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A new coal mine floor rating system and its application to assess the potential of floor heave
International Journal of Rock Mechanics & Mining Sciences 128 (2020) 104241
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International Journal of Rock Mechanics and Mining Sciences journal homepage: http://www.elsevier.com/locate/ijrmms
A new coal mine floor rating system and its application to assess the potential of floor heave Sungsoon Mo a, Hamed Lamei Ramandi a, Joung Oh a, *, Hossein Masoumi b, Ismet Canbulat a, Bruce Hebblewhite a, Serkan Saydam a a b
School of Minerals and Energy Resources Engineering, UNSW, Sydney, NSW, 2052, Australia Department of Civil Engineering, Monash University, Melbourne, VIC, 3800, Australia
A R T I C L E I N F O
A B S T R A C T
Keywords: Rock mass classification Coal mine floor rating Floor heave Buckling
Several underground coal mines have reported floor heave associated with the buckling mechanism. However, studies on the coal mine floor, especially the buckling failure mechanism in the floor and relevant analysis techniques, are still insufficient. This paper introduces a new floor classification system, the Coal Mine Floor Rating, based on the underlying failure mechanisms. The new floor mass rating system quantifies the stability of floor strata by considering two main factors: uniaxial compressive strength and discontinuity spacing of the floor units. The Coal Mine Floor Rating is incorporated into an empirical method, the Floor Heave Index, developed by statistical analysis of a database obtained from actual floor failure cases. The floor heave and non-floor heave cases are determined depending on the Coal Mine Floor Rating and the Horizontal Stress Rating, a proxy that represents the magnitude of horizontal stress. While obtaining underground floor data is challenging, the new floor classification system and the empirical method help assess the potential of significant floor heave on development in underground coal mine roadways for new mining projects or future workings.
1. Introduction Excessive deformation of roadways is occasionally encountered in underground coal mines. The excessive floor deformation is known as floor heave in the mining industry. Floor heave is also described as either “breaking or lifting up of immediate floor strata” or “extrusion of floor strata”.1 Managing floor heave in a timely manner is crucial as it can disturb mining operations and thus often cause significant financial losses. Previous studies have indicated three main mechanisms of floor heave in underground coal mine roadways: bearing capacity failure, swelling and buckling.2,3 Many researchers have studied bearing ca pacity failure as the mechanism involves a range of ground control issues including pillar punching, tensile failure of pillars with rib spalling, roof fall and subsidence as well as floor heave.4–9 For bearing capacity failure of the coal mine floor, various bearing capacity formulae modified from bearing capacity theories in civil engineering have been used.4,10–13 Numerical modelling is also used to compute the bearing capacity of the floor by calibrating the models against bearing capacity formulae.14–17
However, compared to the bearing capacity failure mechanism, studies on the swelling and buckling mechanisms and relevant analysis tech niques are still limited. Over decades, rock mechanics problems have been addressed by using rock mass classification systems.18,19 Notable examples of rock mass classification are the Rock Mass Rating (RMR)20 and the Q-sys tem21 developed for the tunnelling industry. A new classification sys tem, the Rock Mass Quality Rating (RMQR), has also been introduced to estimate the mechanical parameters of rock masses.22 In the hard rock mining sector, the modified version of the RMR, the Mining Rock Mass Rating (MRMR), has been mainly used for cave mining applications.23 In coal mining, the Coal Mine Roof Rating (CMRR)24,25 has been exten sively used in numerous empirical methods, such as evaluating the serviceability of gateroads26,27 and ground support requirements.28 The Geophysics Strata Rating (GSR) was developed to quantify coal mine strata for geotechnical applications.29 A risk-based classification system for the excavated coal mine slopes, the Slope Stability Assessment Methodology (SSAM), has been proposed recently.30 Since less attention has been given to the coal mine floor among
* Corresponding author. E-mail addresses: [email protected] (S. Mo), [email protected] (H.L. Ramandi), [email protected] (J. Oh), hossein. [email protected] (H. Masoumi), [email protected] (I. Canbulat), [email protected] (B. Hebblewhite), [email protected] (S. Saydam). https://doi.org/10.1016/j.ijrmms.2020.104241 Received 2 November 2019; Received in revised form 31 January 2020; Accepted 5 February 2020 Available online 12 February 2020 1365-1609/© 2020 Elsevier Ltd. All rights reserved.
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other rock mechanics topics, a rock mass classification system for floor strata is scarce in the literature. A rare example is a roof and floor classification system developed by Buddery and Oldroyd31 for coal mines in South Africa. The floor classification system adopted uncon fined swelling strain and slake durability tests to assess the potential of swelling and degrading due to water. Riefenberg32 attempted to apply the CMRR to the floor of an underground coal mine and indicated that some modifications of the CMRR would work for rating the coal mine floor. Several underground coal mines have reported floor heave associ ated with the buckling mechanism.33–35 As the heaved floor is generally brushed to maintain clearance, predicting the occurrence of floor heave is beneficial in managing floor heave proactively. In the hard rock mining and civil engineering sectors, practical guidelines regarding anticipated levels of deformation of rock around the excavations have been established.36–38 However, it is still challenging to predict the magnitude, location and timing of floor heave with the current state of knowledge, and no study has been found to provide such guidelines for underground coal mining applications. This paper proposes a new rock mass classification specifically developed for the coal mine floor, the Coal Mine Floor Rating (CMFR). An empirical tool using the CMFR, the Floor Heave Index, is also pro posed to assess the potential of floor heave on development in coal mine roadways. The significance of the new rock mass classification system and empirical approach is discussed, and the limitations of the method are also presented.
developed. 2.2. Identification of floor heave mechanism Bearing capacity failure typically involves soft or weak immediate floor units such as claystone and underclay.4,7,9 Although rock can weaken once it is exposed to water or moisture, most of the floor heave areas were not wet. The uppermost floor units of the mines were thus assumed to be relatively strong in terms of UCS. Moreover, whereas bearing capacity failures normally involve pillar punching, roof falls and subsidence, none of these were reported along with the floor heave events. It is therefore unlikely that the floor failures were associated with the bearing capacity failure mechanism. In addition, a simple analysis using bearing capacity formula was carried out to examine the likelihood of bearing capacity failure. Since many floor heave cases occurred on development, the average vertical stress value was estimated using the tributary area theory while the vertical load exerted on the pillar is typically less than the full tributary area load on development 39: � � (1) σ p ¼ 0:025HC1 C2 WL or 0:025HðW þ bÞðL þ bÞ WL ðMPaÞ where σp is the average vertical pillar stress; H is the depth of cover (m); C1 is the centre-to-centre distance of pillar width (m); C2 is the centre-tocentre distance of pillar length; b is the roadway width (m); W is the ribto-rib distance of pillar width (m); and L is the rib-to-rib distance of pillar length (m). Eq. (1) assumes a gravitational acceleration of 10 m/s2 and a density of the overburden of 2500 kg/m3. Taking a roadway width of 5.5 m and a chain pillar dimension with a centre-to-centre pillar width of 50 m and a centre-to-centre pillar length of 100 m, the average vertical pillar stress using Eq. (1) was calculated to be 10.4 MPa at a depth of cover of 350 m. A cover depth of 350 m was chosen because, in general, significant floor heave was noticed in several mines where the depth of cover is greater than 350 m. There are many bearing capacity formulae modified from bearing capacity theories in civil engineering.11 Eq. (2) proposed by Mandel and Salencon,40 which has been used for Australian coal mine cases,12,41 was used to calculate the bearing capacity of the floor:
2. Development process Mark18 detailed a typical development process for empirical methods and pointed out the important considerations such as identifying failure mode, establishing a clear hypothesis for a model, developing appro priate parameters and analysing data. The CMFR and Floor Heave Index were developed through a similar process. In this section, the important considerations through the process of creating the CMFR and Floor Heave Index are highlighted. 2.1. Identification of failure mode
qfloor ¼ ðUCS = 2Þ � ð4:14 þ W = 2tÞ
Fig. 1 exemplifies the typical modes of floor failures observed in the underground longwall mines in Australia. Tensile cracks at the floor surface were commonly seen in such a failure. The separation of floor units along the bedding planes was also observed frequently. The up permost floor units of the mines include tuff with the average uniaxial compressive strength (UCS) value of 40 MPa, siltstone with UCS ranging from 40 MPa to 65 MPa and concrete. While it is hard to conceive that the uppermost floor units are weak materials considering the UCS values, the failure of those units was commonly noticed during the development stage in many cases. Floor heave also took place on the longwall retreat. As the longwall face approached the roadways, the magnitude of floor heave frequently increased or new floor heave
(2)
where qfloor is the bearing capacity of the floor; W is the rib-to-rib dis tance of pillar width; and t is the thickness of the floor unit. Using Eq. (2), a bearing capacity of 10.5 MPa was calculated with a UCS of 2 MPa and a thickness of 3.5 m. Therefore, to have a lower value of bearing capacity than the average vertical stress of 10.4 MPa, calcu lated before, the floor must have a UCS value of less than 2 MPa with a thickness of more than 3.5 m simultaneously. Although bearing capacity formulae are reported to have some limitations,14,39 based on this bearing capacity analysis, it is less likely that significant floor heave occurs in a floor with such strength. Swelling was also presumed to be unlikely to be the mechanism of
Fig. 1. Mode of floor failure with uppermost unit being (a) siltstone, and (b) concrete. 2
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International Journal of Rock Mechanics and Mining Sciences 128 (2020) 104241
floor heave at the mines as the floor generally appeared to be almost dry in the floor heave areas. Above all, the floor failures seemed to be stressdriven phenomena since floor heave at the Australian coal mines was also generally associated with the retreat of the longwall, which induces higher stresses compared to the development stage.42–44 Furthermore, rapid floor heave occasionally occurred right after excavation. For example, a floor displacement of approximately 500 mm occurred during the development which resulted in the bolting rigs on the continuous miner stuck in the roadway.33 Considering that swelling is known to be a time-dependent behaviour,45,46 it is unlikely for swelling to be the cause of the floor heave events. The modes of floor failures observed in the mines (Fig. 1) correspond to buckling. The Euler buckling theory, which is commonly used to analyse the behaviour of the coal mine roof,47–50 was used for the floor analysis. The simplified form of the Euler buckling formula with an assumption of a clamped-ended column is as below 49: � σ cr ¼ π2 Et2 3b2 (3)
