Figure 1. Instability in 4-point wood crib caused

by single soft timber

MISTAKES, MISCONCEPTIONS, AND KEY POINTS

REGARDING SECONDARY ROOF SUPPORT SYSTEMS

Thomas M. Barczak, Research Physicist

National Institute for Occupational Safety and Health

Pittsburgh Research Laboratory

Pittsburgh, PA

ABSTRACT

Roof support systems are necessary to provide stable mine

openings and much research has been conducted to design a variety

of roof support systems that will function in various manners to

ensure that stable ground conditions are achieved. Despite these

advancements in technology, mistakes continue to be made in the

evaluation and/or installation that significantly degrade the support

capability or lead to erroneous determinations of support

expectations. The purpose of this paper is to discuss misconceptions

about how roof supports perform and factors that impact their

performance. The goal is to present practical information that will

assist mine operators and engineers in selecting, installing, and

evaluating roof support systems properly, and help them to avoid

mistakes that can lead to erroneous expectations and potentially

catastrophic results that may lead to roof falls. The paper is limited

to a discussion of secondary roof support systems and powered roof

supports such as longwall shields.

INTRODUCTION

Roof support is an essential component of being able to mine

safely. Some form of roof support is employed in every underground

coal mine, but the support strategy may differ significantly from mine

to mine. Generally, due to economic constraints, the minimal amount

of support necessary to ensure a high probability of roof stability is

accepted as standard practice. As such, this design philosophy

requires a good understanding of the support performance and ground

interaction to achieve an optimum roof support system. Since this

design philosophy leaves little room for error, any mistake in

understanding the roof support capability, or how the support

interacts with the ground to achieve roof stability, or improper

installation procedures can be costly and lead to unsafe conditions.

The scope of the paper is limited to secondary roof supports and

powered roof supports such as longwall shields. Topics which are

analyzed in the paper include: (1) proper ways to assess support

capacity and costs, (2) understanding the difference in material

properties and full scale support performance, (3) understanding

factors that lead to uncontrollable convergence and support design

issues for this condition, (4) learning how to evaluate support loading

data, (5) understanding the impact of support load density on ground

deformations and how to make equivalent comparisons of support

systems, (6) understanding factors that do and do not increase

support stiffness and/or control the yield capability of the support,

and (7) proper installation practices for various support technologies.

The paper is presented in a form of bulletized items with only a

brief technical explanation for each. Complete theoretical

assessments of the issues are not necessarily provided. The goal is

to provide concise summaries of the various issues with practical

information relative to each topic. This format is provided to

facilitate the use of this paper as a reference guide for roof support

application. A more detailed explanation of the issues can be found

in the references.

SECONDARY ROOF SUPPORT SYSTEMS

1. Remember that the performance of wood cribs can be

controlled by the response of just a single layer of timbers

(figure 1).

For example, you might think that you have an oak crib because

the majority of the timbers in the crib are oak, but a single layer
of a weaker timber such as poplar can degrade the performance of

the crib to that of the weaker timber. In this case, the capacity of

the crib would be reduced by 27% due to the weakness of the

poplar timbers. The consequences of degrading the crib capacity

by employing mixed wood species is shown in table 1. 

Table 1. Comparison of an all oak crib with one constructed

from mixed oak and poplar timbers. Analysis derived from

STOP using 15 tons/ft @ 2 in as design criteria.

Evaluation Parameter

All oak

crib 

(6x6x36 in

timbers)

Mixed

oak/poplar

crib (6x6x36

in timbers)

Support load density, tons/ft . . . . . . . 15.0 15.0

Support spacing, in . . . . . . . . . . . . . . 91.1 61.3

Installation rate, ft per shift . . . . . . . 84.4 56.7

Installation cost, $/ft of entry . . . . . . 28.90 42.90

Savings per 10,000 ft of entry ..... $140,000 $0 (Baseline)

2. Another mistake that is made with conventional crib

construction is to place the wide side of the timber in the

vertical orientation during crib construction. 

Again, this does not necessarily have to be every layer, a single

layer with this orientation can significantly degrade the crib

performance. Table 2 summarizes the consequences of wideside-up, 4-pt crib construction using 5x6x36-in (oak) timbers.

Table 2. Comparison of an all oak crib constructed from

5x6x36 in timbers in a wide-side-down and wide-side-up

orientation. Analysis derived from STOP using

15 tons/ft @ 2 in as design criteria.

