New fracture models for the progressive failure of rock slopes

An improvement to previously developed constitutive FISH User-Defined-Model subroutine by Venticinque (2013) is demonstrated here to simulate the initiation and progressive propagation of fractures through rock structures. This model is based on the amalgamating failure and fracture mechanics theory applied to the finite difference FLAC code. The prior validation of fracture propagation in isotropic rock has been modified to simulate fracture propagation in anisotropic rock. It is shown that the model is capable to accurately simulate fracture distributions in both isotropic and anisotropic rock mass. Furthermore, application of the model to study rock slope stability highlights several characteristics relevant to the progressive failure process of hard rock dry wall slopes. Moreover the model introduces new potential insight towards the effectiveness of rock and cable bolt supports. This work contributes towards improving safety in mines through an increased understanding of key fracture and progressive failure characteristics within geological structures

Dynamic analysis of fault slips and their influence on coal mine rib stability

Historical data indicate that in deep coal mines the presence of faults in close proximity to excavations affect the frequency of coal bursts. A number of researchers have attempted to correlate the fault geometries to the frequency and severity of coal bursts but dynamic numerical modelling has not been used to show how faults can affect coal ejection from the rib side. The dynamic numerical analysis presented here show how different orientations of fault slips may affect coal bursts. To prove the concept, 89 cases of slipping fault geometries were modelled using the FLAC3D software and their effect on rib stability investigated. The results indicate that there is a simple and logical correlation between the fault location, its slip velocity and the ejection of the yielded coal rib side. The seismic compressive wave generates rock/coal mass velocities that directly impact the rib side. If the coal rib is relatively disturbed and loose, these velocities can cause its ejection into the excavation. The slip direction typically impacts one side of the mine roadway only. A 1 m thick loose coal block attached to the 3 m high rib side in mine roadway was ejected at speeds ranging from 2.5 to 5 m/s depending on the fault location, its orientation and the maximum fault slip velocity modelled at 4 m/s

Improvement of Rock Bolt Profiles using Analytical and Numerical Methods

The anchorage capacity of fully grouted bolts has been studied for many years, however the bolt rib profile and its effect on bolt shear resistance is poorly understood. A new development in calculating load transfer capacity between two rib profiles of varying geometries is discussed. The derived mathematical equations presented calculate the stress distribution adjacent to the fully grouted bolt and bolt pull out force needed to fail the resin. The Fast Lagrangian Analysis Continua (FLAC) program was used to verify the calculations of stress within the resin. The novel idea of coupling the bolt geometry with the calculated stress provides another powerful tool to investigate the bolt profile configuration and its effects on the load transfer mechanism for the benefit of the mining industry

Compressive Strength Testing of Toughskin Thin Spray-On Liner

Thin spray-on liners (TSLs) have been attracting more and more attention as an alternative to the steel mesh in underground roadway support. In order to investigate and compare the compressive strength of glass fibre reinforced ToughSkin TSL developed at the University of Wollongong, a compression test was developed using the cube samples of 40 mm in size. The effect of a small amount of glass fibre in the polymer matrix was tested. The test results indicate that the compressive strength and the material stiffness of the cube samples increased with the increase of glass fibre. All of samples exhibited ductile stress strain curve as they had a yield point and a fracture point. The ductile ToughSkin yield characteristics are very important as sudden brittle failure is considered unsafe for mining practices

Using coal rejects and tailings as infills for standing supports in underground gateroads

Laboratory tests were conducted to investigate the uniaxial compressive strength (UCS) of two types of potential infill materials for standing supports. While one type of infill material was made from coal reject fines and a cementitious grout, the other was a mixture of tailings and a cementitious grout. 81 cylindrical specimens with a 50 mm diameter and 100 mm height were prepared and tested. The effect of various water-to-grout (w/g) ratios and grout-to-coal reject fines/tailings mix ratios on the UCS of the infills were investigated. Test results indicated that the strength of both infills was adversely affected by the w/g ratio. In addition, when the volume ratio of the coal reject fines in the infill was not greater than 50%, the strength of the infill was similar to that of the control group specimens. Interestingly, almost all the infills made of tailings and grout had a greater UCS when compared with the control group. The infill made from 50% tailings and 50% grout with the w/g ratio of 1.2 achieved the highest strength enhancement ratio, being 1.92 times the UCS of the control group