the uppermost floor units ranging from 58 MPa to 99 MPa, which must have a greater value of deformation modulus.8,52,53 Although the critical buckling stress calculated by the Euler buckling formula can be the upper bound value for the buckling to occur,54 it is still questionable whether strong rock units with a high deformation modulus would buckle at the given depth of cover of 350 m. It was therefore concluded that there are other contributing factors to the buckling failures of the floor. 2.3. Hypothesis about mechanism It appears that the observed floor heave in the mines could fall into a simplified lithology configuration, a strong uppermost floor unit over lying weak floor strata, as shown in Fig. 3. The underlying floor units of the floor heave areas include coal, coarse-grained sandstone with UCS typically less than 10 MPa, and laminated siltstone with variable UCS. Although the laminated siltstone has a higher range of UCS values than the typical weak rock units, the strength of the rock can be significantly lower in the horizontal direction due to strength anisotropy.55–58 A hypothesis was put forward that the buckling failure of the up permost strong floor unit is associated with the failure of underlying weak floor strata as shown in Fig. 3. A numerical study by Mo et al.59 showed that the failure of the underlying weak floor units induced the deformation and subsequent failure of the strong uppermost unit, and the floor deformation was minimal if the strong uppermost unit was thick enough to confine the failure of the underlying units. This is similar to the failure mechanism that explains the deformation of rock mass around excavations in hard rock mining.60 In other words, the floor heave cases could be stress-driven phenomena, and a high degree of deformation was caused by rock mass bulking due to the failure of the floor materials. The fact that floor heave kept reoccurring many times after the lifted floor was brushed is in line with this hypothesis. The coal mines also experienced floor heave where the uppermost floor unit was coal, as seen in Fig. 4. This indicates that the uppermost unit does not need to be strong for floor heave to occur. The main driving force is assumed to be horizontal stress. This rec ognises that significant floor heave was mostly observed where the depth of cover was greater than 350 m; the depth of cover is correlated with the magnitude of horizontal stress.51 The angle between the di rection of major horizontal stress and the roadways was found to be critical in the occurrence of floor heave,33 which also indicates the role of horizontal stress in the floor heave cases.61
where σ cr is the critical buckling stress; E is the deformation modulus of the column; t is the thickness of the column; and b is the length of the column. Fig. 2 describes the critical buckling stress using Eq. (3) depending on the deformation modulus and thickness of the column, which is the floor unit in this case. A roadway width of 5.5 m was used for the length of the column. As the applied load to the floor unit is assumed to be the in situ horizontal stress, Eq. (4) was used to estimate the magnitude of the major horizontal stress 51:
σ H ðMPaÞ ¼
4:4 þ 0:04 � H ðmÞ þ 0:56 � E ðGPaÞ
(4)
where σH is the magnitude of major horizontal stress; H is the depth of cover; and E is the deformation modulus of the unit. It is of note that Eq. (4) was developed using only measurements from coal mines in New South Wales, Australia. Similarly, a cover depth of 350 m was assumed for this analysis. The estimated horizontal stress magnitudes are also included in Fig. 2. According to Fig. 2, a high potential for the unit to buckle at a cover depth of 350 m exists only if the unit is 0.2 m thick and has a defor mation modulus of less than approximately 2.5 GPa simultaneously. Furthermore, the calculated major horizontal stress magnitudes assume a stress orientation normal to roadways. Since the orientation of the major horizontal stress is not always normal to roadways, the actual magnitude of horizontal stress, at a cover depth of 350 m, could be less than the values plotted in Fig. 2. Even in the case of the 0.3-m-thick unit, it is unlikely that the unit fails by buckling with any value of deforma tion modulus greater than about 1 GPa. This contradicts previous studies that have reported buckling-type floor failures in floors with the UCS of
2.4. Parameter development, data collection and analysis Based on the hypothesis, the quality of the floor was considered to be one of the major parameters in the occurrence of floor heave. Coal mine strata generally consist of multiple units; thus, a rock mass classification appeared to be appropriate for developing simple and practical design methods. Thus, the CMRR was slightly modified to develop a new rock classification system for the coal mine floor, the CMFR, which would be easily understandable in the mining industry. Mark18 stated that the outcomes of an empirical approach need to be clearly defined. For the prediction of floor heave, “success” and “failure” were chosen as the outcome parameters. The typical mining heights of the underground coal mines range from 2.9 m to 3.5 m. From an oper ational point of view, a floor displacement of greater than 300 mm, which is about 10% of the mining heights, was defined as failure. The failure cases can include both rapid floor heave and gradual floor heave that is noticed within typically three months after excavation. Minor floor heave with a displacement of less than 300 mm or no floor heave was defined as success. Of note is that the deformation of the coal mine floor is mostly observed by visual inspection and thus the reported displacement values are typically not very accurate. Therefore, the determination of the outcome can be subjective to some extent. Considering this, floor heave cases with a displacement ambiguously
Fig. 2. Critical buckling stress with major horizontal stress at a cover depth of 350 m calculated using Eqs. (3) and (4). 3
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Fig. 3. Postulated failure mechanism.
Fig. 4. Failure of coal floor. (a) crack development at the centre of roadway, and (b) fractured floor with a high degree of displacement.
recorded, such as “between 200 mm to 400 mm”, were excluded to include only the significant cases of floor heave. Also, this empirical method is only concerned about significant floor heave on development, which is more detrimental to the mining operations than floor heave on longwall retreat. A database was established from five underground coal mines in Australia. Since underground floor exposures were rarely available, data on drill core adjacent to the locations of floor heave on development were collected to calculate the CMFR. As one of the mines showed a considerable variation of floor lithology, the exploration borehole data were not used, and the data from additional floor coring conducted in the gateroads were used. Floor data of the success cases were also collected. Data on the depth of cover and the orientation of major hor izontal stress for both the success and failure cases were gathered. As the outcome parameters were not continuous but binary, logistic regression was conducted to calculate the likelihood of the binary outcome. The details of logistic regression analysis can be found in McQuillan et al.30 and Peng et al.62 The parameters for the statistical analysis included the CMFR, depth of cover and direction of major horizontal stress. Through many iterations of the regression analysis, the ratings of the CMFR were refined. Furthermore, the iteration process revealed that a combination of the depth of cover and the angle between major horizontal stress and roadways yielded better results than using the depth of cover and the horizontal stress angle separately. As a result, a new parameter named the Horizontal Stress Rating (HSR) was created by combining the two factors.
characteristics are similar. Second, a thickness-weighted average of all the Unit Ratings is calculated. Whereas the CMRR considers the units within the bolted horizon, this does not apply to the CMFR as bolts are rarely installed in the floor. The CMFR takes into account the floor units from the floor surface to 3 m below the surface for the following reasons: Kaiser60 mentioned if the depth of failure was 1 m deep, it would result in rock displacement of 0.3 m–0.4 m. Considering the magnitude of significant floor heave was greater than 0.6 m, and in some cases even more than 1 m, this would imply the depth of floor failure could be probably greater than 3 m below the floor surface. Sheffield and Corbett34 reported the results of in situ floor monitoring at an Australian coal mine and indi cated that the greatest movement of the floor was detected below 3 m into the floor. While this may suggest the need to look deep into the floor, the numerical study by Mo et al.59 indicated that weak floor units below 3 m were less likely to fail if the upper units above the weak units were strong enough. Also, it appears to be more practical to quantify only within a 3 m floor horizon rather than a greater depth into the floor. Finally, a Strong Unit Adjustment (SUA) is applied to the thicknessweighted average to compute the final value of the CMFR. Fig. 5 shows a flowchart of the procedure for the calculation of the CMFR. Technically, the CMFR ratings can range up to 100, which is the same as the CMRR. The following sections present the Unit Ratings, thickness-weighted average and SUA in detail. 3.1. Uniaxial compressive strength (UCS)
3. Calculation of the CMFR
The UCS of the rock material in the roof can be an indicator on whether new fracturing can take place.24 It is considered that this also applies to the floor; hence, the UCS values of floor units compose the Unit Ratings. The UCS is also the main factor used in estimating the rock deformation around roadways in hard rock mines.36,38 The UCS rating accounts for approximately one-third, and the shear strength together with the intensity of discontinuities account for the rest of the total CMRR. Riefenberg32 claimed that the material strength
The CMFR is calculated in several steps like the CMRR. First, the coal mine floor is divided into geotechnical units, and a Unit Rating is computed for each unit. A geotechnical unit is generally at least 150 mm thick as suggested by Molinda and Mark63 unless thinner units are known to affect the floor deformation in specific mine sites. Several lithological units can be taken as one geotechnical unit if their 4
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International Journal of Rock Mechanics and Mining Sciences 128 (2020) 104241
Fig. 5. Flowchart for the CMFR.
appeared to be the most critical factor in the deformation of the floor and, thus, a greater weighting to the UCS was suggested for a rating system for coal mine floor compared to the CMRR. Considering this, the UCS rating of the CMFR constitutes 65% of its total rating with the calculation of the rating using Table 1. It is known that the lower the material strength, the higher the likelihood of failure. To reflect this, the CMFR UCS rating is not linear as lower weightings are given to the lower values of UCS. Fig. 6 shows the relationship between the UCS and the CMFR UCS rating. As it has become a common practice to estimate the UCS values of coal mine strata using sonic velocity data from exploration boreholes,64,65 the UCS derived from sonic logs is recommended. 3.2. Discontinuity spacing Fig. 6. Chart for CMFR UCS rating.
