A case study of roof bolting tasks to identify 

cumulative trauma exposure

Kim M. Cornelius and Fred C. Turin

National Institute for Occupational Safety and Health

Pittsburgh Research Laboratory

Mining Injury Prevention Branch

Pittsburgh, PA 15236

Abstract

Work in underground coal mines is repetitive and physically demanding. Workers commonly report a wide range of

aches and pains. Management at one mine was concerned about increased reporting of aches and pains by roof bolting

machine operators. An analysis of roof bolting tasks was conducted at this mine to identify early warning signs of

cumulative trauma. Health and safety risk factors were identified and recommendations to reduce cumulative trauma

exposure were provided. Recommendations addressed job procedures, equipment design, and worker awareness of risk

factors.

Keywords

ergonomics, mining, cumulative trauma, risk factors

1. Introduction

Underground mining in the United States has undergone significant change in the past 20 years. Two key elements have

been increased mechanization and a more educated work force. In spite of these changes, many jobs continue to be labor

intensive and repetitive in nature. They entail tasks that, performed over time, can take a toll on the soft tissues and

joints. The problem may be compounded by an aging mining workforce. In 1986 the mean age of the coal mining

workforce was 39 years and the median total years of experience was 11 [1]. By 1996 the mean age of the coal mining

work force was 45 years and the median total years of experience was 20 [2]. As a person ages, the body's resilience

to chronic wear and tear is reduced, which may cause a worker to pay an increasingly higher health price for performing

the same task. Mining companies, like many other companies, are becoming more aware of cumulative effects to the

worker as reports of these types of injuries rise.

Conducting a job analysis is an important step when considering a job redesign or modification in order to reduce worker

cumulative trauma exposure. A basic approach to job analysis is to examine the types of aches and pains reported, the

tasks performed, and work site conditions. A study was conducted to evaluate roof bolting tasks performed at an

underground coal mine concerned about early warning signs of cumulative trauma. This evaluation will comprise the

primary focus of this article.

2. Roof bolting and cumulative trauma exposure

In an underground coal mine, after an area is mined, it is necessary to support the roof to keep it from collapsing. Since

1950, the primary method for supporting the mine roof has been installation of roof bolts. Long bolts installed into the

roof compress the layers of strata and suspend the weaker layers from a stronger layer above. Roof bolts, typically 6 -

8 feet long, are installed by workers using large roof bolting machines. There are different types of machines used for

high, medium, and low coal seams. Typically in medium seam heights, 43 to 60 inches, and low seams, less than 43

inches, the operator works in a small area between the machine and walls of coal called ribs. A roof bolting machine

operator working in a low seam mine is illustrated in figure 1.
Figure 1. Roof bolting in a low seam mine often requires workers to maintain awkward postures.

In high seams (greater than 60 inches) workers often perform tasks from a platform on the bolting machine, as illustrated

in figure 2. In general, roof bolting machine operators work in tight spaces. Roof drilling and bolt installation in

underground coal mines is labor intensive, repetitive, and exposes operators to many hazards which can result in injury.

In fact, drilling holes and installing bolts were reported as the leading sources of injury to roof bolting machine operators,

accounting for 34.2 and 24.2 percent of accidents, respectively, based on an analysis of accidents in West Virginia [3].

This case study examined roof bolting injuries and associated activities at a high seam mine.

3. Methods

There were three data collection activities for this case study: analysis of lost time incident descriptions, interviews with

mine site personnel, and observation of roof bolting tasks. The goal of the incident analysis was to identify roof bolting

activities and operator injuries having characteristics consistent with cumulative trauma exposure. Interviews were

conducted with roof bolting machine operators, supervisors, and the staff nurse. The objective of the interviews was to

learn about bolting tasks and working conditions, to identify safety hazards, and to discuss the details of aches, pains,

and reported injuries. The interview data was analyzed to identify similarities in injuries and pains; tasks that may

contribute to cumulative trauma; and aspects of the working environment that may contribute to cumulative trauma.

Finally, roof bolting machine operators were observed performing tasks, bolting activities were video taped, still

photographs were taken of bolting equipment and mine conditions, and an experienced operator explained the layout

and operation of their roof bolting machines.
Figure 2. Workers roof bolting in a high seam mine.

