Fracturing, Caving Propagation and Influence of Mining on Groundwater Above Longwall Panels – A Review of Predictive Models
Bruce Hebblewhite University of New South Wales
ABSTRACT
Historically there have been a number of different hypotheses and empirical models developed in an attempt to describe the nature of fracturing above longwall panels in underground coal mining. The motivation for such models varies, ranging from understanding the impact of mining on surface subsidence; to back-analysis of caving behaviour in the immediate roof behind the longwall face. One of the most critical motivating factors that is taking on increased importance in many coalfields, is the need for better understanding, and hence prediction of the impact of mining on overlying strata, particularly strata units acting as aquifers for different groundwater horizons.
This paper reviews some of the major prediction models in the context of observed behaviour of strata displacement and fracturing above longwall panels in the Southern Coalfields of New South Wales, south of Sydney. The paper discusses the parameter often referred to as “height of fracturing” in terms of the critical parameters that influence it, and the relevance and appropriateness of this terminology in the context of overlying sub-surface subsidence and groundwater impact. The paper proposes an alternative terminology for this parameter that better reflects what it is and how it is used. The paper also addresses the potential role of major bedding shear planes mobilised by mining and their potential influence on overlying subsidence and groundwater interference.
INTRODUCTION
Knowledge of the detailed nature of rock deformation and failure above any form of large-scale underground mining is always going to be limited to interpretation from a very incomplete set of data. It is extremely difficult, if not impossible, to directly measure the detailed nature of the rock failure, fracture networks and deformational behaviour above an extracted mining area. Limited techniques such as borehole extensometry can provide some evidence of relative or incremental deformation in the direction of the borehole (usually vertical). However, such data cannot assist below the horizon where full caving has caused major rotation and dislocation of rock blocks and effectively destroyed the instrumentation borehole. Above such a horizon, the data is only valid along the axis and in the direction of the borehole, and to the level of detail defined by the extensometer anchor spacing intervals. It is often also restricted by major shear planes intersecting the extensometer borehole and cutting off the instrumentation below the level of the shear horizons.
Other direct measurement techniques include borehole inclinometers which can assist with measuring shearing across the line of the instrumentation borehole, usually, but not always associated with bedding plane horizons. Coupled with an extensometer to provide movements in the borehole axis direction, the combination of extensometers and inclinometers provides a “coarse” level of deformation measurement along the axis of the instrumentation borehole. This direct borehole monitoring data can also be complemented by down-hole geophysical and calliper logging to provide further fracturing information along the axis of the borehole, together with various forms of borehole wall inspection or scanning devices. However, none of these different borehole techniques assist with detection of the laterally dispersed deformation and failure taking place away from the individual instrumentation boreholes. The result is therefore a very incomplete dataset that relies heavily on in-fill estimation and interpretation.
HEIGHT OF FRACTURING CONCEPT
Why then is there a need for knowledge of deformation and failure above the mining location – in particular, above underground longwall mining panels? The answer can relate to a number of important issues:
(a) To consider the effect of mining taking place at one horizon on a higher horizon within the overburden (either mined previously, or planned to be mined in the future); (b) To assist in developing predictive models for estimating surface subsidence; (c) To develop an understanding of, and predictive model for the impact of underground mining on groundwater present within the overburden.
It is this third issue that has taken on increased importance in recent years and is the primary focus of this current review. In fact, the reason for trying to define regions of fracturing above a longwall panel is not typically about defining the deformation and fracturing specifically, but actually about interpreting the potential impact of such deformation and fracturing due to mining on the groundwater regimes.
Furthermore, at the present time, it is often the measurement of groundwater data which is used to infer the different fracture zones – so the whole argument becomes a circular one. We measure groundwater pressures and related data to infer overburden fracture zones in order to estimate groundwater impact levels and regions. Why not simply refer to the parameters we can measure – groundwater pressures and properties – rather than making arbitrary distinctions regarding the level of rock fracturing that in any case is not clearly defined? This question of improved definition and terminology is revisited later in this paper.
