OPTIMUM MINE DESIGNS TO MINIMIZE COAL BUMPS: 

A REVIEW OF PAST AND PRESENT U.S. PRACTICES 

ABSTRACT 

Mr. Anthony T. Iannacchione 

U.S. Bureau of Mines 

P.O. Box 18070 

Pittsburgh, PA 15236 

and 

Mr. Matthew J. DeMarco 

U.S. Bureau of Mines 

P.O. Box 25086 

Denver Federal Center 

Denver, CO 80225 

Coal bumps have presented serious 

mining problems in the U.S. throughout 

the 20th Century. Fatalities and 

injuries have resulted when these 

destructive events occur at the working 

face. Persistent bump problems can 

resul t in abandonment of 1 arge reserves 

or lead to premature mine closure. 

Through the years, alternative 

techniques such as artificial supports, 

extraction sequencing, destressing, 

pillar design changes and specific 

pi 11 ar retreat practices have been 

successfully implemented to mitigate 

coal mi ne bumps. Several techniques 

have evolved for room-and-pillar 

operations that control the way the 

roof rock breaks, regul ati ng the manner 

in which stresses are redistributed in 

the mined section. Special mine 

1 ayouts empl oyed i n 1 ongwall mines have 

also proved to be successful in safely 

redistributing or containing excessive 

loadings. However, with ever 

i ncreasi ng producti on rates, greater 

overburdens and new mining systems, the 

need to evolve even more effective bump 

control designs will continue to 

challenge the U.S. coal mine industry. 

INTRODUCTION 

An understanding of past bump 

control methods provides a useful , 

perspective for eval uat i ng present 

designs and may show how future 

innovative engineering sol utions will 

evolve. This paper reviews various 

design techniques in chronological 

order to illustrate how the bump hazard 

in the U.S. has changed as mining 

methods have evolved and where the U.S. 

may direct future bump hazard reducti on 

efforts. In some cases the solutions 

have been site specific; while in other 

cases the sol utions have been appl ied 

over a wide range of geologic and 

mining conditions. 

Background 

Almost a1 1 U. S. coal mine bumps have 

occurred either in the southern 

Appalachian Basin of West Virginia, 
Kentucky and Virginia or the Uinta and 

Piceance Creek Basins of Utah and 

Colorado as shown in Figure 1. One of 

the principal mining districts within 

the southern Appalachian Basin is the 

Pocahontas Field shown in Figure 2. 

Coalbeds within this field occur in the 

Pocahontas, Lee and New River 

Formations of the Lower to Middle 

Pennsylvanian System. Massive 

sandstone members, many with 

compressive strengths ranging from 140 

to 210 MPa, are prevalent. Many of the 

coalbeds found in the southern 

Appal achi an Basin are under 

considerable overburden and have high 

horizontal and vertical stress 

conditions. Over the last 13 years, 

bumps have occurred at eight different 

mines with four fatalities and several 

injuries in both room-and-pillar and 

longwall mining systems. Prior to this 

point in time, most bump occurrences 

were restricted to three locations, the 

Gary mining district of West Virginia, 

deep Pocahontas No.4 Coals near Grundy, 

Virginia and the Cumberland mining 

district of Kentucky and Virginia. 

Many of these same geologic and 

mining conditions occur within the 

Uinta and Piceance Creek Basins in the 

western portion of the U.S. shown in 

Figure 1. Bumps have been principally 

experi enced in several coal f i el ds 

within the Book Cliffs and Grand 

Hogback structures of Utah and Colorado 

(Figure 3). The Upper Williams Fork 

and the Blackhawk Formations of the 

Mesaverde Group, Upper Cretaceous 

System contain the principal bump prone 

coalbeds. Both of these areas have 

significant quantities of massive, 

stiff sandstone, 1 arge and small 

displacement faults and intrusions 

caused by igneous dikes. These 

geologic structures have been 

associated with bumps which are related 

to past fatalities and injuries. The 

potenti a1 for further bump problems 

from the southern Appalachian, Uinta, 

and Piceance Creek Coal Basins is 

extremely high due to the extensive 

reserves of deep, high quality (low 

sulfur) coal . 

