Origin of Mining-Induced Fractures Through Macroscale Distortion 

White, B. G., Larson, M., and Iverson, S.R. 

National Institute for Occupational Safety and Health, Spokane Research Laboratory, Spokane, WA 

ABSTRACT: NIOSH researchers have identified a pattern of fracture zone development that suggests an explanation for fracture 

formation around rectangular openings in underground mines. This pattern is characterized by shearing and dilation that result in either 

faults or in the repeated formation and propagation of en echelon fractures from sites of tension. Two computer modeling codes, Fast 

Lagrangian Analysis of Continua (FLAC) and Particle Flow Code (PFC), were used to model different aspects of this pattern. Use of 

very small elements with FLAC enabled the identification of sites of initial tension near the skin of square-cornered rectangular 

openings, while PFC allowed the initiation and progressive development of fractures from these sites to be followed as the fractures 

evolved. 

Such studies can lead to a greater understanding of how roof support can be better selected and installed for specific conditions in 

underground mines prone to roof falls and rock bursts. These studies may also lead to modifications of corner and opening shapes that 

could be incorporated into mine designs to produce more stable mine openings and reduce the risks of rock falls and rock bursts. 

1. INTRODUCTION 

Mining-induced fractures are typically either hidden 

from view or become obscured when fractured rock 

around mine openings collapses or is ejected in a rock 

burst. Consequently, knowledge of the distribution, 

geometry, and extent of mining-induced fractures has 

been limited. However, fracture patterns that suggest 

mechanisms of fracture formation are occasionally 

seen where new crosscuts intersect old ones. The old 

fractures are exposed in the walls of the new opening, 

but well-described examples are rare in the literature. 

Most investigators agree that extension fractures form 

parallel to the direction of maximum principal stress. 

Recently favored explanations for how decimeter and 

longer (macroscale) fractures develop have mainly involved the proliferation, interaction, and coalescence 

of smaller fractures. 

Fairhurst and Cook [1] proposed that stress-induced 

microcracks first form an incipient cleavage parallel 

to the surface of a mine opening. These microcracks 

then grow into larger fractures as a result of the 

applied stress. These fractures shorten with depth, and 

their ends define a shape concave toward the opening 

that represents the limit of breakage in the case of a 

rock burst, roof collapse, or pillar hour-glassing. 

However, in a field example where thin layers were 

displayed at the margin of a rock burst breakout, 

White [2] concluded that the closely spaced fractures 

had not extended across the entire volume of the 

ejected rock, but were present only near the periphery 

of the breakout. Other examples that support this 

scenario are described in White et al.[3] 

Stacey [4] proposed that failure about mine openings 

will occur if extension strain reached a certain value, 

which he considered a material property. He noted 

that extension strain is highest near the corners of 

rectangular openings and suggested that failure begins 

at these locations. He proposed that the extent of 

failure is determined by how much rock around the 

opening exceeds the requisite extension strain. For 

rectangular openings, critical extension strain is 

deepest at the midpoint between corners and its limit 

duplicates the concave shape identified by Fairhurst 

and Cook [1] and commonly seen after roof falls and 
rock bursts. However, Stacey did not differentiate 

extension resulting from tension from Poissonresponse extension. He thought that extension strain 

causes fracturing even where all normal stresses are 

compressive, but he did not identify a consistent 

fracture pattern or a specific mechanism that would

cause fracturing to take place. 

Diederichs [5] confirmed that tension microfractures

form within generally compressive environments and 

suggested that microfractures may propagate some

distance as a result of small, localized, possibly 

randomly distributed regions of tension. Diederichs 

considered that interaction of medium-sized fractures 

leads to large extension fractures. He recognized no 

specific pattern of large-fracture development about 

simulated mine openings. 