Evaluation Parameter

All oak crib

Wide-sidedown

(6x5x36-in

timbers)

Wide-side-up

(5x6x36-in

timbers)

Support load density, tons/ft . . . . . 15.0 15.0

Support spacing, in . . . . . . . . . . . . 91.1 63.6

Installation rate, ft per shift . . . . . 74.7 58.6

Installation cost, $/ft of entry . . . . 28.90 37.30

Savings per 10,000 ft of entry . . . $83,000 $0 (Baseline)

3. Do not use the same crib design at different mining heights and

expect to have the same ground control capability. 

The stiffness of a conventional crib and most timber supports

constructed in piecemeal fashion from several layers of timbers

decreases as the height increases (1). Hence, if the same crib

design is utilized at higher mining heights, the convergence will

generally increase resulting in the potential for degraded ground

stability. As an example, a 10-ft-high, 4-point crib constructed

from 6x6-in cross section timbers will have 62% of the capacity

at 2 in of convergence compared to that of the same crib

constructed to a 5 ft height. This means that 40% more cribs

will be required at the 10-ft mining height to provide an

equivalent support load density.

4. For unconfined concrete supports, do not use the full material

strength to estimate the support capacity.

A common mistake in estimating the capacity of concrete

supports constructed from donut or rectangular sections is to

multiply the contact area by the material strength as measured in

the laboratory under ASTM specifications. In full-scale crib

constructions, the concrete fails from stress concentrations, such

as block discontinuities, that are not present in the laboratory

test samples. These and other factors significantly degrade the

achieved capacity of the concrete support. A conservative

measure of strength is to take 50 % the measured laboratory

strength to predict the full scale support capacity of an

unconfined concrete support structure (2).

5. When following Mine Safety and Health Administration

(MSHA) guidelines for selecting the minimum diameter

requirements for timber posts, the posts will generally fail

from buckling at loads less than their crushing strength.

There are two ways in which a timber post can fail. Either the

wood can crush as the load exceeds the compressive strength of

the timber, or the post will buckle, either near the middle of the

post or near some defect such as a knot that weakens the wood.

Buckling is a form of instability that is controlled by the

slenderness ratio, or ratio of the height of the prop to its

diameter. Using the MSHA guidelines for the determining the

minimum diameter for a given mining height, a post will

develop applied stress less than half of its crushing strength (3).

Table 3 shows an example using green poplar posts. The

achieved capacity relative to the crushing strength is even less

for dry posts. 

Table 3. Expected buckling loads of minimum MSHA

recommended post diameters (made from poplar wood)

as a percentage of the maximum (crushing) load

capability of the post.

Post

length, ft

MSHA recommended

minimum post

diameter, in

Buckling load

Tons Pct of crushing load

5 4 8 47

7 5 12 45

9 6 16 43

11 7 22 43

13 8 29 44

15 9 37 44

17 10 46 44

6. A common mistake in comparing passive roof support systems

is to make comparisons at some constant convergence and

ignoring the fact the achieved convergence will be controlled

by the installed load density of the support. 
 

Roof support systems should be compared on an equivalent basis

in terms of their capability to provide roof control. A common

mistake is to evaluate different roof support systems which are

installed in the same support layout (i.e., spacing and number of

rows of supports), despite differences in their stiffness and

performance characteristics.. Only when the supports are installed

with the same support load density, can an equivalent functional

comparison of roof support capability be provided.

Here’s why. The convergence and hence ground stability will be

determined by the installed support load density. Higher support

load density will generally result in lower convergence and hence

more stable ground conditions. In order to achieve equivalent

support load density for different roof support systems, the

installation (number of rows and support spacing) must be adjusted

according to the performance characteristics of the individual

support (4). 

Failure to examine supports at the same installed load density can

cause you to avoid considering supports with higher unit costs but

superior roof support capability. Here is an example (table 4). A

longwall tailgate entry is supported with two rows of 4-point cribs

constructed from 6x6x36-in mixed hardwood timbers on a 71.4 in

spacing. This system provides a support load density of 16.7

tons/ft, which in this mine controls the convergence to 3 in. If a

24-in Link-N-Lock crib is employed in this same arrangement, the

Link-N-Lock system will provide a support load density of 23.3

tons/ft and reduce the convergence to 1.4 in. Comparing the

installed cost of the two systems, the conventional wood crib

system can be installed at relative cost of $32.50/ft while the LinkN-Lock would require a relative cost of $46.30/ft to install.

