Fracture Propagation in Layered Sandstones with ...

9th Asian Rock Mechanics Symposium, Bali, 18 Oct 2016 ‐ 20 Oct 2016.

Fracture Propagation in Layered Sandstones with Varying Saturation W Timmsa*, P Caia, B Davida, H Masoumia, N Melkoumianb and J. Heob a

b

School of Mining Engineering, University of New South Wales (UNSW), Australia School of Civil, Environmental and Mining Engineering, University of Adelaide (UoA), Australia * [email protected] Abstract

Fracture behaviour of stratified and heterogeneous rock mass is of critical importance for sustainable development of energy and mineral resources. Efficient extraction of resources from such rock masses can depend in part on inducing a well-connected fracture network, whereas, in other situations, the inhibiting of vertical fracturing could limit impacts on water resources and improve environmental sustainability. However, little is known on stress induced fracturing across lithological boundaries in sedimentary rocks, despite considerable research on fracturing in rocks that are homogeneous at matrix and formation scale. The objective of this research, part of a project on fracture continuity and water seepage within stratified rock masses, was to 1) establish empirical functions for strength (UCS)-saturation for sandstone with a heterogeneous matrix and 2) investigate the behaviour of fracture propagation across the interface of different sandstones comprising a stratified rock mass under various test conditions. UCS tests were conducted on Gosford Buff sandstone (homogeneous) and Wondabyne sandstone (heterogeneous) cores of approximately 2:1 length:width ratio. Both sandstones were stronger dry than wet; the UCS of saturated cores was between 43 % and 76 % of the UCS of dry cores, and heterogeneity effects were apparent. UCS tests of layered specimens comprising of Gosford Buff and Wondabyne sandstones (n = 15) showed that the Wondabyne sandstone failed first for both wet or dry rock specimens, and fractures rarely propagated across the interface of rock layers. The implications of these results for extraction of resources, underground excavation and sustainability are discussed, with recommendations to establish the range of conditions that inhibit or promote cross-layer propagation of fractures. Keywords: Fracture Behavior; Strength; Saturation; Heterogeneous Matrix; Sandstone 1. Introduction As part of a project on fracture continuity and water seepage within stratified rock masses, this research aims to establish empirical functions for strength (UCS)-saturation for heterogeneous sandstone rocks and to investigate the behavior of fracture propagation across the interface of different sandstones comprising a stratified rock mass. The paper provides, for the first time, tests of fracture propagation across stratified sedimentary rocks, along with UCS-strength-saturation relationships. The research findings are discussed along with implications of these results for sustainability of resource extraction and a number of recommendations. 1.1. Layered rock and strength 1.1.1 Sources of fracturing in a rock mass Fracture networks are one of the defining parameters for water flow in rocks with otherwise low permeability. Fractures can be defined as complex shaped cavities filled with fluids and solid materials. It can refer to any discontinuity including joints, faults and cracks. A large-scale fracture network is formed by many single fractures. Natural fracture networks can develop after the rock has formed, due to tectonic movement, in-situ stresses, hydrostatic forces, pore pressures and heat stresses (Indraratna and Ranjith, 2001). The moisture status of rocks can effect expansion and relaxation of rock that occurs due to removal of confining rock (Jaeger et al., 2009).

9th Asian n Rock Mechanics Symposiu um, Bali, 18 O Oct 2016 ‐ 20 O Oct 2016.

