STABILITY OF UNDERGROUND OPENINGS ADJACENT TO THE SINK 

HOLE AT THE NIOSH LAKE LYNN RESEARCH LABORATORY

D. R. Dolinar, T. E. Marshall, T. M. Barczak, T. P Mucho 

NIOSH 

Pittsburgh, PA

ABSTRACT 

 Over a portion of the older limestone mine workings at the 

NIOSH Lake Lynn Laboratory, a mining health and safety 

research facility, a large sinkhole formed caused in part by the 

intersection of several tightly spaced joint sets and unusual 

weather conditions. The anticipated propagation of the 

associated roof failures threatened to encroach upon a portion of 

the facility’s secondary escapeway. To protect the escapeway, 

two cribs walls and over 80 cable bolts were installed. In addition, 

extensometers were installed to monitor roof movement in the 

escapeway. However, a large roof fall associated with the 

propagating roof failures in the sinkhole area partially destroyed 

one crib wall where the adjacent escapeway was not reinforced 

by cable bolts. Recent roof movement information indicates that 

this area is still active. Based on these measurements, steps 

have been taken to stabilize this portion of the escapeway. This 

paper discusses roof monitoring, the roof movement, the large 

roof fall, propagation of the roof failures, and the recent support 

measures undertaken to stabilize the escapeway. 

BACKGROUND 

 The National Institute for Occupational Safety and Health 

(NIOSH) Lake Lynn Laboratory, a mining health and safety 

research facility is located along Chestnut ridge on the 

Pennsylvania-West Virginia border near Uniontown, PA. 

Stratigraphically, the facility is in the Greenbrier limestone of the 

Mauch Chunk Formation. This unit is composed primarily of a 

hard gray massive limestone that is interbedded with shales. The 

underground portion of the facility consists of the old mine works 

and the new research galleries. 

 The old mine works were part of an underground limestone 

mine developed in the mid 1960’s and early 1970’s from a 

highwall of a surface quarry escapeway (Mattes, et al., 1983 and 

Triebsch and Sapko, 1990). Approximately 2,300 m of entry was 

developed with rooms 15.2 m wide by 9.1 m high and 15.2 m 

wide pillars. In 1979, new galleries were developed from the 

older works to provide representative coal mine-sized openings 

for research. These galleries consist of about 2,300 m of 5.5 m 

wide by 2.0 m high entries. To provide access to the original 

underground quarry, four portals were driven into the highwall of 

the surface quarry. Portal 1 is used as the primary mine entrance 

while portal 2 functions as the secondary escapeway (figure 1). 

In the late 1990’s two hydrostatic chambers were constructed 

at the furthest extent of the old workings to allow for full scale 

testing of mine seals. In addition to providing access, ventilation 

and escapeways for the new research galleries, portions of the 

old mine are now being used for research studies. As 

underground limestone mining industry continues to expand, the 

importance of the old mine workings for conducting health and 

safety research related in large opening mines will also increase. 

Therefore the stability of the old workings is critical to facilitating 

this planned research. 

ROOF COLLAPSE AND SINK HOLE FORMATION 

 On January 25, 1994 a large roof collapse occurred in the 

old workings near portals 3 and 4 and in close proximity to the 

highwall. The roof fall encompassed approximately 2,550 m2

 of 

entries and crosscuts. Over the collapsed area, the overburden 

thickness was approximately 30.5 m. Given the relatively shallow 

depth and the intersection of two significant joint sets observable 

through most of the roof rock units a sinkhole formed during the 

initial collapse. With time, the underground failure and sinkhole 

have expanded. Figure 1 shows the location of the original roof 

collapse and sinkhole as well the extension of the failure. 

 Several factors contributed to the roof collapse and the 

sinkhole formation. These factors have been discussed in 

previous reports but will also be summarized as follows below 

Zelanko, et al., 1996 and Iannachione et al., 1995). Two key 

elements were the joint systems and water. The dominant joint 

set is oriented at N 70E E nearly parallel with the highwall. Two 

less significant joint sets are oriented at N 20E E and N 45E W. 