However, if the Link-N-Lock was installed on a 157 in spacing, it

would provide the same 16.7 tons/ft of support capacity at 3 in of

convergence, and could then be installed at a cost of only

$21.10/ft. 

Table 4. Examining supports at same load density.

Analysis parameters

Identical Layout Equivalent Load

Density

4-Pt crib 

(6x6x36

in)

Link-NLock 24-

in

4-Pt crib

(6x6x36

in)

Link-NLock

24-in

Number of rows . . . . 2 2 2 2

Center to center

spacing, in . . . . . . . . 71.4 71.4 71.4 157

Support load density,

tons/ft . . . . . . . . . . . . 16.7 23.3 16.7 16.7

Convergence, in . . . . 3.0 1.4 3.0 3.0

Installed cost, $/ft . . . $32.50 $46.30 $32.50 $21.10

7. When considering the cost of a support system, the full

installation cost for the application of the support system

including the material handling efforts should be considered,

and not simply the material cost of an individual support.

Many of the emerging roof support technologies provide both

superior roof support as well as superior material handling

advantages (5). If the benefits of these additional factors are not

adequately considered in an analysis, an unfair advantage is

given to conventional supports which may have lower unit

material costs, but provide less support capability and are more

costly, time consuming, and difficult to put in.

As an example (table 5), a comparison is made between a

pumpable crib support and conventional 9-pt cribbing. The

pumpable crib support developed and marketed by Heintzmann

Corporation has one of the most expensive material costs, while

a conventional wood crib has one of the lowest material costs.

Yet, despite nearly twice the unit costs, the Pumpable crib can

be installed at a lower total installed cost and provide twice the

support capacity at 1 in of convergence because the higher

capacity of the punmpable crib allows it to be installed at lower

support density and the pumpable support can be transported and

installed with considerably less effort. 

Table 5. Comparison of pumpable support and C. Point crib.

Pumpable

Crib

30 in 

9-Point Wood

Crib (6x6x36-in

timber) 

Number of rows . . . . . . . . . . . . . 1 2

Support spacing, in . . . . . . . . . . . 120 110

Installed load density @ 1 in of

convergence, tons/ft . . . . . . . . . . 24 12

Unit support costs, $/support . . . $316 $154

Installed support cost, $/ft ..... $31.60 33.70

Installation rate, ft per shift . . . . 555 70

Construction labor, $/ft . . . . . . . $3.51 $8.54

8. Preloading does not change the stiffness or the overall

performance characteristics of a roof support system, but it

may improve the apparent stiffness and stability of the roof

structure.

The benefits of preloading is not that it changes the stiffness or

performance of the support, but rather it may improve the

condition of the rock structure by building a more competent

roof beam much like a pretensioned roof bolt. The load

development in the support itself as a function of continued

ground movement does not change because of the preload. It’s

stiffness is the same with or without preload, just as a roof bolt

does not change stiffness due to preload. Since preload causes

deformation of the support, its loading cycle essentially begins

at a different point on the load-displacement curve, but it is the

same curve that illustrates its load-displacement profile. 

9. Increasing the density or number of supports does not improve

their individual yield capability (figure 2).

Another common misconception is that the yield capability of

support systems can be increased by increasing the number of

supports or density of supports used in an application. Using

timber posts as an example, it does not matter if there is one post

or ten posts, they will all fail at less than one in of convergence.

The confusion is generally related to the improved ground

control that is often provided by increasing the number of

supports. Increasing the number of supports will increase the
 

Figure 2 - Increasing the number of timber posts does not change

their individual yield characteristics

Figure 3. Rigid-body analysis of a cable truss

Figure 4. Standing supports,

even something as strong as a

Can, cannot eliminate or control

floor heave

support load density, which in turn will tend to reduce the

convergence in the area. The reduced convergence allows supports

such as posts to function in areas where fewer supports would fail,

but if the convergence was not controlled by the additional support

capacity provided by the extra supports, then they would still fail

as the convergence increased beyond their yield capability, which

remains a constant regardless of the support density.

10. The capacity of a cable truss is not twice the capacity of a cable

bolt of the same diameter.