Mechhanical effeccts from underground eextraction caan result in formation oof extensive fracture networkks in the subsurface, add ding or enhanncing naturaal fractures (Z Zoorabadi ett al., 2014). Fracture developm ment in an underground u d rock mass can be classed in three broadly diffferent termss: natural fracturinng which deevelops as a result of normal forcces acting on o the rockk mass undeerground; mechaniical and blassting disturb bance which is the direct result of mining m activiities; and su ubsidence which iss the gravity--driven collaapse of rock iinto an undeerground void d which is m most relevantt in terms of voidss left by miining activitty. Fracturinng caused by y subsidencee is especiallly relevant for coal mining, in particularr for long waall mining w which relies on o the naturaal caving meechanisms off the host rock to ffill in the gooaf left by ex xtracting coaal. Subsidencce causes inccreased fractture intercon nnectivity in the hoost rock all the t way to th he surface (L Leavitt and Gibens, G 1992), and can reesult in the closure or upsidencce of valleyys, compressive strain, and regional horizontaal movemennt (Mine Su ubsidence Engineeering Consulttants, 2007). l rock k masses 1.1.2 Jooints and fraacturing in layered The jjoint networrk in a layeered rock maass is depen ndent on thee orientationn of the layeers, their geomechhanical propperites, and confining prressures actiing on the rock r mass. T The most co ommonly observedd fracture tyype in layered rocks are open-mode fractures, wh hich propagaation path is affected by the roock propertiees and in situ u stress of thee bedding planes (Chang g et al., 2015)). The propagation of fracturess is characteerized as perrpendicular in orientatio on to the lay yer boundariies (Bai and d Pollard, 2000), innitiating prim marily in thee more brittlee and stiffer layer (Schöp pfer et al., 20011) and terrminating in the m more ductile laayer of the tw wo (Chang eet al., 2015). Vlasoov et al. (19990) uses th he representaation of strattified rock mass m as a trransversally isotropic medium m to measurre its effecttive characteeristics. Theese characteeristics are measured using u an asymptootic averaginng process off the actual ccharacteristiccs of the sing gle rock masss layer to determine d the stresss-strain statte. This metthod has beeen applied to t compresssion moduli,, shear modulus and Poisson’’s ratio, in diirections both h parallel andd perpendicu ular to the strratification. The thickness off layers is th he primary ffactor used for f indicating the facture re density off a layer. Fracturee saturation is i the criticaal fracture sppacing which h relies on th he concept oof frictional coupling present between thee fractured layer and th the ambient material. This T point iss reached du ue to an inadequaate amount of tensile sttress, and reesults in no further fraccture propaggation irrespeective of increasees in strain inn the materiaal (Schöpfer et al., 2011)). The impacct of low joinnt frequency y on rock masses iis known to have an insignificant efffect on the peeak strength of the materrial when a confining c pressuree is added (H Hagan and Sh herpa, 2013; H Hagan et al.,, 2013). 1.2. Rocck strength and a permeability

Figure 1:: Revised streength-moistuure content curves c (Vásárrhelyi and V Ván, 2006) It is oof note that many researrchers have ddemonstrated d that moistu ure content ccan have a significant influencce on the sttrength of ro ocks (eg. O Ojo and Bro ook, 1990; Hawkins H andd McConnell, 1992; Vásárheelyi and Ván, 2006). Haw wkins & McC Connell (199 92) studied the t influencee of water co ontent on the strenngth of 35 British B sandsttones in detaail and concluded that thee sensitivity of strength is highly variable across diffeerent types of o sandstonees. In some cases sandsttones with tthe UCS hig gher than


