Foreword to the the Mine Subsidence issue, Volume 59, Number 4

Mine subsidence, particularly that occurring due to coal mine workings, has a long and complex history in Australia, with subsidence issues being recorded as early as the 1820’s.

The management of subsidence by early Australian mining operations generally centred on mine design and strata control and as with many other engineering disciplines, began with rules of thumb and empirical relationships used in Britain being adopted and modified to suit Australian conditions.

Above ground, building practices and infrastructure design followed a similar route, where overseas practices in accommodating the effects of mine subsidence were employed until either unacceptable performance or external conditions required that these practices change.

As with most issues relating to the interaction between the ground and built infrastructure there is a wide range of engineering disciplines involved in the management of subsidence. Those involved in the construction, design and approval of surface infrastructure as well as those involved in the operation and regulation of underground mining frequently interact with each other, albeit with different priorities and historically with different levels of acceptable risk tolerance.

Whilst many advances have been made in dealing with the effects of subsidence in the civil and structural engineering fields in recent decades, within the last ten years geotechnical engineering has advanced considerably in this area, a factor which prompted the compilation of the technical papers contained in this special issue of the Australian Geomechanics journal.

The first two papers in this issue both involve advances made in the assessment and remediation of abandoned or legacy mine workings, an increasingly relevant area for geotechnical engineers particularly in areas of New South Wales, Queensland and Western Australia as old mining areas are developed due to urban expansion.

Johnston, Twomey and Fityus provide a paper detailing the analysis of a 34-year database of sinkhole events occurring in the Northern Coalfield, as well as a probability model to aid in assessing sinkhole risks for future infrastructure and housing. As with most things geotechnical, a commonly applied empirical relationship is shown to work very well for the area in which it was developed but fails when applied elsewhere.

Fityus, West, Semmler, Broadbridge and Johnston outline the results of a large scale mine grouting trial, where a large test vessel was used to simulate a flooded mine drive/bord. Of note are the observations regarding variability in UCS results in the placed grout, with water entrainment during placement appearing to have a large influence on the final strength results. Importantly the permeability of collapsed roof/rubble is also investigated, with the proportion of fines predominantly influencing the grout permeability of the rubble material.

Seedsman provides two papers, both of which deal with issues in the assessment and design of current mine workings, particularly in areas of environmental or subsidence constraints. The first paper outlines the challenges and uncertainties associated with chain pillar design and behaviour where limited geotechnical data and appropriate methods are available. It is proposed that foundation engineering concepts adopted from civil geotechnical engineering may aid as a framework for assessment and provides commentary on their use in modern mine design. The second paper deals with potential changes in hydraulic conductivity due to fracturing induced by longwall mining, the high level of complexity and challenges involved when estimating the potential effects of subsidence on groundwater aquifers.

Kay, Barber and Talbert provide a case study of a 15-year mining project involving a series of longwalls mined adjacent to a major fault complex. This provides an overview of the ways in which contemporary mine subsidence is managed in an area where both significant surface infrastructure and significant geological features are present and highlights some of the difficulties involved in predicting subsidence movements.

Whilst not strictly related to the theme of subsidence, Goodall, Todd and Merifield outline a proposed alternative method for accommodating the effects of soil shrinkage and crack depth on the performance of laterally loaded solar farm piles founded in reactive clay soil. In a familiar theme found in other papers in this issue, rapid development of infrastructure has potentially outpaced a full consideration of the geotechnical conditions and design assumptions used.

Whilst mine subsidence as an issue in Australia is a very old problem, the management of the risks associated with it will continue to adapt to changing conditions as they have done for nearly two centuries. It is notable that the papers contained in this issue are relatively evenly spread between those dealing with the design or management of operating mines and those involving remediation of abandoned mines to facilitate future urban development. Whilst it is a small sample size, this is a trend that is replicated in other forums and one which I believe provides an indication of the high level of importance being placed in appropriately dealing with legacy mining issues.

In all stages of this process, it is crucial that geotechnical engineers remain continuously engaged at every level in the advancement of techniques and methods that improve our ability to deal with subsidence. On behalf of Australian Geomechanics, I hope you find these papers informative and useful.