Roof Span Design for Underground Stone Mines

Esterhuizen, E.

Dolinar, D.

Ellenberger, J.

Essie Esterhuizen, Senior Research Engineer

Dennis Dolinar, Mining Engineer

John Ellenberger, Geologist

NIOSH Ground Control Branch 

Pittsburgh, PA

ABSTRACT

Underground stone mines in the United States use the roomand-pillar method of mining. The stability of the roof between 

pillars determines the mining dimensions and size of equipment 

that can be used. The findings of a survey of roof span stability 

issues and design practices are presented, and the observed factors 

contributing to roof instability are discussed. Identified stability 

issues are evaluated, such as maximum roof span, horizontal 

stress, and roof beam stability. The results of field observations, 

roof monitoring, and numerical analyses are used in the evaluation. 

The importance of the first roof beam thickness and its impact 

on roof stability and support requirements is presented. An 

evaluation of the current experience with stone mine roof spans 

relative to international experience is presented. A step-by-step 

design procedure is suggested which emphazises the need to 

collect adequate geotechnical data, understand the immediate roof 

stability, select a mining direction, and meet support requirements.

INTRODUCTION

In room-and-pillar mines, the roof between the pillars is required 

to remain stable during mining operations for haulage as well as 

access to the working areas. Roof and rib falls account for about 

15% of all reportable injuries in underground stone mines (Mine 

Safety and Health Administration, 2009). Therefore, the stability 

of the roof is an important safety consideration. The size of the 

rooms in underground stone mines is largely dictated by the size of 

the mining equipment. Large mining equipment is used, requiring 

openings that are on average 13.5 m (44 ft) wide by approximately 

7.5 m (25 ft) high to operate effectively (Esterhuizen et al., 2007). 

The dimensions of the roof spans are largely predetermined by 

the equipment requirements, and design is focused on optimizing 

stability under the prevailing rock conditions. If the rock mass 

conditions are such that the desired stable spans cannot be 

achieved cost effectively, it is unlikely that underground mining 

will proceed. In addition, large roof falls can extend across the 

full width of a room and may extend more than 5 m (16 ft) above 

the roof line. These large falls are a safety hazard and can have 

a significant impact on mine access and production. The National 

Institute for Occupational Safety and Health (NIOSH) research 

into stone mine roof stability has focused on identifying both the 

causes of roof instability and techniques to optimize stability 

through design.

OBSERVED ROOF STABILITY ISSUES IN STONE MINES

Observations were carried out at 34 operating stone mines to 

identify the factors that contribute to roof instability. Data were 

collected on rock strength, jointing and other geological structures, 

room and pillar dimensions, roof stability, and pillar performance 

(Esterhuizen et al., 2007). Two to five data sets were collected 

at various locations at each mine site. A data set describes the 

stability of the roof and pillars in an area of approximately 100 x 

100 m (300 x 300 ft). The range of roof spans observed is shown 

in Figure 1, which shows that the 92% of the mine openings are 

more than 10 m (33 ft) wide. Of the mines visited, 41% were 

installing support in a regular pattern, while the remaining 59% 

did not install any roof support, or only rarely installed spot bolting 

if required.

0 1

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40%

50%

60%

70%

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90%

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20-25 25-30 30-35 35-40 40-45 45-50 50-55 55-60

Cumulative percent

Frequency

Roof span (ft)

Frequency

Cumulative 

Figure 1. Distribution of roof span dimensions measured at 34 

different underground stone mines.

Thirty of the 34 mines visited had experienced small-scale or 

larger roof falls. The smaller-scale falls were typically less than 1 

m (3 ft) in length and were categorized as follows:

•  Thin slabs/slivers of rock that spall from the roof

•  Joint bounded blocks that are released from the roof
•  Bedding defined slabs that fail by separating along 

bedding planes

Figure 2 shows an example of a 13-m (43-ft) wide, naturally 

stable excavation with excellent roof conditions. Most of the 

smaller scale instabilities are addressed by scaling, rockbolting, 

or screen installation as part of the normal support and 

rehabilitation activities.

Figure 2. Naturally stable 44-ft (13-m) wide roof span in a 

stone mine.

Large falls, which typically extend over the full width of the 

opening, were observed at 19 of the 30 mines that experienced 

roof instability. Large falls were categorized by identifying the 

most significant factor contributing to each fall. A summary of 

these factors and the relative frequency of occurrence of each are 

presented below:

•  Stress: High horizontal stress was assessed to be the main 

contributing factor in 36% of all large roof falls observed. 

