An Overview of Geomechanics Safety Research On Mobile Roof Supports 

Hamid Maleki; Maleki Technologies, Inc.; Spokane, WA 

John Owens and Wayne Howie 

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

Keywords: Mobile roof supports, load rate monitoring, 

Abstract

After an analysis of the hazards of room-and-pillar 

retreat mining systems, it became apparent that 

safety could be significantly improved by 

considerations of (1) human factors, (2) remotely 

controlled mobile roof supports (MRS=s), (3) mine 

layout designs, and (4) ground monitoring systems. 

Initial studies of the effectiveness of MRS=s 

focused on their interaction with mine strata and 

evaluations of suitable measurements for detecting 

roof stability problems during pillar extraction. 

These studies indicated that overall stress 

distributions and strata movement were most 

influenced by the stiffness of coal-measure rocks 

and the design of mining layouts. Thus, to improve 

worker safety, mine layouts should be carefully 

designed and a pillar extraction method chosen for 

specific geologic and stress conditions. 

Pillar failure was often associated with an increase 

in pressure on the hydraulic gauges of the MRS, 

and roof failure was often preceded by rapid 

changes in the rate of roof-floor convergence. 

These studies led to development of a monitoring 

system that displays loading rate on an MRS in real 

time. A major MRS manufacturer cooperated in 

installing and testing the system on an MRS. New 

field studies focus on evaluating the performance 

of the system, measuring roof-floor convergence, 

and optimizing the safety of MRS operations 

through proper mine layout design. 

Introduction

Room-and-pillar mining is one of the oldest 

methods used for the extraction of tabular ore 

bodies. In this method, a series of rooms are driven on advance using continuous miners and shuttle 

cars while the roof is bolted a short distance behind 

the face. During the retreat, the same equipment is 

used to mine the pillars, which allows roof rocks to 

cave behind the face. To control the cave line, a 

series of secondary support systems are installed as 

mining continues within the pillars. 

The room-and-pillar mining method is at a disadvantage when compared to other mining techniques, such as longwall mining. Because of economies of scale, the productivity of room-and-pillar 

mining is significantly lower. The longwall method is also much safer because the retreat is completed under the protection of self-advancing hydraulic support systems at the face. However, 

during the last two decades, federal laboratories, 

mining companies, equipment manufacturers, and 

geomechanics consultants have cooperated to improve the understanding of strata mechanics and 

develop a remotely controlled, self-advancing 

support system called a mobile roof support 

(MRS). This cooperation has resulted in improvements in the safety and productivity of room-andpillar retreat operations. 

Figures 1 and 2 illustrate generic panel layouts and 

pillar extraction sequences for two typical roomand-pillar retreat systems. The first is three-entry 

access and retreat to one side, while the second is 

nine-entry access with full retreat within the panel. 

 In the first system, mining starts by driving a 

three-entry panel access to the boundaries of the 

room-and-pillar panels. A three-entry system using 

narrow rib pillars is developed to the side and 

retreated. After pulling one row of pillars, another 

row is driven into the solid coal block, and the 

sequence is repeated until the panel coal is extracted. Pillar recovery operations consist of 

splitting the pillars and fenders. Figure 1A presents 

the mine layout at four stages of pillar recovery. 

Figure 1B shows the sequence of the pillar cuts, 

typical position of posts, and the location of unmined stumps for the extraction of a pillar using 

the split-and-fender method. 

In the second system, a nine-entry access is 

developed on advance to panel boundaries. The 

pillars are then extracted until the entire panel is 

mined. Figure 2A presents the panel layout and the 

location of MRS=s at three intermediate stages of 

pillar recovery using the AChristmas tree@ method. 

Figure 2A also shows the sequence of cuts taken 
from two pillars where MRS=s are used as secondary support. Many variations of these two panel 

layouts and excavation sequences are practiced in 

U.S. coal mines. New applications of the threeentry system involve use of MRS=s instead of posts 

and eliminates fenders completely. 

After completing an analysis of the hazards of 

room-and-pillar retreat mining systems, it became 

apparent to the authors that safety could be 

significantly improved by considerations of (1) 

human factors, (2) remotely controlled MRS=s, (3) 

mine layout designs, and (4) ground monitoring 

systems. A significant effort was directed to 

studying the above factors both in the laboratory 

and in the field. 

Human Factors

Several human factors considerations were 

identified during a geomechanics field study 

(Maleki 1981) in which the main objective was to 

identify causes of roof stability problems and 

develop practical monitoring techniques for detecting these problems (Maleki and McVey 1988). 