Discontinuity refers to all fractures or features that have zero or very low tensile strengths such as bedding planes, joints, faults and shears.66 The most significant discontinuity in coal measures that affects the behaviour of the coal mine roof is bedding planes, as weak bedding can lead to roof instabilities from delamination or roof spalling by high horizontal stress or gravity.24,67 Bedding planes also appear to play a significant role in floor failures due to strength anisotropy. Molinda and Mark55 reported the results of the point load tests on many coal measure rocks, suggesting that highly bedded rocks were susceptible to horizontal stress with a high degree of anisotropy. The laboratory tests conducted by Whittles et al.58 also showed the anisotropic behaviour of the laminated siltstones that the strength of the rocks significantly decreased as the direction of the bedding planes was sub-parallel or parallel to the axis of loading. Since horizontal stress is attributed to floor heave events, the spacing of dis continuities in floor units composes the Unit Ratings to consider strength anisotropy. In this context, vertical or sub-vertical discontinuities are not regarded as a discontinuity in the CMFR. Discontinuity spacing ac counts for 35% of the total CMFR with the calculation of the rating using Table 2. The CMRR considers the shear strength of discontinuities as well as the spacing and intensity of them. However, the CMFR only considers
Table 2 CMFR rating scale for discontinuity spacing (S).
UCS (MPa)
Rating 10 2 � UCS – 10 UCS þ 10 0.3 � UCS þ 31 0.125 � UCS þ 45 65
<20
20 � S < 60
60 � S < 200
200 � S < 600
�600
Rating
0
5
15
25
35
the spacing of discontinuities. While the shear strength of discontinuities is an important factor, strength testing, such as the diametral point load test, on floor strata is not a common practice. Thus, the inclusion of the shear strength of discontinuities in floor units appears to be impractical. Furthermore, the spacing of discontinuities itself is a relatively reliable indicator based on several examples. The numerical studies on stratified rock mass by Wang et al.68 indicated that the smaller the spacing of joints was, the relatively lower the shear strength was. Zhou et al.69 mentioned less failure is expected in underground caverns where the spacing of bedding planes is greater. The design guidelines on under ground excavations by Vakili et al.57 and the Hard Rock Squeezing Index by Mercier-Langevin and Hadjigeorgiou38 also do not include other properties of discontinuities but only consider the spacing of disconti nuities along with the material strength. One feature on discontinuity spacing in the CMFR compared to the other rock mass classification systems is that a very low weighting is given to a unit that is laminated throughout.
Table 1 CMFR rating scale for UCS.
<10 10 to 20 20 to 30 30 to 80 80 to 160 >160
S (mm)
3.3. Calculation of the Unit Rating and the thickness-weighted average rating The Unit Rating is the sum of the UCS rating and discontinuity spacing rating. If the UCS of the floor units is less than 10 MPa, a Unit 5
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Rating of 40 is assigned regardless of the discontinuity spacing of the units. This assumes that the competency of the units with the UCS of less than 10 MPa is not governed by discontinuities as the rock matrix is significantly weak. Also, a minimum Unit Rating of 25 is applied to all cases. After the Unit Rating is determined, the thickness-weighted average of the Unit Ratings of all the floor units within the 3 m floor horizon is calculated. An example of the calculation is provided in Table 3. Note that although the thickness of Unit 3 is 1.2 m, only the upper 0.6 m was included in the calculation, as the CMFR considers the floor strata within the 3 m floor horizon.
Table 4 Angle rating for HSR. 20� �θ<30�
θ>30�
Angle Rating
0
1
3
5
Z¼
0:13ðHSRÞ þ 0:25ðCMFRÞ
According to Eq. (6), p ranges from 0 to 1. In this study, failure is taken as 0 while success is taken as 1. Therefore, the more positive the value of Z, the greater the likelihood of an outcome as success with p close to 1. The more negative the value of Z, the greater the likelihood of an outcome as failure with p close to 0. If Z equals zero, p is 50%. By substituting zero for Z and re-arranging Eq. (5) in terms of the CMFR, Eq. (7) is obtained: HSR ¼ 1:91ðCMFRÞ
(7)
43:47
Fig. 7 shows a chart named Floor Heave Index that consists of the Australian database including the 28 failure and 30 success cases. Sta tistically, the zone above the line of Eq. (7) is classified as failure (i.e., significant floor heave), while the zone below the line of Eq. (7) is classified as success (i.e., minor floor heave or no floor heave). Eq. (7) successfully classified 44 cases out of the total 58 cases. Table 5 Australian database for floor heave on development. Failure
4.1. Horizontal Stress Rating (HSR)
No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28
Other than the quality of the coal mine floor, the horizontal stress also appeared to be contributing to the occurrence of floor heave; thus, the HSR was created to represent the magnitude of horizontal stress. Since in situ stress measurements are not always available, it is more practical to use the HSR as a surrogate. The HSR is the sum of the Depth Rating and the Angle Rating. The Depth Rating is simply calculated by taking the depth of cover divided by 10. The Angle Rating is determined using Table 4. For example, a cover depth of 350 m with a major horizontal stress angle of 25� to the roadway gives an HSR of 38, which is the sum of 35 and 3.
Table 3 Example of calculation of the thickness-weighted average in CMFR. Discontinuity spacing rating
1.0 40 5 1.4 30 15 1.2 15 35 ½ð1:0 � 45Þ þ ð1:4 � 45Þ þ ð0:6 � 50Þ� ¼ 46 3
(5)
5:62
where Z is the regression discriminative equation. The likelihood of an outcome (p) can be simplified to Eq. (6)30, 62: � � p¼1 1þe Z (6)
4. Assessment of the potential of floor heave
Unit 1 (Uppermost) Unit 2 Unit 3 Thicknessweighted average
10� �θ<20�
The Australian database consists of 28 failure cases and 30 success cases from four underground coal mines in New South Wales and one mine in Queensland. The CMFR and HSR were calculated for each case. Table 5 lists the database with the CMFR and HSR values. Then logistic regression was conducted to obtain Eq. (5):
In the CMRR, the impact of the strongest unit within the bolted in terval is acknowledged, and the relevant adjustment is made. Similarly, the CMFR also recognises the effect of the strong immediate unit and thus an adjustment called the Strong Unit Adjustment (SUA) is made. An Australian longwall mine reported that moderate floor heave, equal to or less than a floor displacement of 300 mm, occurred where the thickness of the uppermost floor unit with UCS from 40 MPa to 65 MPa was greater than 0.6 m while significant floor heave was recorded where the thickness of the uppermost unit was less than 0.6 m.34 The thickness of the uppermost floor units associated with the buckling-type of floor failure at a US coal mine and two Australian coal mines was 0.3 m with the UCS of 99 MPa, 40 MPa and 58 MPa, respectively.33,53,70 A nu merical simulation suggested that the effect of the uppermost floor unit on confining the failure of underlying floor strata depends on the thickness of the uppermost unit.59 In the CMFR, the SUA is determined by first identifying a floor unit of the highest Unit Rating with a minimum thickness of 0.7 m within the first 1 m floor interval. Second, the Strong Unit Difference (SUD) is calculated by subtracting the Unit Rating of the selected strongest unit within the 1 m floor horizon from the thickness-weighted average of the Unit Ratings within the 3 m floor horizon. Finally, if the SUD is greater than 20, five points are added to the thickness-weighted average rating.