These falls are equally likely to occur in shallow or deep 

cover. A roof fall related to stress-induced damage was 

observed at a depth of as little as 50 m (160 ft) in one case. 

The characteristics of stress-related roof falls are described 

in Iannacchione et al. (2003) and Esterhuizen et al. (2007). 

Figure 3 shows a typical ellipsoidal stress-related fall, and 

Figure 4 shows an example of how such a fall progressed 

through the mine workings in a direction perpendicular to the 

major horizontal stress.

•  Weak bedding planes: The beam of limestone between the 

roof line and some overlying weak band or parting plane 

failed in 28% of all observed large roof falls.

•  Discontinuities: Large discontinuities extending across the full 

width of a room contributed to 21% of the large roof falls.

•  Weak roof rocks: The remaining 15% of the roof falls was 

attributed to the collapse of weak shale inadvertently exposed 

in the roof or progressive failure of weak roof rocks.

Figure 3. Example of a large elliptical-shaped roof fall that was 

related to high horizontal stress in the roof.

July 14

July 26

Oct. 3

Pillars

Major horizontal stress direction

Figure 4. Plan view showing the development of a stress-related 

roof fall in the direction perpendicular to the direction of the 

major horizontal stress, after Iannacchione et al. (2003).

Although the large roof falls only make up a small percentage of 

the total roof exposure, their potential impact on safety and mine 

operations can be very significant. Most cases of large roof falls 

required barricading-off or abandonment of the affected entry. 

When large roof falls occur in critical excavation areas, the repair 

can be very costly.

MAXIMUM STABLE ROOF SPAN

The majority of roof spans in operating mines fall within a 

narrow range of 10 m to 17 m (35 ft to 55 ft) and are related to the 

space needed to effectively operate large loaders and haul trucks. 

A small number of mines used roof spans wider than 15 m (50 ft), 

so it is not clear whether the stability limit is approached at 17 m 

(55 ft) or whether it simply satisfies the practical requirements for 

equipment operation. Given that a large proportion of the mines 
are able to mine without installed support, it seems to indicate that 

wider spans can be achieved if additional supports are used.

One way of assessing the potential maximum span is to compare 

the stone mine data to experience in other mine openings around 

the world. The Stability Chart, originally developed by Matthews 

et al. (1980) and then modified by Potvin (1988), Nickson (1992), 

and Hutchinson and Diederichs (1996), was used as a basis for 

comparison. The Stability Chart plots a modified Stability Number 

N ́, which represents the rock mass quality normalized by a stress 

factor, an orientation factor, and a gravity adjustment. Stability 

zones have been indicated on the chart based on 176 case histories 

from hard rock mines around the world. The following stability 

zones are indicated:

•  Stable – Support generally not required.

•  Stable with support – Support required for stability, the 

support type is cable bolting.

•  Transition – Stability not guaranteed, even with cable 

bolt support.

•  Unsupportable – Caving occurs, cannot be supported with 

cable bolts.

The stability number for each stone mine case history was 

calculated using the procedure described by Hutchinson and 

Diederichs (1996) and was based on the rock classification data 

collected at each site. Figure 5 shows the Stability Chart with the 

stone mine case histories and stability categories. In this chart, the 

actual heading width is shown instead of the “hydraulic radius” 

which is customarily used. The hydraulic radius, in this context, 

is the ratio of the area to the circumference of the roof of the 

excavation. For a parallel-sided excavation, the hydraulic radius is 

50% of the excavation width. It was assumed that the headings in 

stone mines are parallel-sided excavations. The chart also indicates 

the average Stability Number of the stone mine case histories as a 

horizontal dashed line.

It can be seen that the majority of stone mine case histories plot 

in the region of “stable” to “stable with support,” and only one is 

located in the transition zone. This agrees reasonably well with 

the observed stability and support usage in stone mines, although 

stone mines have been able to achieve stability with light support 

compared to cable bolting used in the hard rock mine case histories. 

Based on the average Stability Number for stone mines, it would 

appear that stable, supported excavations can reliably be achieved 

with spans of up to about 20 m (65 ft) using cable bolt supports. 

Cable bolt lengths in the hard rock mines are typically greater than 

half the excavation span, which would imply 10-m (33-ft) long or 

longer cable bolts to achieve stability in a 20-m (65-ft) wide stone 

mine entry. Unsupportable and caving conditions are indicated 

when the span increases to about 27 m (90 ft). These results are 

in line with current experience. It appears that stone mines are 

working near the span limit that can reliably be achieved using 

rock bolts as the support system. Increasing the spans beyond the 

15–17 m (50–55 ft) range is likely to incur considerable cost and 

productivity implications as cable bolting would become necessary.

