Safer mine layouts for underground stone 

mines subjected to excessive levels of 

horizontal stress 

A.T. IANNACCHIONE, T.E. MARSHALL, L. BURKE, 

R. MEVILLE AND J. LITSENBERGER 

Abstract 

Excessive levels of horizontal 

stresses cause ground-fall hazards 

in underground mines in the Appalachian Basin. At an underground 

stone mine in Pennsylvania, a 

modified stress-control mine layout 

is reducing the hazardous conditions associated with excessive 

horizontal stresses. A microseismic 

monitoring system is in place to 

measure levels of rock stability and 

provide information on the effectiveness of the design technique. 

The microseismic data is 

supplemented with frequent and 

extensive mapping of roof falls and 

roof rock damage. Findings to date 

show that the stress-control layout 

provides more stable conditions, 

resulting in a safer environment for 

the mine workers. 

Introduction 

The U.S. National Institute for 

Occupational Safety and Health 

(NIOSH) continues to study the 

impact of different mine layouts to 

control the damaging effect' and resultant hazardous conditions of 

horizontal stress in underground 

stone mines, Successful stable mine 

designs need to be identified and 

communicated to the underground 

stone industry, so that safer 

working conditions for miners can 

be achieved. As underground stone 

production increases, so will the 

need to develop and use safe mine layouts to minimize 

hazardous ground conditions. Excessive levels of horizontal stress are present in many parts of the earth’s 

crust and have often been produced by tectonic plates 

pushing towards one another. Residual stresses from 

these events have been retained in rock, even at low 

overburden (Bickel, 1993). 

The study was conducted at an 

underground stone mine in southwestern Pennsylvania. There, 

excessive levels of horizontal stress 

were known to exist and such 

stresses have been observed to 

cause roof falls. At this mine site a 

traditional room-and-pillar and a 

new stress-control layout were in 

use. The ability of these two mine 

layouts to control ground was 

accomplished by mapping roof falls 

and roof damage areas and by 

monitoring seismic activity with a 

12-channel microseismic system. 

How can horizontal stress 

affect roof stability? 

At the mine, the measured 

N60°E horizontal stress field 

(Iannacchione et al., 1998) matched 

reasonably well with the regional 

east-northeast stress field found in 

other mines within this region (Fig. 

1). Additionally, maximum values 

of horizontal stresses ranged from 

about 10 to 35 MPa (1,400 to 5,000 

psi), with an average of about 25 

MPa (3,600 psi). This range in values of horizontal stress is 10 times 

greater than the stress produced from 

the overburden weight in this part of 

the mine. 

The effects of excessive levels 

of horizontal stress on roof rock 

stability have been discussed by 

several researchers (Emery 1964;

Parker, 1966; Parker, 1973; Gale 

1986; Mark and Mucho, 1994; 

Mucho and Mark, 1994; Iannacchione et al., 1998). 

Mining perpendicular to the orientation of the maximum 

levels of horizontal stress can concentrate that stress in 

the immediate roof rock. When stress levels exceed the 

strength of the roof rock, beams within the roof can 

buckle and fail in shear (Fig. 2). Traces of failure planes 

often appear as low-angle shears approximately 150

from horizontal and 

oriented perpendicular to the direction of the maximum levels of horizontal stress. 

In some mining situations, the 

shear planes can occur with enough 

frequency to coalesce, forming a 

semilinear trend tens-to-hundreds of 

meters in length across a mining 

section. Major shear zones can result 

in large roof falls that are often oval 

in shape, with the long axis perpendicular to the orientation of the 

maximum levels of horizontal stress 

(Fig. 3). Typically, these falls show 

stress concentrations along the axial 

ends of the oval, oriented perpendicular to the maximum stress 

direction (Fig. 3). Elevated stress 

levels are also found in the solid 

rock above the caved area. It is not 

uncommon for this oval-shaped type 

of fall to grow vertically and 

horizontally over the course of days, 
weeks and even years, depending on conditions. 



FIGURE 1 

Measured horizontal-stress conditions in mines in western Pennsylvania, eastern Ohio and northern 

West Virginia (Mark and Mucho, 

1994; lannacchione el al., 1998). 

FIGURE 2 

Low-angle shear failure in limestone roof beam caused by excessive levels of horizontal stress. 

Another recognized characteristic of a roof fall 

caused by horizontal stress is the zone of reduced stress 

adjacent to the long axis of an oval-shaped roof fall. 

These areas of stress reduction are commonly referred to 

as stress shadows (Fig. 3). The directional nature of the 

roof falls and adjacent stress shadow zones provide essential information for determining layouts aimed at reducing this type of ground fall hazard. The potential for 

additional roof instability is highest when mining perpendicular to the direction of maximum horizontal stress 

and lowest when mining either parallel to the direction of 

maximum horizontal stress or ill the stress shadow areas 

adjacent to the long axis of a roof fall. 