200MPa showed greater strength loss than sandstones below 60 MPa. The influence of water content on the sensitivity of rock strength is further supported by a mathematical interpretation of the findings on the effect of moisture on the strength of fifteen British sandstones (Vásárhelyi and Ván 2006) showed how rock strength varies as a function of saturation. Figure 1 illustrates the strength-water content curves of the 15 different rock types calculated using proposed expression (Eq. 1) by Vásárhelyi and Ván (2006): ∗ σ ω a∗ c ∗ e (1) is the uniaxial compressive strength (MPa), is the water content (%) and a*, b* and c* where are material constants. 2. Experiments 2.1. Core sample sourcing and specimen preparation Sandstone samples used in this study included Wondabyne sandstone (WB) and Gosford Buff (GB) sandstone from the Gosford Quarries, which is about 70 km north of Sydney, Australia. Several individual blocks were obtained to examine variability between blocks of the same sandstone. Core specimens were drilled from a cubic sandstone block and cut to approximately 2:1 length:width ratio according to ISRM (2007) methods. At the UNSW, core specimens of nominal diameter of 51 mm were drilled from rock blocks; and at the UoA, core specimens of a diameter of 55 mm were prepared. 2.2. Saturation and drying protocol Full saturation of rock specimens is difficult to achieve in practice, therefore this paper discusses approximate saturation at which the pore spaces are 90 to 100 % filled with water. In order to test the cores under saturated conditions, a vacuum chamber was utilized to saturate the cores. Cores were soaked in deionised water in an acrylic chamber, and a vacuum pump was used to minimize the air in the core. Saturation process was considered to be complete when limited amount of air bubble coming out of the core was observed, and the core was assumed to be saturated. Saturated specimens were stored in deionised water before they were tested. In order to prepare cores at intermediate saturation levels between 0 % and saturation, cores were air-dried or dried in an oven (< 60C to prevent mineral changes) with regular weighing of cores. 2.3. Laboratory studies on geomechanical properties of sandstone rocks To investigate the effect of rock saturation on its geomechanical properties, uniaxial compressive strength (UCS) tests were conducted in this study. The UCS of a rock is a measure of the maximum load a rock specimen can withstand in unconfined conditions. UCS tests on Wondabyne and Gosford Buff sandstone specimens were conducted both at the UNSW and the UoA. At both the UNSW and the UoA, specimens were sealed in plastic after drying and after saturation to maintain the specimens’ conditions accordingly. The core specimens were loaded at the controlled rate of 0.5 MPa/s until failure (UoA) and using a servo-controlled system for strain rate (UNSW). To achieve statistical significance of the results, for each UCS test sets of three specimens were tested at the UNSW and sets of five specimens were tested at the UoA. The uniaxial compressive strength of the specimen was calculated from: (2) where is the uniaxial compressive strength of the specimen, Pmax is the maximum load at failure, and A is the cross sectional area of the specimen. The UNSW team conducted the tests using the servo controlled MTS815 machine at the Laboratory at the School of Mining Engineering, UNSW. The UoA team conducted the tests using the Seidner D7940 compressive testing machine at the Geomechanics laboratory at the School of Civil, Environmental and Mining Engineering, UoA.


3. Resu ults and Disccussion 3.1. Geoological charracterics of samples Two specimens, GW1 (Gosfo ford Buff sanndstone) and d WS1 (Won ndabyne sanddstone) weree selected for geollogical invesstigations. Th he geologicaal analysis of o core thin-sections shoows that Wo ondabyne sandstonne has higher iron conten nt (in the forrm of limoniite), and low wer clay conttent than the Gosford Buff sanndstone. Theese propertiees, and the reelative mineeral particle size directlyy affect rock strength and perm meability chaaracteristics. Gosford d Buff sandsstone The GW1 sub-saample of Go osford Buff sandstone core c is a quaartz arenite tto quartz su ubarenite, mature, well sorted with subang gular to rounnded shaped grains. The quartz grainns (Qtz) appeear black through shades of grrey to white in i colour. Soome of the su ubangular grains are due to silica cem mentation of overggrowths, visiible beyond their originnal edges. Su utured, interlocking appeearance is related to compacttion. Some grains g demon nstrate embaayments whicch occurred during melt formation, and a most are monnocrystalline. Rimming, probably p an iiron oxide, iss present on some grainss. Approximaately 5 % of the saample is micca, mostly muscovite, m annd about 15 % are clays formed by cchemical weeathering. There arre some infi filling opaque minerals ppresent, mosstly likely an n iron oxidee and some isotropic mineralss, possible gaarnet. Figuree 2a-b shows these resultss.

a.

b.

c.

d.

