LOAD AND DEFLECTION RESPONSE OF VENTILATION STOPPINGS TO 

LONGWALL ABUTMENT LOADING - A CASE STUDY

David C. Oyler, Mechanical Engineer

National Institute for Occupational Safety and Health

Pittsburgh Research Laboratory

Pittsburgh, Pennsylvania USA

Gregory Hasenfus, Geomechanical Engineer

CONSOL Energy

South Park, Pennsylvania USA

Gregory M. Molinda, Research Geologist

National Institute for Occupational Safety and Health

Pittsburgh Research Laboratory

Pittsburgh, Pennsylvania USA

ABSTRACT

Few studies have specifically measured and documented the

large-scale loading behavior and durability of ventilation stoppings

to mining induced movements, particularly from longwalls.

Ventilation stoppings are more commonly the concern of ventilation

engineers, and ventilation stopping response to ground movement has

only been documented in an incidental manner. This paper presents

the investigations of underground measurements that have been

conducted to determine the loading response of stoppings constructed

from lightweight aggregate concrete masonry units (CMU). These

investigations have produced some interesting results that may prove

beneficial, not only for developing and assessing alternative stopping

construction techniques, but also for designing and selecting standard

construction methods for use in varying mining conditions. For

instance, the interface friction that results from wedging a CMU

stopping during construction plays a substantial role in its ability to

resist both lateral and vertical loads. Although they are not intended

for ground support, the study showed that block stoppings can resist

vertical loads of at least 2,700 to 3,000 kN. Asymmetric loading of

a stopping may result in localized failure of blocks within a stopping,

which, depending on the severity, can be a precursor of impending

stopping failure. In addition to measurement results and implications,

the paper will also present details of field measurement methods used

to assess stopping response. Stopping stiffness and material strength

characteristics will also be presented.

INTRODUCTION

In conjunction with a project designed to evaluate alternative

stopping construction materials, CONSOL Energy (CONSOL)

recently initiated an underground field study to assess the behavior of

concrete block ventilation stoppings at a southwestern Pennsylvania

mine. With the assistance of the Pittsburgh Research Laboratory of

the National Institute for Occupational Safety and Health (NIOSH),

several gateroad stoppings were instrumented with convergence

sensors and load cells. The data collected from the field study are

being used, along with test data from the NIOSH’s Mine Roof

Simulator (MRS) and from CONSOL=s modified E-72 test apparatus,

to provide insight into the response of concrete block ventilation

stoppings to mining-related ground movements, particularly those that

occur during longwall retreat. This study will primarily discuss the

field study and is intended as a first step toward improving

understanding of the strength and deformation properties of

ventilation stoppings. Ultimately such understanding could lead

toward the development of design criteria to help maximize the

efficiency of stoppings with respect to ventilation, ground control,

economic, and safety considerations.

UNDERGROUND STUDY SITE

The study site is located in a southwestern Pennsylvania coal

mine operating in the Pittsburgh coalbed. The site, as shown in

figure 1, is located between crosscuts 33 and 35 of a longwall

gateroad, approximately 485 m from the panel startup room and

2,700 m from the panel recovery area. The gateroad is the first

headgate of a new block of panels being developed. It was driven

utilizing three entries, with the track situated between the belt and

return (future tailgate) and crosscuts driven at right angles to the

entries. Entries and crosscuts were both driven 4.9 m wide by 2.1 to

2.4 m high. Entry centers were 18.3 and 42.7 m and primary

crosscut centers were 84 m. Development was oriented at a 23E

angle clockwise from the east west direction.

A total of four stoppings were instrumented at the study site, two

between the headgate (belt) and track entries and two between the

track and (future) tailgate entries. Two of the instrumented

stoppings were constructed of trial alternative lightweight concrete

blocks and two of lightweight aggregate concrete masonry units

(CMUs) commonly used at the mine. Due to logistical problems and

proprietary considerations, the alternative concrete block stoppings

will only be discussed to a limited extent in this paper. In addition,

and as anticipated, the stoppings between the headgate entry and

track entry were removed 30 to 80 m ahead of the face, thereby

limiting their value toward this study.

