USING FAULT TREE ANALYSIS TO FOCUS MINE SAFETY RESEARCH

S. Iverson, J. C. Kerkering, and P. Coleman

Spokane Research Laboratory, National Institute for Occupational Safety and Health, 

Spokane, WA
ABSTRACT

Fault tree analysis is a systematic safety analysis tool

that proceeds deductively from the occurrence of an

undesired event (accident) to the identification of the root

causes of that event. One recurring mine safety problem—a

dozer falling into a void over a drawpoint on a coal surge

pile—was analyzed using available, inexpensive fault tree

programs on a personal computer. The analysis identified

basic and intermediate events that led to the burial of the

dozer and graphically depicted the interrelationship between

these various subordinate events as well as the various chain

of events leading up to the primary event. A sensitivity

analysis on these probabilities showed which events had the

greatest influence on dozer burial in a coal surge pile.

INTRODUCTION

Researchers at the Spokane Research Laboratory of the National Institute for Occupational Safety and Health (NIOSH)

are investigating the use of fault tree analysis to find root

causes of mining accidents and fatalities. This paper

describes the development of a fault tree as applied to one

mining system, that of the high number of injuries and

fatalities that occur when a dozer falls into a void in a coal

surge pile. Current research is focused on detecting voids,

studying the dynamics of void formation inside the coal pile,

and developing a better way of guiding the dozer operator

on the pile. Fault tree analysis applied to a surge pile

system (or any other mining system) may suggest root

causes and refine the focus of safety research.

Fault tree analysis (see Lambert, 1973) is one of many

systematic safety analysis methods developed in the last 40

years to promote the safety of highly complex technical

systems. Bell Telephone Laboratories first used fault tree

analysis in 1962 to study the safety of the launch control

system for Minuteman missiles. Since that time, fault tree

analysis has been used by the Boeing Company to enhance

the safety of airplanes and by the nuclear industry to

improve the safety and reliability of nuclear reactors.

Fault tree analysis is a systematic deductive procedure

used to identify the basic causes of a fault event. The

method is deductive because it starts from a single fault at

the top of a flow chart and expands out and downward to

identify the many contributing causes to that single top fault.

Thus, the method proceeds from one event to many events.

If the system were to be studied inductively, the starting

point would be at the bottom. Many fault conditions would

be identified and then these conditions would be evaluated

to find how they might connect to generate an undesired

event. This is a tedious and perhaps impossible task.

Fault tree analysis starts with a top fault event and

proceeds deductively by asking "How can this event have

happened?" Immediate contributing causes to this top fault

event are identified and then listed as part of the next lower

level of analysis (subfaults). These subfault events are then

connected as influencing inputs to the upper-level event by

means of either "and" gates or "or" gates. An "and" gate

requires that all subfault events are necessary for an upperlevel event to occur. An " or" gate says that each of the

input subfaults in and of itself is sufficient to generate the

upper-level event. Inputs to an "and" gate are necessary

conditions for the upper-level event, and if these inputs are

all present, then the upper-level event must of necessity

occur. It also means that if one or more of the “and” inputs

are missing, then the top fault event will not occur. Inputs

to the "or" gate are sufficient conditions for the occurrence

of the upper-level event. This means that if any one event

occurs, then the upper-level fault event will occur.

This same deductive procedure is applied to each of the

identified subfaults of the top fault event. Answers are

linked to each of the upper-level events by means of the

"and" or "or" gates. This procedure then continues from

level to level to create an upside-down tree; that is, a tree

with the narrowest end at the top (containing the top fault

event) with branches outward and downward. Branching

continues until a state of resolution is reached that identifies

the root causes and admits no further analysis. The analyst

may choose to terminate the study in one or more branches

of the fault tree if he or she thinks that the analysis will not

result in any more meaningful information or if that branch

has already been satisfactorily resolved in another branch.

The ultimate result is a qualitative fault tree terminating in

a set of basic faults and undeveloped events. This fault tree

in and of itself is revealing in terms of identifying contributing factors to the undesired top fault event and in showing

how they are interrelated. 

The analysis, however, can be carried further by

establishing or assigning probabilities of occurrence for each

of the basic faults and undeveloped events. This results in

a quantitative fault tree. Such a quantitative fault tree

allows the analyst to calculate the probability of occurrence

of the top fault event given the probabilities of occurrence

of each of the root causes and undeveloped events. 

