ABSTRACT 

 The roof rock in underground limestone mines in Northern 

Appalachia can be subject to high horizontal stresses in spite of the 

shallow depth of the workings. The high stresses can cause roof 

stability problems. The National Institute for Occupational Safety 

and Health staff have observed a distinctive asymmetrical mode of 

failure at the pillar-roof contact in two underground stone mines, 

which is different from the typical failure mode observed in 

response to excessive levels of horizontal stress. A dip of greater 

than 5° was identified as a possible cause of this mode of failure. 

Numerical model experiments showed that an increase in the dip of 

the workings can cause an increase in the stress at the up-dip corner 

of the roof beam. A case study is presented in which failure at the 

pillar-roof contact was observed where the dip of the workings was 

7° in a high horizontal stress field. Numerical modeling showed 

that the relatively small dip of the workings could have induced 

stresses changes in the immediate roof that would explain the 

failures. The model results also showed that this mode of failure is 

less likely to occur if the limestone pillars contain weak bedding 

planes that provide stress relief. In addition, the results showed that 

for the conditions at the case study site, the stress in the roof beam 

was not sensitive to the thickness of the roof beam or to the 

excavation span. The high horizontal stresses at this site are an 

important contributing factor to the observed failures.

INTRODUCTION 

 Northern Appalachian stone mines operate in limestone 

formations ranging from 7 m (22 ft) to over 20 m (70 ft) thick 

which are easily accessible by room and pillar mining methods. 

The roof stability in these mines may be affected by high horizontal 

stresses in spite of the low overburden depth (Iannacchione et al., 

2002). The horizontal stresses can cause excessive deflection and 

buckling of the layered limestone roof beams and may result in 

collapse of the roof. The National Institute for Occupational Safety 

and Health (NIOSH) has observed a distinctive asymmetrical mode 

of failure at the pillar-roof contact in two underground stone mines, 

which is different from the roof failure mode commonly observed 

in response to excessive levels of horizontal stress. The cause of 

the asymmetrical roof failure is not apparent and has consequently 

resulted in unexpected ground control problems in stone mine 

workings. Roof failures have been the cause of 73% of all fatalities 

and 32% of lost work days in underground stone mines from 1983 

through 2002 (Mine Safety and Health Administration (MSHA), 

2004). The Pittsburgh Research Laboratory of NIOSH has 

therefore undertaken a program of research to better understand the 

conditions that result in this type of failure and the mechanisms 

involved, with the objective to facilitate safer mine layout designs 

and improve the safety of mine workers. 

OBSERVED ROOF FAILURES 

Geotechnical Setting 

 Failure at the pillar-roof contact has been observed at several 

underground limestone mines, two of which are reported on in this 

paper. The two mines are located in Pennsylvania, one in the 

Loyalhanna Limestone Formation along the Chestnut Ridge and the 

other in the Valentine Formation, in Central Pennsylvania. The 

geotechnical characteristics of these two formations are 

summarized in table 1. The table shows that the limestone 

formations are strong and in the case of the Loyalhanna, overlain 

by weak roof rocks. In the Loyalhanna Formation, roof stability is 

achieved by maintaining a stable roof beam to support the 

overlying weaker rocks. 

Table 1. Summary geotechnical characteristics. 

Formation Dip Thickness, 

m (ft) 

Uniaxial 

compressive 

strength, 

MPA (psi) 

Characteristics 

of roof rocks 

Loyalhanna 

Flat 

lying 

to 20° 

12 up to 28 

(39 to 92 ) 

54 to 256 

(7,800 to 

38,500) 

Low strength 

shale, siltstone 

and calcareous 

sandstone, 23-

43 MPa (3,300-

6,200 psi) 

Valentine 

40° to 

near 

vertical 

Approximately 21 

(69) 

100 to 145 

(14,500 to 

21,000) 