Identifying stress-related damage 

Large, directional roof falls that are often oval in 

shape typically indicate excessive levels of horizontal 

stress. Horizontal stress also causes roof falls of lesser 

magnitude. Mapping falls in relation to the direction of 

mining provides some of the information necessary to 

determine if a series of fails were induced by horizontal 

stress, 

Figure 4 demonstrates how a progression of roof 

falls can occur as mining advances. Roof falls associated 

with excessive horizontal stress typically occur as mining 

advances into virgin rock perpendicular to the orientation 

of maximum stress. Once the fall occurs, stress is 

temporarily relieved. The process is illustrated in fig. 4. 

During mining in Area 1, a fall occurs after mining advanced to the east in the direction perpendicular to the 

stress field. As mining advances to the south towards 

Area 2, the stress relief or shadow from the fall in Area 1 

provides temporary stability. Then, a mining continues 

southward, the stress levels again build or concentrate 

until relieved by Fall 2. The pattern is repeated in Areas 3 

and 4. 

NIOSH personnel have observed this pattern or 

progression of falls at a number of mines. With this 

knowledge, decisions on changing mine layouts can be 

made to minimize the instabilities induced by horizontal 

stress. Conversely, without this information, it is often 

difficult to recognize a pattern because damaged roof 

zones appear to occur in a random fashion across a mining section. 

How can mine layouts control 

stress-related damage? 

One potentially safer way to room-and-pillar mine in 

a directional stress field is to advance a greater percentage 

of faces parallel to the direction of maximum horizontal 

stress, while minimizing the percentage of faces driven 

perpendicular to this direction (Fig. 5). In a stress-control 

layout, headings are oriented in a more favorable direction 

while crosscuts are in the less favorable direction. 

Mining in this fashion should help to lessen the overall amount of roof rock damage by: 

 Maximizing the number of headings driven parallel 

to the direction of maximum horizontal stress. The 

roof rocks in these headings have lower stress levels. 

 Lessening the number of crosscut driven perpendicular to the stress field minimizes the amount of 

mining where stress levels are the greatest. 

 Driving the crosscut faces only into an existing 

heading. This reduces the potential for high stress 

concentrations in those faces. 

 Offsetting crosscuts to create only three-way intersections. This is helpful in that if failures develop, 

the roof falls developing perpendicular to the 

maximum stress direction will quickly encounter the 

barrier formed by the rib of an adjacent pillar. 
 Maintaining a wedge-shaped 

mining front parallel to the direction of maximum horizontal 

stress. This will help to evenly 

distribute the stress concentrations along the entire mining 

front. 

FIGURE 3 

Probable stress redistribution patterns around 

oval-shaped roof falls, concentrating stress above and 

along the axial ends and reducing stress (stress 

shadow) adjacent to the long axis of the rooffall. 

FIGURE 4 

The likely location of successive roof failures at 

four face positions during room-and-pillar mining 

in a directional stress field. 

How can a mine layout be 

evaluated for safety and 

effectiveness? 

As discussed, the orientation 

and spacing of headings and crosscuts will affect the stability of the 

roof rock when excessive levels of 

horizontal stress are encountered. It 

is critical to use the layouts that provide the safest working conditions 

for miners in the underground stone 

industry. Unfortunately, there are 

currently no widely accepted guidelines available for evaluating mine 

layouts in these conditions. 

Methods to evaluate safe mine 

layouts consist of the following two 

categories: observational or qualitative and measurement or quantitative. Observational technique' 

consist of mapping roof rock instabilities through time. Measurement 

methods consist of assessing 

characteristics that are indicative of 

rock failure. These characteristics 

could include stress changes, rock 

deformations and/or microseismic or 

acoustic emissions. 

In this study, observation and 

measurement techniques were used 

to evaluate the effectiveness of two 

different mine layouts subjected to 

excessive levels of horizontal stress. 

The observational technique used 

consisted of mapping stress-related 

failures and roof fall at regular 

intervals (usually every two weeks) 

and placing this information on 

working mine maps. The measurement technique consisted of monitoring the quantity of seismic 

emissions emanating from the mine 

as faces were advanced. It is known 

that as rock fails in tension or shear, 

it emits audible sounds. In 1975, 

Hardy examined this concept when 

he stated, "In geologic materials, the 

origin of acoustic emissions/

microseismic activity is not well 

understood, but it appears to be 

related to processes of deformation 

and failure, which are accompanied 

by a sudden release of strain 

energy." Microseismic monitoring 

systems can be designed and placed 

in such a way as to locate the source 

of energy release associated with 

roof failure 


(Fig. 6). With this tool, the relative quantity of roof 

damage can be assessed continuously as mining 

progresses. 