The most useful data collected were from the lightweight CMU

stoppings built successively between the track and tailgate entries at

crosscut 33 (figure 1). Although the original project plan was to

leave all instrumented stoppings between the track and tailgate

entries in place through both phases of longwall mining, mining

logistics made it necessary to remove these stoppings slightly less

than 100 m before the first longwall pass. They were subsequently

replaced with new stoppings several days after the face passed

beyond their respective crosscuts.
33

32

34

35

Face advance

N

F H

Scale, m

0 100

Tailgate entry

Track entry

Headgate entry

Instrumented

stopping

Figure 1. Study site

Inby Section view Outby

9

10

11 12 vertical & 13 lateral.

 Pole mounted

Propsetters

Plan view

11 10 9

13

Foam crush layer

Load cells

middle inby middle outby outby

Floor

Roof

Cutter

12

Rib RibRib Rib

wood blocks & wedges

Figure 2. Stopping at crosscut 33 showing construction and

instrument locations

GEOLOGY, STRESS AND GROUND CONDITIONS

The Pittsburgh coalbed averages approximately 2.1 m in the study

area, including a 0.2 m parting and a 0.3 m upper split. The roof rock

generally consists of 1.8 m of thin-bedded silty shale overlain by

interbedded sandstone and sandy shale. The floor geology at the site

was not directly observed, although nearby corehole data suggest that

the immediate floor consisted of 0.15 to 0.6 m of shale and fireclay

underlain by 0.3 to 1 m of limestone and limey shale. The study site

lies below a stream valley, with overburden ranging from 190 to

240 m. The maximum principal horizontal stress direction at the

study mine had been previously measured at approximately N90°E.

Although the entries and crosscuts, which were oriented at N23°E

within the study site, were generally observed to be in good

condition, cutters were observed in many of the crosscuts,. In most

instances, these cutters migrated toward the outby rib of the crosscuts

and occasionally progressed into the tailgate entry, where they soon

terminated. Cutters were found to have a strong influence upon the

convergence and loading patterns observed in the stoppings at this

site. Some spalling was noted on highly stressed ribs near the study

site, typically initiated at the parting. As indicated by the apparently

competent floor geometry, little to no floor heave was observed in the

vicinity of the study site.

STOPPING CONSTRUCTION

The instrumented stoppings were built by mine masons using drystack methods considered standard practice at the mine (figures 2 and

3). The only significant deviation from normal dry-stack practice was

the installation of the flatjack load cells above the ninth block course.

The stoppings were built by laying an initial course of blocks

lengthwise and flat (shortest dimension perpendicular to the floor)

across the entry in a previously leveled area of the crosscut. Rock

dust was used to help provide an even bed for the blocks. The

remaining courses were laid upright, without mortar. A 50 mm thick

crush block, made of phenolic foam, was placed above the fourth

course. As construction proceeded, the stoppings were periodically

wedged along the ribs and eventually at the roof line. Wood blocks

and wedges were used as needed to fill the remaining openings.

After the stopping was in place, it was sealed on one side with an

appropriate thickness of sealant, in this case a surface sealant that, in

part, contained fiber and latex. All exposed wood was also covered

on both sides with sealant. At the study site the stoppings were

approximately 2.5 m high, 4.8 m wide and required 11 or 12 courses

of block.
Figure 3. Photo of the replacement stopping at 33 crosscut

CONCRETE BLOCK PROPERTIES

Three of the lightweight aggregate CMUs used in the stoppings

were tested in the laboratory to determine their dimensions, density,

water absorption, and strength characteristics. The blocks, collected

from the mine, were tested as per ASTM procedure C140 - 96b for

testing Concrete Masonry Units (1). The block dimensions, weights,

densities, and moisture absorption characteristics are shown in

table 1. Compressive strengths, stiffness values, and maximum loads

are also shown in the table, along with their averages.