All three sets of joints are near vertical. The three near vertical 

joint sets outlined large blocks of limestone in the roof of the 

collapsed area. Furthermore, surface and ground water had 

proculated through the joints and eroded the calcareous rock. 

These eroded joints were then in filled with low strength sandy 

clay, thereby, reducing the rock mass strength. Also, the shallow 

depth of the site and the proximity of the highwall resulted in low 

normal stresses across the joints (Iannachione et al., 2002). 

Figure 1. Sinkhole, roof collapse and roof reinforcement 

area at Lake Lynn. 
 Mine design in the form of excessive roof spans for the 

geologic conditions may also have been a factor. The rock mass 

rating using the Norwegian Geotechnical Institute Q system was 

28.13 and using Bieniawski’s Rock Mass Rating (RMR) was 67. 

These ratings indicate a good roof where the maximum span for a 

permanent opening based on the Q-system rating is 15.2 m. 

Although the entry spans were 15.2 m, intersection spans 

approached 22.9 m. The staggered pillar design employed also 

resulted in significant roof spans. 

 Finally, weather conditions may have triggered the roof 

collapse. Extremely cold weather resulted in a significant ice 

build up along the highwall and in the portal areas. A sudden 

warming trend then allowed snow to melt above the mine thus 

increasing the water level in the overburden. The highwall ice 

barriers could have restricted water flow from the overburden 

causing a build up of water pressure in the overburden and 

reducing the effective stress along the near vertical joint sets to a 

level that allowed the roof to collapse (Iannachione et al., 1995). 

 Although the roof collapse and sinkhole formed in the old 

workings, there was the potential that the failure could expand 

along the dominant joint set toward the nearby secondary 

escapeway. Expansion of the failure into the escapeway would 

seriously impact the operation of the facility. Therefore, a 

decision was made to reinforce and protect the secondary 

escapeway and to install instrumentation in the roof to monitor 

roof behavior. The design of the reinforcement was based on the 

structural and geologic conditions in the roof of the escapeway. 

The instrumentation would be used to determine whether the 

reinforcement maintained stability, and would ultimately identify 

the need for further action to protect the secondary escapeway. 

GEOLOGY AND STRUCTURE IN VICINITY OF SECONDARY 

ESCAPE WAY

 To evaluate the geology and structure, two drill core holes 

(one core hole was angled 30 degrees from vertical) and several 

borescope holes have been drilled in and near the secondary 

escapeway (figure 2). A geologic column of the roof in the 

secondary escapeway based on the drill core obtained 

underground is shown in figure 3. The immediate roof consists of 

a competent gray limestone to a depth of 4.3 m. Above the 

limestone there are a series of red claystones and gray shales to 

a depth of 6.1 m. The claystones and shales are then overlain by 

2.7 m of fined-grained limestone. 

 Several fractures and joints were observed in the core from 

both the lower and upper limestone sections. At a roof depth of 

between 0.9 and 1.2 m, sections of core were missing. This zone 

appears to correspond to a major separation along the bedding 

that was also noted in several observations holes in the 

secondary escapeway. In these holes the bedding separations 

and even open breaks in boreholes were observed at depths of 

1.52 to 1.68 m in the mine roof (Molinda et al., 1996). In an 

adjacent area, the open fractures were noted at depths of 1.83 

and 2.44 m in the roof. 

Figure 2. Location of roof reinforcement and instrumentation in the vicinity of the secondary escape way. 

