1. BACKGROUND 

There are 118 operating underground stone mines 

listed in the Mine Safety and Health 

Administration’s list of stone mine workings [1]. 

All of these mines may not be operational at any 

particular point in time. The mines generally use 

room and pillar methods to extract limestone 

formations at depths of cover that range from 

outcrop to approximately 900 m (3,000 ft). About 

80% of the mines operate at a depth of cover of less 

than 90 m (300 ft). The formations are usually flat 

dipping, but some mines that are located along 

synclinal or anticlinal structures can operate at 

moderate dips of between 5° and 20°. A few cases 

exist where mines operate at dips of between 20° 

and 90°. 

1.1. Horizontal Stress 

Horizontal tectonic stresses have been identified as 

one of the causes of roof instability in stone mines 

[2] and can be associated with microseismic activity 

[3]. The mines are located in the mid-North 

American plate where elevated horizontal stresses 

have been measured [2] in the limestone formations 

and in many of the area’s coal mines [4]. Latest 

research has shown that the horizontal stress may be 

explained by the effect of plate tectonics [5, 6]. 

Tectonic loading is related to the movement of the 

North American plate as it is pushed away from the 

mid-Atlantic ridge. Elevated stresses are not 

necessarily present in all the limestone formations 

because local features such as outcropping and 

folding may have relieved the stresses over 

geological time [7]. The tectonic stresses can cause 

roof failure by progressive shearing of rock beams 

in the roof of the excavations. Failure of the intact 

rock has been observed in dipping excavations, 

where tectonic stresses were present [8]. 

In lower stress environments the dip of the 

workings can also affect the stability of the roof 

beam by mobilizing shearing along specific joint 

sets, without failing the intact rock. Failure in low 

stress conditions is characterized by movement of 

large blocks that are bounded by joint surfaces. 

 

 

Effect of the Dip and Excavation Orientation on 

Roof Stability in Moderately Dipping Stone Mine Workings 

Esterhuizen, G.S. 

NIOSH, Pittsburgh Research Laboratory, Pittsburgh, Pennsylvania, USA 

Iannacchione, A.T. 

NIOSH, Pittsburgh Research Laboratory, Pittsburgh, Pennsylvania, USA 

 

ABSTRACT: Underground limestone mines typically use the room-and-pillar method of mining in the generally flat-lying 

limestone formations. In some cases the dip may exceed 5° which can result in unique roof instability problems. Stability may be 

further exacerbated by the presence of horizontal tectonic stresses. Field studies and numerical model analyses are combined to 

assess the affect of the dip in various stress conditions. The results showed that where the horizontal tectonic stresses are sufficient 

to cause intact rock failure, increasing dip results in increased failure potential. The effect of the dip is greater when the maximum 

horizontal stress is parallel to the dip. Two case studies are presented and evaluated against the findings of stress analysis results. 

The study showed that the empirical rules of orienting headings parallel to the maximum stress are generally applicable to dipping 

excavations, except when mining in the up-dip direction. The importance of accurately determining the orientation of the 

maximum horizontal stress is emphasized because the window of favorable orientations relative to the stress is small.
1.2. Mine Layout 

Experience has shown that poor roof conditions can 

be mitigated by changing the mine layout so that the 

rooms and pillars are aligned more favorably to the 

prevailing field stress and local joint orientations, 

[2, 4, 9, 10]. An example of the successful reorientation of stone mine workings is presented in 

Kuhnhein and Ramer [11]. In general, the empirical 

guidelines advocate aligning the direction of 

heading advance parallel to the direction of the 

maximum horizontal stress. The experience and 

empirical guidelines for optimizing layouts are, 

however, largely based on mining in near horizontal 

formations. 

At moderate dips it is not always possible to orient 

the mining layout in the most favorable direction for 

rock stability. The direction of the workings can be 

dictated by the operational requirements of a 

relatively shallow dipping floor for the large rubber 

wheeled mining and hauling equipment. One 

solution to the problem is to develop the headings 

and cross-cuts at an inclination to the strike of the 

formation, providing access at a more favorable 

“apparent dip”. This orientation may not be the 

most favorable for rock stability. 

1.3. Objectives 

In the case of a dipping formation, the complex 

interactions between the dip and the field stress, 

joint orientations and mine geometry are not readily 

apparent, making it difficult to design a mine layout 

that optimizes roof stability. This paper presents 

the results of numerical model studies and field 

observations that were carried out in an 

investigation of the effect of the dip on roof 

stability. The study was limited to cases where the 

field stresses are sufficiently large to affect roof 

stability. An additional objective was to determine 

whether the empirical guidelines for layout design 

in flat dipping formations are also applicable to 

room and pillar workings at moderate dips. 