However, if there is a need to understand, predict or monitor the impact of underground mining on overlying groundwater aquifers, through to surface subsidence impacts by way of understanding rock fracturing regimes, it is important to assess several rock deformation and fracturing parameters in the overburden, specifically:
• The height of connective cracking (or fracturing) • Extent of surface cracking • Potential connections with and impacts of mining on horizontal bedding planes.
It is therefore important to have a clear understanding of what is meant by these terms and how they relate to each other and to the mining process.
Firstly, fracture patterns associated with overburden rock strata subjected to longwall mining can be extensive and quite variable, ranging from complete rock failure in the immediate caving zone above the coal seam, through to some level of near-surface tensile cracking within the subsidence impacted zones of curvature. It must be understood that these two extremities of the fracturing regime are normally isolated from each other, and subject to quite separate or independent behavioural mechanisms. (This distinction is commonly misunderstood, especially by non-geotechnical personnel).
It is simply not possible to fully analyse or characterise all fracture patterns throughout the overburden – either pre- or post-mining. It is considered more important to focus on what is commonly referred to as the “height of connective cracking, or height of fracturing”, which are widely-used terms in the context of describing groundwater connectivity, or potential drainage pathways through the strata. The issue of surface cracking is also of interest, but as a separate fracture region within the overburden, as noted above.
Even the concept of height of fracturing is difficult to fully and accurately analyse and characterise and remains a subject of some considerable debate amongst the geotechnical and hydrogeological community (see Sullivan & Swarbrick (2017), Galvin (2017) & Mackie (2017)). However, it is accepted as being very important to gain a meaningful understanding and best-estimate analysis of such a region of fracturing and “connective cracking” within the overburden, using whatever practical means available.
Conceptual Models
The actual nature of the fracturing above a longwall panel cannot be directly measured but can generally only be inferred from indirect observations and measurements, as discussed above. The conceptual model of the fracture zone has been discussed internationally by many authors through the use of a number of simplified conceptual models which describe a series of zones of different types of rock failure, fracturing and deformation above longwall panels.
Work on this topic from the USA includes Peng and Chiang (1984), Kendorski (1993) and Mark (2007), all of whom proposed either four or five zones of overburden behaviour, based on US experience and observations.
The zones defined by Peng and Chiang (1984) and Mark (2007) include:
• Complete and partial caving zones (A) - where roof rock falls into the void created by the longwall. Complete caving zone is typically 2-4 times extraction height (h). In the partial caving zone beds are completely fractured but remain in contact with one another. Total thickness of caving zones typically ranges from 6-10 times the extraction height; • Fracture zone (B) - where subsidence strains are significant enough to cause new fracturing and create hydraulic connection to the seam below. The fracture zone typically extends up to 24 times the extraction height; • Dilated zone (C) - where permeability is enhanced but little new fracturing is created, extending up to 60 times the extraction height; and • Confined zone (D) - where subsidence normally causes no change in strata properties other than occasional bed slippage. The upper limit of this zone can extend to 15m below the surface.
Kendorski (1993) defined five zones by splitting what some authors describe as the constrained zone into two – a lower dilated zone and an upper constrained zone.
A number of models have been developed by authors in Australia based on a combination of experience and numerical and physical modelling studies. Mills (2012) developed a six-zone model for caving behaviour based on ground displacement, as part of a study to predict mining inflow due to subsidence- induced fracturing. Mills’ model was developed based on surface extensometers, borehole cameras, piezometers, packer testing, microseismic monitoring and stress change monitoring.
Zones within the Mills model are described as:
• Zone 1: chaotic disturbance immediately above mining horizon (0-20m height) • Zone 2: large downward movement, including cross-measure, open fracturing (height up to 1.0 x panel width(W)) • Zone 3: vertical opening of bedding planes (1.0W – 1.6W) • Zone 4: vertical stress relaxation (1.6W – 3.0W) • Zone 5: no disturbance from sag subsidence (>3.0W but shear along bedding planes for subsidence of multiple panels) • Zone 6: compression above chain pillars.