STABILITY OF THE IMMEDIATE ROOF BEAM

The stability of excavations in bedded deposits is closely 

tied to the composition of the first beam of rock in the roof. An 

assessment of the data collected showed that 25 of 34 mines were 

attempting to maintain a specific thickness of limestone beam in 

the immediate roof. In some cases, the upper surface of the beam 

was a pronounced parting plane while in others it was a change in 

lithology, typically when the limestone beam is overlain by weaker 

rocks. A constant thickness of roof beam is achieved either by 

probe drilling to determine the thickness of the roof beam or by 

following a known parting plane or marker horizon.

The average roof beam thickness in mines that were able to 

mine without regular support was 2.25 m (7.4 ft), while the average 

beam thickness in the mines that were using regular roof support 

was 1.3 m (4.3 ft). Several of the mines that used regular support 

did so to alleviate the effects of high horizontal stress, which is not 

related to beam thickness. If these mines are removed from the 

data, the average beam thickness in mines that use regular support 

drops to 0.8 m (2.6 ft). These results seem to indicate that mines 

with a relatively thin beam of limestone in the immediate roof are 

more likely to encounter unstable roof and regular roof bolting 

becomes necessary. There was no correlation between roof beam 

thickness and excavation span.

The beam thickness is obviously not the only factor to consider 

when deciding on roof reinforcement. Other aspects such as roof 

jointing, bedding breaks, blast damage, groundwater and high 

horizontal stress can contribute to roof instability resulting in the 

need for rock bolt support. However, experience seems to indicate 

that a roof beam thickness of less than about 1.2 m (4 ft) is highly 

likely to be unstable and a regular pattern of rock bolt supports will 

be required to maintain the roof stability.

HORIZONTAL STRESS CONSIDERATIONS

This study showed that horizontal-stress-related roof instability 

can occur at any depth of cover. This is not unexpected, given 

that the horizontal stresses are caused by tectonic compression 

of the limestone layers, which is not related to the depth of 

typical limestone mines (Dolinar, 2003; Iannacchione et al., 

2003). Observations show that the tectonic stresses in limestone 

formations that outcrop may have been released over geologic 

time by relaxation towards the outcrop (Iannacchione and Coyle 

2002). Consequently, outcropping mines can have highly variable 

horizontal stress magnitudes that depend on the amount of 

relaxation that occurred over geologic time and the distance from 

the outcrop.

A review of horizontal stress measurements in limestone and 

dolomite formations in the eastern and midwestern U.S. and eastern 

Canada (Dolinar, 2003; Iannacchione et al., 2002) has shown that 

the maximum horizontal stress can be expected to vary between 

7.6 MPa (1,100 psi) and 26 MPa (3,800 psi) up to depths of 300 m 

(1,000 ft). Limited information is available at greater depths. The 

orientation of the maximum horizontal stress is between N60°E and 

N90°E in 80% of the sites. This agrees with the regional tectonic 

stress orientation as indicated by the World Stress Map Project 

(2009). The minimum horizontal stress is approximately equal to 

the overburden stress.

An analysis of the impact of horizontal stress on beams of rock 

that may exist in the roof of stone mine workings showed that 

horizontal stress can be expected to cause buckling of thinly 

bedded roof strata (Iannacchione et al., 1998). Further analyses 

using numerical models showed that brittle spalling (Kaiser et al., 
2000) of the roof rocks under near-uniaxial loading conditions can 

explain roof failure at the stress levels encountered in stone mines 

(Esterhuizen et al., 2008).

0

1

10

100

1,000

0 10 20 30 40 50 60 70 80

Stability Number N'

Entry span (ft)

Average stability 

number in stone mines

Figure 5. Stability chart showing stone mine case histories and stability zones, modified after Matthews et al. (1980), Potvin (1988), 

Nickson (1992), and Hutchinson and Diederichs (1996).

Once a stress-induced roof fall has occurred, it can be costly 

and difficult to arrest the extension of the fall into adjacent areas. 

