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The terms weak rock or hard soil commonly refer to the same type of earth material that has low mechanical properties. This type of material is the transitional material between soil and rock, therefore has the properties of both. However, this does not allow the application of both soil and rock mechanics because the material represents the boundary behaviour of these two important branches of geomechanics. The fundamental differences between rock and soil are the scale and effect of discontinuity networks. This study advances the understanding of weak rock properties by investigating the effect of discontinuities at small and large scales and develops a tool to help practitioners estimate the in-situ properties of weak rocks. A comprehensive review was conducted to classify the type of weak rocks, review laboratory methods for the investigation of weak rock and rock mass properties, and identify available approaches for the field estimation of weak rock mass properties. The review indicated that understanding of low strength rock is lagging compared to other weak rock types, therefore the studies focused on this type of weak rock. A laboratory program was conducted to study the effect of micro-fractures. By conducting size effect experiments on both limestones and artificial weak rocks, this program showed that weak rocks still retain the lithified mineral structure similar to hard rocks but the intact behaviour is significantly affected by the micro properties such as binding between mineral particles and shape, size and spatial variation of mineral particles. At the larger discontinuity scale, the behaviour of weak rock mass was simulated in the laboratory. By simplifying and scaling prominent features of in-situ discontinuity networks, this simulation showed that weak rock masses demonstrated some soil-like properties such as a lower level of downgrading compared to hard rocks. Based on the laboratory work and reviews, a weak rock mass downgrading system, GSIw

Coalburst is a general term, which is commonly used in the coal mining industry for the violent failures of coal in the ribs and face of roadways and panels in underground coalmines. Due to lack of interest in the industry to reveal the causing source of the event, or due to uncertainty about the source, they happily use this term. The term by its own does not reveal the source of the energy, which causes the event. There are three sources of energy that can cause a burst event in underground coalmines: 1) store elastic strain energy, 2) seismic events and 3) gas expansion energy. This paper presents the fundamentals about these sources of energies and discusses our known and unknown facts about the mechanisms. Additionally, it discusses the reliability and effectiveness of stress relief holes and gas exhaust holes as controlling measures to prevent burst events.

When the distributed rock stress around the roadways exceeds the strength of the rock, the rock is failed and a softening zone is formed. Roof deformation developed in the roof and ribs of the roadways are highly controlled by the depth of softening zones. The rock failure process starts from a point ahead of the face and grows into the roof, floor and ribs by advancing roadway. The maximum stress that can be transferred through the failed rocks would be equal to its residual confined strength. Therefore, rock stress is moved above failed zone and will create new failure zone if it is higher than the confined strength of rock at that depth. This process continues until the confined strength of the rock becomes higher than stress components. Bedding and lamination planes play a big role into the failure pathway of rocks around roadways. The thickness of softening zone is significantly influenced by the shear and tensile strength of bedding planes and laminations. This paper presents a 3D numerical assessment of the bedding and lamination planes impacts to the forming and extension of the softening zones. It highlights the requirements for better characterisation of bedding and lamination planes for reliable simulation of roadways.

Deformability modulus of jointed rocks is a key parameter for stability analysis of underground structures by numerical modelling techniques. Intact rock strength, rock mass blockiness (shape and size of rock blocks), surface condition of discontinuities (shear strength of discontinuities) and confining stress level are the key parameters controlling deformability of jointed rocks. Considering cost and limitation of field measurements to determine deformability modulus, empirical equations which were mostly developed based on rock mass classifications are too common in practice. All well-known empirical formulations dismissed the impact of stress on deformability modulus. Therefore, these equations result in the same value for a rock at different stress fields. This paper discusses this issue in more detail and highlights shortcomings of existing formulations. Finally it presents an extension to analytical techniques to determine the deformability modulus of jointed rocks by a combination of the geometrical properties of discontinuities and elastic modulus of intact rock. In this extension, the effect of confining stress was incorporated in the formulation to improve its reliability.

The horizontal hydraulic conductivity anisotropy of coal seams is a controlling parameter for designing gas drainage boreholes. The ratio between the maximum and minimum horizontal hydraulic conductivity (RkH-kh) and the orientation of maximum horizontal conductivity defines this anisotropy in horizontal plane. This paper presents a new analytical procedure based on the field stress data and geometrical properties of coal cleats to calculate these two parameters. The application of this procedure for a real case in Eastern of Australia resulted in an average ratio of 20.9 for RkH-kh and orientation of NE for maximum horizontal conductivity. The comparison between these results with the measured values validated the accuracy of proposed procedure to estimate the anisotropy of horizontal hydraulic conductivity of coal seams.

Hydraulic conductivity is an important characteristic of jointed rocks and methods to quantify this parameter are essential for mining and civil engineering applications. Rock discontinuities play a significant role in the circulation of water through jointed rocks, while the geometral characteristics of joints control the magnitude and orientation of the conductivity tensor. However, because of the complexity of water flow within individual fractures and the complexity of water circulation within discontinuity network, hydraulic conductivity determination is a challenge.Hydraulic behaviour of individual rock fractures, hydraulic conductivity of jointed rocks and depth dependency of hydraulic conductivity for jointed rocks were investigated in this thesis. For individual fractures, an analytical method was proposed to estimate the hydraulic aperture of rough fractures with matched surfaces under linear flow conditions. The application of this method for JRC profiles was verified by precise laboratory experiments, which provided the capability to study the hydraulic behaviour of JRC profiles under linear and nonlinear flow conditions. The unique nonlinear results of these experiments showed a reducing trend between normalised transmissivity and Reynolds number which was modelled satisfactorily by the Forchheimer equation. For the first time, empirical relationships were proposed to calculate the constants in the Forchheimer and Izbash equations. Moreover, a new empirical equation was introduced for the trend between flow friction factor and Reynolds number. The flow friction factor is a function of both Reynolds number and relative roughness, but it is more sensitive to Reynolds number.New analytical formulations were introduced in this thesis to estimate the hydraulic conductivity of jointed rocks. These formulations considered the rock discontinuities to have finite persistence with circular disk shape. Two well-known cases were used to verify the proposed formulat