Figure 3. Roof geology observed in secondary escape way. 
 The roof in the area of the secondary escapeway is heavily 

jointed with near vertical joints spaced between 0.3 to 1.83 m 

apart and orientated mainly N 70 E. These joints are nearly 

perpendicular to the orientation of the entries and parallel to the 

crosscuts and the highwall. However, some of the joints are 

inclined from vertical and intersect the adjacent near vertical 

joints, forming small wedges that are no more than 0.3 to 0.61 m 

wide that can fall out from the roof. Figure 4 shows these 

dominant roof joints crossing an entry adjacent to the secondary 

escapeway. In general, the joint density increases towards the 

portals and highwall. The vertical extent of most of these joints is 

not known but there appears to be several large joints or zones 

with several closely spaced joints with spacing between 6.1 to 

9.1 m that may have a much greater vertical depth, possibly up 

through the shale. These large fractures and joints appear to be 

the main avenues for water seeping through the roof. The 

dominant joint and fracture system as well as the shale zone in 

the roof were taken into account in the design of the original 

reinforcement system installed in the escapeway 

REINFORCEMENT OF SECONDARY ESCAPE WAY 

 Because of the potential for the failure to extend along the 

dominant joint set oriented at N 70  ̊E, the secondary escape way 

was reinforced in July of 1994 (figure 1). The design of the 

reinforcement systems developed to protect the secondary 

escape way focused on three elements: (1) isolating the 

secondary escape way from the collapse, (2) reinforcing the 

secondary escape way roof structure and (3) reducing the inflow 

of water through the roof. Three support remedies were then 

used that included crib walls, cable bolts and polyurethane grout 

(Zelanko et al., 1996). 

 Two concrete crib walls were built in crosscuts between the 

sink hole and secondary escape way and across the dominant 

joint set to act as breaker walls (figure 2). Each wall was about 

15.2 m long, 9.1 m high, and 1.8 m wide, and composed of six 

four-point concrete cribs. The walls consisted of pre cast 

reinforced concrete sections that were stacked as four point cribs 

interlocked at specified positions. The walls were topped at the 

each of the crib corners with 1.83-meter-high timber crib packs. 

Grout bags were placed on the top of the timber cribs and 

pressurized to lock the crib walls in place resulting in about 

4,270 kN of active force being applied to the roof. 

 To provide reinforcement along the dominant joint set, fully 

grouted tensioned cable bolts were also installed in the secondary 

escape way behind crib wall A (figure 2). The bolts were made 

from a 1.5 cm diameter, 7-strand cable with a minimum breaking 

strength of 258 kN. A total of 81 cable bolts were installed on a 

3-m offset row spacing. To intersect the dominate near vertical 

joint set, the cables were installed at a 30-degree angle from 

vertical with half the cables angled inby and half outby from the 

portal in an alternating row pattern. A cable length of 9.1 m was 

used to allow anchorage into the upper limestone and the 

suspension of the lower limestone and shale layers from the 

upper limestone. Prior to grouting, the cables were pre-tensioned 

to 200 kN. 

 To reduce the permeability and increase the strength of the 

roof, polyurethane grout was pumped into the roof at five 

locations. These locations were the most highly fractured zones 

in the secondary escape way and were near crib wall A. A total of 

2,470 kg of grout was injected. 

INSTRUMENTATION 

 To monitor long term stability in the reinforced secondary 

escape, four multi-point wire extensometers were installed in the 

roof in mid 1995. Three of the extensometers (1, 2 and 4) were 

placed in the reinforced section of the secondary escapeway and 

one (3) in the unsupported entry adjacent to crib wall B (figure 2). 

A fifth extensometer would be installed during 2002, in the entry 

adjacent to crib wall B and inby extensometer 3. The roof 

anchors were placed at depths of 8.23, 7.01, 6.1, 3.05, 2.13, and 

0.3 m. The 8.23-meter anchor was above the cable bolt depth 

and used as the reference point for analysis of roof movement. 

The extensometers were read with a data logger so a continuous 

record of roof movement was obtained. 

1996 ROOF FAILURE 

 In October 1996, a roof fall occurred adjacent to the sinkhole 

near crib wall B that extended along the dominant joint set and 

encompassed an area from 6.1 to 15.2 m wide and 45.7 m long 

(figures 1 and 2). Most of the roof fall was only a few feet thick. 