Interesting measures, such as importance and sensitivity, can then be calculated. These measures indicate how

much each basic fault and undeveloped event influence the

probability of occurrence of the top fault event. In a sensitivity analysis, one can vary the probability of each basic

fault and undeveloped event to see how this change influences the probability of occurrence of the top fault event.

This procedure allows the analyst to detect how sensitive the

top fault event is to changes in the magnitude and variability

of probabilities of occurrence of each basic fault or undeveloped event.

Both qualitative and quantitative analyses result in a set

of critical paths, sometimes called “cut sets” or “min sets.”

These are sets of components that, when they occur, will

cause a top fault event. A minimal cut set is a set of events

that, if any event were removed from that set, would not
Figure 1. Surge pile showing overhead conveyor, stacker tube, feeders, and conveyor tunnel

generate a top fault event. Cut sets show the various paths

to the top fault event. Identification of cut sets is important

for directing efforts to prevent a top fault event.

Fault tree analysis has been traditionally used for deterministic systems, such as aircraft and nuclear reactors.

However, there is no inherent reason why a fault tree analysis cannot be applied, perhaps with lesser expectations, to

nondeterministic systems. In this paper, all possible modes

of occurrence of an undesirable event—a dozer burial in a

coal surge pile—are identified in a systematic fashion so

that a clear and demonstrable record of the process is provided. The paper also provides a baseline for evaluating

possible changes to procedures for using dozers on coal

surge piles. More detailed instructions for constructing fault

trees can be found in references listed in the bibliography

section of this paper.

SYSTEM DESCRIPTION

A coal surge pile may be used at either a surface or an underground mine. The surge pile may be used to stockpile

mine-run raw coal or clean coal from the washing plant.

Raw coal may have been crushed before stockpiling and

may contain some noncoal refuse material. Clean coal has

been sized and washed, and waste material (refuse) separated and removed. Generally, raw coal contains larger particles than does clean coal. 

The need for surge piles is based on the disjunction between mining schedules and washing plant feed schedules.

In addition, the dispatch of coal to customers does not

necessarily match the continuous output from the washing

plant. Mines with large, clean coal stockpiles can better

meet coal sale orders without requiring that their customers

have facilities for coal storage. Thus, a surge pile or some

other type of storage is required to stockpile the coal. 

Surge piles at four coal mines—three in West Virginia

and one in Illinois—were visited. These surge pile sites are

typical of what is used throughout the United States. All

four sites had common components and varied only in

lateral dimensions and height. These basic components are

an overhead conveyor, a stacker tube, a conical pile of coal,

an underlying tunnel with a conveyor and multiple feeders,

and dozers to push coal on the surface of the pile. 

The overhead conveyor is used to move the coal to the

top of the stacker tube. The first portion of the conveyor

may be inclined to attain stacker height while the remainder

of the conveyor is horizontal from the surge pile perimeter

to the stacker tube. The stacker tube is a concrete hollow

cylinder with windows strategically placed throughout its

height. When coal falls into the stacker tube from the conveyor, the tube fills to the height of the external pile and

spills out the windows. The stacker tube acts as a support

for the conveyor and controls flow onto the pile via gravity

through the windows. The capacity of the surge pile is

limited by the height of the stacker tube and the angle of

repose of the cone of coal. Furthermore, because of the

spacing of the underlying feeders, live capacity is less.

Dozers are used to push coal away from the stacker

cone area, creating an extended bench that ultimately

increases the capacity of the pile and is limited only by

available area and the feasibility of pushing coal long distances. When coal is taken from a surge pile, dozers are

used to push coal back to the feeders within the stacker cone

area. Not all feeders operate at once when discharging, but

feeders are chosen selectively in conjunction with the

dozing operation. One or more dozers may be used to push
coal to the active feeders. Figure 1 illustrates the components of a surge pile. 

MSHA has published a safety manual (MSHA, 1993)

for stockpiling that includes a section on proper dozer operation. MSHA recommends keeping the dozer a safe distance

from the feeder. When coal is pushed to the feeder draw

hole, one load should be brought close to the hole, then that

load should be bumped into the feeder with a second load.