4. Results

4.1 Incident Analysis Results

Forty-three lost-time incident descriptions were analyzed. Fourteen of these incidents were identified as having

characteristics consistent with cumulative exposure. The incidents selected appeared to have developed over time and

the associated activity involved a risk factor such as excessive force, awkward postures, or repetitiveness. The following

is an overview of the key characteristics:

Five of the fourteen incidents involved pain in the back, neck, shoulder, or elbow.

< Two incidents occurred while putting a roof bolt into a drilled hole.

< Two incidents occurred while lifting bolting supplies.

< One incident occurred while torquing a roof bolt.

Nine of the fourteen incidents involved a strain or sprain injury to the ankle, knee, or hip resulting from a slip, trip or

misstep.

< Seven incidents involved stepping or kneeling on uneven floor, loose materials on the floor, or equipment cable.

< Two incidents involved an operator stepping into or out of the bolting machine platform.
4.2 Interview Results

Twelve roof bolting machine operators were interviewed. The most common aches, pain, or injuries mentioned were:

< face and arm lacerations and cuts

< shoulder, neck, and arm strains and pains

< ankle sprains and twists, back pain and strains, and knee strains

< leg numbness

Operators said that roof bolting tasks require a considerable amount of lifting, carrying, bending, reaching and stretching.

Common activities cited as contributing to their pain and discomfort included: leg pains while leaning out to see the drill

hole; hand and elbow pain from using the controls; sore knees, back, and shoulders from bending and twisting to install

bolts or lift and position drill steels, wrenches, and bolts; shoulder and elbow aches from picking up and holding drill

steels; and knee and back aches at the end of the shift from standing all day.

The nurse’s description of reported pain and the related activities matched that given by the operators. The most

common ailment reported to the nurse was muscle soreness. The most frequently reported area of pain was the shoulder.

4.3 Observation Results

Work tasks observed were bolting of the roof and ribs, and acquisition and stacking of machine supplies. Key issues

taken into consideration were the work envelope, work postures, the amount of twisting, turning, and reaching, and the

number of repetitions of each sub-task per bolting cycle. Table 1 lists key observations and issues relevant to cumulative

exposure.

Table 1. Observations and issues concerning roof bolting machines.

Observation Issue

Confined operator platform causes operators to twist

and stretch to get drill steels, bolts, plates, and

wrenches.

This places operator in awkward postures

creating stress to the muscles and joints,

particularly in the back and the knees.

Supply trays are positioned at heights well above the

operators’ waists.

Lifting and retrieving tools and bolts is stressful

to the neck, arm, and shoulder.

Tops of control levers are positioned well above waist

height.

The operator must work with the arm and wrist

in awkward postures.

Operators lean against the back rail of operator

compartment and out from under the canopy while

performing drilling and bolting tasks.

This places the operators in awkward postures. 

Also, it is putting them at risk of being hit by

falling rocks.

Operators shift their weight to the side of the body

corresponding to the hand which places the drill steel

into the drill chuck.

The muscles on the opposite side of the body,

particularly the low back muscles, are stressed

and may become fatigued.

Operators frequently extend their arm up and out to

hold onto steels while drilling, and onto bolts while

installing them.

This is stressful to the neck, arm, and shoulder

muscles.

Drill steels are being inserted into the drill chuck

usually at knee level or lower.

The operator must do more bending which

stresses the low back muscles.

Transfer of supplies from the back of a bolting

machine to supply trays involves frequent lifting,

carrying, and twisting.

This places operator in awkward postures

creating stress to the muscles and joints,

particularly in the back and the knees.
5. Discussion

Injuries to the musculoskeletal system are common in the workplace. However, many injuries such as muscle and

ligament strains are not the result of a sudden mishap, but occur over time as a result of repeated microtraumas. This

type of injury develops over weeks, months, or even years. There are three main risk factors that contribute to

cumulative trauma disorders (CTDs): force, repetition, and awkward postures. Any one or combination of these

contribute to development of CTDs.

It is necessary to examine the layout of the work area to help identify tasks which may contribute to cumulative trauma.

Putz-Anderson [4] describes ergonomic concerns that, in general, should be minimized at the work area. These

considerations were used as a guideline for the analysis. Analysis of data obtained from lost-time incidence reports,

interviews, and observations were used to identify roof bolting tasks which increase risk to the development of CTDs.

These issues were arranged into the following categories: materials handling, operator orientation in work space, vision

obstruction, control bank design, and slipping and tripping hazards.

Recommendations given below address three elements which define a system: human, equipment, and environment.