UCS rating
θ <10�
4.2. Floor Heave Index
3.4. Strong Unit Adjustment (SUA)
Thickness (m)
Angle of major horizontal stress to roadway, θ (� )
Unit Rating 45 45 50
6
Success Mine NSW 1
NSW 2 NSW 3
CMFR 38.9 40.7 41.1 51.6 41.5 47.9 43.6 57.9 36.8 36.8 36.8 36.8 36.8 36.8 36.8 36.8 36.8 36.8 37.5 37.5 37.5 43.2 43.2 43.2 43.2 43.2 43.2 43.2
HSR 41.5 41.5 42.0 45.0 43.5 44.5 43.0 58.0 35.0 35.5 36.0 36.5 37.0 38.0 41.5 42.5 43.0 44.5 50.0 53.0 55.0 36.0 37.0 43.0 38.5 39.5 47.0 50.0
No. 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58
Mine NSW 1
NSW 2 NSW 3
NSW 4
QLD 1
CMFR 45.3 52.7 44.6 41.5 36.6 47.2 40.0 46.2 48.2 60.6 46.2 46.2 42.9 42.9 42.9 41.9 55.9 51.0 54.4 60.2 57.5 51.6 62.1 72.1 55.8 54.9 61.7 64.7 61.6 37.9
HSR 43.0 45.0 47.5 47.0 45.5 46.0 44.0 47.0 46.5 61.0 34.0 35.0 30.0 35.0 39.5 23.5 27.5 25.5 28.0 43.0 38.0 43.0 45.0 42.0 34.0 46.0 43.0 42.0 29.0 31.0
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5.2. Other contributing factors to floor heave Apart from the quality of the floor, other parameters seemed to contribute to the occurrence of floor heave. For example, the locations of geological structures such as fault around the floor heave areas were reported to be correlated with the occurrence of significant floor heave.34 The impact of fault on floor heave was previously reported from mines in the UK and Poland.72,73 Geological structures within the floor strata were also found to contribute to floor failures at coal mines in South Africa.74 According to Latilla,74 the dolerite intrusions caused vertical slickensided joints in the laminated floor and, subsequently, the floor failure was manifested by opening the vertical joints in the floor. Also, dykes were reported to weaken the soft and sandy shale floor. However, the geological structures are not included in the proposed Floor Heave Index because it is difficult to detect large-scale features in the floor from drill core data. Moreover, two of the studied Australian mines did not have any fault or other geological structures in close proximity to the floor heave areas, and the locations of fault were generally correlated with floor heave on the retreat of longwall in some of the Australian mines, i.e. the Floor Heave Index was proposed to cater only for floor heave on development. Therefore, it is suggested that the presence of geological structures around the locations of floor heave be site-specific considerations. Water or moisture is known to reduce the strength of rock 75; in particular, the strength of claystone or underclay significantly decreases as the moisture content increases.13,76 Although the current database does not have relevant cases, the effect of water or moisture on the strength of floor units needs to be considered. Thus, if the floor contains the moisture-sensitive rock such as claystone and is wet (saturated with water) due to water from machines or the roof, an adjustment to the Unit Rating is suggested at the discretion of each site. Many researchers have discussed the influence of pillar dimension, and rib and roof support on floor heave. Various pillar designs associated with the use of yield pillar were suggested to control floor heave.77–80 Meanwhile, a field study at a Chinese underground coal mine showed that an increase in pillar width from 19.5 m to 56.0 m led to a significant decrease in the floor deformation.81 The rib and roof support with higher strengths minimised floor heave compared to the other area where the lower strength rib and roof support was used in another Chinese coal mine.82 While those factors appear to be critical in the occurrence of floor heave, the Australian database showed little varia tion in the pillar dimension in terms of width-to-height ratio and the ground support design. Further studies will need to be undertaken to incorporate the impact of pillar size and ground support in the Floor Heave Index.
Fig. 7. Floor heave index.
While 14 cases are misclassified, several misclassified cases are located close to the line of Eq. (7). The misclassified cases are deemed insignificant if they are close to Eq. (7). Thus, an intermediate zone has been incorporated as seen in Fig. 7. The intermediate zone was created with p ranging from 40% to 60%. Substituting 40% and 60% for p in Eq. (6) results in the Z values of 0.405 and 0.405 respectively. By substituting 0.405 and 0.405 for Z and re-arranging Eq. (5) in terms of the CMFR, Eqs. (8) and (9) are obtained: HSR ¼ 1:91ðCMFRÞ
40:34
(8)
HSR ¼ 1:91ðCMFRÞ
46:61
(9)
where Eqs. (8) and (9) represent a p value of 60% and 40% respectively. Finally, the cut-off line around the HSR of 23 was made as it is the lowest value of HSR from the database. Using the Floor Heave Index along with the CMFR and HSR, the potential of floor heave on development in roadways can be assessed at the planning stage for new mining projects or future workings. 5. Discussion 5.1. Guidelines on Coal Mine Floor Rating and Floor Heave Index Any empirical methods should be carefully used with a particular concern about the range of the underlying database.19 The database in this study includes CMFR ranging from 36.8 to 72.1, and HSR ranging from 23.5 to 61.0. Care needs to be exercised in extrapolating the empirical relationship outside the range of the original database. In addition, it is again emphasised that the floor heave cases in the data base are associated with the buckling mechanism, and thus the Floor Heave Index is not applicable to floor heave caused by bearing capacity failure or swelling mechanisms. As the CMFR is determined using exploration boreholes which are typically spaced several hundred metres apart, there is a possibility that the floor lithology of some of the roadways is significantly different from that of the boreholes. Even if the floor lithology is assumed to be the same, the UCS and discontinuity spacing of the floor units can vary as uncertainty exists.71 Considering the uncertainty, the Floor Heave Index is not expected to predict the occurrence of significant floor heave at any given location. Instead, this method suggests the potential for floor heave in roadways within the radius of the location of boreholes.
6. Conclusions A new rock mass classification system for the coal mine floor, the Coal Mine Floor Rating (CMFR), has been developed. The newly developed floor rating system quantifies the features of multiple floor units into a single number with the UCS and discontinuity spacing as the main components. As the horizontal stress is the main parameter for the buckling-type floor heave as well as the quality of the floor units, the Horizontal Stress Rating (HSR), a new parameter that represents the magnitude of horizontal stress, has been developed. The CMFR has been incorporated in a simple empirical tool, the Floor Heave Index, created by statistical analysis on an Australian database of both floor heave and non-floor heave cases. The floor heave and non-floor heave cases are clearly classified by the CMFR and the HSR. The new empirical method helps assess the potential of floor heave on development in underground coal mine roadways for new mining projects or future mining areas. Further data collection is suggested to extend the range of the data. Also, future work is recommended to incorporate floor instability cases associated with bearing capacity failure and swelling to produce a comprehensive CMFR for the coal mining industry. 7
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Declaration of competing interest
24 Mark C, Molinda GM. The coal mine roof rating (CMRR) - a decade of experience. Int J Coal Geol. 2005;64(1-2):85–103. 25 Molinda GM, Mark C. The Coal Mine Roof Rating (CMRR) - a practical rock mass classification for coal mines. In: Peng SS, ed. 12th Conference On Ground Control In Mining. Morgantown. West Virginia: West Virginia University; 1993:92–103. 26 Colwell M, Frith R, Mark C. Analysis of longwall tailgate serviceability (ALTS): a chain pillar design methodology for Australian conditions. In: Mark C, Heasley KA, Iannacchione AT, Robert JT, eds. Second International Workshop on Coal Pillar Mechanics and Design. Vail, Colorado: National Institute for Occupational Safety and Health; 1999:33–48. 27 Mark C, Chase FE. Gate entry design for longwalls using the coal mine roof rating. In: Peng SS, ed. 12th Conference On Ground Control In Mining. Morgantown. West Virginia University; 1993:76–83. 28 Mark C, Molinda GM, Dolinar DR. Analysis of roof bolt systems. 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Practical strength criterion for coal mine roof support design in laminated soft rocks. Trans Inst Min Metall A – Min Ind. 2013;122(4):243–249. 44 Zipf RK. Numerical Modeling Procedures for Practical Coal Mine Design. 41st US Symposium On Rock Mechanics Symposium On Rock Mechanics. Golden, Colorado: American Rock Mechanics Association; 2006. 45 Einstein HH. Tunnelling in difficult ground - swelling behaviour and identification of swelling rocks. Rock Mech Rock Eng. 1996;29(3):113–124. 46 Parsapour D, Fahimifar A. Prediction of swelling rocks strain in tunneling. Geotectonics. 2016;50(3):336–346. 47 Colwell MG, Frith R. Why uniaxial compressive strength and Young’s modulus are commonly poor indicators of roadway roof stability - except in the tailgate. In: Aziz N, Keilich W, eds. Coal Operators’ Conference. Wollongong. New South Wales: University of Wollongong & the Australasian Institute of Mining and Metallurgy; 2006:28–43. 48 Colwell MG, Frith R. Analysis and Design of Faceroad Roof Support (ADFRS) - a Roof Support Design Methodology for Longwall Installation Roadways. ACARP Project C19008 Final Report. Brisbane, Queensland: Australian Coal Association Research Program; 2012. 49 Esterhuizen GS, Bajpayee TS. Horizontal Stress Related Failure in Bedded Mine Roofs — Insight from Field Observations and Numerical Models. 46th US Rock Mechanics/ Geomechanics Symposium. Chicago: American Rock Mechanics Association; 2012. 