Terrill and VandeKraats [6] reported on an unusual 

opportunity to view continuous cross sections of 

mining-induced fractures when a halite roof was 

excavated higher with a roadheader along 275 m of 

drift (Figure 1). They concluded that (1) fractures first 

formed near corners, (2) failure either took the form 

of a fault that angled upward toward the centerline of 

the drift or, more commonly, a zone of en echelon 

extension fractures that possessed similar geometry, 

(3) both the zone of en echelon fractures and the fault 

exhibited a component of slip, (4) dilation of individual extension fractures was pronounced, and (5) the 

faults and/or fracture zones extended upward from 

either or both sides of the roof. The observation that 

the faults and the en echelon fracture zones displayed 

similar geometries and displacements and also alternated with each other in their development suggests 

that both types of structures are closely related, which

is an idea also considered by Peng and Johnson [7] 

Although they documented the essential characteristics of the fractures, Terrill and VandeKraats proposed no specific explanation for them or their consistent patterns. 

Based on field and laboratory examples and on Terrill 

and VandeKraats’ descriptions [6], White et al. [3] 

concluded that a fundamental pattern of fracturing 

around the periphery of rectangular mine openings 

involves dilatory shear zones that originate adjacent to 

corners and propagate obliquely away from the corners and immediate surface toward the centerline of 

the drift, as seen in Figure 1. This paper describes 

continuum and discontinuum models that appear to 

provide a mechanical explanation for this consistently 

observed fracture pattern and for creation of the 

fractures themselves. 

Figure 1.—Representative cross section showing fracture patterns in halite roof. (Modified from [6]: Figure 6.) 

2. CONTINUUM MODELING 

Published continuum models of rectangular mine

openings have generally provided no obvious explanation for mining-induced fracture zones about mine

openings. However, recognizing that actual fractures 

are commonly spaced on a scale much narrower than

the thickness of the modeling elements typically used, 

a series of models was constructed using progressively narrower elements. The goal was to find an explanation for fractures that was not evident in models with 

larger elements. Although various continuum models 

are available, the Fast Lagrangian Analysis of Continua (FLAC) [8] computer program was used 

because of the authors’ familiarity with it. A cross 

section for a square-cornered rectangular drift was 

initially modeled. 
Figure 2.—Narrowly spaced mining-induced fractures 

at the periphery of a rock burst cavity at a mine in the 

Coeur d’Alene Mining District. Fractures are spaced 

0.5 to 3 cm apart. 

In a model with physical properties, stresses, and 

opening dimensions approximating those in the Coeur

d’Alene Mining District (properties of quartzite; 70 

MPa horizontal; 35 MPa vertical; 4- by 4.5-m rectangular opening) and model elements similar in 

thickness to the fracture spacings seen at a Coeur 

d’Alene rock burst site (0.5 to several centimeters) 

(Figure 2), sites of high tension were identified near 

the skin of the opening at short distances from the 

corners (Figure 3A). In this model, the calculated

tension exceeded 24 MPa, more than enough to create 

a fracture by tension alone using the material 

properties and stresses assumed. Tension was directed 

normal to the surfaces so as to produce a fracture 

parallel to the surface such as was considered by 

Fairhurst and Cook [1] and commonly viewed in 

mines. Low-magnitude tension normal to the surface 

was also distributed toward the centerline of the 

opening and close to its edge. 

From companion results obtained using the Particle

Flow Code (PFC) computer program [9], it was 

evident that formation of a fracture adjacent to an 

opening relieves compressive stress in the slab of rock 

created. Therefore, a fracture was simulated by

removing a row of elements containing the initial 

tension site. 

Removal of the immediate element containing the site 

of high tension in Figure 3A created an elongate 

region of tension extending diagonally upward and 

away from the corner (Figure 3B). Within this tensile 

region, two sites of concentrated tension were evident. 

The second site was farther upward and beyond the 

end of the fracture and possessed a lower level of 

tension. 

The effects of propagation of the fracture were then 

simulated by removing additional elements in the row 

(Figure 3). As the fracture was extended, the new 

tension sites described above migrated with the end of 

the fracture after first separating from and revealing a 

third tension site. The third site was located deeper in 

the rock and away from the corner end of the fracture 

by a distance equal to that of the original tension site

from the corner (Figure 3C, 3D). Thus, this tension 

site duplicated the one associated with the corner,

although at a lower level of tension. Yet another site 

of tension appeared more distant and over this corner 

(Figure 3D).