Limestone of 

equal strength or 

stronger than the 

Valentine 

limestone 

Field Stresses 

 Stress measurements and field observations have shown that the 

horizontal stresses in the Northern Appalachian limestone 

formations are typically much higher than the vertical stress. The 

INVESTIGATION OF PILLAR-ROOF CONTACT FAILURE IN NORTHERN APPALACHIAN 

STONE MINE WORKINGS 

Gabriel S. Esterhuizen, Mining Engineer 

Anthony T. Iannacchione, Senior Scientist 

National Institute for Occupational Safety and Health 

Pittsburgh Research Laboratory 

Pittsburgh, Pennsylvania USA 
limestone mines are located in the mid-North American plate where 

high horizontal stresses have been measured in several limestone 

mines (Iannacchione et al., 2002) and in many of the area’s coal 

mines (Mark & Mucho, 1994). Latest research has shown that the 

high horizontal stress may be explained by the effect of plate 

tectonics (Dolinar, 2003; Iannacchione et al., 2002). Tectonic 

loading is related to the movement of the North American plate as 

it is pushed away from the mid-Atlantic ridge. A constant strain 

field of between 0.45 and 0.75 millistrains is associated with the 

tectonic loading, which induces high horizontal stresses in the stiff 

limestone strata. High 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 

(Iannacchione and Coyle, 2002). Table 2 summarizes the range of 

horizontal stresses measured by NIOSH in the Loyalhanna and 

Valentine limestone formations. 

Table 2. Magnitude and direction of the maximum horizontal 

stress measured in Northern Appalachian limestone mines.

Formation 

Magnitude of 

maximum horizontal 

stress , MPa (psi) 

Orientation of 

maximum 

horizontal stress 

Loyalhanna Limestone1 27.4 to 47.7 

(3,970 to 6,910) N60°E to N75°E 

Valentine Limestone2 14.9 to 29.6 

(2,170 to 4,300) N80°E 

1

Iannacchione et al., 2003 

2

Dolinar, 2004 

Typical Failure under High Horizontal Stress 

 The more typical roof failure mode observed in high horizontal 

stress conditions may be described as progressive shearing and 

buckling of individual rock layers in the roof. The failures are 

often preceded by excessive deflection of the roof beams, which 

may be associated with micro-seismic emissions. Collapse of the 

roof beams can be progressive in the vertical direction, with 

individual beds typically failing from the bottom up. Failure may 

progress over days, weeks or months until a stable rock layer is 

reached. Once a portion of the roof has failed, the failed zone tends 

to propagate laterally, in a direction perpendicular to the major 

horizontal stress. The propagation of this type of failure may be 

halted by a barrier pillar or a change in the pillar layout. Cable 

bolting has sometimes been successful in halting the propagation. 

Asymmetrical Failure at the Pillar-Roof Contact 

 Pillar-roof contact failure differs from the typical failure mode 

described above because it is asymmetrical, usually occurring only 

along one edge of each pillar in an array of pillars. The failure is 

initially restricted to a small area around the pillar edge, but may 

propagate to the full collapse of an entire room. Figure 1 illustrates 

a typical example of pillar-roof contact failure. The failures all 

appeared to be caused by stress related damage of the intact rock. 

The mode of failure, whether tensile or compressive, was not 

always readily observable. In several cases the failed rock had 

been removed and the roof had been scaled back to a solid face, 

which precluded direct observation of the failure mechanism. Both 

cases of pillar-roof contact failure were associated with a dip of 

greater than 5°. 

Potential Causes of Asymmetrical Failures at the PillarRoof Contact 

 Asymmetrical failure in the rock surrounding underground 

excavations is often caused by stresses that are not parallel to the 

excavation boundaries. In the relatively flat lying limestone 

formations, stress rotation may occur as the result of variable 

topography or faulting. Rapid changes in the depth of cover can 

result in changes in the orientation of the major principal stress. In 

addition, the dip of the workings can result in rotation of the 

excavations relative to the field stress. From observations, it 

appears that the dip of the workings was a contributing factor in 

both cases of pillar-roof contact failure studied. 