50 Seedsman RW, Gordon N, Aziz N. Analytical Tools for Managing Rock Fall Hazards in Australian Coal Mine Roadways. Australian Coal Association Research Program; 2009. 51 Mark C, Gadde MM. Global trends in coal mine horizontal stress measurements. In: Peng SS, Mark C, Finfinger GL, et al., eds. 27th International Conference On Ground Control In Mining. Morgantown. West Virginia: West Virginia University; 2008: 319–331. 52 Aggson JR, Curran J. 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The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The authors wish to acknowledge the support of the Australian Coal Industry’s Research Program (ACARP) project C26064. The Australian coal producers and their mine sites are appreciated for accommodating site visits and providing data. The authors would also like to thank Christopher Mark at Mine Safety and Health Administration (MSHA) for reviewing this paper and suggesting improvements before publication. References 1 Peng SS. Coal Mine Ground Control. third ed. Morgantown, West Virginia: West Virginia University; 2008. 2 Faria Santos C, Bieniawski ZT. Floor design in underground coal mines. Rock Mech Rock Eng. 1989;22(4):249–271. 3 Mo S, Ramandi HL, Oh J, et al. A Review of Floor Heave Mechanisms in Underground Coal Mine Roadways. The Fourth Australasian Ground Control in Mining Conference. Sydney, New South Wales: The Australasian Institute of Mining and Metallurgy; 2018:196–206. 4 Gadde MM. Weak Floor Stability in the Ilinois Basin Underground Coal Mines. PhD Thesis. Morgantown. West Virginia: West Virginia University; 2009. 5 Kumar C. Effects of Weak Floor Interaction on Underground Room-And-Pillar Coal Mining. Mining Engineering. PhD Thesis. Southern Illinois University at Carbondale; 1990. 6 Park DW, Jiang YM, Morley LA, Keeton W. Stability analysis of a room-and-pillar mine with thinly laminated roof, strong pillar, and weak floor. Min Eng. 1992;44(11): 1355–1360. 7 Seedsman RW, Mallett CW. Claystones of the Newcastle Coal Measures. NERDDC Project C0902. Canberra: National Energy Research Development and Demonstration Council; 1988. end of grant report no. 749. 8 Wang Y. Mechanisms and Control of Mine Floor Heave. Mining Engineering. PhD Thesis. Morgantown: West Virginia University; 1996. 9 Vasundhara. Geomechanical Behaviour of Soft Floor Strata in Underground Coal Mines. PhD Thesis. Sydney: University of New South Wales; 1999. 10 Chugh YP, Pytel WM. Design of partial extraction coal mine layouts for weak floor strata conditions. In: Iannacchione AT, Mark C, Repsher RC, Tuchman RJ, Jones CC, eds. Workshop on Coal Pillar Mechanics and Design. Santa Fe, New Mexico: U.S. Bureau of Mines; 1992:32–49. 11 Galvin JM. The UNSW Pillar Design Methodology and Considerations for Using This and Other Empirical Pillar System Design Approaches. 3rd International Workshop on Coal Pillar Mechanics and Design. Morgantown, West Virginia: National Institute for Occupational Safety and Health; 2010. 12 Seedsman RW. The strength of the pillar-floor system. In: Aziz N, Kininmonth B, Nemcik J, Ren TX, eds. 12th Coal Operators’ Conference. Wollongong. New South Wales: University of Wollongong and Australasian Institute of Mining and Metallurgy; 2012:23–30. 13 Speck RC. A Comparative Evaluation of Geologic Factors Influencing Floor Stability in Two Illinois Coal Mines. PhD thesis. University of Missouri-Rolla; 1979. 14 Gadde MM. Numerical model evaluation of floor-bearing capacity in coal mines. In: Esterhuizen GS, Mark C, Klemetti TM, Tuchman RJ, eds. International Workshop On Numerical Modeling For Underground Mine Excavation Design. Asheville. North Carolina: National Institute for Occupational Safety and Health; 2009:47–54. 15 Gadde MM, Peng SS. Weak floor stability during perimeter mining in the Illinois Basin coal mines. In: Peng SS, ed. 28th International Conference On Ground Control In Mining. Morgantown. West Virginia: West Virginia University; 2009:223–232. 16 Jessu KV, Kostecki TR, Spearing AJS, Zhang J. Influence of varying bedding thickness of underclay on floor stability. Int J Min Sci Technol. 2017;27(3):517–523. 17 Kostecki T, Spearing AJS. Influence of backfill on coal pillar strength and floor bearing capacity in weak floor conditions in the Illinois Basin. Int J Rock Mech Min Sci. 2015;76:55–67. 18 Mark C. Science of empirical design in mining ground control. Int J Min Sci Technol. 2016;26(3):461–470. 19 Suorineni FT. Reflections on Empirical Methods in Geomechanics – the Unmentionables and Hidden Risks. AusRock 2014: 3rd Australasian Ground Control in Mining Conference. Sydney, New South Wales: Australasian Institute of Mining and Metallurgy; 2014: 143–156. 20 Bieniawski ZT. Engineering classification of jointed rock masses. Trans S Afr Inst Civ Eng. 1973;15(12):335–344. 21 Barton N, Lien R, Lunde J. Engineering classification of rock masses for the design of tunnel support. Rock Mech. 1974;6(4):189–236. € Ulusay R, Tokashiki N. A new rock mass quality rating system: rock mass 22 Aydan O, quality rating (RMQR) and its application to the estimation of geomechanical characteristics of rock masses. Rock Mech Rock Eng. 2014;47(4):1255–1276. 23 Laubscher DH. A geomechanics classification system for the rating of rock mass in mine design. J S Afr Inst Min Metall. 1990;90(10):257–273.
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Contents lists available at ScienceDirect
International Journal of Rock Mechanics and Mining Sciences journal homepage: http://www.elsevier.com/locate/ijrmms
A new coal mine floor rating system and its application to assess the potential of floor heave Sungsoon Mo a, Hamed Lamei Ramandi a, Joung Oh a, *, Hossein Masoumi b, Ismet Canbulat a, Bruce Hebblewhite a, Serkan Saydam a a b
School of Minerals and Energy Resources Engineering, UNSW, Sydney, NSW, 2052, Australia Department of Civil Engineering, Monash University, Melbourne, VIC, 3800, Australia
A R T I C L E I N F O
A B S T R A C T
Keywords: Rock mass classification Coal mine floor rating Floor heave Buckling
Several underground coal mines have reported floor heave associated with the buckling mechanism. However, studies on the coal mine floor, especially the buckling failure mechanism in the floor and relevant analysis techniques, are still insufficient. This paper introduces a new floor classification system, the Coal Mine Floor Rating, based on the underlying failure mechanisms. The new floor mass rating system quantifies the stability of floor strata by considering two main factors: uniaxial compressive strength and discontinuity spacing of the floor units. The Coal Mine Floor Rating is incorporated into an empirical method, the Floor Heave Index, developed by statistical analysis of a database obtained from actual floor failure cases. The floor heave and non-floor heave cases are determined depending on the Coal Mine Floor Rating and the Horizontal Stress Rating, a proxy that represents the magnitude of horizontal stress. While obtaining underground floor data is challenging, the new floor classification system and the empirical method help assess the potential of significant floor heave on development in underground coal mine roadways for new mining projects or future workings.
1. Introduction Excessive deformation of roadways is occasionally encountered in underground coal mines. The excessive floor deformation is known as floor heave in the mining industry. Floor heave is also described as either “breaking or lifting up of immediate floor strata” or “extrusion of floor strata”.1 Managing floor heave in a timely manner is crucial as it can disturb mining operations and thus often cause significant financial losses. Previous studies have indicated three main mechanisms of floor heave in underground coal mine roadways: bearing capacity failure, swelling and buckling.2,3 Many researchers have studied bearing ca pacity failure as the mechanism involves a range of ground control issues including pillar punching, tensile failure of pillars with rib spalling, roof fall and subsidence as well as floor heave.4–9 For bearing capacity failure of the coal mine floor, various bearing capacity formulae modified from bearing capacity theories in civil engineering have been used.4,10–13 Numerical modelling is also used to compute the bearing capacity of the floor by calibrating the models against bearing capacity formulae.14–17
However, compared to the bearing capacity failure mechanism, studies on the swelling and buckling mechanisms and relevant analysis tech niques are still limited. Over decades, rock mechanics problems have been addressed by using rock mass classification systems.18,19 Notable examples of rock mass classification are the Rock Mass Rating (RMR)20 and the Q-sys tem21 developed for the tunnelling industry. A new classification sys tem, the Rock Mass Quality Rating (RMQR), has also been introduced to estimate the mechanical parameters of rock masses.22 In the hard rock mining sector, the modified version of the RMR, the Mining Rock Mass Rating (MRMR), has been mainly used for cave mining applications.23 In coal mining, the Coal Mine Roof Rating (CMRR)24,25 has been exten sively used in numerous empirical methods, such as evaluating the serviceability of gateroads26,27 and ground support requirements.28 The Geophysics Strata Rating (GSR) was developed to quantify coal mine strata for geotechnical applications.29 A risk-based classification system for the excavated coal mine slopes, the Slope Stability Assessment Methodology (SSAM), has been proposed recently.30 Since less attention has been given to the coal mine floor among
* Corresponding author. E-mail addresses: [email protected] (S. Mo), [email protected] (H.L. Ramandi), [email protected] (J. Oh), hossein. [email protected] (H. Masoumi), [email protected] (I. Canbulat), [email protected] (B. Hebblewhite), [email protected] (S. Saydam). https://doi.org/10.1016/j.ijrmms.2020.104241 Received 2 November 2019; Received in revised form 31 January 2020; Accepted 5 February 2020 Available online 12 February 2020 1365-1609/© 2020 Elsevier Ltd. All rights reserved.