The fallacy in assuming that a cable truss has twice the capacity of

a cable bolt since it is anchored at both ends stems from the fact

the cable truss is a single piece, and that roof produces loading in

the entire cable. In order for the cable truss to have twice the

capacity of one cable bolt, the center section of the cable truss

along the roof line would have to have zero load, which just isn’t

the case. The actual capacity of the cable truss is a bit more

complicated, being determined primarily by the mechanics of the

truss operation (figure 3), but the primary point to remember is that

the roof loading produces a resultant load at the corner of the truss

which produces tension in both the anchor component of the truss

and the horizontal component along the roof line. Due to friction

at the roof and/or contact block, only about 80% of the load

generated in the angle member is transferred into the horizontal

member of the cable (6). This coupled with the fact that the stress

concentration at the corner where the block and/or rock contacts

the cable creates a stress concentration which causes failure of the

cable at these locations with less than ultimate loading (estimated

to be around 87% of ultimate loading (7)) in other parts of the

cable, decrease the capacity of the cable from this simplified rigid

body analysis. The bottom line is that a cable truss can typically

support a rock load of about 120% of the rated capacity of the

cable (6). However, even this can be misleading in comparison to

cable bolt support, because the truss is such a softer system that is

not really providing roof control as much as it is simply supporting

the weight of damaged and broken roof rock.

11. Truss systems primarily provide containment of damaged roof,

and relatively little control over roof deformation prior to failure

of the rock mass.

Trusses are such a soft support system that they provide little

resistance to roof deformations, and as such, cannot be compared

directly with cable bolts or standing roof support systems. For

example, the stiffness of a cable truss is approximately 5 tons/in.

Even a weak 4-point wood crib constructed from 6x6 in poplar

timbers, which is one of the softest standing roof supports,

would have a stiffness of 5 times this. This means that the roof

must move 5 times as much when a cable truss is installed to

generate the same resistance to vertical (dead weight) loading as

a single 4-point wood crib. Cable trusses act as a means of

containment to damaged roof which acts to provide some

residual stability, but do relatively little in preventing the

damage from occurring.

12. Floor heave and pillar yielding cannot be controlled by

standing roof support systems (figure 4).

The capacity of any standing roof support system is generally too

small to control floor heave and pillar yielding and horizontal

stress to some degree. The forces that are causing these ground

reactions are due primarily to the main roof and overburden

weighting. From the perspective of standing roof support

design, the convergence associated with floor heave or pillar

yield, should be considered as “uncontrollable”. The

consequence of this in terms of designing standing roof support

is that these uncontrollable ground displacements will produce

associated displacement and loading in the support. One of the
 

Figure 5a. Can supports are typically topped off with timbers to

establish roof contact

Figure 5b. If too few timbers

are used, the timbers deform

and control the loading of the

support causing it to be less

stiff than intended

 CONFINED CORE CRIB (3C) SUPPORT

NIOSH SAFETY STRUCTURES TESTING LABORATORY

0

200

400

600

800

1,000

1,200

0 10 20 30 40 50 60

DISPLACEMENT, inches

SUPPORT LOAD, tons

Figure 6. Example of a passive support with over a 1,000 tons of

ultimate capacity, but at a convergence which is unacceptable for

roof control

design issues in floor heave or yielding pillar conditions is that the

support must be able to survive this deformation without being

damaged to the point where it becomes unstable or loses the

required capacity to support the roof. Another concern with

standing roof support structures is that these uncontrollable ground

movements will produce levels of loading in the support that will

exceed the bearing strength of the immediate roof and floor, and

thereby cause damage that can lead to further roof instability and

roof falls. Even if the effect is localized where the support simply

punches into the roof or floor, the support capability is then

degraded and this may lead to more global promotion of roof

instability. 

13. Capping material can dramatically change the performance

characteristic of a roof support (figures 5a, 5b).

The important point to remember here is the weak link principle.