Figure 1 is another conceptual model (Mackie model), quite widely accepted in Australia, for representing or describing the zones of fracturing above a single longwall panel. In this model, four major, different zones are identified (note – this diagram is not necessarily to scale, with respect to the height of the different zones).
• caved zone – where there is complete rock failure and large-scale downward movement and some rotation of rock blocks, resulting in a significant amount of void space. There is obviously complete groundwater depressurisation and dewatering/drainage from within this zone. The height of this zone is affected by the type of immediate roof above the coal seam, and mining height, which will define the available void space created by mining.
• fractured zone – where the rock has undergone significant vertical and horizontal deformation, with dilation of some discontinuities (bedding planes and joints) and also incurred connective cracking of intact rock. The result is a fractured rock mass that allows increased permeability permitting depressurisation of groundwater in this zone. The term “height of fracturing” refers to the height above the mined seam to the top of this zone, where there is enhanced vertical permeability due to the fracturing, allowing groundwater depressurisation. It is also understood that within this fractured zone there may be increased porosity due to the fracture network, which may also contribute to depressurisation.
• constrained zone – contains deformation, but significantly less cracking, without extensive connective fracturing occurring, such that depressurisation does not occur to a significant extent. The deformation includes a significant extent of bedding plane shear as the layers of rock strata bend. (In some conceptual models the constrained zone is divided into two different zones where the upper region consists of much more simple strata unit bending, without any major fracturing, but also with shear movement on bedding planes).
• surface zone – the near-surface region where cavities associated with tensile fractures open up as the surface strains create a limited depth of open vertical fractures and horizontal cavities in the shallow strata. These cavities are often said to lie within a zone 10m to 20m below ground level (this should be validated on a site by site basis).
Figure 1. “Height of Fracturing” concept (Mackie model) (source: Hebblewhite et al, 2008) (original source: Dr C Mackie, 2007)
On the basis of this or any of the other similar forms of conceptual model and definitions, the term “height of fracturing” has been used to refer to the region of connective fracturing and structural deformation (bedding planes, joints etc) leading to increased vertical permeabilities which will result in significant depressurisation of the strata. For this reason, groundwater pressure monitoring can be used as a means of detection of the upper limit of this fracturing zone, rather than relying on direct, but limited deformation and fracture monitoring, which as discussed above, is extremely difficult.
Although the description of the zones and their geotechnical behaviours differ slightly between each model, there are consistencies between them, for example zone (2) in the Mills model is of a similar nature to the above fractured zone shown in Figure 1.
Some important summary points to note in relation to the above concepts, regardless of which particular concept model is used, are as follows:
• These are concepts only, representing hypotheses regarding the nature of fracturing and connectivity above an extracted longwall panel. They have been developed as conceptual artefacts, in order to describe the type of deformation and fracturing of the overburden strata, and how it is made up of different zones or different types and intensities of deformation and fracturing. • These concept models have been developed based only on indirect or very incomplete data sets, be they data from geotechnical monitoring, groundwater monitoring or numerical and physical modelling. • The gradation from one zone to another in any of these models, whilst appearing distinct within the concept diagrams, may well be quite gradual and transitional, rather than distinct boundaries, and may be highly impacted by localised geological factors such as specific strata units present, or other structural defects including bedding planes, joints and major structures (faults, dykes etc.).
Based on the above points, extreme caution is urged in use of these model concepts, without significant qualification, and/or detailed analysis of the underpinning data. The breakdown of the overburden into distinct zones should only be regarded as an artefact or concept, to aid in understanding, rather than an exact definition of what is occurring in the ground.