 A further potential contributing factor to pillar-roof contact 

failure is lateral displacement of the roof relative to the floor. 

Observations have shown that lateral displacements can occur 

between different rock beds in the roof, floor and within limestone 

pillars. Such movements can be expected to cause asymmetrical 

loading of the rock around the excavations. 

ANALYSIS 

 The effect of dip on roof stability was selected for further 

analysis. The mine in the Loyalhanna Formation was selected as a 

case study because the dip appeared to be the only variable that 

differentiated the areas that experienced pillar-roof contact failure 

from areas that did not. The objective of the case study was to 

determine whether the relatively minor dip of the workings could 

have been responsible for the observed failures. 

Figure 1. Photograph showing failure at pillar-roof contact 

after loose rock has been removed and roof has been resupported. 

Failure at pillarroof contact 
 The pillar-roof contact failure observed at the mine in the 

Valentine Formation was complicated by the presence of an 

anomalous geological structure in addition to the dip, and was 

therefore excluded from the analyses. 

Site Description 

 The case study mine is located in the Loyalhanna Formation 

which is nearly horizontal in the greater proportion of the workings 

but increases to 7° in the south-western part of the mine. The 

Loyalhanna formation in this area is between 17 m (55 ft) and 20 m 

(65 ft) thick. The limestone is massive, fine to medium grained 

with cross beds and strong, discontinuous bedding layers in the 

main body of the formation. The upper 3 m (10 ft) of the formation 

contains well developed laminations and alternating layers of 

weaker and stronger limestone, sandstone and shale. The 

immediate roof rocks are fine to medium grained sandstone but 

may also contain interbedded shale and limestone bands of the 

Mauch Chunk Formation. The RQD in the layered upper portion 

of the limestone formation is typically 85-95% increasing to 100% 

in the main body. At the base of the pillars in benched areas the 

floor rocks are a fine to coarse grained Pocono sandstone 

occasionally with a weak shale contact. RQD values in the floor 

are in the range of 60% to 90%. The Rock Mass Rating (RMR) 

(Bieniawski, 1989) of the massive 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. Most of the roof falls are 

oval in shape with the long axis in the N30°W direction. 

 Asymmetrical failures at the pillar-roof contact were observed 

only in the southwestern part of the mine where the dip is gradually 

increasing. Figure 2 shows a plan view of a portion of the 

workings and the locations of pillar-roof contact failures as well as 

other roof failures. At one of the locations, the pillar-roof contact 

failure progressed into a massive directional failure. In this area 

the major horizontal stress appears to be oriented at approximately 

N80°W, parallel to the dip of the limestone. The change in 

orientation is apparent from major roof falls which are oriented 

along the strike of the workings in this area. Further indirect 

evidence of the major horizontal stress parallel to the dip is the dipsymmetry of the pillar-roof contact failures. 

 In the area of interest, the rooms are developed 13.7 m (45 ft) 

wide and the development mining height is typically 8 to 10 m (26 

to 32 ft). The pillars are rectangular, having a length of 27.4 m 

(90 ft) parallel to the dip and a width of 15.2 m (50 ft). 

Approximately 2.4 m (8 ft) of limestone is left between the roof 

and the first weak layer to provide a stable roof beam. However, 

owing to the variable composition of the upper part of the 

Loyalhanna Formation, the roof may consist of alternating layers of 

strong and weak limestone, calcareous limestone and shale and clay 

bands. 

Method of Analysis 

 All the analyses were carried out using the FLAC twodimensional finite difference code (Itasca Consulting, 2000). The 

code allows mining sequences, gravity and tectonic loading as well 

as post failure behavior of rock to be modeled satisfactorily. The 

strain softening and ubiquitous joint features of the code were used 

to simulate post failure weakening of the rock as well as the effect 

of laminations in the limestone and the surrounding rocks. 