These factors were (1) the number of people 

required at the face, (2) the amount of time 

required to work at the cave line, (3) poor footing 

in entries, which influenced timely escape during a 

roof fall, (4) worker reaction at the time of a roof 

fall, and (5) the judgment-based methods used by 

miners to evaluate the stability of the roof and 

determine the optimum time for retrieving miners 

and equipment. 

A large crew is required for conventional roomand-pillar retreat operations because posts must be 

delivered, cut to size, and installed. Each 

installation takes approximately 20 min and 

requires two to three workers. Debris on a mine 

floor can accumulate quickly and create poor 

footing. Miners must judge roof stability continually on the basis of observations of primary and 

secondary support behavior (bending of roof 

plates, crushing of posts, etc.). In the study mine, 

when the roof caved prematurely and trapped a 

miner in a cab, other miners rushed to help. A 

second rock fall could have resulted in serious 

injury to rescuers (Maleki 1981). This could have 

easily happened, considering that many posts had 

already been broken and some had been knocked 

down during the first fall. 

Figure 1. Mine layout (A) and pillar extraction sequence (B) using 

split-and-fender method with posts.
Development and Testing of the MRS

 

To improve the safety of room-and-pillar 

retreat systems, a two-step solution was proposed. First, the mechanics of strata behav-ior 

were studied through extensive field 

measurements, and practical techniques for 

assessing roof behavior were developed. 

Second, a prototype of a remotely controlled 

roof support system was developed to eliminate the need to install posts near the gob. 

The machine was equipped with a dozer blade 

so that floor debris could be cleaned routinely, 

which allowed easier travel and escape. The 

prototype unit was developed by the U.S. 

Bureau of Mines in cooperation with an 

equipment manufacturer and a mining 

company (Thompson and Frederick 1986). 

Commercial units have since been developed 

by U.S. and Austrian manufacturers and are 

being used on two continents. The commercial MRS units are more rugged and have 

higher capacities (5,340 to 7,120 kN [600 to 

800 tons]) (Wilson 1991; Howe 1998) than the 

prototype. They consist of a roof canopy, four 

hydraulic cylinders, a caving shield canopy, 

and associated electro-mechanical systems 

Figure 2. Mine layout (A) and pillar extraction sequence (B) using 

Christmas tree method with MRS’s as support. 
mounted on crawler tracks. The system has radio 

control and self-contained power units. Because of 

their greater mobility and because they allow 

higher resource recovery, they are currently being 

used in 36 U.S. coal mines, as well as a number of 

Australian mines (Shepard and Lewandowski 

1992; Habenicht 1988). 

MRS performance has been monitored both in the 

laboratory and in the field by NIOSH personnel. 

Laboratory investigations focused on an evaluation 

of support stiffness and load-carrying capacity 

under controlled static loading conditions. The 

study quantified system stiffness as a function of 

machine height for both two- and three-stage 

hydraulic cylinders (Barczak and Gearhart 1997, 

1998). Three-stage cylinders are needed in thick 

seams, but reduce support stiffness. Each unit has 

the load-bearing capacity of six posts and the stiffness of two hardwood posts (Barczak and Gearhart 

1997). The study also identified inaccuracies in 

hydraulic cylinder pressure measurements of roof 

loads when the bottom cylinder stages were fully 

extended. 

The mechanics of load transfer from pairs of 

MRS=s to mine strata were analyzed using laboratory results, boundary-element modeling, and analytical solutions. The results showed that MRS=s 

support roof rocks near the machines, but do not 

have the capacity to control overall roof-floor convergence and overall stress distributions because 

the MRS=s are considerably less stiff than coalmeasure rocks. In comparison to posts, however, 

an MRS is capable of maintaining the yield load 

after significant amounts of roof-floor deformation. 

Because the mining cycle is accelerated, MRS=s 

help reduce the potential for time-dependent roof 

falls. 

To study the influence of pairs of MRS=s on the 

mine roof, the authors used analytical solutions for 

two pairs of MRS=s positioned 5.5 m (18 ft) apart 

(figure 3) (Maleki and Owens 1998). Results 

showed that MRS=s form a pressure arch in the 

immediate roof that reduces the potential for roof 

falls in the space confined by the MRS=s. This is 

beneficial for protecting a continuous miner when 

it is operating within this space. It was also found 

that higher MRS capacities and setting pressures 

are useful for stabilizing the upper strata, but may 

contribute to differential loading on the immediate 

roof, failure of mechanical bolts, and reduction in 

the stability of the immediate roof. 

Figure 3. Stress isobars along A-A' for twp pairs of 

MRS’s at 5.5-m spacings. 

