SHEAR MECHANISM FOR MINING-INDUCED FRACTURES
 

APPLIED TO ROCK MECHANICS OF COAL MINES
 

Brian G. White
 

Spokane Research Laboratory, National Institute for Occupational Safety and Heath, Spokane, WA, USA
 
ABSTRACT 

Two examples of en echelon mining-induced fractures seen in hardrock mines provided a basis for inferring that fracture zones and 

bedding plane separations immediately surrounding mine openings 

are promoted by oblique shear into the openings. It is hypothesized 

that initial fractures or separations form at the corners of openings 

as a result of high stress and physical constraint on the rock's ability 

to deform elastically toward the opening. These conditions result 

in a locally preferred direction of shearing. The shearing, in turn, 

generates tensile stress that initiates a progression of systematically 

offset fractures approximately parallel to the direction of greatest 

compressive stress. The fractures or bedding separations create 

tabular rock layers that amplify shearing displacement through 

bending and dilation. Such shearing effectively reduces and 

redistributes the compressive stress, but significant dilation is an 

inevitable consequence. The combination of dilation and shearing 

and the progressive development of fracture zones have important 

implications with respect to ground support. 

The concept of mining-induced fractures forming as a result of 

shear is illustrated by two examples from coal mines. First, fractures seen at longwall faces probably result from shear associated 

with subsidence. The fracture zone that develops approximates or 

possibly defines the draw angle of subsidence. As the face 

advances, fractures extend downward along the lower edge of the 

fracture zone, while upper extensions of the fractures are pressed 

closed. 

Fracture zones in entry roofs provide a second practical 

example. Here, mining-induced fractures typically follow bedding 

planes. The shear zone model suggests that the first bedding 

separations develop near the edges of the roof and successive 

separations progress upward and toward the center. However, ifthe 

direction of greatest stress is inclined with respect to the roof, a 

fracture or bedding separation zone may propagate from one side 

only and also extend higher. Because coal ahead of the face 

provides some support against lateral shear deformation, bedding 

separation is inhibited near the face. Rock bolts installed close to 

the face ultimately become more strained and bent than bolts 

installed a few meters from the face, and bolts installed through the 

more remote part of a separation zone may ultimately experience 

the greatest tensile and bending strains. This model is supported by 

field data documenting progressive bolt failures that rapidly 

propagated downward across the roof during face advance. 

INTRODUCTION 

Although mining-induced fractures are well known in coal mines, 

where they are associated with cave development and roof, wall, 

and pillar instability, our understanding of fractures and fracture 

zone development has been lacking. The classic explanation of 

mining-induced fractures is that they result from stress alone and 

form parallel to the direction of greatest stress. For example, it has 

been proposed that stress progressively splices together microcracks 

until they become continuous fractures, or that fractures propagate 

because compressive stress induces tension at their leading edges. 

Two examples of mining-induced fractures observed in hard-rock 

mines provided a basis for proposing that these fractures actually 

resulted from shear strain that developed as rock was displaced 

obliquely toward mine openings. 

This paper first describes these two examples, which stimulated 

the author’s recent research on fracture development, develops a 

general hypothesis for fracture zone development, and applies the 

hypothesis to the formation of face and roof fracture zones in coal 

mines. I propose that application of the shear zone concept of fracture and bedding plane separation to coal mines may substantially 

aid our understanding of these phenomena and lead to improved 

mining and ground control practices. 

MINING-INDUCED FRACTURES IN HARD-ROCK MINES 

Example 1: Rock Burst in a Development Heading 

A rock burst in an Idaho silver mine took place in the face of a 

development heading that was being driven normal to vertically 

dipping, massive quartzite beds (figure 1), creating a bowl-shaped 

cavity centered in the face. Inspection showed that the quartzite had 

been thinly split on the edges of the cavity (figure 2), but much of 

the broken rock ejected from the cavity was thicker than the 

quartzite at the edge of the cavity. Inspection of the walls after the 

next advance showed no evidence of splitting. Thus, the splitting 

was confined to the edge of the cavity and evidently represented 
many individual fractures that were progressively offset with depth 

along the cavity edge toward the center of the face (figure 3A). 

This contrasts with a classic view presented by Fairhurst and Cook 

(1) that mining-induced fractures extend all the way across a face 

or surface. 

Figure 1. Cavity created by a face-strain rock burst in a 

development heading of a hard-rock mine. 

Figure 2. Edge of burst cavity shown 

in Figure 1. Note closely spaced 

fractures. 

Figure 3. Cross sections of fracture zones in various settings. A, Cross section of face-strain 

burst shown in figures 1 and 2; B, compression test specimen; C, hour-glassing of pillars; D, 

inverted-V failure of entry roof. 