Figure 2: Photomiccrographs of GW1 - a. PP PL image, dissplaying quaartz (Qtz), Qttz overgrowtth (OG), mbayment (E Eb); b. XPL image, i displaaying quartz (Qtz), some opaque Qtz rimming (R)), and Qtz em biotitee (Bt), and clay (C). Phhotomicrograaph of WS1 – c. PPL imaage displayin ng embaymen nts (Eb), sutuured contactss of compaaction (S), sillica cementation overgroowths (OG), an a opaque mineral m (o) annd limonite (L Lm); d. XPL imagge shows t vaarious quartzz (Qtz) minerrals, clays (C C) and muscoovite (Ms). Wondab byne (WB) sandstone s The W WS1 sub-sam mple of Won ndabyne sanddstone core is quartz arenite to quarttz subarenite, mature, moderattely sorted with w subang gular to rounnded shaped d grains. Qu uartz (Qtz) grains appeear black through shades of grrey to white and pale yelllow due to th hin section th hickness. Som me of the su ubangular grains aare due to siilica cementtation of oveergrowths, visible v beyon nd their origginal edges. Sutured,


interlockking appearaance is relatted to comppaction. Thiss is also no oted by curvvatures in muscovite m mineralss. Some grains demonstrate embaym ments which occurred o during melt form mation, and most are monocryystalline. Uppon visual inspection, i aapproximatelly 5 % of the t sub-sam mple is micaa, mostly muscoviite, 10 % aree clays and 10 0 % limonitee; both formeed by chemiccal weatherinng. The hydrrated iron oxide, liimonite (Lm)) is visible th hroughout thhe sub-samplle. There is some s infillingg of opaque minerals present, mostly likelly an iron ox xide and som me isotropic minerals, po ossible garneet. Figure 2c--d shows these ressults. ngth (UCS) ttest results 3.2. Uniiaxial comprressive stren For tthis study a total 77 UCS tests weree completed for Gosford Buff and W Wondabyne sandstone specimeens at both dry, d intermed diate and fuully saturation conditionss to establishh the strengtth of the rocks suummarised inn Table 1 and 2 under thhe Section 3.2 2.3. These UCS U tests inccluded tests on o single and doubble layered cores c at both dry and fullyy saturation conditions. 3.2.1 Sttrength of siingle-layered individuaal rocks Exam mples of postt UCS test single layer G Gosford Buff specimens at a 0 % and appproximate saturation are preseented in Figuure 3, and Wondabyne W sppecimens at 0 % and satu uration are ppresented in Figure F 4. Althouggh both Figuure 3 and 4 illustrated that specim mens in eitheer dried or saturated co onditions developeed fracturingg in a similaar shear direection, it neeeds to be highlighted thhat these co ores were characteerized by diffferent UCS strength s valuues. This is further f demon nstrated in seection 3.2.3.

a b Figuree 3: Gosford Buff post UC CS test speciimens: a) at 0 % saturatio on; b) at apprroximate saturation

a b Figuree 4: Wondabbyne post UC CS test specim mens: a) at 0 % saturation; b) at apprroximate satu uration 3.2.2 Sttrength of tw wo-layered composite c rrocks UCS tests on layeered cores co omprising tw wo different sandstones s used in this sttudy were co onducted. The layeers were not adhered or held h togetherr in any way. The followiing results w were observed d:  For layered sandstone teesting, there was not a co onsistent relaationship as tto whether th he top or bottom sanddstone failed d first. Furthher testing is i required to t confirm tthat the Wo ondabyne sandstone geenerally faileed before thee Gosford saandstone, wh hether it was placed on th he top or bottom (Tabble 1).




There was generally g no o evidence oof fracture propagation p across a the laayers (Figure 5). An unusual exam mple of fracttures to be obbserved acro oss both rock ks in the layerr (Figure 6).

