Roof Fall

1%

Rib

16%

Bump

1%

Roof Skin

82%

Figure 1. Percent of injuries by ground fall type

in underground coal mines

ENHANCED SURFACE CONTROL FOR ROOF AND RIB SUPPORT

Stephen C. Tadolini, Vice President

Excel Mining Systems, Inc.

Cadiz, Ohio USA

Dennis R. Dolinar, Mining Engineer

National Institute for Occupational Safety and Health

Pittsburgh Research Laboratory

Pittsburgh, Pennsylvania USA

ABSTRACT

The risk of injury caused by minor roof falls continues to increase

in underground mining. Most ground control injuries result from

small rock falls that occur in a supported area, but do not involve the

failure of the support system. In the roof, these are the "skin failures"

that occur between previously installed primary or secondary bolting

systems. To minimize skin failures and the associated accidents,

several devices were designed and tested in the laboratory to

maximize stiffness, minimize material thickness and weight, and ease

material storage and handling on roof bolting machines. The goal of

this project was to design and evaluate devices that can be installed

with traditional primary or secondary roof supports, and that are easy

to handle/install under temporary or previously supported roof.

BACKGROUND

Surface control can be a critical component of effective roof and

rib support systems. Proper surface control devices, with adequate

stiffness characteristics, can help minimize or even eliminate

progressive roof and rib failures. While there is no universal method

that can be applied for roof and rib stabilization, combinations of

bolts, mesh (steel and nylon grid) straps and plates vary depending on

the type of fracture patterns/cleating and the immediate roof

characteristics. Full “liner” supports, arguably the most effective, can

be time consuming, cumbersome, and expensive. While these

disadvantages can be partially offset with reductions in personal

injuries or fewer production delays, they are seldom implemented. 

To help illustrate the scope of the problem, injuries from the

Mine Safety and Health Administration (MSHA) data base were

examined from 1995 to 1999. Every year MSHA compiles the

reportable injuries and accidents with information defined and

required by the Code of Federal Regulations, title 30, part 50. During

this 5-year period there were 4,100 reported injuries resulting from

roof falls, bumps, rib falls, and roof skin failures (figure 1). Roof

skin failures are defined as rock falls from the surface of the roof to

a depth of 2 ft but not limited in the other dimensions. A roof fall

encompasses a thickness greater than 2 ft. In each of the five years

there were about 800 reported injuries and 10 to 15 fatalities from

ground falls in underground coal mines. However, 98% of these

injuries, usually including 3 to 4 fatalities, resulted from relatively

minor falls of rock from the roof and ribs in coal mines. In 1997, a

typical year, 671 injuries (82% of the total) were from these small

roof falls, while 128 injuries (16%) were from rib falls. Only 13 of

the injuries in 1997 resulted from large roof falls that involved the

failure of the primary support system. The other 799 injuries

occurred in areas where the roof had been supported and therefore

should have been safe.

The activity that is most at risk from these minor falls is roof

bolting. In 1997, 44% of roof skin injuries occurred during the roof

bolting cycle. Rib falls are more widely distributed, with 19%

occurring during roof bolting and 17% during continuous miner

operation. More importantly, over 75% of both the reported rib and

roof fall accidents occurred at or near the working faces (inby feeder

breakers). Much of the previous ground control research has

concentrated on massive roof falls, and not on the small pieces of

rock between supports. Injuries from large roof falls typically

account for less than 15 injuries per year, while the injuries from

small rock falls have numbered around 800 per year. The mining

machine industry has reacted to the problem of small rock falls by

developing canopies and shields to protect roof bolters and miner

operators. Current surface control systems placed on equipment,

principally roof bolt machines, to protect workers have been credited

for reducing the number and intensity of accidents (1-2). Rib bolting
Figure 4. The 17-in square plate with 4

internal spokes

Figure 3. The 19-in circular plate

Figure 2. A traditional W-strap with a 1-in hole in the center

and other support measures have been successful where properly

applied. Often, however, the hazards from skin failure go

unrecognized, so no support is applied, or when applied the support

is ineffective. 