A cross section through the face (figure 3A) showed that the 

fracture distribution bore a striking resemblance to fractures formed 

in laboratory tests when the ends of the samples are constrained 

against expansion or slippage and en echelon fracture zones extend 

obliquely from opposite corners (figure 3B). Thus, the rock burst 

seemed to be a good field example of the laboratory test. The 

fractures in the laboratory specimens also resembled the fractures 

that cause hour-glassing in pillars (figure 3C) (1). 

Another field example of this fracture pattern was described to 

the author at a western coal mine where a well-lithified, hard, brittle 
mudstone roof occasionally experienced inverted-V-type roof 

failures. The geometry described by mine personnel (figure 3D) 

suggested that the edges of these cavities were also formed of 

closely spaced bedding plane separations systematically distributed 

along what would eventually become the edge of the collapse. 

These conclusions fired the author’s interest in mining-induced 

fractures and led to several years of intensive study. However, a 

second notable observation in a hard-rock mine, described below, 

preceded this study. 

Example 2: En Echelon Fracture-Shear Zone 

This example (figure 4) is in a steeply dipping, narrow vein. 

 

A 

B 

Figure 4. Cut (or drift) in a steeply dipping hard-rock 

vein. A, View along wall. Photo taken while standing 

on sandfill from previous cut. Note mining-induced 

fractures and temporary friction rock bolts from roof of 

previous cut exposed in wall. B, View of wall showing 

bent rock bolt offset by shear displacement along en 

echelon fracture zone. 

The 

vein is mined by the overhand method, which involves mining the 

vein in a horizontal cut or drift, filling the cut with sand, and then 

driving a new cut on top of the sand. Thus, mining advances along 

a longwall front in an ore body that stands on end. 

Split-Set1

1

Mention of specific products or manufacturers does not imply 

endorsement by the National Institute for Occupational Safety 

and Health. 

rock bolts provide temporary support during the 

mining of each cut. Since the vein was not quite vertical, each cut 

was slightly offset from the one preceding, so that bolts installed in 

the roof of the previous cut were sometimes exposed in the next cut 

above (figure 4). High stress around the ends of the mined portion 

created horizontal fractures that became exposed in the walls and 

face of each new cut (figure 4B). In figure 4B, a rock bolt from the 

previous cut intersects fractures that were induced by mining, as 

shown by shearing offset of the bolt. It is also evident that the 

fractures involved are not continuous, but form a zone of systematically offset, or en echelon, fractures. Such fracture zones have been 

found to be common in the deep hard rock mines of Idaho. 

The observation that the fracture zone experienced shear is 

consistent with the fractures having resulted from high horizontal 

stress. Shearing effectively reduced the magnitude of the stress. 

The mechanics of shearing evidently involve bending of the rock 

layers (figure 5). Bending, in turn, causes dilation as the end of 

each layer is lifted away from its substrate. Hence, it may be concluded that the stress that causes the fractures is reduced at the 

expense of shearing and dilation, with a net displacement that is 

oblique to the fracture zone. However, extensive shear displacement only occurs after fractures have formed. Therefore, the fracture zone must actually result from shear displacement that originates prior to fracturing, when conditions are still elastic. There 

appears to be no alternative to this interpretation. 

Gramberg (2), who spent a lifetime studying fractures, concluded that extension fractures result from “induced tension” caused 

by compressive load parallel to the fracture. If so, it may be reasonable to consider that the origin all extension fractures caused by 

compressive stress is the same, even when no discernable en echelon arrangement is evident, and that they all ultimately result from 

shear strain, which is actually responsible for the “induced tension.” 

This leads to a general explanation of mining-induced fractures, 

which may be stated in the form of a hypothesis. 

SHEAR HYPOTHESIS FOR MINING-INDUCED FRACTURES 

A general explanation for the origin of mining-induced fractures is 

stated as follows: 

1.	 Asymmetric elastic distortion about mine openings manifests 

shear; 

2.	 Shear causes diagonal tension, which is directly responsible for 

creating mining-induced fractures; 

3.	 A fracture zone propagates en echelon fractures along the 

direction of shearing. 

Since a rigorous and convincing demonstration of this has not 

been presented and has not even been published, this explanation 

will be offered as a hypothesis, although a strong case can be made 

in its support. 
Figure 5. Displacement mechanics along en echelon fractures. A, En 

echelon fractures with intervening layers; B, combined dilation and shear 

displacement resulting from rock layers rotating away from adjacent 

substrates; C, fractures zone “repaired” with imaginary needle and thread. 

Both stitches and layers are under tension, thereby illustrating the origin of 

the tensile stress that causes fractures. 

This explanation for mining-induced fractures contrasts with 

the prevailing view that such fractures form parallel to the direction 

of greatest stress as a result of this stress alone (figure 6A). The 

explanation advocated here is that the fractures result from shear 

strain caused by load and by distortion resulting from the presence 

of a nearby mine opening. In general, this distortion happens at 

corners or edges of mine openings because the openings create a 

preferred direction of displacement that takes the form of shear 

(figure 6B). The shear, in turn, generates diagonal tension approximately parallel to the direction of least stress, and extension fractures result. 