Historical review 

Watts reported one of the earliest 

incidents of bumps in the U.S. (Watts, 

1918). These bumps occurred at the 

Sunnyside No.1 Mine, Sunnyside Field in 

the state of Utah, in development 

sections under a thick overburden of 

principally sandstone. Watts indicated 

the bumps were apparently greatly 

influenced by faults, not more than 

300 m away. 

Bryson (1936) indicates that bumps 

occurred in the Cumber1 and Field of 

Kentucky and Virginia as early as 1923 

and became very troublesome from 1930 

to 1934. Rice (1934) classified bumps 

as two general types, namely excessive 

pressure bumps and shock bumps. 

According to Rice (1934), pressure 

bumps are caused when pi 1 lar stress 

exceeds bearing strength. Shock bumps 

are induced by the breaking of thick, 

massive strata at a considerable 

distance above the coalbed, causing the 

immediate roof to transmit a shock wave 

to the coal. To this day, these 

categories sti 11 provide a practical 

means of classifying bumps. Rice a1 so 

designed a rock filled crib system to 

control the fracturing of the roof rock 

~hich in turn minimized the occurrence 

of shock bumps. 

Holland and Thomas (1954) examined 

177 instances of bumps from the states 

of West Virginia, Kentucky, Utah and 

Virginia that occurred between 1925 to 

1950. They deduced that the primary 

cause of bumps was unfavorable mining 

practices in abutment areas. They a1 so 

noted the importance of a strong floor 

rock, which does not heave readily. 

Peperakis (1958) noted the 

occurrence of bumps in the Sunnyside 

Field of Utah between 1944 and 1957. 

In particular, the Sunnyside No.1-2-3 

Mines had several large bumps while 

advancing under cover deeper than 600 m 

with mu1 tiple-entry sysle~ii:, ashoc~aicd 

with room-and-pil 1 ir !~.;i?irig. Sevzra I 

of these bumps occurr?;l < n 4;)~ el ;p~r:~~t. 

secti ons as f aul t s werat. zpprvacl..~? I 

Unique control desiqqs attempteff dirring 

this era included: v~lev firinq to 
destress the coal, yielding arch 

supports to minimize a s s o c i a t e d roof 

fall damage and hydraulic backfill to 

support the main roof above the gob. 

During this same period a novel 

barrier pillar splitting technique 

called the thin-pillar method (Talman 

and Schroder, 1958) and an auger 

drilling dest.res;lng technique (Talman, 

1955) were intraduced in the Gary 

Mining District cf southern West 

Virginia. The thin-pillar method is a 

design concept k~hich segments the 

barrier into a series of yield pillars 

too small to iid3 significant stress 

1 eve1 s (> 10 Mpa; . The pi 11 ar 

desttsessing practice consisted of 

drilling 61 cm a.ugcr holes from one 

side of the stressed pillar to the 

other. The use of this system was 

cwtai 1 ed whcn sevzrdl miners were 

injured by a bump induced during a 

destrzssing ctwr35 ion. 

302" eet al . 11983) analyzed the 

reccrds GF S~mp even3 'rom 1950 unti' 

1984 and found that bumps were still 

occurring at disturbing rates. Twentyeight fatalities occurred during this 

time: fourteen in the eastern U.S. and 

another fourteen in the west. 

Continued bump problems were probably 

due to the same unfavorable mining 

conditions and practices di scussed by 

Holland and Thomas (1954); however, the 

advent of the continuous mining machine 

brought about different problems which 

r e q u i r e d new control sol uti ons. The 

mobi 1 i ty and versati 1 i ty of the 

continuous miner led to the developnient 

of novel pi 11 ar spl i tti ng and 

extraction sequencing designs for bump 

control. 