Another area of concern is the remote control operation of a

continuous mining machine since the operator is standing beside the

machine under supported roof but not under a canopy. The operator

is continuously walking or moving near the rib and under freshly cut

but supported roof. Material can fall between the typical roof bolt

bearing plates and thin roof mats, and strike the operator. Injuries to

miner operators caused by small rock falls in such situations,

represent 9% of all ground fall injuries.

This investigation examines systems that can be placed between

the bearing plate and the mine roof to increase and maintain adequate

surface control. The designed systems will enhance surface control

by covering as large an area as possible without sacrificing

performance or stability. The system must be stiff enough to resist

downward motion and subsequent collapse if larger amounts of

material separate and fall, must be easy to install, and cost effective.

Ideally, the same systems should be applicable to support rib areas at

critical horizons and locations to minimize slumping for failing ribs.

Dual-use systems minimize material handling and reduce the costs

associated with storage and handling.

FUNCTION, DESIGN AND PHYSICAL DIMENSIONS OF

TESTED PLATES

Function

The main function of the large surface plates is to maintain and

control the surface or skin area around the roof bolt. Essentially, the

plate is designed to maintain a much larger area around the bolt than

the bearing plate. The surface plates are used in conjunction with the

standard bearing plates. This function is achieved by the stiffness

(resistance to movement) and by the large area covered by the surface

plate.

A normal bearing plate must be designed to form a bearing

surface for the roof bolt, control the rock on the surface in the

immediate vicinity of the roof bolt, allow the bolt to retain tension

and allow the bolt to resist rock movement (3, 4). Essentially, the

bearing plate is an important element in maintaining the function of

the roof bolt. 

Design

The tested plates were fabricated from 18 to 20 gauge (0.055 to

0.036-in thick) non-graded steel. Four types of plates were tested that

include a traditional W-Strap, a 19-in diameter circular plate, and a

17-in square plate with two rib-designs. Figure 2 shows the

traditional W-Strap fabricated from 0.055 in thick material with final

dimensions of 18-in long and 12-in wide with a 1-in diameter hole

punched in the center. 

A circular plate, shown in figure 3, has a 19-in diameter and

0.036-in thickness. A hole is punched in the center to accommodate

bolt insertion. Two different internal structural designs were evaluated for the

17-in square plates, one with 8 internal spokes and one with 4

internal spokes (figures 4 and 5). Both types of plates were made

from 0.048-in thick steel material.
Figure 5. The 17-in square plate with 8 internal spokes

Figure 6. Test apparatus placed in Tinus Olsen machine

Figure 7. The test machine and loading apparatus design. All

plates were tested with the same configuration.

Testing of Large Surface Plates

A laboratory test was used to evaluate performance of the large

surface plates. Figure 6 shows the test frame set up. The test

consisted of setting the plates on two supports, 14 in apart then

applying load to the center of the plate. American Society of Testing

Materials (ASTM) tested and approved bearing plates are required on

all of the systems to comply with CFR Regulations. To simulate this

required condition, the center load was applied through a 6-in by 6-in

bearing plate as shown in figure 7. Both load and deformation were

measured and recorded during these tests. The square plates were

tested with the supports parallel to the edges as well as at a 45° angle

to evaluate the diagonal plate properties of the 17-in square plates.

The load-deformation graphs for each tested plate are shown in

figure 8 through 11 and summarized in Table 1. The peak and yield

strengths are given in terms of the load with the yield strength

determined where the load-deformation curve begins to flatten or

deviates noticeable from a straight line. The stiffness was calculated

as the secant line from no load to the yield strength and is the load

required to produce a given deformation. The stiffness is a measure

of the ability of the support to resist rock movement. The energy is

the area under the load deformation curve up to yield and is a

measure of the support toughness. 

Table 1. Results of center point tests conducted on large surface plates.