Table 1 - Physical Properties of Lightweight CMUs

Dimensions: CMU

S2 S3 S4

Height (mm) 194 194 191

Width (mm) 143 143 143

Length (mm) 396 396 396

Ambient Weight, (kg) . . . . . . . . . 19.10 19.25 18.31

Saturated Weight, (kg) . . . . . . . . 20.33 20.34 19.64

Oven dry Weight, (kg) . . . . . . . . 17.63 17.62 16.76

Ambient Density (kg/m3

) ...... 1869 1743 1756

Oven dry Density (kg/m3)...... 1759 1608 1607

Ambient absorbed water (%) . . . 8.3% 9.2% 9.2%

Max water absorption, (%) . . . . . 25.0% 25.0% 26.8%

Comp. Strength, (MPa) . . . . . . . 17.9 15.4 20.2

Tang. Stiffness, (kN/mm) . . . . . . 602 408 948

Secant Stiffness, (kN/mm) . . . . . 397 300 423

Max load, (kN) . . . . . . . . . . . . . . 1015 870 1144

AVERAGES

Comp. Strength, (MPa): 17.8

Tang. Stiffness, (kN/mm): 652

Secant Stiffness, (kN/mm): 373

Max load, (kN): 1010

INSTRUMENTATION

Convergence sensors were installed on all instrumented

stoppings, while load cells were installed only on the tailgate-side

stoppings (figures 2 and 3). The convergence sensors consisted of

spring-loaded string pot position transducers, which were mounted

to the roof at the nearest roof strap and connected to the base of the

stopping by a wire run to an eyebolt screwed into the second course

of concrete blocks. Two additional sensors, measuring roof-to-floor

convergence and stopping lateral movement were mounted on a

vertical spring-mounted pole, which was located near the center and

approximately 0.6 m laterally from the stopping.

Loads were monitored using specially fabricated steel flatjacks

approximately 1219 mm long by 165 mm wide by 12 mm thick

calibrated in the NIOSH ‘s. Each stopping contained four load cells,

installed above the ninth course of block, and positioned end-to-end,

so that the total stopping load could be measured

INSTRUMENTED STOPPING RESPONSE

During the course of the first longwall retreat, two separate

lightweight CMU stoppings were built in crosscut 33, between the

track and tailgate entries. The original stopping was built during

gateroad development and was removed when the first longwall face

was 115 m inby the crosscut to accommodate transportation of

supplies to the longwall face. The replacement stopping was built

three days later, in a similar manner to the first, when the face was

27 m inby. Due to the unanticipated removal and reconstruction

sequence, load cells used in the original stopping had to be salvaged,

recalibrated, and used in the replacement stopping. One of the cells

was damaged during removal and handling. Since no replacement

was available, the cell was reinstalled in the replacement stopping to

preserve the symmetry of the stopping. The damaged cell was

placed on the inby side of the stopping, since a roof line cutter had

been observed on the outby side of the crosscut, the effect of which

was considered to be significant to stopping response.

Convergence Response

The cumulative convergence of both stoppings built in crosscut

33 is graphed on figure 4. Convergence during the five day period,

from October 18 to 23, after the original stopping was removed and

prior to installation of instruments on the replacement stopping, was

estimated from data trends. During this period, all convergence

sensor locations were estimated to have converged an additional

8 mm, except inby side sensor 11, which was estimated to have

moved about 1 mm.

Up until the removal of the original stopping, with the face

117 m from the center of the crosscut, convergence rates measured

by sensors 9, 10 and 11 (outby, middle and inby) were

approximately constant, at 0.25 mm/day. Recording of data from the

replacement stopping began when the longwall face was almost even

with the crosscut. At that time the convergence rates had increased

to about 3 mm/day. Convergence rates began to increase rapidly

when the face passed 41 m outby the site, with the outby side of the

stopping exhibiting a significantly higher rate. Convergence rates

peaked when the face was approximately 60 m outby the crosscut;

instantaneous convergence rates for sensors 9, 10 and 11 being 68,

33 and 8 mm/day, respectively. The rates then began to decline

rapidly until the face was 175 m outby (approximately 0.8 times the

overburden), to approximately 1.5 mm/day across the stopping.