With the widespread util ization of 

the longwall mining technique over the 

last 15 years, bump problems have again 

surfaced to threaten the safe and 

efficient mining of coal. One fatality 

on a advancing longwall face, several 

injuries on retreating 1 ongwal 1 faces 

and at least one mine closure have been 

attributed to bumps. However, 
LEGEND v mmu 

Mines with recent bumps VDZGIMIA 

1 VP3 i EMnnrr 

2 Beatrice rn 

West Virginia 

@ Location of past bump mining areas 

\ 

\ 

\ 

0 - 50 ' Extent of Pocahontas \ -. \ 

\ 

Field 

scale, km \ 

\ Virginia 

1 

Figure 2. - Location of past and present bump prone areas in the southern 

Appalachian Basin. 

ingenuity and experience have prevailed 

and several innovative designs for 

control 1 i ng bumps i n 1 ongwal 1 mi nes 

have been developed. One technique is 

the advancing longwall method which was 

first used at the Mid-Continent Coal 

Mines in the 1970's. In coalbeds where 

bump problems occur on development of 

gate entries, the advancing longwal l 

design can be effective. Two other 

designs are centered around altering 

the size and shape of the gate entry 

pillars. One method consists of 

increasing the gate entry pillar size, 

so that the pillars will prevent 

abutment load ride-over onto the active 

longwall face. The other method 

consists of reducing the gate entry 

pillar size, so that pillars will yield 

in a controlled fashion, aiding in the 

controlled fracturing of the main roof 

rock. All of these methods have some 

drawbacks but in general represent 

innovative design phi 1 osophies for 

control 1 i ng bumps. 

In the U.S., coal pillar bumps have 

been associated with several primary 

characteristics: 1) stiff, massive 

strata, 2) high overburden stresses, 3) 

full extraction mining, 4) multi-seam 

mining, and 5) mining near geologic 

structures such as faults, igneous 

di kes and sandstone channel s . Unique 

combinations of geol ogy and mining 

systems have required many site 

specific bump control designs. 

Therefore, a continual development of 

improved bump control designs is needed 

as new geologic and mining scenarios 

are encountered. Each of these new 

challenges can be overcome by 

evaluating past experiences, analyzing 

current and projected conditions and 

investigating new design techniques in 

the field. This paper will discuss 

past and present experiences in retreat 

mining, barrier splitting, chain pi1 lar 

extraction and 1 ongwall mining methods. 

ROOM-AND-PILLAR MINING 

When coal pillar bumps first began 

to occur in eastern Kentucky (Bryson, 

1936), local mine officials, workmen, 
I 

Figure 3. - Location of past and present bump prone areas in the Uinta and Piceance 

Creek Basi ns . 

mining engineers and many others 

offered many ideas as to their cause. 

Numerous methods of prevention were 

suggested and tried without success. 

Most of the bumps were occurring along 

the retreating pillar line where 

several of the foll owing conditions 

existed: 1) uneven pillar lines, 2) 

irregular pillar sizes, 3) overburden 

in excess of 300 m, 4) strong roof and 

floor strata, and 5) overhanging or 

canti 1 everi ng gob. Several prom-i nent 

mi ni ng personnel were i nfl uent i a1 i n 

developing recommendations which led to 

a series of rules used to mitigate the 

bump hazards in room-and-pillar mining. 

Rice's recornniendati ons 

Rice (1934) developed the premise 

that two types of bumps, termed 

pressure and shock bumps, caused the 

observed mining problems. Pressure 

bumps are caused by stress on moderate 

sized pillars too great for their 

bearing strength. Shock bumps are 

induced by breaking of thick, massive 

strata above the coalbed, which 

transmits a shock wave through the ro~i 

to the stressed coal pillars. Faulty 

mining methods were then identified 

which included: 1) pillars too small, 

2) leaving projecting pillars behind 

retreat 1 ine, 3) narrowing pillars to 

points, and 4) extracting pillars in 

separate groups without any attention 

to a long continuous retreat line. 

Based on these observations, Rice 

recommended two operational methods for 

controlling bumps: 1) straight retreat 

lines and 2) rock-filled cribs. 

Keeping retreating pi 11 ar 1 i nes 

straight eliminated pillar points 

projecting into the gob. This practice 

was fairly easy to initiate and had 

favorable results. Rock-filled cribs, 

for a cushioned support of the roof 

rock, were also tried with positive 

results. General ly the cribs were 

compri sed of ordi nary mine post 

timbers. These were 1.1 m long, not 
less than 0.2 m thick and generally set 

on 6 m centers. A tight packing of 

each crib was obtained with a fill of 

rock material . The cribs were designed 

and placed in such a manner so that the 

roof was allowed to converge gently 

without rupture of the immediate 

strata. This action lessened the 

potential for sandstone breaks within 

the gob which were thought to be the 

cause of so many shock bumps. 