However, one large block of rock estimated to be 4.3 to 5.6 m 

thick, 5.6 m wide and 9.1 m long weighing an estimated 4,500 kN 

fell from the roof and damaged the crib wall. The roof cavity 

where the large block fell extended to a depth of about 6.1 m 

possibly through the shale. Upon falling, the large block rotated 

upon striking the floor and a corner of the rock impacted the lower 

portion of the crib wall with a blow resulting in the collapse of one 

half or three crib sections of the wall (figure 5). However, the wall 

was not designed to take a dynamic side load from a 4,500 kN 

rock falling nearly 9.1 m. The impact was sufficiently violent to 

knock concrete crib blocks 15.2 m into the entry. Although the 

wall did partially fail, it did break the roof failure and prevented 

any extension of the roof fall into the entry beyond the crib wall. 

 

 Examination of the fall revealed that the block fell out 

between two near vertical joints orientated N 70 E that extended 

up through lower limestone to the shale. Further, the area was 

partially surrounded or isolated by collapsed roof. This partial 

isolation further reduced any confining forces along the joints that 

failed. Water was also present along the joints and roof in the 

area further weakening the shale above the immediate limestone 

roof and reducing the joint cohesion over time. 

 The roof instrumentation gave no clear indications that the 

large block of rock was about to fall as the wall was positioned 

between the failing roof and the extensometers. Also, none of the 

Figure 4. Dominant joint set (N 70  ̊E) that is nearly 

perpendicular to the entry and parallel to the high wall. 
extensometers were located between the joints that outlined the 

large block. Approximately, 0.25 cm of movement had occurred 

at extensometer 3 located in the unsupported zone closest to the 

failure, however this movement had occurred nearly 8 to 12 

months prior to the failure with most of this movement less than 

2.1 m into the roof. 

LONG-TERM MONITORING OF INSTRUMENTATION 

 Of the four original roof extensometers, only extensometer 3 

has shown any significant roof movement. Extensometer 3 is 

located in the unsupported entry near crib wall B and in the 

vicinity of a major joint from which water has continuously 

dripped. 

 Figure 6 shows the roof movement at the various anchor 

horizons for extensometer 3. On the figure, the roof movement is 

for an anchor at a given roof depth with respect to the 8.1 m 

reference anchor. Between November 1995 and August 2002 

over 0.63 cm of total movement of the roofline was detected with 

about 0.25 cm movement between 0.3 and 2.1 m movement and 

0.25 cm between the roofline and 0.3 m. Through 1999, 

accelerated movements were initiated in November 1995, 

between late October 1996 and March 1997 and mid March 1998. 

There appears to be a seasonal pattern to the movement with the 

accelerated movement initiating in the fall or winter. Through 

March 2000, a fairly large amount of movement occurred but the 

movement was relatively shallow in the roof with most of the 

movement below 2.1 m. Since March 2000, there has been less 

than 0.13 cm of roof movement. However, most of this movement 

has been between 3.0 and 6.1 m into the roof, clearly much 

deeper than the previous movement. 

 Through mid 2000, the other extensometers 1, 2, and 4 have 

shown little or no movement. Roof movement, if any, has been 

less than 0.025 cm, which is near the detectable limits of the 

instrumentation. In general, the area that has been supported 

with cable bolts behind crib wall A has remained stable. 

DECISION TO EXTEND THE REINFORCED ZONE 

 Beginning in March and April of 2000 a pattern of roof 

movement was observed at extensometer 3 that would lead to the 

decision to reinforce the entry adjacent to the damaged crib wall. 

Figure 7 shows the roof movement from July of 1999 to August of 

2002. Again, the roof movement is for an anchor at a given roof 

depth with respect to the 8.1 m reference anchor. Although 

nearly an order of magnitude smaller, these roof movements were 

occurring at much greater depths than previously detected 

(figure 6). During this time period, only about 0.12 cm of 

movement was measured. However, most of this movement 

occurred between 3.0 and 6.1 m into the roof, the depth of the 

October 1996 roof collapse that partially destroyed the crib wall. 