Tension at the site at the end of the simulated fracture

diminished in magnitude with further extension of the 

fracture, which would have tended to cause the fracture to stop propagating. In contrast, the amount of 

tension beyond and above the end of the simulated 

fracture decreased more slowly as fracture length

increased and ultimately exceeded tension at the end 

of the fracture. This could have promoted formation 

of a new fracture in an en echelon position with 

respect to the previous fracture. A factor that was not 

considered was the influence of maximum principal 

(compressive) stress on failure, which would also 

favor fracture initiation at the en echelon position. 

Finally, tension at all secondary sites was less than 

tension at the original site, suggesting that formation 

of new fractures might ultimately cease. 

In addition to the work described above, Whyatt [10] 

confirmed FLAC results with a double-precision 

modeling run and also duplicated them with the 

Universal Distinct-Element Code (UDEC) computer 

program by using model elements similar in size to 

those used in this study. 
Figure 3.—Portion of FLAC model showing least principalstress near the corner of a rectangular opening for stress conditions and rock

properties approximating those of the Coeur d’Alene mining district. Untextured pattern along edge of opening reflects tension. A. 

Initial model. Tension reached 24 MPa near the corner, but a maximum of only 1.4 MPa along the edge of the roof farther from the 

corner. B, C, D. Least principal stress for simulated fracture created by removing a progressively longer row of narrow elements, 

starting (3B) with removal of the element containing the highest stress in A). A region of tension extends diagonally upward to a second 

site of concentrated tension. C, D. Third and fourth tension sites are revealed as the simulated fracture is extended to right, tensile

regime expands, and tension sites at the end of the simulated fracture move with progressive extension of the fracture. 

However, although these continuum models identified plausible reasons for (1) initial fracture formation and ultimate stabilization and (2) formation of a

new en echelon fracture, the mechanisms responsible for the localized areas of tension that led to this 

behavior were not obvious. To identify the source of

the tension, a discontinuum model using PFC was 

employed. 

3. DISCONTINUUM MODELING 

PFC models materials as assemblages of elastic 

particles bonded together by breakable elastic bonds. 

Just as with continuum methods, models may be

constructed that simulate conditions about mine 

openings. However, PFC permits failure to take 

place. Bond stress, displacement from the prior 

modeling step, and failure show a progression that 
can illustrate how mining-induced fractures and 

fracture zones develop. 

A PFC2D simulation was used to investigate the 

failure process adjacent to a mine opening. 

Numerous model runs were undertaken using large 

rectangular blocks of particles containing rectangular openings. Results resembling deformation 

observed in the field were obtained. However, for 

the immediate exercise, a model was developed to 

simulate conditions at a single corner. This made it 

possible to use smaller particles compared to the 

scale of the opening while also shortening the run 

time. 

In the simulation, an assemblage of particles was 

first constructed in the form of a rectangular shape 

or block. The particles were tightly packed and 

bonded to simulate a strong homogeneous rock. The 

assemblage was confined by four walls, which 

provided a confining pressure of 1.0 MPa. A 

rectangular portion of the assemblage was then 

deleted so that the remaining particles formed a 

notch representing the corner of a mine opening 

(Figure 4). Opposing walls were then moved inward 

on one axis at 0.2 m/sec, and the walls in the second 

axis were controlled to maintain a confining 

pressure of 1.0 MPa. The inward-moving walls 

induced a deviator stress onto the assemblage. The 

walls continued moving inward as the assemblage

progressively failed. 

Figure 4.—PFC model formed by loading margins of a

rectangular block and then excavating a rectangular area to 

simulate a cross section of a corner of a mine opening.

Horizontal load was then gradually increased so progressive

failure could be observed. 