Avoidance of these falls through layout modifications has proved 

to be very successful in several operating mines (Iannacchione 

et al., 2003). It is first necessary to establish the direction of the 

major horizontal stress, which can be determined by various stress 

measurement techniques or can be inferred from stress-related roof 

failures (Mark and Mucho, 1994). The layout is then modified so 

that the main development direction is parallel to the maximum 

horizontal stress and the amount of unfavorably oriented cross-cut 

development is minimized (Parker, 1973). A further modification 

that has proved to be successful is offsetting the cross-cuts and 

increasing the length of the pillars, so that a continuous path does 

not exist along which a roof fall can progress across the layout. 

Modifying a layout in this manner will not necessarily eradicate 

all stress-related problems, but it has been shown to considerably 

reduce these problems (Kuhnhein and Ramer, 2004).

Figure 6 shows a mine layout that has been optimized for 

high horizontal stress. The main heading direction is parallel 

to the maximum horizontal stress; pillars are elongated so that 

unfavorably oriented cross-cuts are minimized; the cross-cuts are 

narrower than the headings; and the cross-cuts are off-set so that 

potential stress related roof falls will abut against solid pillar ribs, 

rather than snake through the layout.

ROOF REINFORCEMENT

The survey of roof support practices showed that grouted rock 

bolts are the most widely used form of support. Bolt lengths are 

typically between 1.8 m (6 ft) and 2.4 m (8 ft), and where the bolts 

are installed in a regular pattern, the most commonly used bolt 

spacings are either 1.5 m (5 ft) or 1.8 m (6 ft). As with most other 

roof bolting designs in strong rocks and high strength, and stiff 

bolts are more likely to provide the desired rock reinforcement than 

low strength and low stiffness systems (Iannacchione et al., 1998). 
Heading direction 

parallel to major 

horizontal stress

Cross - - cuts are narrower 

than headings

Cross -cuts are offset

Figure 6. Diagram showing room and pillar layout modified 

to minimize the potential impact of horizontal stress 

related damage.

Roof screen or other supplemental supports such as cable bolts are 

rarely used.

Roof reinforcement in the relatively strong-bedded rock 

encountered in stone mine can have one or more objectives. 

Depending on the geological conditions, the support system can be 

expected to do the following:

•  Provide suspension support for a potentially unstable 

roof beam

•  Provide local support to potentially unstable blocks in the roof

•  Combine thinly laminated roof into a thicker, stronger unit

•  Provide surface control when progressive spalling and small 

roof falls occur

The above support functions can usually be achieved by the 1.8-

m (6-ft) and 2.4-m (8-ft) bolts used in the stone mines. When poor 

ground is encountered locally or when horizontal-stress-related 

roof failures occur, intense bolting, steel straps, and cable bolts 

have been used with mixed success to halt the lateral extension of 

these large roof falls.

From a design point of view, a stone mine is unlikely to be 

economically feasible if heavy support such as cable bolts and 

screen would be required on a daily basis. Such rock conditions 

would probably require reduced excavation spans; otherwise, the 

support costs would be prohibitive. Therefore, the first objective 

in designing an underground stone mine should be to confirm 

that the rock mass quality is adequate for creating the typical 10 

to 17 m (35 to 55 ft) roof spans without resorting to elaborate 

support systems.

ROOF SPAN DESIGN GUIDELINES

Designing stable roof spans for underground stone mines 

should be conducted using basic engineering principles. Good 

geotechnical information, combined with a pragmatic assessment 

of the likely modes of instability, along with providing the required 

support is likely to produce a stable initial design. Once an initial 

design has been developed, performance monitoring can be carried 

out to optimize the design.

Useful information can be obtained from neighboring mines 

that are operating under similar conditions. A particularly useful 

piece of information would be to identify whether horizontal-stressrelated roof problems exist and if so, the orientation of the stressrelated damage. This information can go a long way in selecting 

the orientation of the main headings in the proposed mine. The 

following steps should be followed to carry out a mine layout and 

roof span design:

1. Geotechnical characterization: Designing stable roof spans 

for stone mines can be successfully carried out if adequate 

geotechnical investigations are conducted ahead of the design. 

Such investigations are best conducted by experienced ground 

control specialists and are likely to include rock strength testing, 

core logging, bedding layering assessment, joint orientation 

assessment, and rock mass classification. If horizontal-stressrelated issues are expected, stress measurements can assist in 

providing an indication of the orientation and magnitude of the 

maximum horizontal stress.

2. Confirm rock mass quality: Using the results of a rock mass 

classification or direct inspection of workings, confirm that the 

rock strength and rock mass quality is similar to that found in 

eastern and midwestern stone mines. The RMR should exceed 

a value of 60.0 and the rock strength should exceed 45 MPa 

(6,400 psi).