Terminology
It is further proposed that there should be a change in the terminology used in future – for all of the reasons discussed above, relating to both the nature of the deformation and fracturing characteristics, as well as the means of measurement or estimation – at least for application in the groundwater context. For use of this concept for groundwater impacts, it is proposed that the term “height of depressurisation” be adopted in future as a more meaningful descriptor, rather than the terms height of fracturing , or height of connective cracking . This proposed terminology is directly linked to the application of the term for groundwater purposes, as well as being directly linked to the main means of measurement or estimation.
It is also noted that some authors are making reference to this zone as a height of drainage or height of complete drainage. However, a caution is raised with use of such a term, which is also discouraged. Whilst it is acknowledged that some increased level of free drainage will occur in this zone (to enable depressurisation to occur), the word drainage can sometimes be interpreted to refer to total dewatering, which is certainly not always the case within the depressurisation zone. From a groundwater perspective, the height of drainage where complete dewatering is always expected would more commonly be associated with the immediate caving zone, as illustrated in Figure 1.
EMPIRICAL PREDICTION MODELS
On the basis of the above forms of conceptual models, several empirical prediction models have been developed in Australia to estimate height of depressurisation, also referred to as height of fracturing, height of connective cracking, or height of complete drainage. However, none of these models is regarded as being the final answer for prediction, and they all require further development, refinement and validation as further case study data becomes available.
It is important to consider the parameters that could influence height of depressurisation.
Important Parameters
The factors that could be considered to impact on such a height are as follows:
• Panel width (the total width of the mining excavation, including adjacent gate roads), W • Mining extraction height (which may or may not be the full seam height), h • Depth, H • Panel width to depth ratio, W/H • Geology of the overburden (including the role played by thick and competent strata beams that may act to bridge across the span of the void below; and/or act as an aquiclude or major aquitard to vertical groundwater flow; and also the role of discontinuities of various forms) • Time (time taken for drainage/depressurisation to occur after mining and potentially propagate upwards through the overburden; and also, time for subsequent re-charge to occur).
The first two geometric factors are the most common ones that are expected to influence height of depressurisation and are commonly found in most of the empirical prediction models. It is of interest to note that the US models discussed earlier are primarily linked to extraction height, h, whilst the Mills model referred to above is focused on the influence of excavation width, W, and does not refer to mining height at all.
The current understanding of the height of depressurisation is that it is most likely to be a function of both mining excavation width, and mining height, however, the relationship to these two fundamental mining geometry parameters is not necessarily linear.
One of the further difficulties with many of the empirical models in relation to these geometric parameters is the limitation in their underpinning databases, with a lack of data across a broad range of these parameters, especially mining height. Attempts to extend the models for the use of thick seam mining panels, for example, is difficult due to a lack of validated data points from high extraction mining heights, including top coal caving scenarios which further complicate the impact of height on caving.
The next parameter listed above is depth (H), and then associated with this is the important relationship of Width to Depth Ratio, or W/H – used widely in relation to predicting surface subsidence. However, it can be argued that below a certain depth, where proximity to surface is not significantly influencing the mining-induced stress field, an incremental change in depth, for a given panel width, will only have a very slight influence on the more localised “height of fracturing” zone above the mining panel. This rock deformation and fracturing behaviour, for a given set of geological, stress and mining height conditions, is likely to be largely dependent on panel width (and mining height) and is independent of small to moderate incremental changes in depth.
It is therefore considered that W/H has little effect on height of depressurisation, and depth is also of limited interest, other than at extremes (very shallow and deep).
In regard to geology, there is clearly going to be an influence. The question is how to account for this. Some models make assumptions regarding the bridging roles of some strata units at different horizons in the overburden. Other models take no account of geology. There are two means of dealing with this important parameter, which in itself can be multi-faceted, relating to strata properties and discontinuity impacts. One option is to move away from empirical modelling altogether and develop a complex numerical model that honours the localised geological variations in the overburden. This is a desirable intention but is actually very challenging to achieve and is still considered to be a forward research or developmental challenge for this discipline, rather than one that is routinely available today.