The boundary conditions applied to the model are 

considered to represent only a rough approximation 

of conditions about mine openings. However, the 

boundary conditions used for the model resemble 

those of many reported laboratory strength tests for 

which Fairhurst and Cook [1] suggested that the 

resulting fracture patterns simulate those seen in the 

field. The failure patterns obtained with the PFC 

model strongly resemble those seen in the laboratory

samples as well as in the field, suggesting that the 

boundary conditions used are reasonable. 

Results from the PFC model documented initiation

of failure near the corner and propagation of a 

failure zone away from it and deeper into the model 

toward the centerline of the simulated opening. 

Dilation and shear, as seen in the field and reported 

by Terrill and VandeKraats [6], were also conspicuous. 

Although the individual bond failures involved in 

the creation of the fracture zone included shear 

failures, tensile bond failures dominated. For the

square-cornered model in Figure 4, a long, single 

fracture parallel to the edge of the opening did not 

form initially. Instead, an inclined broken zone that 

manifested comminution, dilatence, and shearing 

began to develop (Figure 5A). Furthermore, the 

initial cluster of bond failures did not take place 

close to the edge of the opening, but at a position 

where an en echelon fracture might have been 

expected to form (site X in Figure 5A). As noted in 

the discussion of FLAC results, only minimal 

deformation near the edge of the opening was 

apparently required to cause the tension site that

formed in the offset, en echelon position. Therefore, 

the initial clustering of bond failures at that location 

may have been promoted by localized elastic 

deformation at the edge of the opening. This 

suggests, in turn, that the process zone associated 

with fault formation is somewhat self-propagating. 

Laboratory results by Lockner [11] support this 

interpretation. 

Next, bond failures spread downward and approximately half way to the edge of the opening from the 

initial concentration of bond failures. Additional 

failures then began near the edge of the opening 

(Figure 5A, labeled Y) and spread upward until the

narrow zone began to fail completely, forming a 

discrete fault. 
Figure 5.—Stages of progressive failure of square-cornered model. A. Early stage of bond failure, identified as short lines. The first 

clustered failures occurred at site X and spread downward; subsequent failures began and spread upward from site Y. Complete 

detachment along the lower part of the zone ultimately formed a fault. B. Velocity vectors (displacement from previous modeling step) 

during formation of wing crack a-b. Vectors indicate shear and dilational components along fault portion and also along wing crack. C. 

Velocity vectors as wing crack a-b reached its fullest extent. A new cluster of failures (site of diverging vectors in offset position c)

defines early stage of first en echelon fracture. Note scattered distribution of these early failures at this site. D, E. Velocity vectors 

indicating new en echelon fracture d-c-e and sites of initiation of two subsequent en echelon fractures f and g. In D, fault zone has

propagated to fracture d-c-e and activated it as a wing crack. Each long extension fracture propagated through a series of individual

bond failures at fracture tips. Particles not shown in E. 

When finally a horizontally propagating extension 

fracture materialized, it began only when the fault was 

fully detached and had begun to slip (Figure 5B). This 

extension fracture formed at the temporarily stabilized 

end of the fault. Hence, the fracture was equivalent to

a wing crack forming at the end of a sliding crack, 

two structures that are normally studied in the form of

microcracks. At this point, velocity vectors (displacements from the previous modeling step) (Figure 5B) 

indicated that the fault had become a discrete velocity
discontinuity and also that both extension and shearing were taking place along the fault, as well as at the 

tip of the developing wing crack. 

Here it is of interest to note that wing cracks are 

typically described as tension cracks. In this case, 

however, the wing crack exhibited pronounced shearing displacement along its entire length. Numerous 

PFC model runs indicated that a combination of shear, 

tension, and extension is, in fact, ubiquitous to extension fractures. Fracture propagation halts when shear 

and tension at fracture tips cease. This indicates that 

extension fractures are inherently of mixed mode 

(involving both tension and shear, or modes I and II, 

respectively, in the usual nomenclature). Thus, 

extension fractures cannot form parallel to the 

direction of maximum principal stress, as is generally 

assumed, but at some low angle to it (since a plane 

parallel to the maximum principal stress contains no 

shear stress). 