 For three consecutive years, the roof movements were 

initiated in late January to mid-February. After each period of 

accelerated movement, there was a period with little or no 

movement. This movement pattern suggests that overall roof 

conditions are still stable. However, continued seasonal 

movement could result in a destabilization of the roof and 

eventually a large roof fall (Zavodini, 2000). Therefore, because 

of this pattern of deeper seasonal movement, a decision was 

made to develop and initiate a roof reinforcement plan to support 

the section of entry adjacent to the damaged crib wall. 

Figure 5. Crib wall B damaged by fall of large rock, 

October 1996. 

Figure 7. Roof movement measured at extensometer 

three between July 1999 and August 2002. 

-0.04

-0.02

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

7/27/1999 11/27/1999 3/27/2000 7/27/2000 11/27/2000 3/27/2001 7/27/2001 11/27/2001 3/27/2002 7/27/2002

Date 

Roof movement, cm

7 meter

6.1 meter

3 meter

2.1 meter

0.3 meter

Roof

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

4/11/1995 5/15/1996 6/19/1997 7/24/1998 8/28/1999 10/1/2000 11/5/2001 12/10/2002

Date

Movement, cm

7meter

6.1 meter

3 meter

2.1 meter

0.3 meter

Roof

Roof Fall 10/96 (crib wall damaged)

Figure 6. Roof movement measured at extensometer 3 

between October 1995 and August 2002. Note: the gap 

in the data from March to July 1999 resulted from 

damage to the extensometer. 
ROOF SUPPORT DESIGN 

 Although the area in the entry behind the damaged crib wall 

is technically not part of the secondary escape- way, continued 

extension of the fall along the direction of the dominant joint set 

would extend the failure to the escape- way. Further, the entry 

provides direct access to the hydrostatic chamber area and the 

old mine works. Therefore, a three-phase reinforcement plan was 

devised to support the entry behind the damaged crib wall. This 

plan included the construction of a new crib wall, and the 

installation of both roof bolts and cable support. Figure 2 shows 

the extended support area adjacent to the secondary escapeway. 

Because the reinforced secondary escapeway had remained 

stable and that crib wall B actually broke the roof fall, the design 

of the reinforcement for the new area was based to a large extent 

on the previous successful design. 

 Another component of the plan was the installation of more 

instrumentation during 2002 to monitor roof behavior and add a 

further element of safety as work progressed in the entry that was 

to be supported. A fifth extensometer was placed in the center of 

the entry between the two joints along which the 1996 rock fall 

occurred (figure 2). Further, three roof to floor convergence 

monitors were installed just 1.52 m inby the new crib wall toward 

the roof fall in the crosscut (figure 2). 

Crib Wall

 A new crib wall (C) was built across the entrance of the cross 

cut just behind crib wall B (Figure 2). Figure 8 shows the 

completed crib wall that extends for a length of 21.3 m and covers 

two major joints, spaced 18.3 m apart. The crib wall was built on 

a steel reinforced concrete pad that was either 0.3 or 0.6 m thick 

depending of the floor elevation. A total of 9 interlocking 4-point 

cribs were erected with the height of the concrete crib portion of 

the wall being 6.71 to 7.01 m. The crib blocks were made from 

steel reinforced concrete were 0.61 m by 0.3 m and 1.83 m long. 

 Sixty-centimeter diameter thin-walled steel containers that 

were 1.22 to 1.68 m high were placed on the corners of each fourpoint crib and pumped full of a 1,000 psi compressive strength 

grout. Prestressing cells were then placed on the filled 

containers. The cans are a stiffer system than the wood timber 

packs used on the original walls. Further, the cans will minimize 

any long-term creep and moisture affects that can occur with the 

wood cribs. These prestressing cells were 66 cm in diameter and 

consisted of two thin metal sheets that were welded around the 

perimeter to form a circular pancake cell. An inlet valve allowed 

the cells to be pressurized with a polyurethane resin. The groutfilled wall was then pressurized specially designed flat jacks 

placed on top of each can. Each flat jack was pressurized to 

between 0.34 and 0.69 MPa resulting in a total load of between 

3,560 to 4,000 kN being applied to the roof and crib wall. Once 

the resin hardened, the cells provided a rigid support system. 