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other rock mechanics topics, a rock mass classification system for floor strata is scarce in the literature. A rare example is a roof and floor classification system developed by Buddery and Oldroyd31 for coal mines in South Africa. The floor classification system adopted uncon fined swelling strain and slake durability tests to assess the potential of swelling and degrading due to water. Riefenberg32 attempted to apply the CMRR to the floor of an underground coal mine and indicated that some modifications of the CMRR would work for rating the coal mine floor. Several underground coal mines have reported floor heave associ ated with the buckling mechanism.33–35 As the heaved floor is generally brushed to maintain clearance, predicting the occurrence of floor heave is beneficial in managing floor heave proactively. In the hard rock mining and civil engineering sectors, practical guidelines regarding anticipated levels of deformation of rock around the excavations have been established.36–38 However, it is still challenging to predict the magnitude, location and timing of floor heave with the current state of knowledge, and no study has been found to provide such guidelines for underground coal mining applications. This paper proposes a new rock mass classification specifically developed for the coal mine floor, the Coal Mine Floor Rating (CMFR). An empirical tool using the CMFR, the Floor Heave Index, is also pro posed to assess the potential of floor heave on development in coal mine roadways. The significance of the new rock mass classification system and empirical approach is discussed, and the limitations of the method are also presented.
developed. 2.2. Identification of floor heave mechanism Bearing capacity failure typically involves soft or weak immediate floor units such as claystone and underclay.4,7,9 Although rock can weaken once it is exposed to water or moisture, most of the floor heave areas were not wet. The uppermost floor units of the mines were thus assumed to be relatively strong in terms of UCS. Moreover, whereas bearing capacity failures normally involve pillar punching, roof falls and subsidence, none of these were reported along with the floor heave events. It is therefore unlikely that the floor failures were associated with the bearing capacity failure mechanism. In addition, a simple analysis using bearing capacity formula was carried out to examine the likelihood of bearing capacity failure. Since many floor heave cases occurred on development, the average vertical stress value was estimated using the tributary area theory while the vertical load exerted on the pillar is typically less than the full tributary area load on development 39: � � (1) σ p ¼ 0:025HC1 C2 WL or 0:025HðW þ bÞðL þ bÞ WL ðMPaÞ where σp is the average vertical pillar stress; H is the depth of cover (m); C1 is the centre-to-centre distance of pillar width (m); C2 is the centre-tocentre distance of pillar length; b is the roadway width (m); W is the ribto-rib distance of pillar width (m); and L is the rib-to-rib distance of pillar length (m). Eq. (1) assumes a gravitational acceleration of 10 m/s2 and a density of the overburden of 2500 kg/m3. Taking a roadway width of 5.5 m and a chain pillar dimension with a centre-to-centre pillar width of 50 m and a centre-to-centre pillar length of 100 m, the average vertical pillar stress using Eq. (1) was calculated to be 10.4 MPa at a depth of cover of 350 m. A cover depth of 350 m was chosen because, in general, significant floor heave was noticed in several mines where the depth of cover is greater than 350 m. There are many bearing capacity formulae modified from bearing capacity theories in civil engineering.11 Eq. (2) proposed by Mandel and Salencon,40 which has been used for Australian coal mine cases,12,41 was used to calculate the bearing capacity of the floor:
2. Development process Mark18 detailed a typical development process for empirical methods and pointed out the important considerations such as identifying failure mode, establishing a clear hypothesis for a model, developing appro priate parameters and analysing data. The CMFR and Floor Heave Index were developed through a similar process. In this section, the important considerations through the process of creating the CMFR and Floor Heave Index are highlighted. 2.1. Identification of failure mode
qfloor ¼ ðUCS = 2Þ � ð4:14 þ W = 2tÞ
Fig. 1 exemplifies the typical modes of floor failures observed in the underground longwall mines in Australia. Tensile cracks at the floor surface were commonly seen in such a failure. The separation of floor units along the bedding planes was also observed frequently. The up permost floor units of the mines include tuff with the average uniaxial compressive strength (UCS) value of 40 MPa, siltstone with UCS ranging from 40 MPa to 65 MPa and concrete. While it is hard to conceive that the uppermost floor units are weak materials considering the UCS values, the failure of those units was commonly noticed during the development stage in many cases. Floor heave also took place on the longwall retreat. As the longwall face approached the roadways, the magnitude of floor heave frequently increased or new floor heave
(2)
where qfloor is the bearing capacity of the floor; W is the rib-to-rib dis tance of pillar width; and t is the thickness of the floor unit. Using Eq. (2), a bearing capacity of 10.5 MPa was calculated with a UCS of 2 MPa and a thickness of 3.5 m. Therefore, to have a lower value of bearing capacity than the average vertical stress of 10.4 MPa, calcu lated before, the floor must have a UCS value of less than 2 MPa with a thickness of more than 3.5 m simultaneously. Although bearing capacity formulae are reported to have some limitations,14,39 based on this bearing capacity analysis, it is less likely that significant floor heave occurs in a floor with such strength. Swelling was also presumed to be unlikely to be the mechanism of
Fig. 1. Mode of floor failure with uppermost unit being (a) siltstone, and (b) concrete. 2
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floor heave at the mines as the floor generally appeared to be almost dry in the floor heave areas. Above all, the floor failures seemed to be stressdriven phenomena since floor heave at the Australian coal mines was also generally associated with the retreat of the longwall, which induces higher stresses compared to the development stage.42–44 Furthermore, rapid floor heave occasionally occurred right after excavation. For example, a floor displacement of approximately 500 mm occurred during the development which resulted in the bolting rigs on the continuous miner stuck in the roadway.33 Considering that swelling is known to be a time-dependent behaviour,45,46 it is unlikely for swelling to be the cause of the floor heave events. The modes of floor failures observed in the mines (Fig. 1) correspond to buckling. The Euler buckling theory, which is commonly used to analyse the behaviour of the coal mine roof,47–50 was used for the floor analysis. The simplified form of the Euler buckling formula with an assumption of a clamped-ended column is as below 49: � σ cr ¼ π2 Et2 3b2 (3)
the uppermost floor units ranging from 58 MPa to 99 MPa, which must have a greater value of deformation modulus.8,52,53 Although the critical buckling stress calculated by the Euler buckling formula can be the upper bound value for the buckling to occur,54 it is still questionable whether strong rock units with a high deformation modulus would buckle at the given depth of cover of 350 m. It was therefore concluded that there are other contributing factors to the buckling failures of the floor. 2.3. Hypothesis about mechanism It appears that the observed floor heave in the mines could fall into a simplified lithology configuration, a strong uppermost floor unit over lying weak floor strata, as shown in Fig. 3. The underlying floor units of the floor heave areas include coal, coarse-grained sandstone with UCS typically less than 10 MPa, and laminated siltstone with variable UCS. Although the laminated siltstone has a higher range of UCS values than the typical weak rock units, the strength of the rock can be significantly lower in the horizontal direction due to strength anisotropy.55–58 A hypothesis was put forward that the buckling failure of the up permost strong floor unit is associated with the failure of underlying weak floor strata as shown in Fig. 3. A numerical study by Mo et al.59 showed that the failure of the underlying weak floor units induced the deformation and subsequent failure of the strong uppermost unit, and the floor deformation was minimal if the strong uppermost unit was thick enough to confine the failure of the underlying units. This is similar to the failure mechanism that explains the deformation of rock mass around excavations in hard rock mining.60 In other words, the floor heave cases could be stress-driven phenomena, and a high degree of deformation was caused by rock mass bulking due to the failure of the floor materials. The fact that floor heave kept reoccurring many times after the lifted floor was brushed is in line with this hypothesis. The coal mines also experienced floor heave where the uppermost floor unit was coal, as seen in Fig. 4. This indicates that the uppermost unit does not need to be strong for floor heave to occur. The main driving force is assumed to be horizontal stress. This rec ognises that significant floor heave was mostly observed where the depth of cover was greater than 350 m; the depth of cover is correlated with the magnitude of horizontal stress.51 The angle between the di rection of major horizontal stress and the roadways was found to be critical in the occurrence of floor heave,33 which also indicates the role of horizontal stress in the floor heave cases.61
where σ cr is the critical buckling stress; E is the deformation modulus of the column; t is the thickness of the column; and b is the length of the column. Fig. 2 describes the critical buckling stress using Eq. (3) depending on the deformation modulus and thickness of the column, which is the floor unit in this case. A roadway width of 5.5 m was used for the length of the column. As the applied load to the floor unit is assumed to be the in situ horizontal stress, Eq. (4) was used to estimate the magnitude of the major horizontal stress 51:
σ H ðMPaÞ ¼
4:4 þ 0:04 � H ðmÞ þ 0:56 � E ðGPaÞ
(4)
where σH is the magnitude of major horizontal stress; H is the depth of cover; and E is the deformation modulus of the unit. It is of note that Eq. (4) was developed using only measurements from coal mines in New South Wales, Australia. Similarly, a cover depth of 350 m was assumed for this analysis. The estimated horizontal stress magnitudes are also included in Fig. 2. According to Fig. 2, a high potential for the unit to buckle at a cover depth of 350 m exists only if the unit is 0.2 m thick and has a defor mation modulus of less than approximately 2.5 GPa simultaneously. Furthermore, the calculated major horizontal stress magnitudes assume a stress orientation normal to roadways. Since the orientation of the major horizontal stress is not always normal to roadways, the actual magnitude of horizontal stress, at a cover depth of 350 m, could be less than the values plotted in Fig. 2. Even in the case of the 0.3-m-thick unit, it is unlikely that the unit fails by buckling with any value of deforma tion modulus greater than about 1 GPa. This contradicts previous studies that have reported buckling-type floor failures in floors with the UCS of
2.4. Parameter development, data collection and analysis Based on the hypothesis, the quality of the floor was considered to be one of the major parameters in the occurrence of floor heave. Coal mine strata generally consist of multiple units; thus, a rock mass classification appeared to be appropriate for developing simple and practical design methods. Thus, the CMRR was slightly modified to develop a new rock classification system for the coal mine floor, the CMFR, which would be easily understandable in the mining industry. Mark18 stated that the outcomes of an empirical approach need to be clearly defined. For the prediction of floor heave, “success” and “failure” were chosen as the outcome parameters. The typical mining heights of the underground coal mines range from 2.9 m to 3.5 m. From an oper ational point of view, a floor displacement of greater than 300 mm, which is about 10% of the mining heights, was defined as failure. The failure cases can include both rapid floor heave and gradual floor heave that is noticed within typically three months after excavation. Minor floor heave with a displacement of less than 300 mm or no floor heave was defined as success. Of note is that the deformation of the coal mine floor is mostly observed by visual inspection and thus the reported displacement values are typically not very accurate. Therefore, the determination of the outcome can be subjective to some extent. Considering this, floor heave cases with a displacement ambiguously
Fig. 2. Critical buckling stress with major horizontal stress at a cover depth of 350 m calculated using Eqs. (3) and (4). 3
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Fig. 3. Postulated failure mechanism.