It is always the least stiff material which controls the initial

response of a support. Hence, if the capping material is softer than

the rest of the support material, then the capping material will

control the loading of the support until it is squeezed to the point

where the stiffness in the capping material is equivalent to that of

the other support material. Here are a few examples. When cap

blocks are used on timber posts, the cap blocks, even though they

may be of the same wood type, are softer than the post because

the wood is softer when loaded perpendicular to the grain than

parallel to the grain. Hence, using cap blocks softens the

response of the timber post, resulting in more overall

deformation or yield before the post fails. Another good

example is the Burrell Can. With the Can the goal is to make

sure that there are sufficient timbers on top of the Can to transfer

the loading into the Can without significant deformation of the

wood. When too few timbers are used, the applied loading

causes the wood to deform before the Can reaches its yield load

capacity. In other words, the wood is the softer member and

controls the loading (deformation) of the support rather than the

material in the Can. If sufficient contact area is created with the

use of enough wood, the then applied stress in the wood from

the roof loading will be below the yield strength of the wood,

meaning that the Can will be the softer of the two components

and it will control the response of the support rather than the

wood. 

14. Passive roof supports do not have any load carrying capacity

until they are squeezed from the roof and floor convergence.

The load carrying ability of a passive roof support can only be

defined as a function of displacement of the support. It has no

load carrying capacity until it is squeezed and generally

increases in capacity as the amount of displacement continues,

up to the point where its ultimate strength is reached. Care must

be made at comparing the peak load capacities of different

supports since the peak capacities can occur at widely varying

degrees of convergence. The relationship of load to

displacement is called the stiffness of the support. Stiffer

supports develop load more quickly (with less displacement)

than softer supports. Figure 6 is an example of a support having

a peak load of over 1,000 tons, but this capacity is reached at

nearly 5 ft of displacement, well beyond the useful range of

allowable convergence in coal mines.

15. Headboards and footboards should be used on the Propsetter

support to prevent punching into roof or floor.
 

Figure 7. Full contact supports

such as the Link-N-Lock crib

should be wedged at more than

the corners of the support

STRATA PRODUCTS (USA) -- PROPSETTER TESTS

NIOSH SAFETY STRUCTURES TESTING LABORATORY -- JAN 9, 1995

0

20

40

60

80

100

120

140

160

0 1 2 3 4 5 6 7 8 910

DISPLACEMENT, inches

SUPPORT LOAD, kips

No splits in prop

3/4 in wide split entire 

length of prop

Figure 9. Load-displacement performance of Propsetter with

and without splits in the prop due to drying out of the timber

One of the most common mistakes made in the installation of the

Propsetter is not to use the footboard or headboard. This mistake

is generally made for one of two reasons: either the materials are

not available after having been used elsewhere or they are not used

in order to avoid having to cut the prop to the correct height.

Propsetters are sold with footboards and headboards as a unit. On

occasion, the headboards or footboards are misplaced, but it more

likely that extra footboards will be used in unexpected high areas

where the prop will not reach the reach of the roof without them.

This can be avoided by buying extra footboards and/or headboards.

Without this head and foot board, the chance that the prop will

punch into the roof or floor greatly increases. Both the 8.5 in and

10 in diameter Propsetter produce around 2,000 psi of roof bearing

stress at peak loading without the headboard. The floor loading

can be even greater than this because of the pod formation on the

bottom of the prop as it accepts load and deforms. Hence, the prop

will punch into weak roof or floor materials, and in doing so

causes further damage to the roof and floor materials and seriously

degrades the roof support capability of the prop.

16. Full contact crib supports such as the Link-N-Lock and Tri-Log

should not be wedged only at corners as is typically done in

conventional cribbing (figure 7). 

The purpose of designing a crib to provide full timber contact is to

increase its stiffness and capacity. When these systems are wedged

at the corners only, the contact area at the roof layer is diminished.

Until there is sufficient load to crush the wedging, the initial

loading will be controlled by the wedged contact area and not the

full crib timbers as intended, resulting in considerably less support

resistance during the initial roof movements. As a minimum,

wedging should be done at the corners of the crib as well as in the

middle of the timbers. Ideally, wedging should be applied along

the full perimeter of the crib to preserve the initial stiffness of the

support.

17. Splits in Propsetter typically do not significantly affect the peak

capacity of the prop (figure 8).

A common misconception regarding the Propsetter support is that

splits in the prop body caused by drying out of the timber will

cause the prop to buckle prematurely and destroy the loading

capability of the prop. This is not the case. Test results in the

Mine Roof Simulator have shown that the Propsetter is still able

to provide the rated capacity of the prop (figure 9). It will not

buckle prematurely, since the wedging action is still maintaining

control of the prop capacity. The dryness may degrade the post

failure performance of the prop. On occasion, the neck of the

prop in the wedged section may deform in a more brittle manner

than planned, resulting in more load shedding after reaching the

rated peak loading. When the prop is green, the wood fibers are

more pliable and the brushing action is more reliable.