a b Figure 55: Typical boottom core faailure withouut propagatio on of fracturees through lay ayer: a) C10' and B1'; bb) C11 and B2' B

a b F Figure 6: Fraacture propag gation acrosss layer: a) C1 11' and B3'; b) b other sidee of same corre 3.2.3 Su ummary of results from m UCS tests The ffollowing ressults were ob bserved:  Both sandsttones were stronger s dry than wet. According A to o results obtaained by thee UNSW team the UC CS for saturaated specimeens was on average a betw ween 43 % annd 76 % of the UCS for dry speccimens. At the t UoA, thee UCS stren ngth of saturrated Wondaabyne sandsttone was around 62 % of that for the dry onne, and the UCS U strengtth of the satturated Gosfford Buff sandstone was w around 70 0 % of that fo for the dry on ne.  Variability between b bloccks of sandsttone of the saame type were significannt. As shown in Table 1, cores from m the C blocck of Gosforrd Buff sand dstone were stronger thann cores from m the GB block of the same type in n UCS tests. b of Gosfford Buff san ndstone show wed a relative vely high streength, yet  Dry specimeens from C block this strengthh was reduced by 45 % w when the speecimens weree approximattely 100 % saturated. s Such changees were simillar to the speecimens from m GB block. ~30 %, 32 % and 92 %) Wondabbyne sandsto one were  UCS resultss for partial saturated (~ obtained on a second bllock (Block 2) at UNSW W. These ressults demonsstrated that the USC t specimen ns decreasedd by 13% when w the deg gree of satura ration increassed from strength of the


29.7 % to 91.7 %. The relationships between strength (UCS) and saturation for Block 2 are plotted on Figure 7. The rock strength-saturation test results for both single and double layer specimens obtained by the UNSW team together with notes on observations during the UCS tests on double layer specimens are presented in Table 1. Results obtained from the UCS tests by the UoA team are presented in Table 2. Table 1: Summary of rock strength-saturation test results (obtained at UNSW) SINGLE CORE Saturation n

StDev %

Wondabyne (WB block)

Dry

5

5.9

UCS (Mpa) 25.8

Gosford (GB block)

Dry

5

1.1

36.1

Gosford (C block)

Dry

4

11.7

53.2

Wondabyne (Block 2)

~ 29.7%

6

2.1

32.4

Wondabyne (Block 2)

~ 32.5%

6

2.2

30.0

Wondabyne (Block 2)

~ 91.7%

6

2.9

28.2

Wondabyne (WB block)

100%#

5

2.3

19.5

Gosford (GB block)

100%#

2

0.8

15.7

#

3

4.4

29.4

5

6.2

41.5*

5

6.5

24.6*

5

3.7

32.4*

Gosford (C block)

100%

Comments

LAYERED CORES Top core Gosford (C block) Gosford (C block) Wondabyne (WB block)

Bottom core Wondabyne Dry (B block) Wondabyne 100%# (B block) Gosford Dry (GB block)

All bottom rock failed first All bottom rock failed first All top rock failed first, except 1 test with bottom core failed first; failure at mid UCS strength of both rocks

#

Approximate saturation, close to 100% as discussed earlier. *Note that these values are not standard UCS strength of the individual cores, but simply represent the compression pressures at failure of the layered cores. Table 2: Results from UCS tests (obtained at UoA)

Cores Saturation n StDev % Gosford Buff Dry 5 1.1 Wondabyne Dry 5 3.4 Gosford Buff 100%# 5 3.2 # 5 3.9 Wondabyne 100% # Approximate saturation, close to 100% as discussed earlier.

UCS (Mpa) 39.6 57.8 27.6 35.8


3.3. Empirical relationship between the rock saturation and strength Several empirical relationships were established for UCS strength and degree of saturation based on the data from Wondabyne sandstone specimens. As shown in Figure 7, all relationships were similar in that UCS strength reduced with increasing degree of saturation. The equation developed on UCS was specific to the actual block of sandstone, and to whether the sandstone was drying (from saturation) or wetting (from dry), so a generalised universal equation was not possible to establish. The equation with the highest R2 fit (for n=10) was as follows for a Wondabyne sandstone block, where S is the degree of saturation (0 to approximately 100 %): 22 57.8 (3)

UCS (MPa)

Several empirical relationships between UCS and saturation were established for the Wondabyne sandstone (n=18 with R2 of 0.27; n=10 for R2 of 0.44). The negative linear trends between UCS and saturation were dominated by heterogeneity of this sandstone, with more precise results obtained on a second block at the UNSW by a two stage measurement of water content before and after UCS testing. 80