FIGURE 5 

Mine layout with a high percentage of rooms driven parallel to the maximum stress direction this layout should 

lessen overall roof damage. Staggered crosscuts reduce 

the potential for running roof falls. 

FIGURE 6 

Idealized example of how strain energy is released by 

shear and tensional rock failures, transmitted through 

the rock as a compressive wave and captured by a microseismic system's geophone. 

Working hypothesis

This study evaluated pillar design and room orientation as a tool to reduce the hazardous effects of excessive 

horizontal stress conditions in underground stone mines. 

The study is based on the following working hypothesis: 

Roof strata subjected to excessive 

levels of horizontal stress often fail 

in shear and tension. This failure is 

often of high enough intensity to 

produce seismic energy (acoustic 

emissions) that can be recorded by a 

properly tuned microseismic 

system. If two different mine layouts 

are used in similar geologic and 

overburden conditions, the layout 

that concentrates less stress around 

working faces will produce less rock 

failure and. hence, less microseismic 

activity. Therefore, the layout with 

significantly fewer roof falls and 

less microseismic activity should, in 

general, invite less roof-strata 

damage and less loose rock that 

could potentially injure miners. 

Test mine site conditions 

The southwestern Pennsylvania 

study mine site produces crushed 

stone from the Loyalhanna (limestone) Formation. Roof falls found at 

this mine fit the characteristics of 

those caused by excessive levels of 

horizontal stress as discussed previously. Roof instabilities typically 

start with the development of compression zones. These zones consist 

of low-angle shears oriented approximately 30°W. When roof falls 

occur, many of them are oval in 

shape, with the long axis oriented 

approximately N30°W. In the northeastern section of the mine, these 

roof falls follow this same northwesterly direction (Fig. 7). All of 

these failure patterns indicate a 

strong horizontal stress field oriented at approximately N60°E. Additionally, hydrofracturing tests at 

tile site measured the maximum 

stress direction to be 60°E to N75°E 

(Iannacchione et al., 1998). 

In February 1998, mine management initiated a new mine layout to 

lessen the degree and frequency of 

roof instabilities at the mine site. 

This stress-control mine layout was 

designed to advance the majority of 

the faces parallel to the maximum 

horizontal stress direction in the 

northeast section of the mine (Fig. 

7). Two lead headings were driven 

N60°E in advance of the other headings to the left and right Crosscuts 

were generally driven N30°W or 

S30°E only after the adjacent 60°E 

headings had been advanced past
the breakthrough point. Only 

three-way intersections were used, so 

that the crosscuts were offset. Pillars 

were rectangular in shape with the 

long axis oriented N60°E. Pillars, 

27.4 m (90 ft) long, were designed to 

provide a solid, continuous rib to 

hinder propagation of the directional 

failures in the 30°W direction.

Along the east section of the 

mine, two north-south trending 

headings were driven to outline a row 

of ventilation barriers and to prepare 

set-up entries for turning all headings 

in the N60°E direction. While the east 

section used a mining plan similar to

that used in the southeast section, it 

lacked sufficient crosscut 

development to be considered as 

similar to the southeast layout. 

In the southeast section of the 

mine, mine management decided to 

continue with the traditional roomand-pillar mine layout until the east 

and south mining fronts could be 

straightened to facilitate the turning 

of the entries in that part of the mine. 

Rooms were driven 10°E and N50°W 

on 24.4-m (50-ft) centers outlining 

10.7-m (35-ft) square pillars. The 

decision to continue with the 

traditional mine layout in an area 

close to the new stress control mine 

layout provided a good opportunity to 

compare the two designs. 

NIOSH installed a microseismic 

system during the winter or 

1999-2000 and began monitoring 

microseismic activity on Feb. 9, 

2000. The locations of the faces that 

were mined from February 2000 until 

August 2000 are shown in Fig. 7. The 

microseismic system consisted of 

data-acquisition devices, filtering and 

analysis equipment, 12 geophone 

sites located throughout the area of 

interest, and cables connecting the 

gephones to the instrument trailer. 

FIGURE 7 

Mine layout at the start of the field study on Feb. 9, 2000, and 

the location of roof falls that occurred during the study. Also 

shown are the direction of the maximum horizontal stress and 

the faces mined during the study. 

Roof falls and microseismic activity 

During the study period, four significant roof falls 

occurred. Three of them occurred in the southeast section, one each during February, March and May. The 

fourth fall occurred in the southeast corner of the east 

section during June-July (Fig. 8). The northeast section 

did not have any roar falls and experienced only minor 

instabilities in the immediate roof. 