Figures 8. Splits in the Propsetter support typically do

not decrease its rated capacity, which is controlled by the

wedged section at the bottom of the prop and not the main

body of the post.

18. Shrinkage of crib type timber supports can be detrimental to

long term support applications.

One factor that is often overlooked in timber supports is that all

wood will shrink. The rate of shrinkage is greater when the

wood is initially green as is the case with most mine timbers

when they are first installed. Mine timbers are often saturated at

over 30% moisture content upon delivery to the mine, and as

such may shrink for several months until the moisture content

drops down below 15%. Since each timber shrinks, the more

timbers used in the crib construction, the greater reduction in

height will occur. In other words, as the seam height increases,

the probability of the support shrinking away from roof contact

increases. 
 

Figure 10. Shrinkage in wood is most

severe in the direction of the growth

rings (tangential) and 50% less in the

radial direction

Figure 11. Prestressing of a support, such as the Hercules

crib shown here, can help to offset the effects of wood

shinkage and maintain the support tight against the

mine roof and floor

The orientation of the grain also controls the rate of shrinkage.

Shrinkage is most severe in the direction of the growth rings or in

a tangential orientation (figure 10) and about 50% less across the

growth rings (radially), and only slightly along the grain. Poplar,

a common wood used in crib and prop construction, shrinks

around 8.2% going from a green to a dry state (tangential

shrinkage) (8). Therefore, a 6-in-thick crib timber can shrink to

5.5 in-thick timber when fully dried. Hence, an 8-ft-high crib

would shrink by 6 in. While the natural humidity in the mine is

likely to keep the timber from fully drying, it is not uncommon for

cribs to shrink away from the mine roof in areas of static stress or

in areas where the loading (convergence) rate is less than the

shrinkage rate of the wood.

Prestressing a timber support, such as the Hercules crib shown in

figure 11, can help to offset the effect of the shrinkage. As the

shrinkage occurs, the preload will be diminished but will extend

the time that the support remains in contact with the roof. Since

shrinkage is considerably less along the grain, timber posts are

far less likely to shrink away from the mine roof than are wood

cribs.

19. Standing supports should not be set on loose material.

Setting a roof support on loose material typically reduces the

effective stiffness of the support since the material is softer than

the support. The result is that the support will provide relatively

little resistance to the roof movement until this material is

compacted. The proper installation procedure should be to

prepare the floor by removing all debris to establish a firm and

level foundation to set the support on.

20. Concrete cribs should not be wedged between the layers of

concrete.

Concrete is very susceptible to stress concentrations. Wedging

between the layers of concrete blocks or donuts, which is

sometimes done to level the crib during construction, creates

stress concentrations that will cause the support to fail

prematurely. Proper floor preparation should be used to

establish a level foundation prior to building the crib. Slight

tilting of the support is better than trying to spot level it during

construction.

21. Laboratory testing of pumpable roof supports in bags will not

provide an accurate representation of the peak loading

capacity of the support unless the top is flat.

Questions have arisen regarding the peak loading of these

pumpable supports measured from full scale laboratory tests in

rigid roof frames such as the Mine Roof Simulator. When these

supports are poured on the surface without a rigid roof, the bags

will typically crown as the bag is filled under the pump pressure.

Since the peak loading capability is a function of the contact

area, the crowning reduces the contact area and thereby reduces

the peak loading capacity. If the material is brittle, as is used in

current pumpable support systems as marketed by Heintzmann

Corporation and Fosroc Corporation, the material fractures and

sheds load after the peak loading is reached. Hence, the full

diameter of the support is never realized in the development of

the support loading. In the mine, the bag is likely to conform to

the roof contour as it filled, and hence the full area will be

established to provide support capacity consistent with the bag

diameter and the material strength.

22. The stability of supports is dependent on the aspect ratio of the

support structure. Failure to abide by the design rules for a

particular support will lead to premature failure.