80

60

60 Fit 2: Linear for Block 2 (partial saturated) Equation UCS = ‐5.02S+32.8 Number of data points used = 18 Coef of determination, R2 = 0.27

Block 2 saturated for weeks dried as measured

40

40 Fit 1: Linear for Block @ UoA Equation UCS = ‐22S + 57.8 Number of data points used = 10 Coef of determination, R2 = 0.92

20

20 Block 1 dry as supplied, estimated saturation

0 0%

20%

40%

Block1 Block (@UoA) Block 2 (@UNSW) Fit 3: Linear Fit 1: Linear Fit 2: Linear (partial saturated)

60%

80%

Fit 3: Linear for Block 1 @ UNSW Equation UCS = ‐6.2S + 25.8 Number of data points used = 10 Coef of determination, R2 = 0.44

0 100%

Saturation

Figure 7: Empirical relationships between rock strength and saturation established for the Wondabyne sandstone based on the UCS test data 3.4. Implications for resource extraction and sustainability An understanding of the conditions for fracture propagation across layered sedimentary rocks is important for productive extraction of resources underground, in a manner that is both safe and sustainable for the operation and the environment. For some energy extraction projects (eg. coal and coal bed methane), it would be advantageous that fracture propagation is restricted by overlying sedimentary layers, so the indication from these results that it was unusual for fractures to propagate across sandstone layers warrants further study. For any underground excavations, the possibility of continuous fracturing upwards through multiple sedimentary strata could significantly increase the risk of inrush of water (ie. a potential safety issue), or increase seepage from surface waters to deeper layers (ie. a potential environmental sustainability issue).


These experimental results together with ongoing testing of layered rock cores, demonstrate on step in a combined approach with site studies to improve prediction of groundwater flows. For example, an approach to obtaining a more accurate prediction of groundwater inflow, as proposed by Pinzani and Coli (2011) was to divide the surrounding geology into separate domains based on geomechanical and hydrogeological properties. These areas were individually analyzed and their potential for fracture development and inflow based on the properties of the rocks present were evaluated. This will help to quantify fluid inflow by summing the seepage for the area being considered, ultimately providing reliable estimates of total inflow, and the potential for seepage from surface waters into fracture systems. The frequency at which fracture systems propagate across sedimentary layers with different properties is thus critical to understand. 4. Conclusions UCS tests were conducted on two different sandstones under various test conditions. Empirical functions for strength (UCS)-saturation were established for sandstone with a heterogeneous matrix. Additionally, the behaviour of fracture propagation across the interface of different sandstones comprising a stratified rock mass were investigated. Relationships between strength-saturation would provide a unique and important research outcome for application to underground operations. The UCS tests showed that the wet sandstone strength was on average between 43 % to 76 % of the dry sandstone strength. However, significant variability was observed between blocks of sandstone of the same type, and the actual relationship between UCS and saturation also depended on whether the sandstone was wetting, or drying. Furthermore, it was unusual for fractures to propagate across the layers during UCS tests and the failure appeared to be independent of whether the stronger or weaker sandstone was the top layer. The strength of the Wondabyne sandstone compared with the Gosford sandstone could be attributed to geological fabric and iron content, though further testing is required because of heterogeneity between different blocks of the same sandstone. Additional testing of sandstone specimens at intermediate saturation levels from the same block (ie. homogeneity) is recommended for further development of the strength-saturation relationship, to provide a reliable empirical predictor for underground excavations modelling. Furthermore, investigation is warranted of possible differences in strength-saturation relationships for specimens that are wetting (ie. initially dry), versus specimens that are drying (ie. initially near saturation). Using other types of sandstones and sedimentary rocks from various geological locations around the world together with investigating the influence of temperature on rock saturation and fracture propagation will help to obtain a more comprehensive insight into the fracture propagation in a granular rock materials and further develop the proposed strength-saturation relationships for underground environment. Furthermore, extending the presented research to stronger sandstones with (UCS >200 MPa) is recommended. Testing with synthetic sandstones will allow systematic investigation of the effects of microstructural parameters such as grain size and the degree of cementation. Acknowledgements This research was funded by the Mining Education Australia (MEA) Collaborative Research Scheme, and for additional research, by UNSW School of Mining Engineering. Kanchana Gamage is thanked for UCS test operations and Dayna McGeeney of UNSW Connected Waters Initiative is thanked for photomicrographs and geological analysis of rock cores.