Early field evaluations focused on a comparison of 

ground movements in two room-and-pillar retreat 

sections using the split-and-fender method with 

posts (figure 1), and the Christmas tree method 

with MRS=s as the secondary support system (figure 2). In addition, the history of hydraulic pressure was analyzed for all four MRS legs (Hay and 

others 1995). Deformation measurements indicated generally higher strata movement at the intersections in the section using the Christmas tree 

method. Because of differences in geologic conditions and mining practices, it was not possible to 

make a direct comparison. We recommended that 

numerical modeling of these geometries address 

mine layout designs while keeping geologic conditions constant. 
Panel Layout Design

Field studies identified the importance of mine 

layout designs and revealed the dangers of overconfidence concerning the ability of MRS=s to support the entire area. Such overconfidence contributed to workers choosing unsafe operating locations. Thus it became apparent to the authors that 

to improve stability, layout designs that control 

convergence and stress should be developed. To 

illustrate this point, boundary-element analyses 

were completed in which stress distributions were 

calculated in both single and multiple seams. 

These analyses were also helpful in tailoring the 

type of monitoring required to assess changes in 

the stability of the mining system. 

The first study compared stress distribution and 

convergence patterns for two pillar recovery plans: 

split-and-fender and Christmas tree. Model input 

was based on extensive laboratory and field 

measurements in one mine (Maleki 1981), and 

modeling procedures were based on a methodology 

developed for coal mine excavations (Maleki 1990; 

Maleki and Owens 1998). The analyses were 

completed for a typical depth of 305 m (1,000 ft). 

Figure 4 presents the calculated roof-floor convergence for a point in the intersection for two pillar 

recovery methods (point B in figures 1A and 2A) 

and provides guidance for selecting monitoring 

systems. Note that calculated deformation significantly increases within a mining step, which is 

associated with the failure of fenders and stumps. 

MRS=s will therefore experience an increase in 

both vertical and lateral support loading as fenders 

fail. Since fender failure induces differential 

movement in the mine roof, a roof fall may be 

triggered. Such a roof fall may be sensed through 

monitoring either convergence rate or possibly 

MRS leg pressures. The change in convergence 

that occurs as a result of failure of the fenders is 

large enough to cause a change in leg pressure. 

Other changes in convergence rate, however, may 

best be detected by monitoring roof-floor convergence in view of the insensitivity of leg pressure to 

roof loads if the bottom stages are fully extended. 

Roof-floor convergence is at least 10% higher 

using the Christmas tree method, as illustrated in 

figure 4. To control convergence, a stump (figure 

2) left in the model (no pushout). Further 

improvements in stability and convergence can be 

achieved by changing the size of the stumps and 

pillars left behind. 

 

Figure 4. Calculated closure for split-and-fender 

and Christmas tree methods at location B. 

MRS=s are used often when mining difficult 

reserves, such as where there are earlier workings 

in adjacent seams. To assist a mine operator with 

design of an MRS layout in a two-seam reserve, the 

authors completed a multistep analysis. The stress 

distribution was calculated for an MRS section 

recently placed in a bed (No. 2) 12 m (40 ft) below 

a partially mined seam (No. 1). Model input was 

based on extensive measurements at the mine 

(Maleki 1988a). Overburden averaged 360 m 

(1,200 ft) over the entire modeled area. 

Figures 5 and 6 present mining geometry and 

vertical stress distribution for both seams. Step 1 

included longwall mining in the southern portion of 

the reserve and simultaneous room-and-pillar 

mining with partial retreat in the northern part of 

seam 1 directly above the areas of interest. Step 2 

consisted of limited development work in seam 2. 

Results from step 1 (figure 5) show the stress distribution for both the longwall and room-and-pillar 

panels. Stresses are elevated near the edge of the 

caved areas because of the low stiffness of the 

caved rocks. This change in stress pattern influenced load transfer to the lower seam and thus should 

be considered in development of layouts below. 

Results from step 2 (figure 6) clearly demonstrate 

load transfer (stress footprint) to bed 2 because of 

mining in the top seam (note that the only mining 

that had occurred in bed 2 was a small amount of 

pillar development, as shown toward the right 

boundary of the model). Stresses were lower under 
Figure 5. Vertical stress distribution for upper seam 1, step 1. 

the gob and higher near the periphery of the fullextraction areas. Results were used to define the 

optimum location for room-and-pillar layouts in 

bed 2 for pillar extraction using MRS=s. 

