P. Hagan

Numerical Study of the Mechanical Behavior of Nonpersistent Jointed Rock Masses

AbstractEstimating the mechanical properties of nonpersistent jointed rock masses is one of the most challenging problems in practical rock engineering due to the complex interaction of rock joints and intact-rock bridges. In this paper, the effect of joint geometrical parameters of nonpersistent rock mass on uniaxial compressive strength (UCS) and the deformation modulus was studied by using the discrete-element particle flow code PFC3D. In this numerical approach, the intact material is represented by an assembly of spherical particles bonded together at their contact points, and the joint interface is explicitly simulated by slip surfaces that are applied at contacts between the particles lying on the opposite sides of the joint interface. The failure process is simulated by the breakage of bonds between particles. A previous study of the authors has shown that this approach is capable of reproducing the mechanical behavior of nonpersistent jointed rock masses by a comparative study against physical ex...

Load Transfer Behavior of Fully Grouted Cable Bolts Reinforced in Weak Rocks Under Tensile Loading Conditions

To enable a better understanding of the load transfer performance of fully grouted cable bolts, a new laboratory short encapsulation pull test (LSEPT) unit has been designed and constructed. With this unit, both peak and residual capacity can be assessed over a relatively large pull-out displacement range of 100 mm. Various cable bolt failure modes, including relative slippage along the cable/grout and grout/rock interface, can be studied. Both a modified and a plain strand cable bolt confined in a material having a low strength of 8 MPa were tested. Within this testing process, the impact of the design of the bearing plate was considered that was found to alter the failure mode of the modified cable bolt. Other factors are considered, including the influence of borehole diameter, cable surface geometry, and sample strength on the axial performance of cable bolts.

Unified Size-Effect Law for Intact Rock

AbstractA suite of laboratory testing was performed on Gosford sandstone samples having a range of sizes, including point-load and uniaxial compressive tests. A unified size-effect law (USEL), based on the work by Zdenek Bazant, involving fracture energy as well as fractal theories, was introduced. It was shown that USEL correlates well with the ascending and descending uniaxial compressive strength trends obtained from Gosford sandstone as well as five other rock types reported by Brian Hawkins. Fractal characteristics found to be the primary mechanism for ascending strength trends and surface flaws could be considered as a secondary mechanism. The influence of the contact area on the size-effect behavior of point-load results was investigated using a new approach. This approach was novel in the way it incorporated the load contact area. Determination of the point-load strength index using this new approach led to opposite size-effect trends compared with those observed using a conventional point-load st...

Size-Dependent Hoek-Brown Failure Criterion

AbstractThe size dependency of intact rock is of importance to different disciplines, such as civil and mining engineering. One example relates to the design of structures on or within a rock mass for which an estimation of the strength of the intact rock blocks within the mass is essential. Despite a large number of studies on size effects in rock, less research has investigated size effect under triaxial conditions. Thus, a suite of advanced triaxial compressive experiments was conducted on Gosford sandstone samples with diameters of 96, 50, and 25 mm. A size-dependent Hoek-Brown failure criterion was developed by incorporating a unified size-effect law into the original Hoek-Brown failure criterion. The model was calibrated against the triaxial data obtained from Gosford sandstone. It was shown that there is good agreement between the proposed model prediction and the experimental results. Finally, an example of application of the size-dependent Hoek-Brown failure criterion was presented to demonstrate...

Modification to Radial Strain Calculation in Rock Testing

The complete uniaxial or triaxial stress–strain behavior of a rock sample can be achieved in compressive testing using a servo-controlled testing system. Initially, the load-deformation data is recorded during an experiment, and then to eliminate the scale dependency of force, this data are converted to the stress–strain curve for final reporting. It has been found that because of the particular design of the current sample instrumentation system used to measure the circumferential or lateral deformation during the uniaxial or triaxial compressive tests, a modification is required to be included in the conventional method of radial strain calculation. As a result, a new analytical solution has been developed to improve the accuracy of the radial strain calculation. The modification procedure differs depending on whether the test-sample diameter is greater or less than 50 mm. A comparative study was conducted based on visual observation and quantitative analysis to show the variation in a number of rock parameters including elastic modulus, Poissons ratio, peak stress, critical strain, residual stress strain, softening fracture energy, and residual stress when this modification was implemented. For a 50-mm-diameter sample, which is the suggested size for rock testing according to the International Society for Rock Mechanics, the effect of this modification on the parameters obtained from the post-peak region, such as critical strain, residual stress strain, and softening fracture energy varied from 5 % to 9 %. It was concluded that the impact of this modification procedure is more significant with smaller-diameter samples used in rock testing.