Fig. 4. Failure of coal floor. (a) crack development at the centre of roadway, and (b) fractured floor with a high degree of displacement.
recorded, such as “between 200 mm to 400 mm”, were excluded to include only the significant cases of floor heave. Also, this empirical method is only concerned about significant floor heave on development, which is more detrimental to the mining operations than floor heave on longwall retreat. A database was established from five underground coal mines in Australia. Since underground floor exposures were rarely available, data on drill core adjacent to the locations of floor heave on development were collected to calculate the CMFR. As one of the mines showed a considerable variation of floor lithology, the exploration borehole data were not used, and the data from additional floor coring conducted in the gateroads were used. Floor data of the success cases were also collected. Data on the depth of cover and the orientation of major hor izontal stress for both the success and failure cases were gathered. As the outcome parameters were not continuous but binary, logistic regression was conducted to calculate the likelihood of the binary outcome. The details of logistic regression analysis can be found in McQuillan et al.30 and Peng et al.62 The parameters for the statistical analysis included the CMFR, depth of cover and direction of major horizontal stress. Through many iterations of the regression analysis, the ratings of the CMFR were refined. Furthermore, the iteration process revealed that a combination of the depth of cover and the angle between major horizontal stress and roadways yielded better results than using the depth of cover and the horizontal stress angle separately. As a result, a new parameter named the Horizontal Stress Rating (HSR) was created by combining the two factors.
characteristics are similar. Second, a thickness-weighted average of all the Unit Ratings is calculated. Whereas the CMRR considers the units within the bolted horizon, this does not apply to the CMFR as bolts are rarely installed in the floor. The CMFR takes into account the floor units from the floor surface to 3 m below the surface for the following reasons: Kaiser60 mentioned if the depth of failure was 1 m deep, it would result in rock displacement of 0.3 m–0.4 m. Considering the magnitude of significant floor heave was greater than 0.6 m, and in some cases even more than 1 m, this would imply the depth of floor failure could be probably greater than 3 m below the floor surface. Sheffield and Corbett34 reported the results of in situ floor monitoring at an Australian coal mine and indi cated that the greatest movement of the floor was detected below 3 m into the floor. While this may suggest the need to look deep into the floor, the numerical study by Mo et al.59 indicated that weak floor units below 3 m were less likely to fail if the upper units above the weak units were strong enough. Also, it appears to be more practical to quantify only within a 3 m floor horizon rather than a greater depth into the floor. Finally, a Strong Unit Adjustment (SUA) is applied to the thicknessweighted average to compute the final value of the CMFR. Fig. 5 shows a flowchart of the procedure for the calculation of the CMFR. Technically, the CMFR ratings can range up to 100, which is the same as the CMRR. The following sections present the Unit Ratings, thickness-weighted average and SUA in detail. 3.1. Uniaxial compressive strength (UCS)
3. Calculation of the CMFR
The UCS of the rock material in the roof can be an indicator on whether new fracturing can take place.24 It is considered that this also applies to the floor; hence, the UCS values of floor units compose the Unit Ratings. The UCS is also the main factor used in estimating the rock deformation around roadways in hard rock mines.36,38 The UCS rating accounts for approximately one-third, and the shear strength together with the intensity of discontinuities account for the rest of the total CMRR. Riefenberg32 claimed that the material strength
The CMFR is calculated in several steps like the CMRR. First, the coal mine floor is divided into geotechnical units, and a Unit Rating is computed for each unit. A geotechnical unit is generally at least 150 mm thick as suggested by Molinda and Mark63 unless thinner units are known to affect the floor deformation in specific mine sites. Several lithological units can be taken as one geotechnical unit if their 4
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Fig. 5. Flowchart for the CMFR.
appeared to be the most critical factor in the deformation of the floor and, thus, a greater weighting to the UCS was suggested for a rating system for coal mine floor compared to the CMRR. Considering this, the UCS rating of the CMFR constitutes 65% of its total rating with the calculation of the rating using Table 1. It is known that the lower the material strength, the higher the likelihood of failure. To reflect this, the CMFR UCS rating is not linear as lower weightings are given to the lower values of UCS. Fig. 6 shows the relationship between the UCS and the CMFR UCS rating. As it has become a common practice to estimate the UCS values of coal mine strata using sonic velocity data from exploration boreholes,64,65 the UCS derived from sonic logs is recommended. 3.2. Discontinuity spacing Fig. 6. Chart for CMFR UCS rating.
Discontinuity refers to all fractures or features that have zero or very low tensile strengths such as bedding planes, joints, faults and shears.66 The most significant discontinuity in coal measures that affects the behaviour of the coal mine roof is bedding planes, as weak bedding can lead to roof instabilities from delamination or roof spalling by high horizontal stress or gravity.24,67 Bedding planes also appear to play a significant role in floor failures due to strength anisotropy. Molinda and Mark55 reported the results of the point load tests on many coal measure rocks, suggesting that highly bedded rocks were susceptible to horizontal stress with a high degree of anisotropy. The laboratory tests conducted by Whittles et al.58 also showed the anisotropic behaviour of the laminated siltstones that the strength of the rocks significantly decreased as the direction of the bedding planes was sub-parallel or parallel to the axis of loading. Since horizontal stress is attributed to floor heave events, the spacing of dis continuities in floor units composes the Unit Ratings to consider strength anisotropy. In this context, vertical or sub-vertical discontinuities are not regarded as a discontinuity in the CMFR. Discontinuity spacing ac counts for 35% of the total CMFR with the calculation of the rating using Table 2. The CMRR considers the shear strength of discontinuities as well as the spacing and intensity of them. However, the CMFR only considers
Table 2 CMFR rating scale for discontinuity spacing (S).
UCS (MPa)
Rating 10 2 � UCS – 10 UCS þ 10 0.3 � UCS þ 31 0.125 � UCS þ 45 65
<20
20 � S < 60
60 � S < 200
200 � S < 600
�600
Rating
0
5
15
25
35
the spacing of discontinuities. While the shear strength of discontinuities is an important factor, strength testing, such as the diametral point load test, on floor strata is not a common practice. Thus, the inclusion of the shear strength of discontinuities in floor units appears to be impractical. Furthermore, the spacing of discontinuities itself is a relatively reliable indicator based on several examples. The numerical studies on stratified rock mass by Wang et al.68 indicated that the smaller the spacing of joints was, the relatively lower the shear strength was. Zhou et al.69 mentioned less failure is expected in underground caverns where the spacing of bedding planes is greater. The design guidelines on under ground excavations by Vakili et al.57 and the Hard Rock Squeezing Index by Mercier-Langevin and Hadjigeorgiou38 also do not include other properties of discontinuities but only consider the spacing of disconti nuities along with the material strength. One feature on discontinuity spacing in the CMFR compared to the other rock mass classification systems is that a very low weighting is given to a unit that is laminated throughout.
Table 1 CMFR rating scale for UCS.
<10 10 to 20 20 to 30 30 to 80 80 to 160 >160
S (mm)
3.3. Calculation of the Unit Rating and the thickness-weighted average rating The Unit Rating is the sum of the UCS rating and discontinuity spacing rating. If the UCS of the floor units is less than 10 MPa, a Unit 5
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Rating of 40 is assigned regardless of the discontinuity spacing of the units. This assumes that the competency of the units with the UCS of less than 10 MPa is not governed by discontinuities as the rock matrix is significantly weak. Also, a minimum Unit Rating of 25 is applied to all cases. After the Unit Rating is determined, the thickness-weighted average of the Unit Ratings of all the floor units within the 3 m floor horizon is calculated. An example of the calculation is provided in Table 3. Note that although the thickness of Unit 3 is 1.2 m, only the upper 0.6 m was included in the calculation, as the CMFR considers the floor strata within the 3 m floor horizon.
Table 4 Angle rating for HSR. 20� �θ<30�
θ>30�
Angle Rating
0
1
3
5
Z¼
0:13ðHSRÞ þ 0:25ðCMFRÞ
According to Eq. (6), p ranges from 0 to 1. In this study, failure is taken as 0 while success is taken as 1. Therefore, the more positive the value of Z, the greater the likelihood of an outcome as success with p close to 1. The more negative the value of Z, the greater the likelihood of an outcome as failure with p close to 0. If Z equals zero, p is 50%. By substituting zero for Z and re-arranging Eq. (5) in terms of the CMFR, Eq. (7) is obtained: HSR ¼ 1:91ðCMFRÞ
(7)
43:47
Fig. 7 shows a chart named Floor Heave Index that consists of the Australian database including the 28 failure and 30 success cases. Sta tistically, the zone above the line of Eq. (7) is classified as failure (i.e., significant floor heave), while the zone below the line of Eq. (7) is classified as success (i.e., minor floor heave or no floor heave). Eq. (7) successfully classified 44 cases out of the total 58 cases. Table 5 Australian database for floor heave on development. Failure
4.1. Horizontal Stress Rating (HSR)
No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28
Other than the quality of the coal mine floor, the horizontal stress also appeared to be contributing to the occurrence of floor heave; thus, the HSR was created to represent the magnitude of horizontal stress. Since in situ stress measurements are not always available, it is more practical to use the HSR as a surrogate. The HSR is the sum of the Depth Rating and the Angle Rating. The Depth Rating is simply calculated by taking the depth of cover divided by 10. The Angle Rating is determined using Table 4. For example, a cover depth of 350 m with a major horizontal stress angle of 25� to the roadway gives an HSR of 38, which is the sum of 35 and 3.