The simplest illustration of this idea can be seen when a 

reference shape, such as a square, is deformed into a parallelogram 

by shearing along two opposite sides. The diagonal distance 

between opposed acute corners increases and represents extension. 

Near mine openings, the extension becomes tension. Alternatively 

(figure 5C), we can imagine “repairing” the open fractures of 

Figure 5B with an imaginary needle and thread. If so, it is evident 

that both thread and rock layers are in tension. 

Along originally straight edges of mine openings, the 

fundamental shape is a flattened S (figure 5B, C). The direction of 

greatest stress crosses the S at a low angle, so a fracture that forms 

parallel to the opening actually experiences a little bit of shear 

stress. Consequently, mining-induced fractures don’t result strictly 

from tension. A little shear displacement occurs, not enough to see 

without careful measurment, but enough to aid significantly in the 

formation and propagation of each fracture. 

Typical modeling exercises that identify compressive and shear 

stress concentrations near corners of openings suggest that “shear 

zones” are initially highly foreshortened. Hence, mining-induced 

fractures do not all form at once, but in succession. Once a fracture 

forms, it isolates a layer of rock, and stress is redistributed and 

reduced within the layer. Stress continues to be redistributed 

deeper into the rock, and a new fracture is formed, and so on. The 

geometry of the situation is such that each new fracture is translated 

sideways from the previous one, tending to propagate a zone of 

fractures that are distributed en echelon. 

One way of looking at fractures that is especially relevant for 

roof rock in coal mines is by comparing them to the geologic 

structures known as kink bands (figure 7). Empirically, kink bands 

form in layers that possess high interlayer friction (3). A kink band 

solves the friction problem by dilating the layers within the kink 

band. The dilation is a simple consequence of the geometry, the 

dimension normal to the layers increasing within—as compared to 

outside—the band, so that the layers separate enough to reduce or 

eliminate friction. In a similarly deformed, unlayered material, the 
same strains would create en echelon fractures and a fracture zone 

with a geometry similar to that of a kink band. In deep coal mines, 

this situation is relevant to vertically oriented loads that cause new 

fractures that cut across bedding. 

Figure 6. Contrasting views of fracture formation. A, 

Conventional view that principal stresses cause either 

conjugate faults or extension fractures parallel to greatest principal stress (F1) through compressive stress 

alone. B, Alternative view as expressed in this paper, 

that extension fractures, the most common mininginduced fracture type seen in hard-rock mines, result 

from elastic shearing displacement that generates 

tension approximately parallel to the direction of least 

stress. (Note: Distortion is highly exaggerated.) 

 

 

Figure 7. Representation of kink band, a familiar 

geologic structure. A, Dimension B-B’ is increased with 

relative to original thickness A-A’. B, Geometry of 

deformation reduces high interlayer friction by 

separating layers within band, thus enabling 

deformation to take place. 

SHEAR MECHANISM APPLIED TO FAMILIAR
 

FRACTURES IN COAL MINES
 

Formation of fractures at two locations in coal mines can be partially or entirely accounted for by the shear mechanism described 

above. In examples described here, it is important to bear in mind 

that fractures are likely to exploit preexisting flaws such as bedding, 

cleats, and joints. Hence, mining-activated preexisting structures 

may not be readily distinguishable from inactive flaws. However, 

the geometries of activated structures are likely to be as illustrated 

in the examples. 

Example 1: Face Fractures and Subsidence 

A familiar fracture type seen in longwall coal mines, as well as in 

South African longwall gold mines and Idaho silver mines, is one 

that forms parallel to the face. This type is instrumental to cave 

development (figure 8). These fractures apparently form in the 

region of high, nearly vertical stress that is concentrated ahead of 

the face (figure 8B). 

Longwall mines are also characterized by subsidence that 

extends beyond the face a distance partially defined by the draw 

angle n (figure 8B). In addition, horizontal stress changes 

associated with subsidence increase compressive stress over the 

longwall panel and create tension adjacent to the edge of the draw 

(4). 

The opportunities for shear as a result of the high vertical stress 

near the face are limited. A direction describing gravitationally 

assisted movement toward the gob is strongly favored. Thus, a 

preferred direction of shearing extends upward along the angle of 

draw. As the face advances, the fractures are pressed closed along 

the upper edge of the active shear zone and opened below. 