The aspect ratio is defined as the ratio of the support height to its

width. For example, a 24 in Link-N-Lock constructed to a 6-ft

height would have an aspect ration of 3.0. The required aspect

ratio to maintain stability is different for each support. Proper

aspect ratios are best determined from full scale testing in the

laboratory, although it should also be recognized that stability

can also be affected by other factors such as the symmetry of the

loading and the roof and floor contact. Below are some rules of

thumb for several commonly used support systems. (1)

Conventional wood cribs – 4.3, (2) Link-N-Lock cribs – 4.0:, (3)

Tri-Log cribs – 4.0:, (4) Link-N-X cribs – 3.0:1, (5) Concrete
 

0

200

400

600

800

1,000

1,200

500 Ton 800 Ton 1000 Ton

Support load, tons

Convergence load

Setting load

Figure 13. Due to increases in stiffness, higher capacity shields

develop more loading for the same convergence than softer,

lower capacity shields

Figure 12. Determination of the aspect

ratio for wood crib

cribs – 6.0:1, (6) Can Support — 5.0:1, and (7) Pumpable crib –

4.0:1. The aspect ratio for conventional wood cribbing is with the

width measured from the distance between the centerline of the

crib timbers (not the actual length of the timber) (figure 12), or the

width equals the length of the timber minus twice the overhang

distance plus the timber width. For example, a 36-in length timber

that is 6 in wide with a 3-in overhang would have an effective

width W = 36 - (2 * 3) + 6 = 24 in.

23. When supports of different stiffness are mixed, the stiffer support

will do the majority of the work.

Stiffer supports will develop loading more quickly in response to

convergence then will softer supports. Hence if softer supports are

mixed in between stiffer supports, the stiffer support will be doing

most of the work. For example, if a longwall tailgate is supported

with a single row of alternating 4-point and 9-point cribs, each on

10 ft center-to-center spacing, the 9-point cribs (on a 20-ft spacing)

will be doing most of the work. It is possible that the 4-point cribs

may not provide adequate control in the spans between the 9-point

cribs. Another scenario is when some supplemental support is

used to alleviate roof control problems. If the supplemental

support is significantly stiffer than the roof support system that is

normally used, it can improve ground control. However, if the

stiffer support system is installed at large spacings, two things may

unintentionally happen. Either the support capacity will not be

distributed far enough to improve ground conditions over the total

span which is likely in weaker roof conditions, or in stronger roof,

the stiffer support may get overloaded and fail prematurely since

it is doing the bulk of the work. A unfortunate consequence of this

second action is that the support may be banned from further

consideration, when in fact had it been used as the main

secondary roof support on a normal spacing it would have

performed well.

POWERED ROOF SUPPORT SYSTEMS

24. Staging of hydraulic leg cylinders will affect the stiffness of the

support.

Staging of multi-stage cylinders can have a significant affect on

the stiffness and response of the support to ground movements.

Generally, the stiffness of a particular stage will increase as the

extension of the stage decreases, or as the height of the support

decreases (9). This is one reason why two longwall shields,

sitting side by side one another, will have different levels of

loading at the completion of a mining cycle. Another

consequence of this behavior is that it can complicate the use of

the support loading as a measure of roof loading and instability.

25. Increasing the capacity of longwall shields does not guarantee

that they will yield less often or last longer.

An often overlooked consequence of increasing the capacity of

longwall shields by increasing the size of the leg cylinders is a

proportional increase in stiffness. The increased stiffness means

that the shield will develop more loading with less ground

movement. This can be beneficial if the shield capacity is

controlling the ground movements of the immediate roof, but in

conditions where main roof weighting is the controlling factor,

then it is likely that the convergence will not be fully controlled

by the shield capacity. In this case, the increased stiffness

simply results in increased shield loading. Under this type of

load behavior, an 800-ton shield is just as likely to load to yield

after setting as would be a 1,000-ton shield (figure 13).

Consequently, the life of a 1,000-ton shield is not likely to be

any longer than that of an 800-ton shield, assuming similar

structural margins of safety in the component designs (10).
 

2,500

3,000

3,500

4,000

4,500

5,000

5,500

6,000

6,500

0 50 100 150 200 250 300 350

ADDITIONAL SHIELD LOADING AFTER SETTING, tons

LEG PRESSURE, psi

Full extension of bottom 

stage

Partial extension of 

bottom stage

Figure 14. Whenever the bottom stage is fully extended when

 the shield is set, there will be a period in which there is no

measured changed in hydraulic pressure in the bottom 

stage when roof loading occurs

Figure 15. Corrosion of a lemniscate link bore

can significantly increase the stress from link

loading and increase the probability of failure

26. No change in leg pressure is generally assumed to mean there is

no increase in roof loading. This is not the case whenever the

bottom stage of the leg cylinder is fully extended (figure 14).