 During wall pressurization the data from the roof to floor 

convergence monitors and extensometers 3 and 5 were 

examined. When the wall was pressurized, no movement was 

detected on any of the instruments. 

Roof Bolts and Mesh

 The roof bolts and mesh system installed prior to the cable 

bolts was designed to provide support to the immediate roof and 

prevent the fall of surface material from the roof. An area of 

approximately 280-m square of entry adjacent to the new crib wall 

were supported with a 2.22 cm, grade 60, fully grouted torque 

tension bolts installed on a 1.52 m by 1.52 m pattern. A two-piece 

resin system was used with a fast set on top and a slow set resin 

on the bottom. This resulted in the upper 0.91 m providing 

anchorage for the lower tensioned portion of the bolt. The applied 

bolt torque was between 28 and 41 m-kg. Depending on the 

depth of the major bedding plane separation in the immediate 

roof, the length of the bolt varied from 2.44 to 3.05 m as the 

system was designed to be anchored above the main bedding 

fractures that ranged in depth from 1.52 to 2.44 m into the roof. In 

conjunction with the bolts, a 4-gauge welded wire mesh with a 

10.2 by 10.2 cm opening size was installed to provide surface 

control and to minimize the risk of injury to workers from minor 

rock falls. 

Cable support 

 Thirty fully grouted cables bolts were installed in the entry 

adjacent to the new crib wall. A Dywidag Z-Bulb cable bolt was 

used that consisted of a no. 6 (1.52 cm diameter), 7-wire strand, 

ASTM Grade 270 cable with a minimum breaking strength 

260 kN. To minimize corrosion, the cable bolts were galvanized. 

This cable bolt was different than that used in the original 

installation where a thread bar was used to tension the cable 

because the manufacturer no longer makes the older thread bar 

type of cable bolt. With the new system, a barrel and wedge 

anchor head were used to tension the cable and to secure the 

bearing plate. 

 The cable bolts were 9.1 m long and installed on a 3-m row 

spacing with alternate rows offset by 1.52 m. This pattern 

resulted in 5 rows of cables across the entry with 6 cables per 

row. Approximately 18.3 m of entry adjacent to the new crib wall 

was reinforced with cables. The cable bolts were angled at 30 ̊

from vertical with alternating rows of the cables angled inby and 

outby from the portal. Again the system is designed to reinforce 

the dominant joint set with at least 2 cables bolts intersecting 

each joint. Essentially, the design of the cable bolt pattern is 

similar to the pattern used in the original cable bolt installation in 

the secondary escapeway. 

 The cables were installed in a 4.13 cm hole. However, the 

initial anchor was developed from a resin grout cartridge placed in 

a 3.49 cm hole along the upper 2.4 m of the bolt. Three Z-bulbs 

anchors created along the upper 1.5 m of the cable were used to 

increase the anchorage capacity in section where the resin 

cartridge developed the initial pre-tensioned anchor. A 1.2 m long 

steel tube was placed along the lower portion to stiffen the cable 

during insertion of the cable through the grout cartridge. The 

resin anchor was allowed to set over night before the cable was 

tensioned to between 89 to 98 kN using a cable-tensioning unit. 

The cables were tensioned against 20.3 by 20.3 cm, high capacity 

Figure 8. New crib wall C covering the crosscut in front 

of damaged crib wall. 
(grade 5), domed bearing plates. Two small holes were drilled in 

the dome of the bearing plates to allow for the breather and grout 

tubes to be installed for the grouting operation. Once the cable 

had been tensioned, the remainder of the hole and cable were 

fully grouted using a pumpable cement grout. 

DISCUSSION 

 A large roof fall did occur in the fall of 1996 that destroyed a 

portion of crib wall B. The entry adjacent to the crib wall was then 

reinforced about six years later. However, during this time, the 

data from the instrumentation installed in the roof was 

continuously evaluated to determine if any changes occurred in 

the roof behavior. 