When the face was 474 m outby the crosscut (approximately

2.2 times overburden depth) convergence rates stabilized at
655

605

530

470

369

272

197

76

-19

-140

-175

-175

-271

-340

-472

-559

-618

654

589

508

429

294

216

131

-20

0

20

40

60

80

100

120

140

160

180

8/17/00 9/14/00 10/12/00 11/9/00 12/7/00 1/4/01 2/1/01 3/1/01 3/29/01 Date

Convergence, mm

-10

-5

0

5

10

15

20

25

30

35

40

Lateral movement, mm

9 (outby)

10 (middle)

11 (inby)

12 roof

13 lateral

First panel face distance, m

Second panel face distance, m

 Lateral movement:

 Positive - Toward Headgate

 Negative - Toward Tailgate

9

12

11

13

13

10

Figure 4. Total convergence at crosscut 33 location. Distances from the face to crosscut 33, in meters, are indicated.

approximately 0.25 mm/day, which was essentially the original

convergence rate prior to the first longwall pass.

Visual observations suggest that the convergence across crosscut

33 resulted from a combination of typical longwall side abutment

load and the presence of a cutter roof condition on the outby side of

the crosscut. It is surmised that the cutter allowed the roof to rotate,

as if cantilevered about a hinge on the inby rib. Although this cutter

was observed shortly after development mining, it did not show signs

of significant movement until the approach of the longwall face. It is

not clear from the data whether side-abutment loading or the

horizontal stress concentration ahead of the face (which, because of

the east west direction of the maximum stress, would not have had an

effect on crosscut 33 until the face had passed) caused the increase in

convergence rates. However, the large difference between the

convergence rates on the inby and outby sides of the stopping

suggests that the presence of the cutter had as much, if not more

impact on stopping loading than did the vertical loading of the pillars.

It should be noted that, although the instruments could not

differentiate the floor and roof movements, few visual signs of floor

heave were observed in this or the nearby crosscuts.

Lateral Movement

The lateral movement for both the original and replacement

stoppings at crosscut 33 is shown in figure 4. The original stopping

bowed only a few millimeters toward the tailgate. The replacement

stopping began to bow rapidly toward the headgate at the same time

as the stopping began to converge and take load. The bowing reached

31 mm after a week and then reversed, stabilizing around 22 mm,

before increasing again in late January 2001. On April 11, with the

second panel only 140 m away, the bowing reached a maximum of

36.5 mm. The instruments were then removed prior to stopping

removal to facilitate tailgate ventilation.

Although a small portion of the bowing of the stopping could

have resulted from differential convergence between the stopping

blocks, wood wedges, and the floor bearing surface, the majority is

believed to be the result of squeezing of the crush block layer, which

was intended to protect the stopping from entry convergence

damage. Most of the measured bowing (31 of 36.5 mm) took place

during the period of maximum vertical convergence, which

coincided with the squeezing of the crush block. Although stopping

failures caused by rotation of concrete blocks about soft-inclusion

layers have been observed in the field, installation of these layers

near the bottom of the stopping typically minimizes the magnitude

of the rotation. Since neither the stopping at crosscut 33 nor any

other stopping near the study site was observed to fail as a result of

instability, it may be assumed that lateral displacements on the order

of 20 to 30 mm are not sufficient to cause an instability failure to

concrete block stoppings.

Loading Response

Figure 5 shows the load cell data for the replacement stopping,

beginning on October 23. No data are presented for the load cells in

the original stopping, all of which measured loads less than 9 kN,

which is only slightly greater than the zero error of the load cells.

Significant loading on the outby load cells of the replacement

stopping began when the first longwall face was approximately 60 m

outby the crosscut, about 12 hours after the convergence rates began

to increase rapidly. The outby and middle outby cells loaded

rapidly, reaching loads of 500 and 400 kN, respectively, by the time

the face was 175 m outby the crosscut. The middle inby load cell

indicated almost no load change during this period. As with the

convergence, the loading rates declined rapidly from their peaks

within a short time after the face had passed.
17

-77

-150

-175

-211

-300

-369

-472

-546

-569

-657

654

572

495

417

284

199

131

-100

0

100

200

300

400

500

600

700

800

10/22/00 11/19/00 12/17/00 1/14/01 2/11/01 3/11/01 4/8/01 Date

Load, kN

Middle inby

Middle outby

Outby

Middle inby (from convergence)

Outby (from convergence)

First panel face distance, m

Second panel face distance, m

Middle inby 

(from convergence)

Outby (from

convergence)

Middle inby 

Middle outby 

Outby

Figure 5. Loading curve for replacement stopping at crosscut 33. Distances from the face to crosscut 33, in meters, are indicated.