A common misconception is that no increase in leg pressure in

supports with hydraulic leg cylinders must mean that there is no

increase in roof loading. In multi-stage leg cylinders, such as those

used in all longwall shields and mobile roof supports, the bottom

stage is where the pressure measurements are taken. When this

bottom stage is fully extended (to its full stroke) when the support

is set against the mine roof and floor, there will be no increase in

the measured pressure in the bottom stage until the pressure in the

top stage exceeds the setting force generated by the bottom stage

against the mechanical stops (9). Hence, the greater the setting

pressure, the longer the period will be when the bottom stage will

not record an increase in roof loading following the setting of the

support. An example of the measured pressure for this condition

on a longwall shield is shown in figure 14. 

27. Corrosion can cause structural failures in longwall shields

without any load being applied.

Corrosion is one the most overlooked factors in shield design and

is a reason for premature failures. Corrosion causes stress

concentrations which lead to microscopic and eventually

macroscopic failures of the steel. Since it does not require load to

be present, corrosion can be a reason why aging shields with

relatively low operating cycles still experience structural failures

(11). The effects of corrosion are also overlooked in the design of

shield components. One common example is the design of link

clevises, which are subject to both wear and corrosion. Figure15

shows the effects of corrosion on a lemniscate link clevis in a

longwall shield. As seen in the figure, the clevis has numerous pits

in the steel from the corrosion. These pits dramatically reduce the

surface area of the contacting pin, resulting in significantly

increased stress levels. Since poor assumptions are often made

regarding the contact arc of these pin and clevis areas in the

original design phase, then the decreased contact area caused by

the corrosion is enough to reduce the margins of safety and can

cause localized yielding of the steel. Since corrosion effects are

not simulated as part of the performance testing program during

the procurement of the shield, the impact of corrosion is often

overlooked in the design phase.

28. Internal leakage in hydraulic leg cylinders reduces the load

carrying capacity of a longwall shield and can result in no

load carrying capacity if the leak is large enough.

A common misconception is that internal leakage (i.e., from

worn seals) will not result in loss of load carrying capacity since

the fluid is still contained in the cylinder. This is not true.

When the seals leak, the hydraulic fluid travels back through the

return line to the supply tank. Fluid loss in either the top or

bottom stage results in a pressure drop in both stages, and a

proportional reduction of support capacity. Even if the staging

valve leaks and the fluid is still contained within the cylinders,

there will be a loss of support capacity due to a readjustment of

the staging caused by a drop in pressure in the top stage (9).

29. Preloading does not increase the stiffness of a support.

Another misconception regarding preloading concerns whether

it increases the stiffness of a support. It does not. This is true

for standing roof supports as well as powered roof supports such

as longwall shields or mobile roof supports. Once the support

is set at whatever preload, the load development as a function of

convergence will be the same as if the support was not

preloaded. Hence, in terms of longwall shields, any additional

ground movement will produce the same increase in shield

loading when the support is set with 2,500 psi leg pressure as it

would when it is set with 4,500 psi leg pressure. In other words,

the shields capability to resist additional ground movements is

not improved by a higher setting force. 

CONCLUDING REMARKS

A wide variety of secondary roof supports are routinely used to

provide additional roof support in almost every mining application.

Many of these roof supports systems have been used for years while

new ones are being developed to provide superior roof control.

Despite their widespread use, some of the basic principles that
 

govern their performance and resulting ground support capability are

not always well understood. 

This paper provides an overview of practical issues that can affect

support performance and provides useful information to make sure that

the support is properly used and constructed. This is accomplished by

providing a series of bulletized topics that address specific issues. The

format allows the user to quickly scan the list of topics and review ones

that are relevant. 

The National Institute for Occupational Safety and Health

(NIOSH) Support Technology Optimization Program (STOP),

although not discussed in this paper, provides a forum to properly

design standing roof support systems, taking into account the many

issues addressed in this paper (12-14). An updated version of this

software, Version 2.3, was released in May 2001 and is discussed in

detail in another paper in this proceedings.

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Considerations in Longwall Face and Gate Road Support Selection

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U.S. Department of Health and Human Services, Public Health

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