3. Selection of mining direction: The direction of the headings in 

the production areas should be favorably oriented to potentially 

high horizontal stress and the prevalent jointing. As with any 

underground excavation layout, it is preferable to intersect the 

main joint strike direction by more than 45° to limit the size of 

potential blocks formed in the roof and ribs of the excavation. 

Since room-and-pillar mines have two orthogonal directions of 

mining, the heading direction should be favored over the crosscut direction when selecting the orientation of the layout. If the 

orientation of the maximum horizontal field stress is known, and 

stress related problems are anticipated, the heading direction 

should be oriented parallel to the direction of major horizontal 

stress, with due consideration of joint orientations and crosscut stability. Often it will be a compromise to select the final 

heading orientation. Other modifications to the pillar layout 

should be considered, as discussed below.

4. Modification of pillar layout: A simple square pillar layout 

with headings and cross-cuts of equal width is sufficient in 

most cases. However, if horizontal-stress-related instability 

is expected, the pillar layout can be modified to improve the 

likelihood of success. Possible layout modifications were 

shown in Figure 6.

5. Selection of mining horizon: The location of the roof-line 

relative to pronounced bedding planes or lithology changes 

should be identified next. Experience has shown that if the 

immediate roof beam is less than 1.2 m (4 ft) thick, it is very 

likely to be unstable. Thicker roof beams may be required if 

excessive horizontal stresses are encountered. Persistent 

parting planes can be selected to form the roof-line if they are 

present at a convenient location in the formation being mined. 

Using a pre-existing parting plane as the roof line helps to act 

as a marker and usually provides a clean breaking surface for 

blasting operations. Many of the mines that do not use roof 

supports have a natural parting as the roof line.

6. Selection of roof span: Past experience has shown that stable 

roof spans in the range of 10 m to 15 m (35 ft to 50 ft) have 
been regularly achieved in underground stone mines. NIOSH 

studies have shown little correlation between mining roof spans 

and rock quality, mainly because there is such a small range 

of rock qualities in operating mines. For an initial design, 

it might be prudent to design for no more than 12-m (40-ft) 

spans. The spans can be increased incrementally, if warranted 

by monitoring of actual roof performance. There is little 

experience with spans that are greater than 15 m (50 ft).

7. Support considerations: Depending on the characteristics of the 

immediate roof, basic support in the form of patterned rockbolts 

may be required. The importance of the thickness of the first 

beam in the roof, the orientation of excavations relative to the 

maximum horizontal stress, and the characteristics of rock joints 

will determine whether and how much support is required. 

Mines that do not use bolting are located in formations 

with a favorable combination of geological conditions, and 

they conduct blasting practices that maintain an unbroken 

roof horizon.

8. Monitoring and confirmation: Once a roof design has been 

finalized and mining is underway, monitoring should be 

implemented to verify the stability of the roof. Monitoring 

results can be used to identify potential stability problems 

before they occur and may indicate that a change in the design 

is required. Monitoring technologies that are available include 

borehole-video logging (Ellenberger, 2009), roof deflection 

monitoring (Marshall et al., 2000), roof stability mapping using 

the Roof Fall Risk Index (RFRI), (Iannacchione et al., 2006), 

and microseismic monitoring of rock fracture (Iannacchione et 

al., 2004; Ellenberger and Bajpayee, 2007).

DISCUSSION AND CONCLUSIONS

A study of pillar and roof span performance in stone mines that 

are located in the eastern and midwestern United States showed 

that various stability issues can be addressed by appropriate design. 

A roof span design procedure is proposed that systematically 

addresses each of the main stability issues. The procedure 

focuses on selecting an appropriate mining horizon and mining 

direction. The importance of the thickness of the first bed in 

the roof and the likelihood for added rock bolting is described. 

Layout modifications are described that can be made to reduce the 

incidence of horizontal-stress-related instability.

Roof span design requires a good understanding of the 

geotechnical characteristics of the formation being mined. The 

essential data are the uniaxial compressive strength of the rock, 

characteristics of the discontinuities in the immediate roof, 

and the rock mass classification. Knowledge of the magnitude 

and orientation of the stress field can assist in orienting the 

layout appropriately.

The design procedure is based on observation of the actual 

performance of pillars and roof spans in stone mines within the 

eastern and midwestern United States. The guidelines should only 

be used for design under similar geotechnical conditions.

DISCLAIMER

The findings and conclusions in this paper have not been 

formally disseminated by the National Institute for Occupational 

Safety and Health and should not be construed to represent any 

agency determination or policy.

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