Fractal Characteristics of Sandstone Cutting Fracture under Mechanical Shock Loading Conditions

It is the key to guide rock-breaking design and engineering practice for how to obtain a reasonable test indicator to assess the cuttability of the rock. Some sandstone samples were tested by using the linear rock cutting machine in the school of mining engineering, University of New South Wales (UNSW), Australia. The curves characteristics for the weight percent of the broken debris with the mesh size distribution were obtained through the screening statistics. Furthermore, the fractal dimension of the specimen broken debris was derived through theoretical calculations and statistical analysis. The results showed that the rock cutting fragmentation is of significant fractal features under the mechanical shock loads. The broken debris fractal dimension of the structural integrity specimens is bigger, the range of the fractal dimension is smaller and the broken debris size distribution is more even than that of the poor structural integrity specimens. The fractal dimension is the ideal test indicator to assess and analysis the rock-breaking degree.

Environmental influence on mesh corrosion in underground coal mines

Abstract The effect of corrosion on different steel mesh types was examined in a Controlled Mine Environment laboratory which recreated underground mine environmental conditions. Corrosion rates of mesh at different testing conditions were calculated and weld strengths of the corroded mesh were examined. Mesh specimens in ‘dry’ conditions had no significant corrosion effects over 12 weeks. However, ‘wet-dry cycle’ mesh specimens exhibited significant corrosion rapidly, with estimated corrosion rates of around 0.01 mm/yr for galvanised mesh, whether sprayed once or three times daily, 0.70 mm/yr for three times daily sprayed black mesh and 0.48 mm/yr for once daily sprayed black mesh. The mesh corrosion mechanism was dramatically changed by wet-dry cycle. The results are indicative of the behaviour of installed mesh.

Defects Visualization Using Acoustic Emission Tomography Technique

In this study, some experiments were conducted for different types of defects embedded in grout specimens. Based on the comparison between the actual condition of the defects and tomograms, the feasibility of acoustic emission tomography for defects visualization was investigated. Influences of inclusions, crack size and inclination, and infills were also studied. Experimental results indicated the location, geometry, and inclination of inclusions and cracks. It is concluded that a series of factors influence wave propagation and the accuracy of tomograms, and assessing tomography resolution and target defects before carrying out a test is likely to improve the effectiveness of tomography.

Rock-Arch Instability Characteristics of the Sandstone Plate under Different Loading Conditions

Under the concentrated loading and the uniform loading, the tests on the brittle fracture and the hinged arching until the rock-arch instability of the sandstone plate were conducted using self-developed loading device, and the sensitivity of influent factors on the rock-arch failure was analyzed by numerical test based on the particle flow code (PFC). The results showed that sandstone plate instability presented four phases: small deformation elastic stage, brittle fracture arching stage, rock-arch bearing stage, and rock-arch instability stage. Under the uniform loading, the maximum vertical force of the rock-arch instability was much higher than that under the concentrated loading condition, but the maximum lateral force was almost the same. The number of acoustic emission (AE) and its positioning results of the sandstone plate showed that the extent of the plate damage under the uniform loading was higher than that under the concentrated loading condition. The friction coefficient effect, size effect, loading rate effect, and the initial horizontal force effect on the rock-arch instability were analyzed by the PFC3D numerical experiment.

COMPORTAMIENTO DE UN ANCLAJE TOTALMENTE EMBUTIDO EN ROCAS ADYACENTES, SOMETIDO A ENSAYOS DE DOBLE CORTADURA (CIZALLADO)

Para analizar los resultados de un anclaje completamente embutido en rocas unidas se efectuaron ensayos de doble cizallado y se llevo a cabo la deduccion analitica de la relacion entre la fuerza axial, la fuerza de cizallado y el desplazamiento de cizallado del anclaje completamente embutido usando el modelo mecanico simplificado. Finalmente se analizaron las sensitividades de los parametros influyentes del anclaje. Los resultados muestran que la fuerza de cizallado predomina en el ensayo de cizallado que la fuerza de cizallado es mucho mayor que la fuerza axial. La fuerza de cizallado del anclaje completamente embutido aumenta cuando el incremento del desplazamiento del cizallado aumenta y la curva que relaciona la fuerza de cizallado y el desplazamiento de cizallado puede ser simplificada a cinco pasos: fuerte aumento, ligera disminucion, suave aumento, suave disminucion y muy fuerte disminucion. Del mismo modo, las curvas de fuerza axial y de desplazamiento por cizallado pueden ser simplificadas a tres pasos: aumento estable, mantenimiento y fuerte disminucion. Los resultados de la simulacion muestran que el limite de tension de la resistencia a cizalla del anclaje es el factor mas significativo, la resistencia de las rocas unidas es el segundo y los parametros de la superficie unida tienen el ultimo efecto. Las conclusiones obtenidas en el estudio son de un valor teorico importante para la practica de ingenieria similar. Palabras clave: Ensayo de doble cizallado, Anclaje completamente embutido, Fuerza axial, Fuerza de cizallado, Simulacion numerica.