 A series of three-dimensional analyses were initially carried out 

to determine the validity of using two-dimensional models. It was 

found that two-dimensional models provide acceptable results of 

roof behavior near the center line of the pillars, which is the zone of 

interest of this study. The two-dimensional models do not provide 

any information on roof stability in the intersections. Twodimensional models allow greater detail of roof layering and stress 

changes to be modeled than would be practical using threedimensional models. 

 A parametric study of the effect of dip on stability of the roof 

was first carried out using the two-dimensional models. This was 

followed by a more detailed back analysis of the conditions in the 

case study area. Further analyses were carried out to investigate 

the effect of changes in the roof beam thicknesses and changes in 

the room width on roof stability. Finally an analysis was carried 

out in which the limestone pillars were assumed to contain two 

weak bedding planes, which allowed the horizontal stresses to be 

relieved. Since the immediate roof beam is essentially subject to 

uniaxial loading by the horizontal stress, the horizontal stress was 

used as an indicator of the potential for failure at the pillar-roof 

contact. 

Parametric Study of Dip Effects 

 The parametric study was carried out to assess the effect of an 

increase in the dip on roof stability and the potential for pillar-roof 

contact failure. A base case two-dimensional FLAC model was set 

up to simulate the mining of a series of rooms in a 20 m (66 ft) 

thick limestone bed with a nominal depth of cover of 100 m 

(330 ft). The contact between the top of the limestone and the 

overburden was simulated as a weak interface, which is free to 

slide if the frictional resistance is overcome. The model layout, 

Figure 2. Plan showing layout of pillars and location of pillarroof contact failures at case study site. 

N 

7o dip 

0 100 m 

Scale 

Shallow roof falls 

less than 1.7m 

(5 ft) deep 

Pillar-roof contact 

failures 

Pillar-roof 

contact failure 

that developed 

into a large roof 

fall 
showing the room, pillars and location of the weak interface is 

presented in figure 3. The material properties were selected to 

represent typical Loyalhanna limestone and the surrounding weaker 

rocks. Table 3 lists the material properties used in the basic model 

setup. The model was loaded in the vertical direction by gravity. 

The horizontal stresses were initialized to simulate the effects of 

constant tectonic strain, varying between 0.1 and 0.4 millistrain. 

The horizontal stress in the stiff limestone was always higher than 

in the softer surrounding rocks owing to the constant tectonic strain 

field. In these models the rock was assumed to be linearly elastic, 

so that shedding of stresses due to rock failure would not occur. 

Table 3. Base case material properties used in FLAC models. 

Material Bulk modulus, 

GPa (psi) 

Shear modulus, 

GPa (psi) 

Loyalhanna 

Limestone 49.2 (7.1 x 106

) 37.0 (5.4 x106) 

Overburden 2.2 (320,000) 1.6 (232,000) 

Floor rock 7.6 (1.1 x 106) 5.7 (826,000) 

 The effect of an increase in the dip of the workings on stresses in 

the roof was assessed by changing the dip of the limestone in the 

model to 5, 10 and 15°, see figure 4. 

 The results of an analysis in which the horizontal stress in the 

limestone was 25 MPa (3,620 psi) (corresponding to a tectonic 

strain of 0.36 millistrain) are presented in table 4. These results 

show that the stress at the down-dip corner of the roof tends to 

decrease with increasing dip, while the stress at the upper corner 

increases. The changes in stress are relatively small. This implies 

that failure will only be caused by an increase in dip if the roof is 

already highly stressed and is near the point of failure. Under such 

a scenario, an increase in dip may initiate asymmetrical failure at 

the pillar roof contact on the up-dip side of a room. 

Table 4. Effect of dip on horizontal stresses in the immediate 

roof. 

Dip of 

limestone 

formation 

Percentage change in 

horizontal stress at A1

(negative indicates 

decrease) 

Percentage 

change in 

horizontal stress 

at B1

5° -4.0% +4.6% 

10° -10.4% +7.9% 

15° -18.7% +9.4% 1

Refer to figure 4 for location of points A and B. 