The numbers are centered on the time when the face was that distance away. Distances to the first longwall face are at the 

top of the graph, to the second at the bottom. Negative numbers indicated that the face was outby the crosscut.

Position numbers are staggered for readability.

The lag between stopping convergence and loading resulted from

the presence of the soft crush blocks. During the lag periods the

outby side of the stopping converged 33 mm and the middle of the

stopping converged 16 mm. Once the crush blocks converged 55 to

60% of their original thickness, the stopping began to resist

significant loads. This is consistent with laboratory tests, which show

that the strength of these crush blocks is only 0.1 to 0.17 MPa, and

that significant load increases occur at about 70% of the block

thickness.

The loading data were complicated by damage to two cables on

November 8, when the face was 260 m outby the crosscut, and by a

leak in the middle-outby load cell, which appears to have begun

around November 10. Data from the leaking middle-outby load cell

after November 10 are invalid and are not shown in figure 5. The two

power cables to the middle-inby and outby cells were disconnected

by a roof fall on November 8 and were not repaired until December

12. Stiffness values calculated from the load-to-convergence ratios

were combined with convergence data to estimate the loads during

this period. These convergence-based load estimates are plotted for

the outby and middle-outby cells. For the period after December 12,

the estimates are within 16% of the measured loads.

These load data show that the stopping attained considerable load

resistance without failing. Based upon the performance of the outby

load cell, located where the highest loads were developed, the

ultimate load capacity of the stopping appears to be in excess of

2,700-3,000 kN. This is the equivalent of about three, large

(1.5 x 1.8 x 0.76 m), 4-point wood cribs loaded to maximum capacity.

Note that tests conducted in NIOSH’s MRS show that the strength of

6-in wide normal and lightweight aggregate CMU stoppings,

constructed in a typical dry-stack manner, can reach as much as 5,000

and 3,750 kN, respectively, under ideal conditions.

Stiffness Estimates

Figure 6 shows the load versus displacement curves for several

combinations of the crosscut 33 stopping load cells and convergence

sensors. As graphed, the stiffness is the slope of the curves, with the

units being kN/mm. Although these curves appear quite variable, they

all consist of two linear regions: an initial, near-horizontal region

through 20 to 50 mm of convergence, and a second region with a

much higher stiffness at larger displacements. The horizontal region

indicates the initial stopping stiffness due to the presence of the crush

block layer. For all practical purposes, the initial stopping stiffness,

which was essentially zero, represents the stiffness of the crush block.

The second stiffness, which occurred after the crush block was

completely flattened, represents the stiffness of the CMUs, with a roof

line wooden end constraint. After the crush blocks were completely

flattened, the calculated stiffness values ranged from 6 to 32 kN/mm.

For comparison, tests conducted under ideal conditions in NIOSH=s

MRS produced stiffness values for 6-in wide dry-stacked normal and

lightweight aggregate CMU stoppings, of 450 and 310 kN/mm,

respectively. Both the wide range in the computed stiffnesses and the

large difference between the field and laboratory-derived stopping

stiffness estimates suggest this is a subject that warrants additional

study.
-100

0

100

200

300

400

500

600

700

0 10 20 30 40 50 60 70 80 90 100

Convergence, mm

Load, kN

9 vs Outby

10 vs Middle Outby

10 vs Ave. Middle Inby & Outby

9 vs Middle Outby

Ave 10 & 11 vs Ave Middle Inby & Outby

12 vs Middle Outby

Figure 6. Load displacement graph for replacement stopping at

crosscut 33. Curve slopes indicates stiffness

There are at least two likely reasons for the wide range of

calculated stiffness values, both attributable to the stopping

instrumentation. First, while the load cells were 1.2-m long sensors

that measured average loads, the convergence sensors measured at

discrete locations, not always aligned with the center of the load cells.