Recommendations directed at the human element are intended to increase worker awareness of risk factors. This

knowledge can then be motivation for workers to modify their behavior to reduce exposure. Equipment

recommendations address modifications to existing equipment, which can be performed at the mine site or retrofitted

by the manufacturer, and recommendations that would require more significant changes that should be addressed in the

design of future roof bolting machines. Environmental factors play an important role in human-machine interfaces.

Environmental conditions addressed in the recommendations include space restrictions, visibility restrictions, and

housekeeping. The underground mining environment is particularly challenging for equipment designers.

Evaluation teams that examine roof bolting tasks in underground mines should consider the following recommendations

when looking at modifications to work procedures and equipment.

< Increase worker awareness of the risk factors associated with developing CTDs.

< Examine activities which require high force, high repetition, and awkward postures to determine if the task or

equipment can be modified.

< Modify materials handling tasks to carry supplies as close to the body as possible, restrict the size of the load,

and minimize lifting distances.

< Eliminate barriers in the path which require the operator to lift supplies up and over.

< Improve supply tray design and position, and methods for stacking and retrieving supplies.

< Design bolting machine tasks and equipment to minimize shoulder abduction.

< Design operator work areas considering reach and visibility requirements.

< Reduce force required to activate controls.

< Increase spacing of controls to accommodate a gloved hand.

< Make height of control banks adjustable to accommodate each operator.

< Consider a height-adjustable, padded rail at the back of the operator platform.

< Evaluate thresholds between machine walkways and operator platforms with special consideration given to

slipping and tripping hazards.

< Improve housekeeping practices and implement an active program to evaluate.

< Increase worker awareness of slipping and tripping hazards.

Working environments in underground mines are dynamic and there can be large differences between mines. The

recommendations provided were intended to be used as a guide for more comprehensive examinations of roof bolting

activities. Each mine should conduct a mine-specific evaluation due to varying conditions, equipment, and workforce.

An evaluation team with diverse members including roof bolting machine operators, first-line supervisors, engineers,

and safety personnel is an effective approach for developing solutions [5],[6],[7]. Additional information on roof bolting

hazards and injuries, work routine risks, and interventions is available [8], [9]. Specific information is available

concerning human factors considerations for designing underground mobile mining equipment at

www.cdc.gov/niosh/mining/.
6. Summary

The information presented is intended to provide the reader with an awareness of factors which may contribute to

cumulative trauma injuries to roof bolting machine operators. A case study which analyzed roof bolting tasks was

conducted at a mine to identify early warning signs of cumulation trauma. Three types of data were analyzed: lost

time incident descriptions, interviews with mine site personnel, and observation of roof bolting tasks. Forty-three

lost-time incidents were analyzed, of which fourteen were identified as having characteristics consistent with

cumulative exposure. Five of the fourteen incidents involved pain in the back, neck, shoulder, or elbow, and nine of

the incidents involved a strain or sprain injury to the ankle, knee, or hip resulting from a slip, trip, or misstep.

Twelve roof bolting machine operators were interviewed. Operators said that roof bolting tasks require a

considerable amount of lifting, carrying, bending, reaching and stretching. The most common aches, pain, or injuries

mentioned were face and arm lacerations and cuts, shoulder, neck, and arm strains and pains, ankle sprains and

twists, back pain and strains, knee strains, and leg numbness. Work tasks observed were bolting of the roof and ribs,

and acquisition and stacking of machine supplies. Key issues considered were the work envelope, work postures, the

amount or twisting, turning, and reaching, and the number of repetitions of each sub-task per bolting cycle. Results

were used to develop recommendations to address the elements of the system: human, equipment, and environmental

considerations. Recommendations directed at the human element are intended to increase worker awareness of risk

factors, equipment recommendations address modifications to existing equipment or in the design of new roof

bolting machines, and environmental recommendations address space restrictions, visibility restrictions, and

housekeeping. The recommendations developed should be useful to equipment manufacturers and to the

management and workforce at underground coal mines. Because there can be significant difference between mines

in terms of environment, geology, workers, equipment, and processes, it would be useful to develop or acquire

ergonomic expertise to evaluate the issues described and tailor solutions to the conditions. 

Biographical Sketch

Kim M. Cornelius is an Industrial Engineer and Certified Professional Ergonomist currently working for the

National Institute for Occupational Safety and Health (NIOSH) at the Pittsburgh Research Lab. Her current work

focuses on musculoskeletal disorders and ergonomic interventions in mining. Kim holds B.S. and M.S. degrees in

Industrial Engineering from the University of Pittsburgh. 

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