Assessment of Longwall Mining Induced Connective Fracturing of Overburden Strata

As the Australian mining industry operates under strict environmental accountability, the ability to reliably predict and manage mining-induced water inflows and aquifer interference is critically important. Estimating the height of longwall mining-induced fracturing and the height of complete groundwater drainage is a contentious issue, with some misconception and misunderstanding apparent among practicing engineers and groundwater modellers. With the introduction of longwall top coal caving in several mines in Australia in recent times, this knowledge gap in estimating the extent of strata fracturing has significantly widened - creating extra demand for rigorous study and the development of more sophisticated and reliable assessment tools.

The extent of mining-induced subsurface deformation, fracturing, surface subsidence and aquifer interference is largely controlled by local lithological conditions, mining methods and mine layouts. Thus, to effectively assess the impact of mining on groundwater, it is imperative to develop methods and tools that - unlike empirically based strata desaturation height-estimation methods - can properly consider all influence factors and accurately predict strata deformation and subsequent fracturing, particularly connective fracturing, in varied hydrogeologic environments.

The project has developed a method and suite of post-programming tools for the particle-based distinct-element method (DEM) code known as PFC, developed by Itasca Corp. This approach allows the simulation of the initiation and propagation of fractures, allowing the caving of overburden strata to be represented in a realistic manner, and the permeability enhancement of the strata overlying the goaf to be calculated from the first principles. Permeability enhancement in these models is a direct function of the fractures induced in the deformation/caving process; fracture location, extent, aperture and connectedness.

We selected Crinum North and Springvale collieries as project sites for the availability of extensive hydrogeological data, comprehensive monitoring data and past studies.

Key project results are summarised in the following discussion areas:

· Particle-based DEM capabilities in simulating strata failure;

· Post-processing tool development;

· Practical considerations when simulating longwall mining with DEM;

· Estimating permeability from DEM results; and

· Permeability models for Crinum North and Springvale.