength, and bolt failure mode. Two double shear apparatuses of different size were used for the study. The tensile strength, the shear strength and the deformation modulus of bolt control the shear behavior of a sheared bolted joint. Since the strength and deformation modulus of FG bolt, rock bolt and cable bolt obtained from uniaxial tensile tests are different, their shear behavior in reinforcing joints is accordingly different. Test results showed that the shear stiffness of FG bolted joints decreased gradually from the beginning to end, while the shear stiffness of joints reinforced by rock bolt and cable bolt decreased bi-linearly, which is clearly consistent with their tensile deformation modulus. The bolted joint shear stiffness was highly influenced by bolt pretension in the high stiffness stage for both rock bolt and cable bolt, but not in the low stiffness stage. The rock bolt contribution to joint shear strength standardised by the bolt tensile strength was the largest, followed by cable bolts, then FG bolts. Both the rock bolts and cable bolts tended to fail in tension, while FG bolts in shear due to their low shear strength and constant deformation modulus.

Effects of cyclic loading on the shear behaviour of infilled rock joints under constant normal stiffness conditions

The variation of the shear strength of infilled rock joints under cyclic loading and constant normal stiffness conditions is studied. To simulate the joints, triangular asperities inclined at angles of 9.5° and 18.5° to the shear movement were cast using high-strength gypsum plaster and infilled with clayey sand. These joints were sheared cyclically under constant normal stiffness conditions. It was found that, for a particular normal stiffness, the shear strength is a function of the initial normal stress, initial asperity angle, joint surface friction angle, infill thickness, infill friction angle, loading direction and number of loading cycles. Based on the experimental results, a mathematical model is proposed to evaluate the shear strength of infilled rock joints in cyclic loading conditions. The proposed model takes into consideration different initial asperity angles, initial normal stresses and ratios of infill thickness to asperity height.

Numerical Modeling of Fully Grouted Rockbolts Reaching Free-End Slip

AbstractIn this study, numerical simulation was conducted to model the behavior of rockbolts with free-end slip in tension. A nonlinear bond–slip relationship was input to the numerical model to represent the interaction mechanism of the interface between rockbolt and grout. The results of the model were validated against laboratory tests. The numerical model matched the experimental results in terms of load–displacement relationship, rockbolt strain distribution, and interfacial shear-stress distribution. This paper indicates that (1) the rockbolt element in the numerical model is capable of describing the strain-softening behavior of the rockbolt–grout interface; and (2) the pullout tests of rockbolts with free-end slip can be used to derive the bond–slip relationship, and thus provide insights into the mechanical behavior of rockbolts with free-end slip.

A New Equation for the Shear Strength of Cable Bolts Incorporating the Energy Balance Theory

The application of cable bolts as a secondary support system is an increasing trend in underground coal mines worldwide. The performances of cable bolts have been evaluated under both axial and shear loading conditions. Two methods of testing cables for shear, single and double shear, have been recognised. This paper examines the shear behaviour of a variety of cable bolts under different pre-tension loads by double shear testing. Plain, spiral and the combination of both cable types were used in this study. The initial axial load and the type of cable bolts are the main factors affecting their shear strength. By increasing the axial pre-tension load, the peak shear load occurs at lower shear displacement. The failure angle due to cable bending across the joint at different pre-tension loads varied between 41° and 49°. This demonstrates that the ratio of axial and perpendicular displacements is almost constant and on average the failure occurs at about 45°. A novel analytical model is proposed to evaluate the shear behaviour of pre-tensioned fully grouted cable bolts subjected to double shearing. Energy and Fourier Series methods were incorporated in the model to simulate the shear behaviour of cable bolts. The comparison of the experimental results with the proposed model shows a good agreement.