Bryson (1936) descri bed a detai 1 ed 

field test of this design method at a 

deep mine in the Harl an Coal bed, 

Cumberland Mining District under 430 m 

of cover. Bumps had killed five men at 

this particular site. Figure 4 shows 

the test area located off two support 

entries, approximately 210 m wide, 

adjacent to a 1 arge gob area. Rooms 

were driven approximately 91 m between 

the support entries about 10.7 m wide. 

Prior to the extraction phase, the 

study area was comprised of a series of 

narrow (10.7 m wide) and large (43 m 

wide) pillars. As the section was 

mined, sixteen roof-to-floor 

convergence stations and sixty-four 

rock filled cribs were instal led. 

Once the area was extracted, 

convergence began and continued almost 

continuously (Figure 4). Bryson 

reported that a few roof rock cracks 

gradually widened to as much as 41 cm 

without causing roof collapse. 

Convergence continued until the roof 

and floor came almost in contact. In 

general the strata settled by cracking 

and grinding with much noise but 

without developing many large breaks. 

Bumps did not occur during the 

extraction of coal from this section 

except for one incident when the pillar 

line was not kept straight. 

Holl and and Thomas's recornmendati ons 

Hol 1 and and Thomas (1954) expanded 

on Rice's general recommendations 

concerning pillar extraction procedures 

and produced the following ten rules to 

minimize pillar bumps: 1) Recover all 

coal in a pillar operation; 2) Avoid 

pillar-line points; 3) Keep roof spans 

projecting over the gob as short as 

o 20 40 60 ao Dirtanca from axtractrd zona, m 

v-7 r-7 r-7 r-7 r-7 7-7 r-T\ (r-? 7--; I I 

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. I I . 1, I I ua b--\ I I 

I I ]I ;:-I L --------- > ----------- J L ------ ----A L-a 

roof-to-floor convergenc 0 30 60 90 

---- 

rock-filled cribs 

pillars extracted durrng test 

- 

scale, m 

I 

Figure 4. - Location of roof-to-floor 

convergence stations and rock-fill ed 

cribs placed during retreat mining. 

Graph shows the convergence trend from 

one station. 

possi bl e or el se provide support so 

that the roof beds do not fracture; 4) 

Do not do development work in abutment 

areas; 5) Do not split pillars on or 

near the extraction line; 

6) Use the open-ending extraction 

technique with lifts of not more than 

4.3 m; 7) Leave one to two rows of 

pillars adjacent to old gob areas; 8) 

Maintain pillars the same size and 

shape; 9) Keep development entries 

narrow, approximately 4.3 m; and 10) 

Note areas of rolls, change in dip, 

change in coal thickness and hardness. 

Use this information in designing the 

mining system. 
Although many of the rules still 

apply to modern room-and-pi 11 ar 

operat i ons , several rul es no 1 onger 

apply by today's standards but may 

again be useful as new mining methods 

are developed. For instance, 

continuous mining machines require 

regular mining patterns and entries 

larger than 4.3 m to operate. It is 

also necessary to conduct development 

work in abutment areas during the final 

stages of a mines life. This is a 

necessary procedure as the mine pulls 

back along main entries, extracting the 

remaining large barriers. The practice 

of a retreating longwall through 

existing openings may change this 

assumpt i on. 

Peperakis's recommendations 

Peperakis (1958) summarized the 

cumulative experience ut i 1 izi ng novel 

engineering designs at the Sunnyside 

Mines prior to the introduction of 

longwall mining techniques in the early 

1960's. Many of Sunnyside's bumps 

initiated roof falls of the immediate 

shales and thin laminated sandstones 

beneath the massive main sandstone roof 

rock. Bumps i ni ti ated on development 

were associated with a series of faults 

trending along strike with 

displacements ranging from 1 to 8 m. 

Peperakis identified the following 

seven measures taken to mi ninii ze the 

bump hazards: 1) 1 ong hole shooting; 2) 

cutting up large blocks into smaller 

more uniform pillars ahead of the 

retreating pillar line; 3) do not split 

1 arge bl oc ks on devel opment ; 

4) break 1 arge development blocks ahead 

of retreat pillar 1 ines into uniform 

size blocks; 5) substanti a1 

supplemental support can reduce the 

severity of bumps; 

6) steel yieldable arch supports 

minimize roof fa1 1 s foll owing bumps; 

and 7) hydraul ic backf i 11 reduces 

stress transfer during bump events. 