After the wing crack was well developed, a new concentration of tension failures appeared at a site 

upward and beyond the end of the wing crack in an 

offset, en echelon position (Figure 5C). Meanwhile, a 

near-by area containing some earlier-formed bond 

failures (labeled X) remained essentially inactive, 

which suggested that failure development was strongly influenced by imposed, local displacements and 

that pre-existing areas of weakness that were somewhat removed from such sites might not be involved 

in the immediate deformation. The newly active site 

initially formed a rather chaotic cluster of failed 

bonds, but ultimately developed into a discrete 

fracture that propagated toward the centerline of the 

opening in a progression of individual bond failures 

with no conspicuous process zone development. This 

fracture ultimately spawned a second en echelon 

fracture (Figure 5D), which, in turn, led to a third 

(Figure 5E). 

Displacement associated with individual bond 

failures, as well as failures that extended individual 

extension fractures, involved lateral convergence of 

particles toward the tips of propagating fractures. 

Outward divergence from the actual bond failures and 

central portions of longer extension fractures reflected 

extension and tension. However, displacement components parallel to extension fractures on opposite 

sides of fracture tips and along central portions of 

active fractures were unequal or were opposite in 

direction, reflecting the existence of shear in the plane 

of the fractures. 

As the fracture zone developed, PFC revealed continuously evolving displacements associated with 

shear along the entire fracture zone, which resulted in 

localized shear and in divergence that caused extension or actual tension. The PFC model identified fracture formation about the square-cornered opening as 

resulting from a progression of failures and readjustments that produced additional failures that ultimately 

extended the greater shear zone. 

4. DISCUSSION AND CONCLUSIONS 

Field observations have identified a fundamental 

pattern in mining-induced fractures about rectangular 

openings. This pattern is characterized by shearing 

and dilation and the formation of en echelon fractures. 

An opening surrounded by very small elements was 

modeled using FLAC. The FLAC models identified 

sites of high tension near the skin and corner of an 

opening where an extension fracture would be initiated, propagated away from the adjacent corner, and 

ultimately stopped as tension at the fracture tip diminished. Meanwhile, a new tension site would develop 

above and beyond the distal fracture tip in a position 

that could form a new en echelon fracture. This 

process would then be repeated until displacements 

were no longer extensive enough to produce new 

fractures. 

Using PFC, the fracture process was simulated and 

showed that (1) individual microfractures formed as a 

result of combined shear and tension and (2) macrofractures propagated by the same basic mechanism. A 

fault generated with PFC spawned a wing crack, and 

the wing crack, in turn, spawned an en echelon 

fracture. Several additional en echelon fractures followed as the final development of the youngest 

extension fracture in the fracture zone coincided with 

the creation of a new region of tension where a new 

en echelon fracture formed. 

Incidental observations of interest are that individual 

macrofractures modeled with PFC propagated in their 

own plane in combined crack-tip tension and cracklength extension and shear. Based on the current 

example and on other modeling, such mixed-mode 

behavior is characteristic of extension fractures. In 
general, such fractures form at a low angle to the 

greatest compressive stress, not parallel to it. 

PFC modeling demonstrated minimal process-zone 

development at the leading edge of extension 

fractures. No significant direct contribution to fracture 

formation by interaction and coalescence of microfractures (bond failures) was seen, which suggests 

that this process is not essential for formation and 

propagation of individual extension macrofractures. 

However, coalescence of microfailures was fundamental to fault development. 

The concurrence of both FLAC and PFC models with 

field observation indicates that these methods 

reasonably simulate the mechanisms of formation of 

common mining-induced fractures. In subsequent 

FLAC modeling, it was found that the magnitude of 

tension near the skin of the opening could be greatly 

reduced, but not eliminated, by changing the shape of 

the corner. Rounded corners produced little tension. 

However, PFC models duplicated the types of failures 

seen with a square corner, indicating that the 

fundamental failure patterns and mechanisms remained unchanged. These studies suggest that modifications of corner and opening shapes could be 

incorporated into mine designs to produce more stable 

mine openings and reduce the risks of rock falls and 

rock bursts. 

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