The second option is to restrict the application of empirical models to areas of similar geology from which the underpinning database has been derived. In such a situation, the database itself takes account of the local geology and there is no need to incorporate geological parameters into the prediction model. This approach is the recommended one – at least for the immediate future – however, it again suffers from the limited size and range of parameter values within the database. Therefore, regardless of the approach to be taken, expansion of the database is of critical importance.
The final parameter in the list above was “time”. This is widely regarded as a very important parameter, but to date, has not been included in any of the major empirical models adopted. Once again, this is a challenge for future development of improved prediction models.
Current Empirical Models
Two empirical prediction models are currently used quite widely in the Southern Coalfield, and elsewhere, and are known as the Tammetta and Ditton (or DGS) models.
The Tammetta model (Tammetta, 2013) is a function of geometric parameters only, including mining depth, but there is no consideration of geology. Whilst there have been a number of minor adjustments to the Tammetta model equation in recent years, the fundamental relationship for estimating the so- called Height of Fracturing, or Complete Height of Groundwater Drainage (H cd ), represented by the connective cracking regime, is as follows:
-5 0.2 1.4 Hcd =1438 ln(4.315 x 10 * H * h * W + 0.9818) + 26 (m) Eq. 1
Where H = depth (m) h = extraction height (m) W = void width (m)
The Ditton model is also widely used (Ditton & Merrick, 2014) and includes the same geometric parameters, but also makes provision for the role of stiff or strong strata units in the overburden. There are a number of different equations within the Ditton model, to take account of both geometric and geological factors.
As part of the recent Independent Expert Panel on Mining in the Catchment (IEPMC, 2018), the IEPMC concluded that both of these empirical models were of value, with limitations, but should continue to be used. The IEPMC also referred to work by Galvin and Mackie, who had reprocessed the Tammetta database without regard to maintaining dimensional accuracy and concluded that, for practical purposes, the height of complete drainage derived from it could be simplified further to:
= 0.3 ∗ℎ∗ ′ ( ) Eq. 2
Where h = extraction height (m) W ′ = effective void width (being the maximum void to reach the point of critical surface subsidence on the surface (m)
This simplified equation did not include depth, and as a result, has accepted the fact that there is no direct connection between height of complete drainage (depressurisation) and W/H ratio.
EMPIRICAL DATA
Height of Depressurisation
In order to develop and apply the above empirical prediction models, it is important to have an appropriate and well-populated database. This requires the collection of both pre- and post-mining groundwater data taken from above the mining panels, to compare the level of depressurisation that has occurred. One of the difficulties in expanding the database for empirical predictions is that in a given area or geological regime, there are unlikely to be a wide range of mining panel widths or different extraction heights.
Figure 2 is an example of such a pre- and post-mining groundwater dataset taken from a centreline above an isolated longwall panel with solid coal on both sides, at a depth of 380m below ground level (mbgl).
The pre-mining pressure head data (June 2013) shows values from depths of 50m to below 250m, increasing with depth, as is expected, in line with the hydrostatic pressure gradient. The post-mining data (February 2014), obtained from a new hole drilled after longwall extraction, directly adjacent to the original borehole, was only able to obtain data in the depth range of 50m to just approaching 150m.
Comparison of the two sets of data shows the impact of mining on the pressure head reduction in this region of the overburden, with similar head at a depth of just above 100m, but a significant reduction in head approaching the 150m head level, coinciding with the base of the Hawkesbury Sandstone. This point of change in head (between 100m and 150m) represents the height of depressurisation above this longwall panel. If a depth of 100m is used, this would represent a height of depressurisation of 280m (above a 305m wide panel).
Figure 2. Pre- and post-mining pressure heads above the centreline of a longwall panel.
Bedding Plane Shear
A further important consideration in relation to the overall fracture and deformation regime within the overburden associated with the impact of longwall mining on groundwater is that of bedding plane shear (also referred to by some as basal shear planes).