Osterwald (1962) noted that many 

other oriented structural features, 

such as shatter zones, cleavage, pyrite 

veins, and c y l i n d r i c a l and smooth 

fractures, were found in the bump prone 

areas. He suggested that mine layouts 

could take advantage of these features 

to reduce stress concentrations thereby 

decreasing the incidence of bumps. 

BARRIER SPLITTING OPERATIONS 

Coal bumps can often occur during 

extraction of the large barriers 

adjacent to the main entries. Violent 

bumps during barrier splitting appeared 

to be especially troublesome during the 

1950's in southern West Virginia. 

Engineers working for the US Steel Coal 

Corp., a major coal producer in the 

region, developed a method of splitting 

large barriers adjacent to main entry 

systems (Talman and Schroder, 1958). 

They had found through experience that 

pillars smaller than 14 ni or larger 

than 49 m almost never bumped. An 

extraction method known as thin-pillar 

mining was developed which 

systematically cuts the large barriers 

into pillars with widths smaller than 

14 m, leaving a barrier pillar remnant, 

which was either destressed or left in 

place. 

When implementing a th-in-pillar 

mining system for barrier extraction, 

multiple entries are first driven 

within the barrier directly adjacent to 

the main entries. The remaining sol id 

barrier located between the newly 

advanced headings and the stabi 1 ized 

I Old gob Barr~er L 

I Old gob 

Figure 5. - Typical mining sequence 

utilizing the thin-pillar method. 

Numbers indicate cut sequence order. 
gob. Mining of barrier and 

predeveloped chain pillars proceed 

simultaneously (Figure 5). Splitting 

of the barriers occurs from the 

recently driven headings adjacent to 

the active gob and back towards the 

next solid barrier. These headings are 

very cl ose together, i sol at i ng yi el d 

pillars,about 6.1 m wide. These yield 

pillars fail in a controlled manner, 

shedding the high stresses both to the 

active gob areas and further into the 

solid barrier. When the remaining 

barrier approaches 49 by 49 m, a 

critical size pillar is formed. This 

large abutment pillar is called a bump 

block and left to avoid a potential 

bump. These 1 arge blocks aid in 

breaking the roof at the pillar line 

and protect the remainder of the 

section from excessive convergence. 

The thin pillar mining system has 

many forms but is generally employed 

when extracting the barriers left to 

protect main entries. The smaller 

pillars tended to yield to the high 

stresses imposed on them by the 

overburden and normal mining. The 

adoption of this technique greatly 

reduced the occurrence of bumps. 

CHAIN PILLAR EXTRACTION 

Even with the previously discussed 

designs, numerous bumps occurred in 

room-and-pillar sections throughout the 

1980's (Campol i et a1 , 1988). Many 

recent bump occurrences have occurred 

on continuous miner sections where rows 

of chain pillars (15 to 30 m wide) are 

extracted next to the gob. Individual 

chain pillars are extracted very 

rapidly, causing the loads to shift 

swiftly without allowing the adjacent 

pillars to redistribute the load in a 

controlled fashion. These pillars have 

difficultly accommodating excessive 

amounts of strain energy, increasing 

their potential to bump. In response 

to this problem, a novel pillar 

splitting method utilizing the stressreduction principles of thin-pillar 

mining was devised by the operations 

staff at the Olga Coal Company, near 

Welch, West Virginia. 

enter solitting 

7 to 15, 19 to 27 

L scale, m 

Figure 6. - I d e a l i z e d pi1 1 ar extraction 

sequence for bump control during roomand-pillar mining. 

This novel retreat mining technique 

involves the sequential mining of 

numerous places over three to four rows 

of pillars in order to gradual ly direct 

the overburden loads away from the 

pillar line, where most of the men and 

machines are located. An idealized 

schematic of the extraction sequence is 

shown in figure 6. By design, all coal 

pi1 1 ars three rows away from the 

retreating pillar line have at least a 

"bump" cut. This bump cut is a 6.1 by 

6.1 m cut of coal taken from a regular 

sized chain pillar (18.3 by 21.3 m). 