Plate Type Strength, lbs Deformation

yield, in

Stiffness, yield,

lbs/in

Energy, yield,

in-lbs Area, in2

Peak Yield

Square (17 in) 4-spoke, parallel 886 800 0.60 1,400 220 289

Square (17 in) 4-spoke, diagonal 1,375 1,290 1.70 760 1,100 289

Square (17 in) 8-spoke, parallel 550 450 0.40 1,133 94 289

Square (17 in) 8-spoke, diagonal 825 725 1.40 520 500 289

Circular (19 in) 520 400 0.20 1,531 80 284

W-Strap 633 633 0.29 2,180 92 216
Figure 8. Results of load tests performed on W-strap

Figure 9. Results of load tests performed on 19-in diameter

circular plates

Figure 11. Results of load tests performed on 17-in square plates

with 4 spokes (parallel and diagonal)

Figure 10. Results of load tests performed on 17-in square plates

with 8 spokes (parallel and diagonal)
Analysis of Laboratory Tests

From the tests curves, it appears the deformation, plate yield and

ultimate failures result from bending the plates. Some deformation

may have been the result of punching of the bearing plate into the

center of the larger plates. 

Results

The test results indicate that the 4-spoke square plate has the

highest yield and peak load while the circular plate has the lowest

strength. The square plate design appears to be fundamentally

stronger than the circular plate; especially the 4-spoke plate, though

both plates have nearly the same surface area. The area of the circular

plate is 283 in2

 and the square plates are 289 in2. The W-strap has an

intermediate strength between the circular and square plates even

though the plate is thicker than the other plates. When comparing the

square plates, the 4-spoke plate is significantly stronger than the 8-

spoke plate. The four extra spokes parallel to the plate edges act to

weaken the plate essentially by providing a weak point (crease) where

the plate bends. This proves that careful attention must be given to

the internal structural design of the plates. The square plates are

much stronger when tested across the diagonals. This is the result of

more material and structure in the plate corners, adding strength to

the plate across that direction of bending. 

The yield strength should be used and not the peak strength for

design and evaluation of the large plates. Once the roof surface

begins to fail and the plate goes into yield, because the rock will act

as a dead weight load on the plate, the plate will continue to deform

with little or no increase in the load. The rock load will be transferred

from the plate only when the rock falls from the roof caused by

excessive deformation of the plate. When this happens, the support

has failed and its only function is to retain loose material. Therefore,

the plate performance beyond yield is not considered important. 

When considering the stiffness of the plates, the circular plate is

1,000 lbs/in and the 17-in square, 4-spoke plate is 1,400 lbs/in. Much

of this stiffness appears to come from the external or edge structure

of the plates. The stiffness of a square flat plate without any structure

is only 75 lbs/in (calculated). For the 8-spoke square plate the added

spokes reduce the stiffness to less than that of the 4-spoke plate. The

four spokes connecting the raised circular section surrounding the

plate center with the center edge of the plate added no stiffness to the

structure, because they did not extend across the entire plate width as

do the edge deformations and, therefore, they do not increase

resistance to bending. Further, these four added spokes altered the

edge structure at the critical point of the center edge of the plate

where the largest bending moment occurs and thus reduced the

stiffness. The W-strap has a higher stiffness, than either the circular

or square plates but only in one direction because of their design.

This increased stiffness may result from a smaller width of flat steel

plate between the edge ridges than the other two plates. Some of the

plate displacement results from the deformation of this flat section of

the plate. The greater the distance between the edge ridges, the less

rigid the flat section of the plate becomes.

EFFECTS OF PLATE SIZE ON PERFORMANCE

From the laboratory test set up, the strength and stiffness of the

plates are determined across a standard length of 14 in. However, it

is the full plate dimensions that will determine the actual stiffness and

strength and govern how the plates will perform in situ. With plate

dimensions larger than 14 in, the bending moment generated from

the load will increase. Further, the expected rock load will also

increase with the plate dimensions. With the larger bending

moment, the plate deflection will increase while the plate stiffness

will decrease unless compensated by the plate structure through the

moment of inertia. Essentially, the larger the plate, the greater the

bending moment developed by the rock load which will result in a

reduction in plate strength and stiffness. 