Table 3 Example of calculation of the thickness-weighted average in CMFR. Discontinuity spacing rating
1.0 40 5 1.4 30 15 1.2 15 35 ½ð1:0 � 45Þ þ ð1:4 � 45Þ þ ð0:6 � 50Þ� ¼ 46 3
(5)
5:62
where Z is the regression discriminative equation. The likelihood of an outcome (p) can be simplified to Eq. (6)30, 62: � � p¼1 1þe Z (6)
4. Assessment of the potential of floor heave
Unit 1 (Uppermost) Unit 2 Unit 3 Thicknessweighted average
10� �θ<20�
The Australian database consists of 28 failure cases and 30 success cases from four underground coal mines in New South Wales and one mine in Queensland. The CMFR and HSR were calculated for each case. Table 5 lists the database with the CMFR and HSR values. Then logistic regression was conducted to obtain Eq. (5):
In the CMRR, the impact of the strongest unit within the bolted in terval is acknowledged, and the relevant adjustment is made. Similarly, the CMFR also recognises the effect of the strong immediate unit and thus an adjustment called the Strong Unit Adjustment (SUA) is made. An Australian longwall mine reported that moderate floor heave, equal to or less than a floor displacement of 300 mm, occurred where the thickness of the uppermost floor unit with UCS from 40 MPa to 65 MPa was greater than 0.6 m while significant floor heave was recorded where the thickness of the uppermost unit was less than 0.6 m.34 The thickness of the uppermost floor units associated with the buckling-type of floor failure at a US coal mine and two Australian coal mines was 0.3 m with the UCS of 99 MPa, 40 MPa and 58 MPa, respectively.33,53,70 A nu merical simulation suggested that the effect of the uppermost floor unit on confining the failure of underlying floor strata depends on the thickness of the uppermost unit.59 In the CMFR, the SUA is determined by first identifying a floor unit of the highest Unit Rating with a minimum thickness of 0.7 m within the first 1 m floor interval. Second, the Strong Unit Difference (SUD) is calculated by subtracting the Unit Rating of the selected strongest unit within the 1 m floor horizon from the thickness-weighted average of the Unit Ratings within the 3 m floor horizon. Finally, if the SUD is greater than 20, five points are added to the thickness-weighted average rating.
UCS rating
θ <10�
4.2. Floor Heave Index
3.4. Strong Unit Adjustment (SUA)
Thickness (m)
Angle of major horizontal stress to roadway, θ (� )
Unit Rating 45 45 50
6
Success Mine NSW 1
NSW 2 NSW 3
CMFR 38.9 40.7 41.1 51.6 41.5 47.9 43.6 57.9 36.8 36.8 36.8 36.8 36.8 36.8 36.8 36.8 36.8 36.8 37.5 37.5 37.5 43.2 43.2 43.2 43.2 43.2 43.2 43.2
HSR 41.5 41.5 42.0 45.0 43.5 44.5 43.0 58.0 35.0 35.5 36.0 36.5 37.0 38.0 41.5 42.5 43.0 44.5 50.0 53.0 55.0 36.0 37.0 43.0 38.5 39.5 47.0 50.0
No. 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58
Mine NSW 1
NSW 2 NSW 3
NSW 4
QLD 1
CMFR 45.3 52.7 44.6 41.5 36.6 47.2 40.0 46.2 48.2 60.6 46.2 46.2 42.9 42.9 42.9 41.9 55.9 51.0 54.4 60.2 57.5 51.6 62.1 72.1 55.8 54.9 61.7 64.7 61.6 37.9
HSR 43.0 45.0 47.5 47.0 45.5 46.0 44.0 47.0 46.5 61.0 34.0 35.0 30.0 35.0 39.5 23.5 27.5 25.5 28.0 43.0 38.0 43.0 45.0 42.0 34.0 46.0 43.0 42.0 29.0 31.0
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5.2. Other contributing factors to floor heave Apart from the quality of the floor, other parameters seemed to contribute to the occurrence of floor heave. For example, the locations of geological structures such as fault around the floor heave areas were reported to be correlated with the occurrence of significant floor heave.34 The impact of fault on floor heave was previously reported from mines in the UK and Poland.72,73 Geological structures within the floor strata were also found to contribute to floor failures at coal mines in South Africa.74 According to Latilla,74 the dolerite intrusions caused vertical slickensided joints in the laminated floor and, subsequently, the floor failure was manifested by opening the vertical joints in the floor. Also, dykes were reported to weaken the soft and sandy shale floor. However, the geological structures are not included in the proposed Floor Heave Index because it is difficult to detect large-scale features in the floor from drill core data. Moreover, two of the studied Australian mines did not have any fault or other geological structures in close proximity to the floor heave areas, and the locations of fault were generally correlated with floor heave on the retreat of longwall in some of the Australian mines, i.e. the Floor Heave Index was proposed to cater only for floor heave on development. Therefore, it is suggested that the presence of geological structures around the locations of floor heave be site-specific considerations. Water or moisture is known to reduce the strength of rock 75; in particular, the strength of claystone or underclay significantly decreases as the moisture content increases.13,76 Although the current database does not have relevant cases, the effect of water or moisture on the strength of floor units needs to be considered. Thus, if the floor contains the moisture-sensitive rock such as claystone and is wet (saturated with water) due to water from machines or the roof, an adjustment to the Unit Rating is suggested at the discretion of each site. Many researchers have discussed the influence of pillar dimension, and rib and roof support on floor heave. Various pillar designs associated with the use of yield pillar were suggested to control floor heave.77–80 Meanwhile, a field study at a Chinese underground coal mine showed that an increase in pillar width from 19.5 m to 56.0 m led to a significant decrease in the floor deformation.81 The rib and roof support with higher strengths minimised floor heave compared to the other area where the lower strength rib and roof support was used in another Chinese coal mine.82 While those factors appear to be critical in the occurrence of floor heave, the Australian database showed little varia tion in the pillar dimension in terms of width-to-height ratio and the ground support design. Further studies will need to be undertaken to incorporate the impact of pillar size and ground support in the Floor Heave Index.
Fig. 7. Floor heave index.
While 14 cases are misclassified, several misclassified cases are located close to the line of Eq. (7). The misclassified cases are deemed insignificant if they are close to Eq. (7). Thus, an intermediate zone has been incorporated as seen in Fig. 7. The intermediate zone was created with p ranging from 40% to 60%. Substituting 40% and 60% for p in Eq. (6) results in the Z values of 0.405 and 0.405 respectively. By substituting 0.405 and 0.405 for Z and re-arranging Eq. (5) in terms of the CMFR, Eqs. (8) and (9) are obtained: HSR ¼ 1:91ðCMFRÞ
40:34
(8)
HSR ¼ 1:91ðCMFRÞ
46:61
(9)
where Eqs. (8) and (9) represent a p value of 60% and 40% respectively. Finally, the cut-off line around the HSR of 23 was made as it is the lowest value of HSR from the database. Using the Floor Heave Index along with the CMFR and HSR, the potential of floor heave on development in roadways can be assessed at the planning stage for new mining projects or future workings. 5. Discussion 5.1. Guidelines on Coal Mine Floor Rating and Floor Heave Index Any empirical methods should be carefully used with a particular concern about the range of the underlying database.19 The database in this study includes CMFR ranging from 36.8 to 72.1, and HSR ranging from 23.5 to 61.0. Care needs to be exercised in extrapolating the empirical relationship outside the range of the original database. In addition, it is again emphasised that the floor heave cases in the data base are associated with the buckling mechanism, and thus the Floor Heave Index is not applicable to floor heave caused by bearing capacity failure or swelling mechanisms. As the CMFR is determined using exploration boreholes which are typically spaced several hundred metres apart, there is a possibility that the floor lithology of some of the roadways is significantly different from that of the boreholes. Even if the floor lithology is assumed to be the same, the UCS and discontinuity spacing of the floor units can vary as uncertainty exists.71 Considering the uncertainty, the Floor Heave Index is not expected to predict the occurrence of significant floor heave at any given location. Instead, this method suggests the potential for floor heave in roadways within the radius of the location of boreholes.
6. Conclusions A new rock mass classification system for the coal mine floor, the Coal Mine Floor Rating (CMFR), has been developed. The newly developed floor rating system quantifies the features of multiple floor units into a single number with the UCS and discontinuity spacing as the main components. As the horizontal stress is the main parameter for the buckling-type floor heave as well as the quality of the floor units, the Horizontal Stress Rating (HSR), a new parameter that represents the magnitude of horizontal stress, has been developed. The CMFR has been incorporated in a simple empirical tool, the Floor Heave Index, created by statistical analysis on an Australian database of both floor heave and non-floor heave cases. The floor heave and non-floor heave cases are clearly classified by the CMFR and the HSR. The new empirical method helps assess the potential of floor heave on development in underground coal mine roadways for new mining projects or future mining areas. Further data collection is suggested to extend the range of the data. Also, future work is recommended to incorporate floor instability cases associated with bearing capacity failure and swelling to produce a comprehensive CMFR for the coal mining industry. 7
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Declaration of competing interest
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