2. FIELD OBSERVATIONS OF DIP EFFECTS 

ON ROOF STABILITY 

Two underground stone mines in the Northern 

Appalachian region have been studied as examples 

of the effects of dip on roof stability. The mine 

layouts, observed failures and the issues related to 

roof stability in dipping formations are discussed 

below. 

2.1. Case Study 1: A mine in the Loyalhanna 

Limestone Formation 

The mine extracts high quality limestone for 

construction purposes from the Loyalhanna 

Limestone Formation in the Chestnut Ridge area, 

Western Pennsylvania. The workings located on an 

anticlinal structure and are nearly horizontal along 

the crest of the anticline but the dip increases to 7° 

in the south-western part of the mine as the mine 

expands down the side of the anticline. The Rock 

Mass Rating (RMR) [12] of the Loyalhanna 

limestone is in the range of 85 to 90 units. 

The mine exhibits many of the characteristic signs 

of excessive horizontal stress conditions. Mapping 

of roof damage has confirmed the presence of 

sporadic roof shearing and occasional massive 

directional roof falls. In addition, asymmetrical 

failures at the pillar-roof contact were observed 

only in the southwestern part of the mine where the 

dip is gradually increasing. Figure 1 shows a plan 

view of a portion of the workings and the locations 

of pillar-roof contact failures as well as other roof 

failures. One of the failure areas progressed into a 

major directional fall, with a result that the entire 

area had to be abandoned because of safety 

concerns. Figure 2 shows a roof collapse that was 

initiated by failure at the pillar-roof contact. Based 

Pillar-roof 

contact failures 

Pillar-roof contact 

failure that 

developed into a 

large roof fall

N

Shallow roof falls 

less than 1.7m (5ft) 

deep

0 100m

Scale 

7o dip 

Maximum Horizontal Stress 

Direction

Fig. 1.Sketch plan of Case Study 1 showing mine layout, 

location of roof instability and direction of maximum stress. 
on stress mapping and numerical modeling of the 

failures, it was concluded that the major horizontal 

stress was parallel to the dip of the workings in the 

area. An analysis of the effect of the dip on roof 

stability in this case [8] showed that the high 

tectonic stresses were concentrated in the up-dip 

corner of the rooms and could explain the observed 

failures. However, the study did not assess the 

optimal direction of the workings relative to the dip 

and the high field stresses. 

2.2. Case Study 2: A Mine in the Linden Hall 

Formation 

The second case study mine extracts the Valentine 

Limestone Member along the Nittany syncline 

within the Linden Hall Formation, in Central 

Pennsylvania. The Valentine member is about 25 m 

(80 ft) thick and dips at 10 to 15° to the south-east. 

The mine extracts approximately 8 m (25 ft) of the 

limestone by initial development followed by 

benching operations to a height of about 20 m 

(65 ft). Rooms are 15m (50 ft) wide. The heading 

and cross-cut directions of the workings were laid 

out so that the floor dips at an apparent inclination 

of 5-10% for operational reasons. As a result, the 

headings and cross cuts are approximately 30° offstrike. The rock mass rating (RMR) [12] is between 

57 and 69 in the areas where roof failures were 

observed. Two prominent, steeply dipping joint 

sets exist within the rock mass that strike at N57oE 

and N33oW. The joint sets are respectively parallel 

to the strike and dip of the formation. Less well 

developed, shallower dipping joints were also 

observed in the rock mass. 

Various types of roof failure were observed in the 

mine. In the shallower areas, typical joint delineated 

failure was observed, in the absence of significant 

Fig. 2. Roof failure at Case Study 1, view looking down dip.

N

150m

Dip 10-13°

Large roof fall

Roof fall < 6ft deep

Roof fall > 6ft deep

Joint structure

Roll in roof

Figure 3 : Sketch plan of case study site 2 showing room and pillar layout, location of roof instability and orientation of 

maximum horizontal stress 
field stresses. As the depth of cover increases, signs 

of stress related failure was observed. The mine 

layout and the location of the observed roof failures 

in the deeper part of the mine are shown in figure 3, 

where the depth of cover is between 150 and 240 m 

(500-800 ft). Failure of the intact rock at the pillarroof contact was observed at the up-dip side of one 

of the rooms, while failure along the down dip side 

was observed in five of the headings. Some of the 

failures ran across the rooms. Figure 4 shows a 

typical roof failure that occurred along the down dip 

side of the headings. The local jointing appeared to 

have contributed to the failure of the immediate roof 

beam in some of these headings. A large scale 

collapse in one of the intersections at the deepest 

part of the mine, also shown in figure 3, was a 

further confirmation of the presence of horizontal 

tectonic stresses. At this location the failure mode 

was similar to that described by Iannacchione et al. 