Back Analysis of Observed Failures 

 A detailed FLAC model of three rooms and the four adjacent 

pillars was used to back analyze the observed pillar-roof contact 

failures. The strain-softening ubiquitous joint constitutive model in 

FLAC was used to simulate the effect of layering of the rocks. 

This allows initial rock yield to occur and stresses to be shed as the 

failure progresses. A portion of the FLAC model is presented in 

figure 5, which shows the detail of the layering of the limestone 

and weaker overburden materials. The strength properties of the 

limestone rocks and the weak and strong roof beds are listed in 

table 5. The large scale rock strengths of the limestone and 

surrounding rocks were reduced by 0.58 from the laboratory 

determined strength values to account for the difference in strength 

of laboratory size specimens relative to the field scale, after Hoek 

and Brown (1980). The main body of the limestone was modeled 

as a massive unit, without any bedding weaknesses. The effect of 

weak bedding layers within the limestone pillars was evaluated 

separately. 

20 m thick 

limestone bed 

Ground surface 

Low strength 

floor rocks 

250 m 

275 m 

100 m depth 

Fixed 

boundaries 

Weak interface 

between limestone 

and roof rocks 

Figure 3. Numerical model layout showing location of rooms 

and pillars in the limestone formation. 

Figure 4. Detail of numerical model showing dipping 

limestone formation and location of Points A and B for 

assessment of stresses in the roof beam. 

20m (65 ft)

Scale

Overburden 

Floor

Limestone

Point A 

Point B 

Excavated 

room 
Table 5. Properties of limestone and bedding in the upper 3 m 

(10 ft) of the limestone formation used in the FLAC models. 

Material 

Rock 

cohesion, 

MPa (psi) 

Rock 

friction 

angle 

Rock tensile 

strength, 

MPa (psi) 

Bedding 

cohesion, 

MPa (psi) 

Bedding 

friction 

angle 

Massive 

limestone 18 (2,610) 45° 8.0 (1,160) - - 

Strong beds 12 (1,740) 35° 2.5 (360) 6.0 (870) 25° 

Weak beds 4 (580) 25° 0.1 (14) 1.0 (145) 25° 

 The stresses in the model were initialized so that the horizontal 

stress depended on the stiffness of the rock layers and the vertical 

stress was dependent on the depth below the ground surface. The 

limestone horizontal stress of 35 MPa (5,070 psi) was selected to 

simulate a horizontal tectonic strain of 0.5 millistrain. 

Stresses in the Roof Beam 

 For the initial analysis, the thickness of the solid limestone roof 

beam over the room was set to 1.2 m (4 ft), resulting in a total 

thickness of limestone and overlying interbeds of 4.2 m (14 ft). 

 The horizontal stresses in the rock surrounding the room are 

presented in figure 6 which clearly shows the asymmetrical nature 

of the stress distribution. The average horizontal stress in the roof 

beam increased considerably above the tectonic stress in the 

limestone, being in the range of 50-60 MPa (7,250-8,700 psi) at 

center span while the tectonic stress was 35 MPa (5,070 psi). The 

maximum horizontal stress values in the roof beam are found near 

the corners of the room. The horizontal stresses adjacent to the 

downdip corner (Point A) and updip corner of the roof (Point B) 

were 73.5 MPa (10,660 psi) and 88.5 MPa (12,830 psi), 

respectively. 

Figure 6. FLAC plot showing horizontal stress distribution around a room with interbedded roof rocks in massive 

limestone. Contour intervals are 10 MPa (1,450 psi). The tectonic stress was 0.5 millistrain. 

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JOB TITLE : Room - 0.0005 tect - SU/SS model - layered roof 

NIOSH 

Pittsburgh Research Labs 

Excavated room 

Laminated roof 

beds 

Sandstone 

Shale 

Limestone 

Point A 

Point B 

(Pa) 

Figure 5. Detail of the numerical model used to assess the effect 

of dip on roof stability for the case study site. The locations of 

point A and B for assessment of roof stability are shown. 