It is clear from monitoring data, modelling (both numerical and physical), as well as intuitive interpretation of mechanistic behaviour, that there will be extensive presence of shearing on bedding planes (including major strata unit basal planes) at multiple horizons above and adjacent to the caved region of overburden. Bedding plane shear is an essential “pre-requisite” for bending of strata units in the higher overburden zones, as well as cantilevering of bedding planes at the edges of the caved region. This type of behaviour is to be expected and will include both direct shearing on the bedding planes, and also dilation of some bedding planes over the mined area.
In terms of the impact of such behaviour on groundwater, there is no doubt that dilated bedding planes above or within the immediately affected zone of disturbance will have a significantly increased level of permeability – at least horizontally. However, without dilation, beyond the extremities of the mining panel, the permeability of a sheared bedding plane may or may not increase in horizontal permeability, depending on localised properties of the bedding plane surface and the extent of pre-existing and mining-induced clamping forces normal to the shear plane.
Figure 3 is an example of a series of bedding plane shears detected some distance in advance of a longwall face, at depths of 57m and 85m. The major shear (at 57m depth) corresponds to the top of a significant strata unit whilst the lower shear is located within a thick sandstone unit.
Figure 3. Bedding plane shear detected above Dendrobium Mine, near Sandy Creek Waterfall, detected using borehole inclinometer . (source: Walsh et al., 2014)
This type of bedding plane shear is clearly a mining-induced phenomenon and can be measured reasonably accurately where boreholes are drilled, and inclinometers installed ahead or adjacent to mining. However, what is required in future investigations is some pre- and post-mining hydraulic conductivity monitoring in the vicinity of these shear horizons to determine how much, if any, the horizontal permeability of such horizons has changed as a result of mining. It is not appropriate to assume, without evidence, that permeability will increase automatically, whenever shear has occurred. If such permeability increases are detected, the next consideration is the lateral extent of such potential flow paths for groundwater loss or leakage, which may or may not be significant. Further monitoring work is clearly needed.
CONCLUSIONS
• Longwall mining clearly creates both fracturing and various forms of deformation within the overburden strata, including normal vertical subsidence, bedding plane dilation or separation, and also bedding plane shear.
• There are a number of conceptual models put forward internationally to describe the different zones of strata behaviour above a mined longwall panel. The different models have a number of common features, as well as some key differences.
• Such models are useful to conceptualise the overburden behaviour and the impact of mining on the strata integrity and potential groundwater impacts, however, they should be considered with caution, as concepts only, to assist in understanding. In particular, there is no evidence to support hard, or discrete boundaries between such zones within the overburden, which in reality probably involve gradational change, impacted greatly by local geological conditions, and also potentially changing with time.
• The use of numerical geotechnical models to represent overburden fracturing and deformation, hence groundwater impacts from longwall mining, is still under development. Such work should continue but is not considered sufficiently advanced at present to be relied upon alone, for prediction.
• There is a role for well calibrated/validated empirical models to predict groundwater impacts, and in particular, to predict the height of depressurisation – a preferred terminology for what has also been previously described as height of fracturing, height of connective cracking, or height of complete drainage.
• These empirical models should be developed for application within particular or reasonably consistent overburden geological conditions, to avoid the difficult problem of modelling the impact of a range of complex geological phenomena on rock deformation and failure/fracturing.
• The empirical models are in need of expansion of their underpinning databases, particularly in relation to the fundamental mining parameters of excavation width and height. They also need greater data collection on both pre- and post-mining groundwater impacts, to allow for model validation.
• Horizontal bedding plane shear is a phenomenon that has been recognised to occur not only in the immediate proximity of the mining-impacted overburden, but also potentially extending some distance away from the mining activity. Further groundwater monitoring is needed to determine whether such shear behaviour has a material impact on horizontal permeability of such shear planes or not, and the extent of any new flow paths created, if permeability is increased.
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