During this same period, the microseismic system 

recorded approximately 1,443 events, after data associated with blasting, electrical surges or mining activity 

such as Ian, equipment and drill vibrations were filtered 

out. A more complete analysis of all the microseismic 

activity is planned and may help to further determine 

o

how stresses interact with mining to cause roof rock failure. 

While the location and magnitude of any individual 

event can have some degree of error associated with it, the 

accumulated data can and do show significant trends. Of

the 1,443 events, 1,302 were located within one of the 

three sections shown in Fig. K. Of these, 194 (15%) occurred in the northeast section, 275 (17%) occurred in the 

east section and 833 (64%) occurred in the southeast 

section. Much of the activity in the southeast section is 

clustered around the three (February, March and May) 

roof falls. However, significant activity also occurred in a 

wide band extending from behind to well in front of the 

working faces. Microseismic activity in the east section 

was largely evident along the eastern mining front, with 

much of it clustered around the June-July roof fall in the 


southeast corner. The northeast section was characterized 

by little microseismic activity, with a weak clustering in 

the unmined limestone between the new and traditional 

mine layouts. 

Another (actor to consider in conjunction. with microseismic activity is mining face advancement, referred 

to in this paper as the activity rate. At this mine, most 

microseismic activity was associated with excessive 

stress levels that induced rock deformation and failure. 

Increases in rock deformation and failure were, in turn, 

caused by advances in mining. Therefore, normalizing 

microseismic activity to face production provides one 

way of testing part of this study's hypothesis - "the layout 

that concentrates less stress around working faces will 

produce less rock failure and hence. less microseismic 

activity." During the study period, 138 faces were blasted 

in the northeast section, 71 in the east section and 130 in 

the southeast section (Fig. 7). The activity rates for each 

month during the study period and corresponding to each 

section in the study area are shown in Fig. 9. In general, 

the activity rate for the southeast section is highest, 

followed by the cast section. Additionally, the months of 

February, June, July and August show the 

highest activity rates for the southeast section. Except for 

August, these are the months when roof falls were occurring in the east and southeast section. 

The activity rate from the northeast section averaged 1.32 

microseismic events per production blast, with a standard 

deviation of 0.68. In the east section, the activity rate had 

an average of 3.94 with a standard deviation of 3.21, 

while the southeast section had an average of 7.67 with a 

standard deviation of 4.57. Clearly, the roof fall and 

microseismic data show that the amount of roof rock 

instabilities in the new stress control layout was significantly lower than that in the traditional room-andpillar layout. Because of the significantly lower 

seismicity and observed roof falls, the authors believe 

that the working hypotheses are validated - this stress 

control mine layout produced significantly fewer roof 

Calls and less microseismic activity and, in general, less 

roof strata damage. Less failed rock reduces the potential 

for falls of ground and, therefore, makes for a safer 

working environment for the miners.

FIGURE 8 

Locations of 1,302 microseismic events recorded from 

Feb. 9 through Aug. 31, 2000, and contained within the 

northeast, east and southeast sections. 

Summary and conclusions 

Observational methods in the form of roof-rock

damage mapping were conducted 

during the mining of both a stress 

control and a traditional roomand-pillar layout. During the study 

period, (our roof falls were observed. 

All of them occurred within the east 

and southeast sections, where the 

traditional room-and-pillar layout 

was used. Additionally, both the 

frequency and rate of microseismic 

activity were much less in the 

stress-control layout than in the 

traditional room-and-pillar layout. 

The stress-control layout that was 

successfully demonstrated in this 

study has the following attributes: 

• It increases the total proportion of 

headings driven in the favorable 

direction with lower stress levels. 

• It lessens the total proportion of 

crosscuts driven in the less-favorable 

direction with elevated stress levels. 

• It drives crosscuts generally 

into an existing heading . 

• It offsets crosscuts to create more 

stable, three-way intersections • It 

reduces the potential for running roof 

falls by offsetting crosscuts in the less 

favorable direction . 

• It maintains a wedge-shaped mining 

front parallel to the maximum 

horizontal stress field. 

All of these attributes help to 

lessen horizontal stress concentrations in the root and reduce the potential for roof rock failures. 

This NIOSH field study has 

shown that there is less observable 
roof failure, and microseismic activity from the northeastern stress-control layout than from the eastern or

southeastern traditional room-andpillar layout. The amount or mining

from the northeastern portion of the

mine in this study was at least

equivalent to the production from the

eastern and southeastern portions of

the mine. 

Therefore, the observations and

measurements support the working

hypothesis that the stress-control

design employed here does not

damage the roof rock as much as the

traditional room-and-pillar design. It

then follows that it is a safer design

from the standpoint of reducing falls

of ground. The stress-control layout

should have 'widespread application

for protecting underground stone

miners where excessive levels of

horizontal stress exist. • 



FIGURE 9 

Histogram showing the microseismic activity 

rates for the three mining sections. 

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