 Prior to 2000, although there was a fairly large amount of 

movement at extensometer 3, this movement was relatively 

shallow, below 2.1 m. The movement would occur as a sudden 

accelerated movement followed by a period of deceleration and 

finally by a period of little movement. Extensometer 3 was also 

near a major joint and apparently was detecting the loosening of 

the lower roof around the joint. Further, this movement occurred 

seasonally usually in the late fall or early winter. Because of the 

pattern and timing of roof movement, the cause of the movement 

appears to be external. The external stimuli for this movement 

probably is related to the seasonal conditions and changes that 

occur in the late fall or early winter. 

 Beginning in 2000, the pattern of roof movement at 

extensometer 3 changed. Although the movement was an order 

of magnitude less, the movement was much deeper into the roof 

and appears to involve the roof up through the shale-claystone at 

a depth between 4.27 to 6.1 m. Proximity of extensometer 3 to 

the major unsupported joint is also of concern where the deeper 

movement could indicate that the joint was now becoming active. 

Though instability near a major fracture may reflect just the local 

instability in the vicinity of the joint, continued movement near 

such joints could destabilize larger blocks of rock leading to the 

extension of the roof fall into the entry. 

 This latest roof movement pattern has also been seasonal 

with the initial accelerated movement occurring from late January 

to mid February for three consecutive years, from 2000 to 2002. 

The time frame is similar to when the main collapse occurred that 

caused the sinkhole formation. The periods of accelerated 

movement are again probably the result of external stimuli that is 

related to the winter season. However, it is not known whether 

this movement is related to ice and the change in the effective 

stress due to increased water levels such as may have occurred 

in the original failure. With the formation of the sink hole, the area 

should be well drained since there was no ice builds up near the 

collapsed wall or extensometer 3. However, this period does 

correspond to the coldest time of the year and to the coldest time 

in the mine. January/February temperatures near this location 

underground average between 7.2 to 10 o

C while summer 

temperatures average about 18.3 o

C. Although the change in 

temperature within the area underground is seasonally only 8.3 to 

11 o

C, this area appears to have little confining stress across the 

joints. Therefore, any contraction of the rock mass due to lower 

temperatures would further reduce any small confining stress 

across the joints (Hooker and Duvall 1971). For the present, the 

roof movement pattern suggests that the roof conditions are still 

stable. After each period of accelerated movement, the 

movement decelerates and there is a period with little or no 

movement. Essentially, the forces that trigger the movement are 

of limited duration and once reduced, allow the roof to again 

reach equilibrium. 

 Whatever the cause though, continued seasonal application 

of the external stimuli could ultimately lead to the destabilization 

of the main roof and eventually, a large roof fall similar to the one 

that damaged the crib wall. Further, to eliminate or reduce the 

affects of the external stimuli that are causing the roof movement 

would be difficult, whether it is water and ice or temperature. 

Therefore, because of this pattern of deeper movement, a 

decision was made to develop and initiate a roof reinforcement 

plan to support the section of entry behind the damaged crib wall. 

 In designing the reinforcement system for the entry adjacent 

to the secondary escapeway, a 3-component system was 

developed where each component in the reinforcement plan 

contributes to maintaining the stability and function of the entry. 

The new crib wall acts as a breaker to prevent the extension of 

the failure along the dominant joint set into the entry and toward 

the escapeway. The roof bolts and wire mesh provide support to 

the immediate roof in between the cable bolts and also surface 

control. Finally, the cable bolts provide reinforcement along the 

dominant joint set behind the wall. In addition, the continued 

monitoring of the instrumentation will provide a basis for 

evaluating the reinforcement effectiveness and long-term roof 

stability. 

 During installation and pressurization of the new crib wall, 

roof to floor convergence meters and extensometers 3 and 5 

detected no roof movement. This lack of movement indicates that 

there were no major separations that were closed near the wall 

during pressurization. Further, locally the application of a 

3,560 kN force can be significant, but the effects dissipate rapidly 

with distance. At extensometer 3, located approximately 3 m from 

the wall, where significant lower roof movement had occurred, 

there was no roof movement from wall pressurization. 