Point B 

3 m of Interbedded shale 

and limestone 

Laminated shale 

13.7 m 

8 m 

Limestone 

Sandstone 

Excavated room 

Point A 
 The stress near the updip corner of the room is therefore 

approximately 20% higher than at the downdip corner. This is 

consistent with the observations that showed that failure initiated at 

the up-dip corner of the rooms and confirms that failures at the 

pillar-roof contact are likely to be the result of asymmetrical 

stresses owing to the dip of the workings. 

 Two additional sets of analyses were carried out to determine if 

the roof beam stability could be improved by changing the 

thickness of the roof beam or by reducing the width of the rooms. 

Conventional roof beam mechanics, in which beams are loaded by 

their own weight, such as the Voussoir beam method (Beer and 

Meek, 1982) or simple elastic beam formulations (Obert and 

Duvall, 1967), indicate that beam stability is greatly enhanced by a 

reduction in the beam span or by an increase in the beam thickness. 

However, in this case where the beams contain high horizontal 

stresses, the effects of changes in the beam dimensions are 

dominated by re-distribution of the horizontal stress, and were 

found to be contrary to expectations. 

Results for Variable Roof Beam Thicknesses 

 The thickness of the beam between the roof of the workings and 

the first weak interbed was varied between 0.6 m (2 ft) to 2.4 m 

(8 ft) to determine whether stability would be enhanced by 

increasing the beam thickness. The total thickness of the roof beam 

from the upper limestone interbed to the roof was therefore varied 

between 3.6 m (11.8 ft) and 5.4 m (17.7 ft). 

 The results are presented in table 6, which show that the stress in 

the roof beam is not sensitive to the beam thickness until the beam 

is reduced to only 0.6 m (2 ft). Inspection of the model results 

showed that this sudden increase in the stress is related to the 

initiation of failure in the overlying shale which causes downwards 

deflection of the roof beam and induces high stresses at the 

excavation corners. The results illustrate the potentially complex 

behavior of the roof rocks and the difficulty in estimating stability 

without detailed knowledge of the roof beam thickness and strength 

properties of the various rock layers. 

Table 6. Horizontal stress in the roof beam for different roof 

beam thicknesses in massive limestone. 

Limestone roof 

beam thickness, 

m (ft) 

Horizontal stress at 

A, MPa (psi) 

Horizontal stress at 

B, MPa (psi) 

0.6 (2) 95.4 (13,830) 104.8 (15,200) 

1.2 (4) 73.5 (10,660) 88.5 (12,830) 

1.8 (6) 71.1 (10,310) 90.9 (13,180) 

2.4 (8) 69.0 (10,000) 92.5 (13,410) 

Results for Variable Room Widths 

 The effect of changing the room width was evaluated by 

reducing the room width in the model from 13.7 m (45 ft) down to 

8.2 m (27 ft) and comparing the stresses in the roof beam. The 

results are summarized in table 7, which shows that for the high 

horizontal stress conditions at the mine site, a reduction in room 

width does not necessarily result in a reduction in the horizontal 

stress of the roof beam. The reason for this unusual behavior is that 

the roof beam relieves stress by vertical deflection. A thicker beam 

is less able to deflect and is therefore unable to relieve the high 

horizontal stress. This is contrary to the behavior of beams that 

deflect under their own weight. 

Figure 7. FLAC plot showing horizontal stress distribution around a room with interbedded roof rocks in 

limestone containing weak bands. Contour intervals are 10 MPa (1,450 psi). 

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JOB TITLE : Room - 0.0005 tect - SU model - layered roof - bedded LS 

NIOSH 

Pittsburgh Research Labs 

Excavated 

room 

Laminated roof 

beds 

Sandstone 

Shale 

Limestone 

Point A 

Point B 

Weak bands in 

limestone 

(Pa) 
Table 7. Horizontal stress in the roof beam for different room 

widths in massive limestone. 