As such, there were no pairs of convergence and load data that

corresponded exactly to the same location on the stopping. Second,

out-of-plane convergence measurements could also produce

variations in calculated stiffness values, since the roof line

convergence transducers were mounted on roof straps rather than at

the top of the stopping. This is illustrated by the significant

differences in convergence data from sensors 10 and 12, both of

which were located near the center of the stopping, but laterally

separated from it by 0.3 to 0.6 m, respectively. These data suggest

that convergence close to the stopping was reduced by the presence

of the stopping, the adjacent roof support, or both. The variations in

the stopping convergence rates could, as a result of the placement of

the instruments on the stopping, result in a wide range of stiffness

estimates.

The difference between the field and laboratory derived estimates

of stopping stiffness was quite large, with the laboratory estimate 10

to 50 times greater than the field estimates. One possible explanation

for the disparity is that higher convergence on either side of the

stopping, and in particular at the sensor locations on the straps, may

have tended to underestimate the stiffness. The wide range in the

computed field stiffness estimates supports this hypothesis. Another

possibility is that the differences are real and that the field constraint

conditions and construction methods tend to produce a much softer

structure. Laboratory test walls are built on flat, stiff steel platens,

which are more uniform, thus making it possible to construct walls

whose stiffness more closely approaches that of the individual blocks.

Both possibilities may apply.

STOPPING PERFORMANCE

Source of Loads

The primary source of stopping load appeared to have been

convergence caused by cutter roof in the crosscuts, which in turn was

produced by the action of horizontal stress on weak silty-shale roof.

The cutters were observed during development mining and reactivated

as the first longwall face passed. Side-abutment stress may also have

contributed to the stopping loading, but the uneven distribution of the

convergence, with the greatest convergence on the outby or cutter side

of the crosscuts, suggests that the cutter played the predominant role.

The headgate stoppings were removed when the face was between 40

and 100 m inby and experienced only moderate (less than 26 mm)

convergence. Due to the presence of the crush blocks, loads on the

headgate-side stoppings were most likely very small.

Lateral Strength Observations

Although complete data were collected only from the crosscut 33

stoppings, a significant event occurred to the alternative lightweight

block tailgate stopping, located at crosscut 35, when it failed

prematurely. As in the case of crosscut 33, it was necessary to remove

the original stopping in crosscut 35 and replace it several days later.

In this case however, a roof fall occurred between the track entry and

the middle of the crosscut a few hours after the instruments were

installed on the replacement stopping, between 2 and 4 days after the

replacement stopping had been built. At that time, only the outby side

of the stopping had begun to take load, again due to a cutter on the

outby side of the entry. At the time of the roof fall, the outby side

load cell indicated only a 104 kN load, the middle-outby load cell a

43 kN load, and the inby side load cells less than 11 kN. Based upon

the locations of the blocks, the lack of significant convergence, and

the relatively low loads, it is surmised that the force of the air blast

from the fall, combined with the resulting nearby roof disturbance,

pushed down the inby three quarters of the stopping. The remaining

portion of the stopping stood loaded for a period of 6 hours before the

load began to gradually drop to zero. The crosscut 35 stopping was

rebuilt, but the instruments were not reinstalled.

This incident demonstrated that a moderate pre-load, in this case

104 kN, producing a vertical pressure of 0.6 MPa, effectively

increased the lateral strength of the stopping. The validity of the field

observation is also confirmed by theory and laboratory tests. Plate

theory (2) indicates that fixed end-constraints provide greater

resistance to plate rotation, and hence to lateral plate displacement,

than simply supported ends, which allow rotation at the edges. It is

surmised that in stoppings, as small set loads from wedges or loads

produced by mine convergence are introduced, the points of contact

between the stopping and the roof, rib and floor begin to act more like

a fixed support, due to increases in end-support friction and stiffness

(the latter as a result of gap reduction). Additionally, the interface

friction between blocks will increase, thereby increasing their

resistance to sliding.