Development and Testing of Ground Monitoring 

Systems

During field tests in underground mines, the 

authors identified three factors that might adversely 

influence worker safety in an MRS section. 

$ Elimination of posts reduced a worker=s ability 

to assess roof conditions. 

$ Overconfidence in the ability of MRS=s to 

support the entire area caused some miners to 

chose unsafe operating positions. 

$ Use of MRS=s on a routine basis under adverse 

geologic and mining conditions to recover 

reserves that were otherwise unminable. It 

became apparent to the authors that there was a 

need to develop a warning system that would 

alert workers to unstable roof conditions so that 

 miners and equipment could be moved before a 

fall occurs. 

Two monitoring methods were chosen on the basis 

of mine measurements and numerical modeling 

considerations. These were roof-floor convergence 

and load-rate monitoring on the hydraulic legs of 

MRS=s. Convergence pins can be placed over the 

entire area of interest to monitor the stability of the 

whole section (Maleki 1988b). Monitoring the rate 

of load on MRS legs also is believed to provide 

warnings about major events, such as failures of 

fenders and pillars. These events generally trigger 

roof falls. A reliable warning system can be 

developed by combining both convergence and 

load-rate data. 

Convergence measurements were obtained from 

four mines that use different primary and secondary support systems under variable amounts of 

cover (90 to 360 m [300 to 1,200 ft]) and both flatlying and dipping seams (0b to 8b). Figure 7 presents measured total convergence showing that roof 
Figure 6. Vertical stress distribution for lower seam 2, step 2. 

falls occurred generally after 2.5 cm (1 in) of 

convergence (and occasionally up to 50 cm [20 

in]). Total convergence by itself is not a suitable 

indicator of roof stability. 

Rate of convergence is a reliable measure of roof 

stability (figure 8). Note that there were no roof 

falls where the convergence rate was lower than 

0.5 cm/min (0.2 in/min). Minor falls were 

recorded at a convergence rate of 0.5 to 0.65 

cm/min (0.2 to 0.25 in/min). Critical rates exceeding 0.65 cm/min (0.25 in/min) were measured prior 

to roof falls in all four study mines. Results are 

very encouraging, although site-specific convergence measurements should be taken in any new 

mine to verify this critical rate. 

Research continues to refine monitoring and data 

acquisition systems, as well as define critical rates 

that indicate a change in the stability of the system. 

 A prototype load-rate monitoring device has been 

developed for the MRS=s. The device monitors 

pressure in the hydraulic cylinders of the MRS dynamically, calculates changes in pressure over 

time, and converts pressure changes to loading 

rates. Warning lights can then be activated, depending on the rate of change. Field tests of the 

load rate concept will complete identification of 

critical rates of load that indicate imminent roof 

falls. 

Load Rate Monitoring System on MRS=s

Hydraulic supports such as the MRS provide little 

or no discernible audible or visual indications of 

impending roof caving. In MRS retreat mining 

sections, miners rely on the hydraulic gauges on 

the MRS=s to determine when to cease operations 

and leave the area of the active mining face before 

a roof fall. A imminent roof failure is often preceded by a rapid increase in pressure on the dial 

gauges. However, these gages are difficult to read, 

requiring the miners to approach the MRS=s to 

monitor the gages. This requires them to be close 

to the active mining face, an area susceptible to 

roof falls, and in a location with a lot of equipment 
Figure 7. total measured roof-floor convergence prior 

to roof falls at four mines. 

Figure 8. Roof-floor convergence rates prior to roof 

falls at four mines. Key is same as for figure 7. 

activity. As a result, miners do not check the 

pressure gauges often. A load rate monitoring 

system was developed that monitors and displays 

dynamic loading rates on an MRS in real time to 

alert miners to dangerous loading conditions during pillar extraction. The system can easily be 

seen by all miners in the vicinity of the MRS=s. 

Figure 9 is a schematic diagram of the load rate 

monitoring system. The system monitors the two 

hydraulic systems with standard analogue pressure 

transducers at 0 to 34.5 MPa (5,000 psi). This 

pressure is translated to an analog voltage of 0 to 5 

V dc. An analogue-to-digital converter then transforms each voltage to a 12-bit (0 to 4096 level) 

digital value that is input to a dedicated Micro-485 

programmable controller for processing. The controller is based on a highly integrated version of the 

world standard 8051 mocrotroller family. It calculates loading rates in Assemply, Basic, and C languages and controls output signals to the three 

load-rate indicator lamps. A socketed 80C51FA 

CPU is ideally suited to the control and data 

acquisition requirements of the system. 