Room width, 

m (ft) 

Horizontal stress at 

A, MPa (psi) 

Horizontal stress at 

B, MPa (psi) 

13.7 (45) 73.5 (10,660) 95.6 (13,860) 

11.9 (39) 75.4 (10,930) 97.6 (14,150) 

10.1 (33) 79.9 (11,590) 102.5 (14,862) 

8.2 (27) 84.4 (12,240) 108.5 (15,730) 

Results for Limestone Pillars Containing Weak Bands 

 The above analyses demonstrated the importance of layering in 

the roof and stress relief by sliding along weak beds. The fact that 

failure at the pillar-roof contact did not occur at all the pillars was 

thought to be related to a stress relief mechanism, which may be 

associated with weak bands in the limestone. Consequently, a 

model was set up to simulate limestone pillars that contain two 

weak bands near mid-height. Figure 7 shows the horizontal stress 

distribution in such a pillar, and can be directly compared to figure 

6. It can be seen that stress relief along the weak bands in the pillar 

result in a significant reduction in the stress in the roof. The 

horizontal stress at Point A reduces from 73.5 MPa (10,660 psi) to 

54.3 MPa (7,870 psi), which is a 26% reduction. The horizontal 

stress at Point B reduces from 95.6 MPa (13,860 psi) to 65.1 MPa 

(9,440 psi), which is a 32% reduction. 

 The results demonstrate that the presence of weak bands in the 

limestone may act as a stress relief mechanism. Limestone 

containing strong and discontinuous bedding would therefore be 

more prone to stress related roof failures than a limestone with 

weak bedding planes. 

CONCLUSIONS 

 Field observations combined with numerical model studies 

indicate that asymmetrical failure at the pillar-roof contact in highly 

stressed limestone formations can be caused by stress rotation 

relative to the excavation boundaries owing to the dip of the 

workings. An increase in the dip of the workings results in 

increased stresses in the up-dip corner of a room, which may 

initiate failure at the pillar-roof contact, similar to the failures 

observed in the field. 

 A case study demonstrated that failures observed at the pillarroof contact on the up-dip side of rooms dipping at 7° may be 

satisfactorily explained by the elevated stresses caused by the dip 

of the workings. Numerical models showed that, for the conditions 

at the case study site, the stress at the up-dip side of a room are 

typically 20% higher than the down-dip side, in spite of the 

relatively minor dip of the workings. In addition, the results 

showed that: 

1. The horizontal stress in the limestone roof beam was not 

sensitive to the thickness of the beam for beam thicknesses 

of 0.6 m (4 ft) or greater. 

2. Failure of the weak shale that overlies the limestone has an 

unfavorable effect on the limestone roof beam stability. 

3. Reducing the room width could cause an increase in the 

horizontal stress in the limestone roof beam and may bring 

the beam closer to failure. This phenomenon is the result of 

the initially high horizontal tectonic stress in the limestone. 

4. Failure at the pillar-roof contact is more likely to occur in 

areas where the pillars consist of massive limestone than in 

areas where the limestone pillars contain weak bedding 

planes. The weak planes provide shear surfaces which 

relieve the high horizontal stresses. 

 The model results seem to indicate that increasing the room 

width might be a technique to reduce high horizontal stresses in the 

roof beam. However, the model did not consider all the factors that 

could affect roof stability, such as the effect of a loss of horizontal 

stress, the presence of jointing or other structures and the capability 

of the roof support system. 

 The study has shown that failure at the pillar roof contact can be 

linked to the high tectonic stresses in the stiff limestone roof rocks. 

Small changes in the orientation of the field stresses can initiate 

failure in the already highly stressed roof rocks. 

REFERENCES 

Beer, G. and Meek, J.L. (1982). Design Curves for Roofs and 

Hangingwalls in Bedded Rock Based on Voussoir Beam and Plate 

Solutions. Trans Inst. Min. Metall., Vol. 91, pp. A18-22. 

Bieniawski, Z.T. (1989). Engineering Rock Mass Classifications. 

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