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True Triaxial Strength of Coal Measure Rocks and its Impact on Roadway Stability and Coal Burst Assessment

Underground » Coal Burst

Published: September 22Project Number: C27039

Get ReportAuthor: Winton Gale, Gaetano Venticinque and Mahdi Zoorabadi | SCT Operations

Rocks in the ground are subject to a range of stresses. The stresses are defined on the basis of the three principle stresses (σ1,2,3) and shear stresses (τ).  In coal mines the rock and coal are subject to fracture (failure) on a regular basis during development driveage and extraction operations. The strength of the ground during mining operations is a key factor in defining the optimum mining strategy.  Rock strength is one of the basic properties needed to predict rock and rock mass behaviour in geomechanics.

Typically, the definition of rock strength is based on the maximum and minimum stresses acting on the rock mass. This generally relates to a relationship defining at what principle stress (σ1) does a rock fracture. The value of σ1 is termed the strength at a particular confining pressure.

Strength (σ1) =UCS + k * minimum confining stress (σ3).

where k=triaxial strength factor (tsf) and

UCS is the unconfined compressive strength.

This has been the standard in coal mining geomechanics for many years, however, the effect of the intermediate stress (σ2) has been found to significantly influence rock strength when tested under full triaxial stress conditions. The wider use of three dimensional modelling has prompted an assessment of the effects of σ2 on the strength of rock and the resultant modification to the deformation about mining excavations.

Of particular note is the formation of guttering type failure about a roadway when the principal stresses are out of plane, or intersect the roadway at an angle to the heading direction.

In order to take the effect of the intermediate stress into account two factors are required; a three dimensional model and rock strength criteria which takes into account the effect of intermediate principal stress (σ2).

The aim of this project is to undertake full three dimensional stress testing of rock types commonly found in coal mines to assess the effect of σ2 strength variation on rock failure about a mine roadway.

The rock types tested were Hawkesbury sandstone and a well bedded (laminated) siltstone unit from the Hunter Valley. The rock testing requires that at least (ten) 10 samples are required to obtain information of the effect of intermediate stress (σ2) on a single rock type. In fact, many more are required to obtain a full triaxial strength relationship for each rock unit. Therefore, the sampling requirements are quite significant.  Coal was initially planned for testing as well, however it became apparent that the sample requirements of the rock testing approach made sampling of coal very difficult. Coal tends to break up and the potential of obtaining a good sample set of cubes was seen as problematic.  A decision was made to use rock which could be sampled as per the requirements in order to assess the overall true triaxial strength effects.

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