Loading is proportional to internal pressure and 

surface area of the piston head of hydraulic 

cylinder and is determined by the formula 

F = A e P, 

where F = force (N), A = area (cm2

), and P = 

pressure (MPa). The embedded processor reads 

changes in cylinder pressure through two multiplexed data acquisition channels of the load rate 

monitoring controller. These pressure changes are 

converted to loading rates that activate different 

colored lights as the loading rate increases on the 

MRS. Green indicates that there is minimal change 

in load rate on the MRS, yellow indicates that the 

load rate increasing and that additional caution is 

recommended, and red indicates a rapid load rate 

increase and that a roof fall may occur soon. A 

continuously flashing red light indicates that the 

hydraulic cylinder load is approaching the yield of 

the MRS, and that the unit may soon collapse. 

Alternative load rate indicator devices (multicolor 

strobes, LED=s, and audible alarms) can be used 

with the system to meet specific warning requirements as requested by mine operators or MRS 

manufacturers. 

The system will operate as an integral part of the 

MRS and will not require on-site maintenance. 

Necessary calibration can be done prior to installation or periodically as mine conditions change, but 

need not be done by operating personnel at the 

mine. The operating parameters for the system are 

set by connecting the system to a laptop computer 

via an RS-232 null modem cable with the communication terminal emulator acting as the laptop 

client program. This allows a trained user to easily 

change the parameters for triggering the various 

load rate indicator devices to suit conditions at the 

mine. The load rate monitoring system is designed 

tobeMSHApermissible. 
With the cooperation of a major MRS manufacturer, the system was installed and tested on a MRS 

in the laboratory. It is anticipated that the addition 

of this type of device to MRS=s will significantly 

improve the safety of room-and-pillar retreat 

operations using these machines. 

Conclusions and Recommended Work

To eliminate setting and handling posts and reduce 

the number of miners required to work near the 

cave line and other dangerous locations, a remotely 

controlled MRS has been developed and field 

tested. Optimum use of MRS=s depends on careful 

panel designs, mine orientation, and prudent primary support designs geared to expected geologic 

and stress conditions. MRS=s have a limited zone 

of influence around them and thus can best be 

utilized in combination with other MRS=s and in 

conjunction with ground monitoring systems. 

An integrated ground monitoring system is being 

tested in which the simplicity of convergence 

measurements are combined with more elaborate 

load-rate monitoring on MRS leg cylinders. Measurements from four mines with various geologic 

and support conditions have shown that monitoring 

roof-floor convergence enables miners to detect 

unstable roof conditions within the whole area of 

the interest at the face. Monitoring load rates can 

also provide information on the stability of pillars 

and fenders. 

References

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(MO Univ., Rolla, MO, Nov. 2-4, 1987). MO 

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Univ., Morgantown, WV, Aug. 1-3, 1995). Dept. 

of Mining Engineering, WV Univ., pp. 55-62. 

Figure 9. Diagram of load rate monitoring system. 
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Ph.D. dissertation, Colorado School of Mines, 

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by Monitoring the Rate of Roof Movement. Coal,

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Maleki, H., and J. Owens, 1998. Analysis of the 

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in Rock Mechanics-SAR Rocks 98, ed. by L. A. da 

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(Santos, Sœo Paulo, Brazil, Nov. 22-25, 1998). 

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13, 1986). Dept. of Mining Engineering, WV 

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Wilson, H. G., 1991. Mobile Roof Support for 

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Engineering, WV Univ., pp. 103-114. 

Hamid Maleki received both MS and 

Ph.D. degrees from the Colorado 

School of Mines. He has worked as 

rock mechanics engineer for mining 

companies and geomechanics 

consultants for mining and nuclear 

waste industries. He is a principal 

with Maleki Technologies, Inc. Mr. 

John Owens received his BS 

degree from Gonzaga University. 

He is a mechanical engineer with the 

Spokane Research Laboratory 

(NIOSH). Mr. Howie is an electronics 

technician with the Spokane 

Research Laboratory. 

ADDRESSES: 

H. Maleki: maleki.tech@worldnet.att.net

Maleki Technologies, Inc., 5608 S. 

Magnolia, Spokane, WA 99203; (509) 

448-7911; FAX (509) 448-7911 

J. Owens: jd04@cdc.gov

Spokane Research Laboratory, NIOSH, 

315 E. Montgomery, Spokane, WA 

99207; (509) 354-8007; FAX (509) 354-

8099 

W. Howie: wch7@cdc.gov

 Spokane Research Laboratory, NIOSH, 

315 E. Montgomery, Spokane, WA

99207; (509) 354-8003; FAX (509) 354-

8099