Laboratory tests conducted by CONSOL using a modified E-72

test fixture also confirm the effect of this pre-loading. The E-72 test

is an ASTM standard test (3) for determining the lateral strength of

wall segments. It is also used to validate the strength of stopping

walls in coal mines. In the standard vertical test, the wall sections are

not constrained on any side. However, in tests conducted by

CONSOL, the E-72 load frame was modified to allow the walls to be

wedged in the frame, thereby simulating mine roof and floor

constraints (Figure 7). The resulting tests indicated that 6-in wide

dry-stacked walls, typically capable of supporting lateral loads of

approximately 2.7 kPa in a free standing configuration, could support

lateral loads as high as 5.0 kPa with tight roof line wedging

(Figure 8). This indicates that end-constraint and pre-loading are

important factors in controlling the lateral strength of block walls.
Figure 7. E-72 test fixture with test in progress, just prior

to failure. The fixture has been modified to allow

wedging to provide constraint at the top. The air 

bag that generates the lateral load is visible to the 

right of the test wall, near the center of the figure.

0.0

1.0

2.0

3.0

4.0

5.0

6.0

0 10 20 30 40

Lateral displacement, mm

Pressure on wall, kPa

Test conditions: 

 Sealant used, both sides

 Free standing wall

 Test run to failure

Both tests: 6" solid concrete, 25 kg block.

 Dry-stacked wall construction.

Test conditions: 

 Sealant used, one side

 Wall wedged

 Test terminated just before failure

Figure 8. Comparison of the lateral strength of wedged versus

free standing walls, tested in the E-72 apparatus of figure 7.

Practical Stopping Construction Considerations

The east west oriented maximum horizontal principal stress

produced roof cutters in many of the crosscuts that tended to run

toward and then along the outby ribs. The effect of the cutters was to

produce differential convergence rates across the stoppings, with the

greatest convergence on the cutter or outby side. This observed trend

suggests that, in such instances, man doors could be better protected

by moving the doors as far as possible toward the inby sides of the

stoppings. The cutter trend also suggests that the efficiency of the

secondary support occasionally installed adjacent to these stoppings

for protection from the vertical loading, could be optimized by

concentrating supports along the outby crosscut ribs.

Soft Inclusions

The crush blocks used in this study appeared to be fairly

predictable in their effect on stopping stiffness and convergence.

Generally the system stiffness was essentially zero until the

convergence reached 55 to 60% of the crush block thickness, and

reached a maximum when the total convergence reached 70 to 120%

of the crush block thickness (the latter value indicating that

convergence was taking place in stopping components other than the

crush block). The effect of the crush block on the lateral stability was

not determined, although it should be noted that most of the lateral

movement (31 of 36.5 mm) took place during the period when the

crush blocks were being squeezed. After the crush blocks were

squeezed, the bowing actually decreased, possibly indicating that

during the crushing process, concrete CMUs were rotating around the

crush block layer, and that once this layer was crushed flat, the blocks

were forced to rotate back to a more level orientation. This

hypothesis is consistent with the lateral stopping measurements,

which show the bowing reversing after the crush blocks were

completely flattened.

The crush blocks were introduced in order to protect the stoppings

from convergence damage. It is likely that in many instances they

perform this function, especially in places where ground movements

are primarily time dependent, such as in cases of floor heave.

However, in cases of roof movement, there is a question as to whether

crush blocks prolong stopping life or instead permit additional

movement that can be damaging to the roof, thereby accelerating roof

movements, and subsequently, stopping loading. While it is

recognized that CMU stoppings are not intended as roof control

devices, it is obvious that they have the capacity to resist very high

loads. Early development of this capacity, which is not possible when

crush blocks are included in a stopping, may make it possible for them

to prevent this acceleration of the roof movement, thus allowing the

stoppings to protect themselves. The combination of crush blocks in

a stopping, along with stiff secondary support, could produce the

same effect, without causing high loading of the stopping.

The available data are ambiguous concerning the above

considerations, since neither the roof nor the stoppings failed in the

study. Additionally, it was not the intent of the study to assess roof

conditions and roof stability. While the constant rate of convergence,

beginning in December 2000 and continuing until the stopping was

removed in April 2001, suggests that failure of stopping 33 was

inevitable (given sufficient time before its removal), and that the soft

inclusion helped to extend the life of the stopping, the data did not

resolve the question of whether the use of a stopping without crush

blocks could have led to a reduced initial convergence rate, that
would have helped to stabilize the roof, thereby reducing subsequent

convergence.