The plate strength will be determined from the stresses that

develop in the plate from the bending moments. The following

equation can be used to determine the maximum bending moment

generated during the laboratory test (5):

S FLZ

4I max

T T

y

=

where: Smax = Maximum outer fiber stress, psi

 Z = distance from neutral axis, in

FT = maximum plate load (yield), lbs

LT = length between supports, in

Iy = moment of inertia of the plate, in4 .

This equation provides an approximation of the stresses

developed in the plate because this equation assumes a line load

across the surface plate. Also, for the complex structure of the plates,

it is difficult to calculate both the moment of inertia and the distance

of the maximum stress from the neutral axis.

However for the same shape plate, the maximum load for a plate

with a larger dimension between supports can be determined by

equating the maximum stress based on the standard laboratory test

and that expected for the larger dimension. The relationship is:

F F L

L A

T T

A

=

where: FA = maximum load (yield) for actual plate dimensions, lbs

FT = maximum plate load (yield), lbs

LA = plate width, in.

LT = length between supports, in

Essentially the strength of the plate is determined by using the

laboratory strength, the ratio of the laboratory test span (14 in), and

the actual plate dimension. For the 17-in four spoke plate the yield

load from the test was 800 lbs where the calculated yield strength for

the full plate width is 660 lbs. For the circular plate, with a tested

yield strength of 400 lbs, the yield strength across the full width

would be 295 lbs. For the W plate, with a tested yield strength of

633 lbs, the yield across the full length is 490 lbs.

The increased plate dimensions will also affect the plate

stiffness. The change in stiffness can be calculated using the plate

load and deformation at yield. The following equation can be used

to approximate the plate deflection based on a two-point test with a

concentrated line load across the plate: 
D FL

48 EI

3

y

=

where: D = plate deflection, in

F = yield load of plate, lbs

L = width of plate, in

E = elastic modulus of the steel, psi

Iy = moment of inertia of the plate, in4 .

This load-deformation equation can be used to estimate the

moment of inertia from the laboratory test results for the different

plates. Using the load and deformation at yield, the moment of inertia

for the 17-in four spoke plate is 2.7x10-3 in4

, the Circular plate is

2.0x10-3 in4

, and the W-Strap is 4.4x10-3 in4.

 

The calculated deflection at yield for a full width of the 17-in

square plate is 0.9 in. Based on the calculated yield load of 660 lbs,

the plate stiffness is 740 lbs/in as compared to the tested plate

stiffness of 1,400 lbs/in. For the Circular plate, the calculated

deformation is 0.7 in at the yield load of 295 lbs with a resulting

stiffness of 400 lbs/in. This compares to the tested stiffness of

1,000 lbs/in. For the W-strap, the calculated deformation is 0.6 in at

a yield load of 490 lbs resulting in a stiffness of 785 lbs/in where the

measured stiffness was 2,130 lbs/in. The increased dimensions

significantly decrease the surface plate stiffness. 

The stiffness and strength of larger plates of similar structure to

those tested should have significantly lower strengths and stiffness.

For a twenty-four in square four spoke plate the estimated strength is

470 lbs and a stiffness of 260 lbs/in. For a twenty-four in diameter

Circular plate, the estimated strength is 235 lbs and a stiffness of

200 lbs/in. The assumption made for these calculations is that the

distance to the neutral axis is the same as the tested plates and the

moment of inertia of the plates is the same. The first assumption is

correct only if the lateral plate dimensions are altered. The second

assumption is correct if only increasing the lateral dimensions of the

plate increases the moment of inertia. However, the flat portion of

the plates adds little to the moment of inertia. The moment of inertia

for a 0.48 in thick plate, with a 24 in width is 2x10-4 in4

 and with a

17 in width, is 1.5x10-4 in4

. The difference in the moment of inertia

resulting from the increased width is about two orders of magnitude

less than the moments of inertia of the plates. Therefore, simply

increasing the lateral dimensions only have a minimal affect on the

moments of inertia.