The frequent occurrence of audible 

events or thumps during extraction are 

responsible for the terminology. Two 

pi 11 ar mn1.1~ 

-1 -PAP+ to the gob l ine are 

split in half by extending the bump cut 

entirely through the pillar. Finally, 

the pillar wings or fenders are 

extracted in the row closest to the gob 

1 ine. 

This novel design was further 

evaluated by the Bureau of Mines with 

an extensive rock mechanics 

instrumentation array to determine how 

the strata responded during mining 

(Campoli et al, 1989). The response of 

the strata was measured by 44 coal 

cell s (Borehol e Pl atened Fl at j acks) and 

over 70 convergence stations. 

Observations from this field site 

indicated the technique did indeed 

redistribute stress in an effective 

manner (Figure 7). The pressures were 
transferred greater distance than 

normally expected, up to eight pillar 

rows away from the pillar line. This 

redistribution effectively transferred 

the load over a very large area, 

greatly minimizing the bump hazard. 

extracted coal \ 

~~;W:;II~IIIIIIIO BIBIEmIdl~Lo n 

mm~mmmmm~ scae3 - 

U~CI~[L]~~~C~OO 

Figure 7. - Isopach map of the pressure 

changes around an instrumented portion 

of a multi-place retreating pillar line 

over a four week period. 

Recently, an attempt has been made 

to evaluate this extraction philosophy 

using numerical modeling techniques 

(Zipf and Heasley, 1990). Several 

idealized mining scenarios were modeled 

by a boundary element program developed 

by the Bureau of Mines with non-linear 

material types and an energy release 

rate subroutine. The study found this 

gate systems and, in some instances, 

along the faces of several U.S. 

longwall s. Currently, several high 

production, deep 1 ongwall mines (two in 

Virginia, two in Utah, one in Colorado 

and one mine in Kentucky) have 

experienced bumps. In several cases 

these bumps have occurred where the 

gate entry pillars were unable to 

prevent abutment loads from "ridingover" onto the mined longwall panel 

(Iannacchione, 1988). In other cases, 

the burr~ps occurred in response to 

either gob caving characteristics 

adjacent to the longwall face or in 

association with excessive gas 

pressures. It should a1 so be noted 

here that at one mine the bump problems 

on development were severe enough to 

warrant the elimination of gate entries 

by empl oyi ng the advancing 1 ongwall 

system. 

As a result of the gate entry bump 

problems, two different design 

philosophies have emerged in the U. S. 

based primarily upon regional geologic 

conditions and mi ning preferences, to 

address these 1 argely s t r e s s - r e 1 a t e d 

probl ems. Standard gate entry designs 

in the southern Appalachian Coal Basins 

consist of three or Inore entries with 

at least one row of abutment pillars; 

whereas, two and three entry systems 

with yield pillars are a more common 

gate entry design in the Uinta and 

~iceance creek Basins. Many of the 

mines in the southern Appalachian Basin 

require mu1 tip1 e gate entries because 

of methane emission problems. Three of 

the bump prone longwall mines operating 

in the southern Appal achi an Basin 

employ three and four entry designs 

with a combination of yield and 

abutment pillars. The most common 

novel pi 11 ar spl i tting and- extraction designs used in the Uinta and Piceance 

sequencing method superior in reducing Creek Basins consist of one or two 

the potential for bump occurrence than yield pillars. 

more traditional techniques such as 

single spl i t-and-fender, pocket-and- Abutment gate entry design 

wing and open-ending. 

A well designed abutment gate entry 

design will support a considerable 

LONGWALL GATE ENTRY DESIGN amount of the abutment loads generated 

from both the adjacent gob and the 

More recently, coal bumps have approaching longwall face. This method 

occurred within the multiple entry tail is well suited for longwalls of 
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rrrrn rrrrn rrrn Cmn rrrrn Inn c17 c17 bumped coal 

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Figure 8. - Examples of abutment gate entry layouts: a) experienced bumps ahead of 

the passage of the face, b) experienced bumps as the face passed, while c) did not 

experience bumps until after the face had passed. 

moderate depth (300 to 600 m) with 

substantial gas emission problems. In 

bump prone ground massive strata often 

cause excessive stress 1 eve1 s, causing 

the gate entry pillars t o f a i l prior to 

the passage of the longwall mining 

face. As a means of controlling the 

abutment load ride-over problem, 

engineers at the Island Creek Coal 

Company in southwestern Virginia 

redesigned the gate entry pillars so 

that they would not fail during panel 

extraction. Redesign of the gate entry 

was accomplished by increasing the 

width of the center abutment pillar 

from 25 to 36.6 m (Figure 8). An 

extensive field study was conducted by 

the Bureau of Mines at this mine to 

identify the effects of different 

pillar sizes and configurations on the 

occurrence of bumps (Campol i et al. 