Effects of shear rate on cyclic loading shear behaviour of rock joints under constant normal stiffness conditions

The presence of joints and discontinuities within a rock mass can significantly affect its mechanical behaviour and therefore the stability of structures constructed at close proximity. Several studies have been carried out by previous researchers to understand the mechanical behaviour of joints under both constant normal load (CNL) conditions, in which the normal load remains unchanged during shearing, and constant normal stiffness (CNS) conditions to imitate the stiffness of the surrounding rock mass (Patton 1966; Ladanyi and Archambault 1969; Barton 1973, 1976; Seidel and Haberfield 1995; Indraratna and Haque 2000; Buzzi et al. 2008). The importance of CNS conditions to simulate the actual shear behaviour of rock joints in the field has been described by Johnston and Lam (1989), Skinas et al. (1990) and Indraratna et al. (1998). The above-mentioned studies focussed only on the monotonic loading shear behaviour of rock joints. The effects of cyclic loading on the shear behaviour of rock joints in earthquakes and blasting were investigated in detail by Plesha (1987), Hutson and Dowding (1990), Lee et al. (2001), Stupkiewicz and Mroz (2001), Grasselli and Egger (2003), Jafari et al. (2003) and Belem et al. (2009). As the shear rate might vary depending on the source of the load and rock media, Crawford and Curran (1981) carried out a series of experiments on artificial rock joints with various shear rates and normal stresses under CNL conditions. Based on the measured data, they concluded that the shear rate may influence the shear strength of hard and soft rock joints differently. In another study, Jafari et al. (2004) verified the results of Crawford and Curran (1981) for shear rates between 0.05 and 0.4 mm/min under monotonic loading. None of these researchers investigated the effects of shear rate on the cyclic loading shear behaviour of rock joints under CNS conditions, which is a critical issue in stability analysis of underground structures subjected to seismic events. Accordingly, three sets of cyclic loading shear tests with various shear rates and initial normal stresses were conducted on artificial triangular joints under CNS conditions. In this study, the experimental data are critically analysed.

The effects of installation procedure on bond characteristics of fully grouted rock bolts

The bond characteristics of fully grouted rockbolts installed in steel tubes were investigated by bolt push tests. Steel tubes were inserted in a mine roadway roof to represent the confinement of rock boreholes. Rockbolts were installed in tubes using the installation technique of Australian underground mines. These tubes, with rockbolts inside, were retrieved from the field and brought back to the laboratory to be cut into 100-mm sections, which were then push tested. It was found that each bolt section had a distinct load-displacement profile, and that bond strength varied significantly along the bolt length. The factors influencing the bond strength of rockbolts were identified. The influence of the installation procedure on the bond strength of bolts in tubes was investigated.

An elasto-plastic constitutive model for rock joints under cyclic loading and constant normal stiffness conditions

An elasto-plastic constitutive model is introduced for rock joints under cyclic loading, considering the additional shear resistance generated by the asperity damage in the first forward shear cycle and sliding mechanism for further shearing. A series of cyclic loading direct shear tests was conducted on artificial joints with triangular asperities and replicas of a real rock asperity surface under constant normal stiffness (CNS) conditions. The model was calibrated and then validated using selected data sets from the experimental results. Model simulations were found to be in good agreement with the rock joints behaviour under cyclic loading and CNS conditions both in stress prediction and dilation behaviour. In addition, dynamic stability analysis of an underground structure was carried out, using Universal Distinct Element Code and the proposed constitutive model.

Coal sorption characteristics and coal surface tension

Underground coal mining in the Bulli Seam of Sydney Basin, Australia encounters the problems of coal and gas outburst as the coal contains high concentration of CO 2 , where certain parts of the seam have been found to be especially hard-to-drain. In order to better understand these problems, the influence of temperature and moisture on sorption capacity of bituminous coal was investigated at 35°C, 45°C and 55°C and at pressure up to 4 MPa. Based on the principle of surface chemistry, the reduced value of surface tension (RVST) was calculated and analysed. The theory of coal surface free energy was found to be capable of explaining coal sorption characteristics. These findings provide the basic knowledge for a better understanding of coal sorption characteristics associated with coal seam gas drainage, which will benefit ventilation and degasification design and hence help to prevent coal mine operations from gas outburst disaster.