[7], having an elliptical shape with shear rupturing 

of the roof beds. Stress mapping [4] was conducted, 

which indicated that the maximum horizontal stress 

is oriented approximately along the strike of the 

workings, at N70oE, this coincides with the regional 

tectonic stress in North-Eastern Pennsylvania [5]. 

The occurrence of asymmetrical roof failures 

indicated that the dip of the workings had an effect 

of the mode and severity of roof failure. The 

presence of increasing tectonic stresses with 

increasing depth was a further complicating factor 

in this case. 

3. ANALYSIS OF DIP EFFECTS ON ROOF 

STABILITY 

Three-dimensional numerical stress analyses were 

carried out to evaluate various combinations of 

excavation layouts, formation dips and stress 

conditions. The analyses were all based on the 

assumption that a relatively high tectonic stress is 

present in the limestone formation, which can 

initiate failure of the rock mass within the roof of 

the workings. Shearing along individual joints was 

not considered, but rather general rock mass failure 

in response to excessive stresses. In practice, roof 

stability can also be affected by individual joints or 

other structures, which were not included in these 

analyses. 

3.1. Modeling Method 

The FLAC3D finite difference code [13] was used 

to conduct the analyses. Several models were set up 

to simulate arrays of rooms and pillars, an example 

of one of the models is presented in Figure 5, which 

shows the element sizes used to model the pillars, 

roof and floor rocks. The basic layout simulated a 

grid of square pillars 15 x 15 m wide by 10 m high. 

The rooms were also 15 m wide, resulting in a 

typical 75% extraction. The excavations were all 

assumed to be located in a uniform, elastic rock 

mass. 

The models were loaded by field stresses of 5 MPa 

in the vertical direction to simulate overburden of 

100 m. The horizontal stresses were set at 20 MPa 

and 10 MPa to simulate a typical tectonic stress 

field. The direction of the maximum horizontal was 

varied in the analyses. The effect of the dip was 

assessed by tilting the models through 10°, 15° and 

Fig. 4. Roof failure in heading at Case Study 2.

Roof failure along 

down-dip side of room 

15m

Figure 5: Cut-away view of a three-dimensional model of a 

room and pillar layout, showing element sizes in the pillars, 

the roof and floor rocks. 
20° while maintaining the stresses in their original 

orientations. 

The relative merits of the different layouts were 

evaluated by comparing the volume of potential 

roof failure. The failure volume was calculated by 

the Coulomb criterion, using a cohesion value of 

12 MPa and a friction angle of 30o. A “stability 

index” was calculated, which is the ratio of rock 

strength to the maximum stress, this is similar to a 

safety factor. Contours of the stability index 

provide a good visual impression of the extent and 

location of potential roof failures. 

3.2. Effect of the Dip on Roof Stability 

Two cases were considered, in the first case the 

maximum horizontal stress was set parallel to the 

dip of the formation, similar to Case Study 1, and in 

the second the maximum horizontal stress was 

parallel to the strike, similar to Case Study 2. The 

effect of the dip on roof stability was evaluated by 

calculating the failure volume in the central part of 

the array of rooms and pillars, so that edge effects 

would not be included. Figure 6 summarizes the 

results, which show that an increasing dip can have 

a significant effect on roof stability if the major 

horizontal stress is parallel to the dip of the 

workings. However, if the maximum horizontal 

stress is parallel to the strike, the formation dip has 

only a limited effect on roof stability. Figures 7 and 

8 show the stability index 2 m above the roof of the 

workings for a dip of 0 and 15°, respectively. It can 

be seen that the increased dip causes a greater 

amount of failure in the roof between pillars, note 

that the intersections are not as severely affected as 

the rooms. 

3.3. Effect of Heading Orientation on Roof 

Stability 

A second set of analyses was carried out in which 

the effect of changing the orientation of the 

workings was assessed. The objective was to obtain 

insight into roof stability in cases where mines are 

forced to change the heading orientation to achieve 

a desired apparent dip. This evaluation was carried 

out by modeling a single heading at various 

orientations relative to the dip direction of the 

limestone formation. The same two stress 

conditions described above were evaluated. These 

analyses were all carried out at a dip of 15°. The 

results were evaluated by comparing the potential 

0%

5%

10%

15%

20%

25%

30%

35%

40%

0 5 10 15 20 25

DIp of formation (degrees)

Increase in failure volume 

Maximum stress parallel to dip

Maximum stress parallel to strike

Fig. 6. Effect of dip on volume of failure in the roof of room 

and pillar workings. 