At this point the authors want to note that they recognize that

yieldable stopping designs have been developed and are available to

the mining industry. Some of these designs are capable of significant

convergence without failure and the decision to use such products

will depend upon economic, ventilation, performance and ease of

construction considerations.

CONCLUSIONS AND RECOMMENDATIONS

This study has provided valuable technical data regarding the in

situ response of typical dry-stacked CMU stoppings to longwall

abutment stresses. The data will be useful for future reference in

comparing similar type stoppings under varying conditions or

alternative stopping designs and materials. The authors believe that

improvement in the efficiency of ventilation stoppings, with respect

to ventilation, cost and safety considerations, can best come from an

improved understanding of their response to, and interaction with,

variable underground loading conditions.

Of some significance is the need to reconcile the difference

between the in situ and laboratory stiffness measurements, since

large-scale laboratory testing has great potential for efficiently and

rapidly assessing stopping designs. Toward this end, additional

underground study is warranted. Such in situ testing can provide

insight into the in situ loading mechanisms and the interactions

between the stoppings and the ground, which, in turn, may also be of

value for laboratory testing.

The study confirmed that conventional ventilation stopping

designs are capable of providing substantial resistance to roof

movement. The lightweight aggregate CMU stopping described in

detail in this paper demonstrated a load capacity of at least 2,700 kN,

without failure. Large scale laboratory simulations suggest that the

maximum capacity of 6-in wide stoppings constructed of normalweight aggregate CMUs could approach 5,000 kN. This is equivalent

to three large four-point wood cribs. Although it is not suggested that

ventilation stoppings can or should be considered as roof supports, it

is obvious that under some conditions they do provide substantial

load resistance, and therefore may be capable of controlling roof and

floor weighting in order to protect their own integrity.

This study has raised the question as to how and when yielding

stopping systems or soft inclusions should be designed and/or utilized

in underground block stoppings. In some instances, such as in cases

of floor heave, a soft inclusion may increase the longevity of a

stopping. However, it is possible that in other instances, allowing

such deformation may lead to accelerated roof loading that could lead

to stopping failure unless additional support is used.

The study has also shown that the loads and convergence rates

that stoppings may be required to withstand over the length of their

operating life can vary greatly, particularly when subject to multiple

longwall loading cycles. In this instance, the cutter roof conditions in

the track to tailgate crosscuts, which may have been exacerbated by

horizontal stress concentrations due to the longwall retreat, appear to

have been the major contributor to the convergence and loading of the

stoppings. Additionally, stoppings do not react to average loads, but

rather entry specific loads that may vary significantly across an entry.

Ultimately, to maintain its integrity, a stopping will need to resist, or

otherwise deform with, the worst-case entry condition.

An unusual failure of one of the instrumented stoppings provided

data that suggested the value of moderate loading to the lateral

strength of a CMU stopping. This was further demonstrated by

modified E-72 lateral load tests that proved the significance of

wedging, or pre-loading. It is believed that such pre-loading can be

as significant to CMU lateral strength as the strength of the stopping

component materials and the manner of their construction.

Finally, in addition to furthering the understanding of stopping

response to longwall mining, the results of this study also provided

practical insight for improving stopping construction at sites under

similar conditions. It was observed that cutters on the outby side of

the entries lead to significantly higher convergence rates on that side

of the stoppings. This suggested that critical or sensitive stopping

elements, such as man doors, could be better protected by moving

them to the inby side of the stopping. Additionally, it may be possible

to improve the efficiency of the supplemental supports used to protect

the stoppings, by concentrating the supports in the areas most likely

to be affected by cutter roof or floor movements.

REFERENCES

1. American Society for Testing and Materials. Standard Test

Methods of Sampling and Testing Concrete Masonry Units.

C140-96b. Annual Book of ASTM Standards: Volume 4.05,

1997, pp 1-8.

2. Timoshenko S., Woinowsky-Krieger S. Theory of Plates and

Shells. McGraw-Hill Book Company, Inc., New York, NY,

1959, pp.580.

3. American Society for Testing and Materials. Standard Test

Methods of Conducting Strength Tests of Panels for Building

Construction. E72 - 98 Annual Book of ASTM Standards:

Volume 04.11, Building Constructions., 2000, pp 1-11.