Determination of stress state in rock mass using strain gauge probes CCBO

The strain gauge probes of different construction are typically used for determination of stress state rock mass. The modified overcoring method known as the Compact Conical ended Borehole Overcoring method (CCBO) for stress state determination in rock mass was designed in Institute of Geonics of the CAS (IGN) in cooperation with Kumamoto University in Japan. The implemented adjustment of the overcoring method consists mainly in omitting the overcoring phase (stress relief phase). The probe is glued directly to the conically shaped end of a borehole. The data logger located within the conical probe enables continual strain monitoring directly in the conically shaped end of the borehohole during the overcoring procedure. The conical probe used to monitor stress changes, named Compact Conical ended Borehole Monitoring (CCBM), can continuously monitor rock strain changes in key locations due to mining. Many stress measurements using both strain gauge probes CCBO and CCBM were carried out in the last decade. These measurements were performed in varied rock mass adjacent to mine excavations. Most of the stress measurements were carried out in Carboniferous sedimentary rocks as part of the experimental work in the Czech part of the Upper Silesian Coal Basin (USCB). Several stress measurements were carried out during the mine development operations and associated geotechnical exploration work while constructing the Milasín - Bukov underground gas storage (BUGS) [1], as well as the Bukov Underground Research Laboratory (BURL) [2]. Both underground facilities were designed within Rožná and Olší uranium deposits situated on the north-eastern margins of the Strážek Unit consisting of the metamorphic rock formations. Several measurements were carried out in granitic environments (igneous rocks) as part of the international Large-Scale Monitoring (LASMO) project in Grimsel (Switzerland) and in Josef underground laboratory (Bohemian massif). The article presents the basic principles and the methodology of stress measurements in rock mass using strain gauge probes and the data analysis from the variable rock environments

Why the peak shear load of indented cables increases with increased wire failures?

In shear testing of indented cables it has been found that indented cables peak share load failures behave contrary to the normal failure behaviour. The gradual strength loss with each individual wire failure in an indented cable strand may not lead to subsequent peak shear failure of the remaining strands in decline. This failure behaviour is characteristic of indented cables and occurs irrespective of the test method used (single shear or double shear test). Accordingly in this study all wires in a tested cable strand were instrumented with strain gauges. Each instrumented wire was individually colour coded to assist in determining the location of the wires in the strand circumference with respect to the direction of shear. The location of wires in the perimeter was identified at the sheared joint surface areas. During testing of the cable using a circular MKIV double shear apparatus (Naj Aziz DS Box) the initialisation of wire failure was identified by the strain gauge readings. This data found that the wires failing early were located on the upper segment of the bent strand during shearing process, indicating that the indentations introduced stress concentration spots on the wire, causing the strand wires to fail prematurely with less tolerance to bending than smooth wired cable

Static and dynamic tendon pull-out test research at the University of Wollongong

Tendon technology is widely used for strata control in underground coal mines, in both primary and secondary support systems. The understanding of how they work is crucial to effective strata reinforcement design. Research on tendon technology is an evolving study and this paper is aiming at maintaining this evolution by continuing research on load transfer mechanisms under both static and dynamic conditions, which was reported initially by (Anzanpour, 2021) in ROC2021. This programme of study includes testing of different strength capacity cable bolts, which have been important in the stabilisation of the ground around mining excavations affected by rock bursts and ground seismicity. The aim of the study was to evaluate tendon performance in different loading environments. From a series of tests carried out in the most recent study, it was found that in pull testing, the load transfer characteristics vary with respect to the type of testing. The required dynamic energy for pulling-out a cable bolt can be between 50-80% lower than the static load, based on the cable type and its geometry (Plain or Bulbed). Debonding and pullout mechanisms regardless of loading rate, seem to be similar in both static and dynamic tests, however, plain cable bolts behave differently from bulbed cable bolts in reaction to pull-out load

Variation in load transfer along the length of fully encapsulated rock bolts, based on the installation mixing parameters

The reinforcement quality of the fully resin encapsulated bolt depends on several factors, which are a combination of the bolt design configuration, bolt/hole diameter ratio and installation procedure. Gloving in bolt installation constitutes a challenging problem for effective strata reinforcement and the stability of gate roads and tunnels. A total of ten bolts were installed into ten 1.7 m long threaded steel pipes with different resin spin times, the pipes were retrieved from the installed holes in an underground mine road way roof and then cut into 100 mm long sections. The encapsulated bolts in the tube sections were then push tested. Significant variations were found in bond strength along the installed bolt length in the whole tube, with typically the top 200 mm of bolt being significantly lower (50%) in most cases due to gloving and unmixed resin components

Experimental study on the uniaxial compressive behaviour of a fibre reinforced polymer standing support

Laboratory tests were conducted to investigate the behaviour of an innovative fibre reinforced polymer (FRP) standing support subject to uniaxial compression. The FRP standing support consisted of two major components: 1) the internal cylindrical concrete column made of coal rejects and a cementitious grout, and 2) the external FRP jacket. A total of ten specimens with different water-to-cementitious grout (w/c) ratios and various layers of FRP confinement were prepared and tested. As expected, an increased w/c ratio adversely affected the compressive strength of the internal cylindrical column. The compressive strength of the column decreased from 15.9 MPa to 13.4 MPa when the w/c ratio increased from 1 to 1.2. Test results also indicated that the columns became much stronger and more deformable when confined with an FRP jacket. In contrast to the unconfined control specimens, the columns confined with two layers of FRP experienced an increase of approximately 150% in maximum compressive strength at approximately 500% higher axial deformation. A further growth in strength and deformability was also observed when the columns were confined with four layers of FRP. The maximum strength and deformability achieved were up to 49.6 MPa and 7.2% respectively