Using the standard tests, both the strength and stiffness of the

plates can be determined across a standard dimension. Comparisons

can be made between plates to determine what might be a more

effective design. However, the size of the plates must be considered

in determining how the plates may perform in situ. Increasing the

plate dimensions can significantly decrease both the strength and the

stiffness. The above analysis was developed as an example to

illustrate how the plate dimensions will affect the plate performance.

These examples are somewhat over simplified as to how the plates

may actually perform in situ. The in situ performance will not only

be affected by the plate size but the manner in which the plate is

loaded. 

CONCLUSIONS

The fall of the small rocks or skin material accounts for about

82% of the average 800 reported accidents from ground falls. The

activities of roof bolting and operating the miner have been

identified as the jobs with the most exposure to these accidents. This

investigation examined the systems that can be placed between a

bearing plate and the mine roof to minimize these small failures and

help maintain adequate surface control. 

The test results indicate the 4-spoke, 17-in square plate achieved

the highest yield and peak load while the circular plate had the

lowest strength. The square plate design appears to be fundamentally

stronger than the circular plate; especially the 4-spoke plate, even

though both plates have nearly the same surface area. The area of

the circular plate is 283 in2

 and of the square plates 289 in2. The

tests on the two types of square plates revealed the 4-spoke plate is

significantly stronger than the 8-spoke plate. The four extra spokes

parallel to the plate edges act to weaken the plate, essentially by

providing a weak point (crease) where the plate can bend, illustrating

that careful attention must be given to the internal structural design

of the plates. The yield strength should be used and not the peak

strength for design and evaluation of the large plates. This is

because the skin or surface of the rock is a soft system. Once the

load comes on the plate there may be little load transfer as the

system goes into yield except when the surface rock falls. When this

occurs the support has failed its only function to retain material. 

Even with the same stiffness as the circular plate, the 4-spoke

plate has a much higher energy or toughness at yield . Essentially,

the 4-spoke plate can absorb substantial more load and deformation

than the circular plate and still function while providing the same

resistance to rock movement. Because of the higher toughness but

with the same stiffness, the 4-spoke has to be considered superior.

The W-strap has a higher stiffness than both the 4-spoke spider

and circular plates. However, the energy at yield is less than half

that for the 4-spoke plate but almost three times the energy of the

circular plate. The W-strap is certainly a better support than the

circular plate up to yield. At lower loads, the W-strap will resist

rock movement better than the 4-spoke plate. However, the superior

toughness of the 4-spoke means that this plate will continue to

function well after the W-strap has failed. In this case the increased

stiffness is a trade-off with the superior toughness. It is important to

remember that the W-strap provides substantially less surface

coverage and is more of a one-dimensional support than either the

4-spoke or circular plates.

By using the same amount of material and thickness, an

engineered design of a plate can be significantly improved. The next

phase of this investigation will access the field performance of the

4-spoke plates under actual mining conditions and loading histories.

The ultimate goal remains to provide improved surface control and

protection for the personnel working in mines with spalling roof and

rib conditions.

REFERENCES

1. Bauer E.R., Pappas, D.M., Dolinar D.R., McCall F.E. and

Babich, D.R. Skin Failure of the Roof and Rib in Underground

Coal Mines. In: Peng S. S. and Mark C., eds. Proceedings of
the 18th International Conference on Ground Control in Mining.

Morgantown, WV, West Virginia University, 1999, pp108-115.

2. Bauer E.R. and Dolinar D.R. Skin Failure and Rib Support

Techniques in Underground Coal Mines. Proceedings: New

Technology for Coal Mine Roof Support. Pittsburgh, PA,

National Institute for Occupational Safety and Health IC 9453,

2000, pp 99-109.

3. Stillborg, B. Professional Users Handbook For Rock Bolting.

Series on Rock and Soil Mechanics V 15, Trans Tech

Publications, 1986, 145 pp.

4. Franklin, J. and Dusseault, M. Rock Engineering. McGrawHill, 1989, 600 pp.

5. Roark, R. Formulas for Stress and Strain. McGraw-Hill, 1965,

432 pp.