 The data from the instrumentation in the reinforced 

escapeway shows that little or no roof movement has occurred 

and therefore the area has remained stable since the summer of 

1994 when the escapeway was reinforced. This provides strong 

evidence of the success of the design of the reinforcement 

system. Even though crib wall B failed, the wall did act as a 

breaker to the fall where the wall performed it’s main function, to 

prevent the extension of the fall into the entry. Therefore, the 

design of the new reinforcement system for the entry could be 

based to a large extent on the previous design used in the 

secondary escapeway. 

CONCLUSIONS 

 The original reinforcement system consisting of cable bolts 

and crib walls have maintained the stability of the secondary 

escapeway with little or no movement being detected despite the 

proximity of the sinkhole and major joint structures in the roof that 

extend from the failure. Further, even though a crib wall was 

partially destroyed by a large rock fall, the crib wall did break the 

fall and prevent the fall from extending into the entry. Therefore, 

the original reinforcement design and system can be considered 

successful. 

 Ultimately, based on roof movement detected by the roof 

instrumentation, a decision was made to extend the reinforcement 

into the entry behind the damaged crib wall. If the entry was left 

unprotected, the failure could extend across the entry and up to 

the secondary escapeway. Therefore, the instrumentation 

installed to monitor roof movement was a valuable tool on which 

decisions concerning the stability of the entry and the need for 

further reinforcement were made. Although there was a fairly 
large amount of movement in the lower roof, it was the much 

smaller, but deeper, roof movement that triggered the decision to 

install more reinforcement. 

 Because of the success of the original reinforcement, the 

design of reinforcement in the entry was based to a large degree 

on that original system. Since the performance of the original 

reinforcement was successful, the new system should provide 

adequate protection to the entry and secondary escapeway. 

Further, continued monitoring of the roof instrumentation will 

provide a measure of the performance of the reinforcement and 

an evaluation of the stability of the area adjacent to the sinkhole. 

REFERENCES 

 Delanco, J.C., Mucho, T.P., Iannacchione, A.T. and Barczak, 

T.M., 1996, “Stabilization of a Large Shallow, Potentially Unstable 

Underground Opening,” Proceedings of the 2nd North American 

Rock Mechanics Symposium: NARMS’96, Montreal, Quebec, 

Canada, June 19-21, pp. 155-161. 

 Hooker V. and Duvall, W., 1971. “In Situ Rock Temperature 

Stress Investigations in Rock Quarries,” USBM RI 7589, 11 pp. 

 Iannacchione, A.T., Dolinar, D.R., and Mucho, T.P., 2002, 

“High Stress Mining Under Shallow Overburden in Underground 

U.S. Stone Mines,” Proceedings of the First International Seminar 

on Deep and High Stress Mining, Nov. 6-8, Perth, Australia. 

 Iannacchione, A.T., Mucho, T.P. and Prosser, L.J. 1995, 

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Industry,” Proceedings of the Environment, Safety and Health 

Forum (National Stone Association), Nashville, Tennessee, 

October 22-24, 24 pp. 

 Mattes, R., Bacho, A. and Wade, L., 1983, “Lake Lynn 

Laboratory: Construction, Physical Description, and Capability,” 

USBM IC 8911. 

 Molinda, G.M., Monaghan, W., Mowery, G. and Persetic, 

G.F., 1996, “Using Ground Penetrating Radar for Roof Hazard 

Detection in Underground Mines,” NIOSH RI 9625, pp. 14. 

 Triebsch, G. and Sapko, M., 1990, “Lake Lynn Laboratory: A 

State of the Art Mining Research Laboratory,” Proceedings 

International Symposium on Unique Underground Structures, 

Denver, CO, June 12-15, pp. 75-1 to 75-21. 

 Zavodini, Z, 2000, “Time-Dependent Movements of Open-Pit 

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