1990). Rock mechanics instrumentation 

placed in the gate entries showed the 

redesigned pillars had reduced the 

stress levels. This caused the 

abutment pillars to fail at a much 

later point in the mining sequence 

(Heasl ey and Barron, 1988). 

Observations have shown this design 

eliminated abutment load ride-overs 

from the adjacent gob panels onto the 

actively mined panel . As a result of 

this implementation, the incidence of 

bumps at the mine has been greatly 

reduced. A1 though this method has 

proven successful at this mine, i,t may 

have limitations when the overburden is 

extreme or when the coalbed is thick. 

These conditions may require extremely 

1 arge abutment pillars which may not be ... ." pr-actical . 

Yielding gate entry design 

Yield pillar designs allow the gate 

entry system to deform under the weight 

of the approaching panel abutments, 

thereby diverting substantial load to 

the nearby sol id coal panel. This 

method of stress control for gate 

entries is we1 1 -suited for two-entry 

designs. The first U.S. applications 

of which were pioneered in the early 

1960's at the Sunnyside Mine, Uinta 

Basin (Huntsman and Pearce, 1981). 

Longwall mining had just been 

introduced to the U.S. coal industry at 

that time, and entry design methods for 

bump-prone ground were not well 

developed. Perhaps without fully 
realizing the advantages of a twoversus mu1 tiple-entry yielding system, 

the decision to develop only two 

entries was primarily based on limiting 

the amount of ground to be opened-up 

prior to panel retreat. Nearly thirty 

years later, this system has continued 

to successfully eliminate entry pillar 

bumps duri ng panel devel opment and 

retreat operations, especially in areas 

of the operation overlain by up to 610 

m of mesa-forming sediments. 

Although a yield pillar design has 

worked very well at the Sunnyside 

operation, not all mines have 

experienced this level of success. A 

nearby mine attempting to emulate the 

very profitable yield pillar design had 

difficulties in developing small 

pi 11 ars without serious bumps 

occurring, and routinely lost 

significant portions of tailgate 

entries to large bump events. It soon 

became clear that the successful 

application of yielding designs was 

dependent, in part, on the immediate 

geology surrounding the pi 11 ar system. 

Soft roof and floor units aggravate 

ground conditions in yielding systems 

because 1) the roof cannot sustain the 

1 arge deformations that accompany 

pillar yielding, and 2) the floor 

cannot withstand the additional bearing 

1 oad of suppl emental support required 

to hold the entry. Competent roof and 

floor conditions are necessary to 

maintain stability during entry closure 

and not impai r subsequently instal 1 ed 

secondary support. On the other hand, 

strong roof and floor conditions, like 

those at the neighboring mine described 

above, enhance the strength 

characteristics of the coal by offering 

high degrees of confinement at the 

coalbed interface. This may in turn 

require a very narrow pillar design to 

initiate yielding. It may also not be 

possible to develop such a narrow 

structure under highly stressed 

conditions without some form of costly 

ground preconditioning. 

An additional consideration when 

employing yield pillar designs concerns 

the effect that redistributing stress 

from the gate entries onto the active 

longwall may have on face cutting 

operations, parti cul arly near the 

tailgate. ~mploying a yielding entry 

system may eliminate bumps in the gate 

entries, but may also escalate the 

incidence of bumping during face 

shearing due to increased abutment 

loading. Coal stress measurements at 

the Sunnyside No.1 Mine showed the peak 

face abutment to commonly reside within 

3 to 6 m of the standing longwall 

(Haramy and McDonnell , 1988) . The 

extent of yielding was even greater 

along the panel rib-line, extending to 

nearly 15 m at two measurement 

locations. This characteristic of side 

and front abutment stress distribution 

about the panel explains why bumps are 

very infrequent at this operation. 