This is not the only mechanism likely to be involved with 

subsidence. Cantilevering may also be a factor, and multiple shear 

zones may form and extend below the seam, mirroring the zone or 

zones along the draw line. However, if fractures are well developed 

at the face, their origin by shearing is likely. Also, if cleats and 

joints are present and suitably oriented, they will promote this type 

of deformation. 
B 

Figure 8. Mining-induced fractures parallel to the face in 

longwall mines. A, Hard-rock mine, vertical vein (photo rotated 

90° to approximate geometry of flat-dipping coal); B, proposed 

origin of fractures in coal mines as result of shear displacement 

along angle of draw. 

A 

Example 2: Entry Roof Fracture Zone 

A second example of familiar mining-induced fractures in coal 

mines involves those that form parallel to bedding in entry roofs. 

One example has already been mentioned earlier, in which fracture 

zones inclined upward from both sides are associated with invertedV-type roof failures (figure 3D). Inverted-V fracture zones 

obviously require equal horizontal stresses at each corner (figure 

9A). But, if the major principal stress is inclined with respect to the 

opening, one corner is shielded from this stress. Fractures will only 

develop preferentially from the opposite corener and may also 

extend higher above the back along a steeper plane (figure 9B). An 

inverted mirror image of this zone may also form in the floor 

(figure 9B), but this paper only considers the roof. 

Because the coal ahead of the face provides some support, 

fracture zone development is suppressed at the face. This is an 

important point. 

Both of these situations can by illustrated in plan view (figure 

9C, D). The fractures and fracture zones are fully developed some 

distance from the face, while fractures near the face are limited. As 

a practical concern, rock bolts installed away from the face are 

inserted through already fractured, sheared, and dilated ground. 

These bolts are doing the job they do best: They are subjected to 

tensile load and experiencing only minimal stress, so the roof is 

likely to be stable providing anchorage is adequate. But bolts 

installed close to the face in unfractured ground will be subjected 

to shearing and dilation as soon as the face advances and the 

fracture zone progresses. 

A good example of progressive bolt loading that can be 

accounted for by this mechanism has been documented by Signer 

and Lewis (4) (figure 10). Instrumented bolts installed in the roof 

near the face recorded a buildup of load that progressed downward 

from high on one side of the roof. Ultimately, loads exceeded the 

capacity of the bolts, which failed progressively (deepest failure 

first) until a cave resulted. 

These examples suggest that it may be useful to be aware of 

what kind of fracture zone is forming in the roof and that an 

understanding of fracture zone geometry and process may lead to 

more effective roof bolting practices. 

SUMMARY 

Observations made in hard-rock mines were the stimulus for 

developing a hypothesis concerning the origin of mining-induced 

fractures and fracture zones as a product of elastic shear displacement toward mine openings. It is suggested that this explanation for 

mining-induced fractures may have general application. Thus, 

lessons learned in hard-rock mines also apply to fractures and 

bedding plane separations in coal mines. Understanding the 

mechanics of fractures and their fracture zones can lead to more 

effective mining and ground control methodologies. 
 

 

Figure 9. Fracture zones in entry roofs. A, Horizontal stress 

produces symmetrical fracture zones. B, High subhorizontal 

stress generates asymmetrical fracture zones that are 

steeper and extend deeper than fracture zones shown in A. 

C, Plan view of A in which fractures are viewed through roof. 

D, Plan view showing fractures in B. E, F, Roof bolts installed in area 1 are subjected to dilation and shearing as fracture zone follows face advance; bolts installed in area 2 are 

in ground where fracture zone is already well developed. 

Figure 10. Cross section through instrumented roof 

bolts installed near entry face (after 5). Bolts loaded 

rapidly along a sloping line with subsequent entry 

advance. Bolt failures progressed downward to the left 

until total collapse. 

REFERENCES 

1. Fairhurst, C., and N.G.W. Cook. 1966. The Phenomenon of 

Rock Splitting Parallel to the Direction of Maximum Compression 

in the Neighborhood of a Surface. In Proceedings of the First 

Congress of the International Society of Rock Mechanics (Lisbon, 

Portugal, Sept. 25-Oct. 1, 1966). V. 1, Nat. Lab. of Civil Eng., 

Lisbon, Portugal, pp. 687-692. 

2. Gramberg, J. 1989. A Non-Conventional View of Rock 

Mechanics and Fracture Mechanics. Balkema, Rotterdam, 259 pp. 

3. Patterson, M.S., and L.E. Weiss. 1961. Experimental 

Deformation and Folding in Phyllite. Geol. Soc. Amer. Bull., v. 77, 

no. 4, pp 343-374. 

4. Peng, S.S. 1978. Coal Mine Ground Control. John Wiley and 

Sons, New York, 450 pp. 

5. Signer, S, and J. Lewis. 1998. A Case Study of Bolt 

Performance in a Two-Entry Gateroad. In Proceedings, 17th 

International Conference on Ground Control in Mining, ed. by S.S. 

Peng (Morgantown, WV, Aug. 4-6, 1998). WV University, 

Morgantown, WV, pp. 249-256.