Fig. 7. Plan view showing failure index contours in immediate 

roof of horizontal workings, maximum horizontal stress direction 

shown by the arrows 

Fig. 8. Plan view showing failure index contours in immediate 

roof of workings dipping at 15°, maximum horizontal stress 

direction shown by the arrows.

15
failure volume in the roof within the first 10m 

behind the face. All the results were normalized to 

the failure volume for a heading that is parallel to 

the maximum horizontal stress. 

The results for the case where the maximum 

horizontal stress is parallel to the strike of the 

formation are summarized in figure 9. Here it can 

be seen that a heading parallel to the maximum 

stress will experience the least amount of failure, as 

predicted by the empirical guidelines. However, a 

small deviation from this optimal direction causes a 

rapid increase in the potential roof failure, 

especially if the heading is advanced in the downdip direction. 

For the case where the maximum horizontal stress is 

parallel to the dip of the formation, the results are 

quite different, as shown in figure 10. Here it can 

be seen that the most favorable direction of advance 

is in the down dip direction (0°), which is also 

parallel to the maximum principal stress. Any 

deviation from this direction results in a rapid 

increase in the amount of potential failure. 

Advancing in the up-dip direction (180°) is shown 

to be relatively unfavorable because the horizontal 

stresses are concentrated at the top of the face and 

will cause damage to the roof rocks. When 

advancing down dip, the stresses will be 

concentrated in the floor, while the roof remains 

relatively unaffected. 

4. APPLICATION OF RESULTS TO CASE 

STUDIES 

The above results can be used to comment on the 

stress effects on roof stability at the two case study 

mines. The discussion only addresses failure 

induced by the horizontal tectonic stresses. Other 

issues such as joint directions and other geological 

structures are specifically excluded here, but would 

have to be considered when deciding on alternative 

layouts. 

4.1. Discussion of Case Study 1 

Based on the results of the numerical model 

analyses, the current down-dip direction of advance 

appears to be the most favorable orientation for the 

workings, given that the maximum horizontal stress 

is parallel to the dip direction, refer to figure 10. 

However the results in Figure 6 show that the 

increasing dip can result in increasing roof 

instability. 

4.2. Discussion of Case Study 2 

The headings at Case Study 2 are developed 30° 

off-strike and the maximum horizontal stress is 

nearly on strike. Based on the results in figure 9 it 

can be seen that these orientation are unfavorable 

for stress induced failures and the stress field can be 

one of the causes of roof instability. The 

requirement to mine at an apparent dip has 

indirectly contributed to the roof instability at this 

mine. 

5. SUMMARY AND CONCLUSIONS 

The findings of this study relate to room and pillar 

type stone mining excavations in a highly 

anisotropic stress field, with the major principal 

stress being horizontal. The stresses were assumed 

to be sufficiently large to initiate failure of the rock 

mass in the roof of the excavations. Under these 

conditions, the study showed that: 

• An increase in the dip of the workings will 

cause an increase in roof instability, especially 

when the maximum horizontal stress is 

aligned in the dip direction of the workings. 

• The most favorable direction for heading 

advance is parallel to the orientation of the 

maximum horizontal stress, except when the 

advance is in the up-dip direction. 

• The range of favorable orientations is narrow; 

a deviation of 30° from the optimum direction 

can be just as bad as being 90° off. 

• The empirical guidelines, advocating heading 

development parallel to the direction of the 

maximum horizontal stress are also applicable 

in dipping formations, except when advancing 

in the up-dip direction. 

The study has shown that the dip combined with 

elevated tectonic stresses can have a detrimental 

effect on the stability of stone mine workings, as 

seen at the two case study mines. The results of the 

numerical model analyses reported above can be 

used to assist in orienting mine workings relative to 

the stress field in dipping workings. The 

importance of accurately determining the 

orientation of the maximum horizontal stress by 

stress mapping or direct measurement is 

emphasized, because the window of favorable 

orientations relative to the maximum stress 

direction is small. 
Failure Index

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2

180°

0°

90°

60°

30°

150°

120°

Down 

Dip 

Di i

90°

120°

60°

150°

Maximum horizontal 

stress direction 

30°

Heading Direction 

Figure 9: Roof failure index versus heading advance direction. Maximum horizontal stress is parallel to the strike of the 

formation. The formation dip is 15°. Increasing failure index indicates increased failure potential. 

Failure Index

0.0 0.2 0.4 0.6 0.8 1.0 1.2

0°

30° 

180°

90° 90° 

120° 

60°

30°

60° 

150° 150°

120° 

Dip Direction

Maximum horizontal 

stress direction 

Heading Direction

Figure 10: Roof failure index versus heading advance direction relative to the dip. Maximum horizontal stress is parallel to the dip

of the formation. The formation dip is 15°. Increasing failure index indicates increased failure potential. 
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