Moderately stiff immediate roof and 

floor units, in conjunction with a lowto-moderate strength coal, do not allow 

coal bed confi nement to generate high, 

bump-initiating stresses near the face. 

In contrast, measurements at the 

neighboring Castlegate Mine, which 

employed a non-yielding two-entry gate 

design, showed peak face abutments to 

exist within 0.3 to 0.7 m of the 

standing wall (Barron, 1990). Bumps 

were common to this operation, 

requiring routine face destressing to 

avoid serious injury to mine personnel 

and damage to the longwall equipment. 

Employing a yielding design would 

almost certainly have aggravated the 

occurrence of bumps in both the 

headgate and tailgate entries? as 

corroborated by previous attempts to 

implement yield designs on earlier 

panel gates. T h i s mine was eventual ly 

forced to close, 1 argely due to coal 

bump complications. 

In general, two-entry gate entry 

systems more common1 y empToy pi 11 ar 

designs that yield during or shortly 

after development. By design, the 

narrower gate entries typically 

generate significant side abutment 

stresses, and thereby lower the risk of 

bump occurrence. In situations where 

two-entry systems are not practical, 

yield pi 11 ars have been used 

effectively in mu1 ti -entry systems, but 

are more commonly used in conjunction 

with abutment pillar designs, as shown 
in the previous section. In either 

application, yield pillar designs have 

proven to be an effective alternative 

in mitigating bump hazards in deep U.S. 

coal mines. 

SUMMARY AND CONCLUSIONS 

In summary, many useful designs have 

been developed in the U.S. to reduce 

the severity and occurrence of coal 

mine bumps. This paper has listed the 

most successful of these designs in 

chronological order for both room-andpillar and longwall mining. The 

following is a list of major 

observations and recommendations 

developed from mining in burnp prone 

strata: 

) The potential for bump occurrence 

ncreases when mining in stiff roof and 

'loor rock. Strata of this nature are 

frequently found within the southern 

Appal achi an, Uinta and Pi ceance Creek 

Basins. 

2) Bumps can occur a) in development 

sections when faults and igneous dikes 

are approached, b) in room-and-pillar 

sections when cantilevering gob is 

encountered, c) in longwall sections 

when geologic structures are 

encountered, and d) in either room-andpi1 lar or longwall sections when 

overburden, abutment or shock loads are 

excessive. 

3) Supplemental support has been useful 

in minimizing bump damage. Rockfilled cribs allow gob to converge 

gently without rupture. Combinations 

of cribs, crossbars and props reduce 

the severity of bumps in main entries. 

Wood cribs and yielding arches in 

combination with rock bolts helped 

support weak, immediate roof during 

bumps which reduce the associ ated 

occurrence of roof fall. 

4) The use of straight retreating 

pillar 1 ines and total extraction of 

all coal can el iminate projections of 

bump prone material into the gob. 

5) Developing or spl itting 1 arge blocks 

of coal into smaller, uniform blocks 

ahead of the retreating pillar line, 

causes the coal to yield in a 

control 1 ed fashion prior to extraction 

and allows the roof to bend gently. 

6) Sequential splitting of pillars away 

from the retreating pillar line can 

effectively move excessive stress 

condi tionr in a control led manner away 

from the working face. 

7) Sizing gate entry pillars 1 arge 

enough to contain induced stresses can 

effectively reduce bump occurrence. 

8) Sizing gate entry pillars to yield 

in a controlled fashion can assist 

fracturing of the main roof and in some 

cases lessens the magnitude of abutment 

and/or shock loads onto the longwall 

face. 

Most of the past and present U.S. 

bump control designs have helped to 

control the way the roof rock breaks 

and regulated the manner in which 

stresses are redistributed. For the 

most part, these techniques have been 

very successful, but have not been 

appl i ed over a wide range of geol ogi c 

and mining conditions. As production 

rates and overburdens increase and new 

mining systems are developed, the 

mining industry will be required to 

develop new bump control designs. 

Evaluating past experiences, analyzing 

current and projected conditions and 

investigating innovative design 

techniques in the field will help 

develop the needed technology to keep 

bump prone U .S. mines safe and 

productive. 

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