References Bai, T. & Pollard, D. D., 2000, Closely spaced fractures in layered rocks: initiation mechanism and propagation kinematics, Journal of Structural Geology, 22, 1409-1425. Chang, X., Shan, Y., Zhang, Z., Tang, C., Ru, Z., 2015, Behavior of propagating fracture at bedding interface in layered rocks, Engineering Geology, 197, 33-41. Hagan, P., Sherpa, M., 2013, The Effect of Joint Properties on a Discontinous Rock Mass, Mining Education Australia, Journal of Research Project Review, 2(1), 75-81.


Hagan, P., Sherpa, M.D. and Masoumi, H.. 2013, The effect of joint frequency on a discontinuous rock mass under experimental compressive strength testing condition. (CD paper No.240). In Proceedings of the 47th US Rock Mechanics Geomechanics Symposium, 23-26 June, San Francisco, US. Hawkins, A. and McConnell, B., 1992, Sensitivity of sandstone strength and deformability to changes in moisture content, Quarterly Journal of Engineering Geology and Hydrogeology, 25(2), 115-130. Indraratna, B. and Ranjith, P. G., 2001, Hydromechanical Aspects and Unsaturated Flow in Jointed Rock, 1st ED, Lisse, Netherlands: A.A. Balkema Publishers. ISRM, 2007, "The Complete Suggested Methods for Rock Characterization, Testing and Monitoring: 1974–2006 ISRM," In: Ulusay R, Hudson JA (eds) Suggested methods prepared by the commission on testing methods, Compilation arranged by the ISRM Turkish National Group, Kozan ofset, Ankara Jaeger, J., Cook, N. G., Zimmerman, R., 2009, Fundamentals of Rock Mechanics, USA, Blackwell Publishing. Leavitt, B. R. and Gibens, J. F., 1992, Effects of longwall coal mining on rural water supplies and stress relief fracture flow systems, United States, West Virginia University. Mine Subsidence Engineering Cconsultants, 2007, Introduction to Longwall Mining and Subsidence, Chatswood. Ojo, O. and Brook, N., 1990, The effect of moisture on some mechanical properties of rock, Mining Science and Technology, 10(2), 145-156. Pinzani, A. and Coli, N., 2011, A practical geostructural approach for the evaluation of tunnel water inflow by means of FEM seepage analysis, The 45th symposium of Rock Mechanics, San Francisco. Schopfer, M. P. J., Arslan, A., Walsh, J. J., Childs, C., 2011., Reconciliation of contrasting theories for fracture spacing in layered rocks, Journal of Structural Geology, 33, 551-565. Vasarhelyi, B. and Van, P., 2006, Influence of water content on the strength of rock, Engineering Geology, 84, 70-74. Vlasov, A. N., Merzlyakov, V. P., Ukhov, S. B., 1990, Effective characteristics of the deformation properties of stratified rock, Soil Mechanics and Foundation Engineering, 27, 22-26. Zoorabadi M; Saydam S; Timms W; Hebblewhite B, 2014, 'New empirical equations to estimate the hydraulic aperture of rock discontinuities at different depth', in Hagan PC; Saydam S (eds.), AusRock 2014: Third Australasian Ground Control in Mining Conference, The Australasian Institute of Mining and Metallurgy, pp. 375 - 382, presented at AusRock 2014: Third Australasian Ground Control in Mining Conference, Sydney, 5 - 6 November