Fatal Surge Pile Accidents

Fatal accidents have occurred on surge piles when a dozer

was driven over a hidden void and fell into it, burying and

suffocating the operator. MSHA has reported 17 fatalities

directly related to a collapsed void that occurred from 1980

through 1998 (MSHA, 2000). Of these, 10 were dozer

operators, one a tractor operator, and six were persons on

foot. One dozer-related surge pile fatality occurred in 1999.

The three most recent coal surge pile fatality reports

were reviewed for the current work. These three fatalities

occurred in 1995, 1998, and 1999. Circumstances of each

accident were slightly different, but in all three accidents, a

dozer was operating directly over or near the feeder, the

stacker tube was feeding onto the pile, the pile heights were

all equal to or greater than 40 ft, and all dozers were

apparently pushing coal away from the stacker tube. Differences in feeder operation at the time of the accidents were

(1) the feeders were on, (2) gravity allowed the coal to be

fed to an operating belt from a nonenergized vibratory

feeder, and (3) the sliding gate gravity feeder and belt were

turned off. Overhead feeder markers were used at two of

the three mines. Two of the three surge piles were clean

coal; the third was raw coal. In two of the three fatalities,

the dozer orientation was blade-up in the void, while in the

third, the dozer was oriented sideways. 

Communication between dozer operator(s) and the control room operating the belts and feeders is important. Lack

of communication may have been the cause for two of these

accidents. The third accident, however, resulted even

though the underlying belt and feeders were off. The void

had formed 10 days earlier, but had been undetected. Figure

1 shows a scenario in which the dozer operator is positioned

directly over an unsuspected void. 

Currently, MSHA, NIOSH, and industry are focusing

on reducing or eliminating coal surge piles accidents and

fatalities. MSHA has tested strengthened glass to prevent

the dozer cab from filling with coal in the event the dozer

fell into a collapsed void, and West Virginia has recently

passed a law requiring the use of strengthened glass in

dozers operating on coal surge piles. According to MSHA,

two mines are investigating the use of remote-controlled

dozers to remove operators from the hazardous area

(Fredland, personal communication, 1999). One remotecontrolled dozer is currently operational. NIOSH is

supporting research in the detection of voids in coal surge

piles using geophysical methods. NIOSH is also developing

a global positioning system (GPS) with an onboard visual

display to track and monitor dozer position with respect to

feeder locations, which would provide real-time positioning

information to a dozer operator.

FAULT TREE ANALYSIS OF DOZER BURIAL IN

COAL SURGE PILES

The fault tree is constructed by first identifying the top

fault event, which, in this case, is a dozer falling into a void

on a coal surge pile. A secondary event (A) that contributes

directly to the top fault event occurs when the dozer operator positions the dozer directly over the hazardous feeder

zone. The only other secondary event (B) required to trigger

the top fault event is the formation of a void within the coal

pile between the feeder and the surface. These two secondary events are further broken down to determine the root

causes. Figure 2 shows the completed fault tree for a dozer

falling into a void on a coal surge pile.

In secondary event A, where the dozer is driven directly

over or near the feeder, the question arises as to why the

operator put him- or herself in this hazardous position.

Either the operator has unintentionally driven over the feeder or feels confident that no void exists at the feeder. For

the former, six reasons were proposed: poor visibility,

inexperience, inadequate training, fatigue, distractions, and

inadequate feeder markers. In this fault tree, only the poor

visibility event was further explored. Six reasons were then

developed for the poor visibility response: that the dozer

was being driven at night, the cab windows were dirty, the

sun caused a glare on the windows, the cab structure

obstructed vision, weather conditions (such as rain, snow, or

blowing dust) were poor, and steam was rising from the pile.

If the operator had intentionally positioned the dozer

over the feeder, confident that no void existed but putting

him- or herself at risk, the only feasible reason was to save

time by taking a more direct route during dozing. Three

types of actions that favor a direct route are driving over or

backing up to the hazardous feeder zone while pushing coal

away from the feeder area when expanding the pile, pushing

coal to an active feeder but passing over an adjacent feeder

or backing up to the hazardous zone, and passing over a

feeder when relocating and moving onto or off the pile. 

In secondary event (B), a void is formed over a feeder

when there is subsurface flow at the same time there is no

surface flow. Subsurface flow or flow from the feeder

happens normally when the conveyor is on and the feeder

allows flow. If the feeder is energized and the belt is on,

flow will occur. Flow may also occur when the feeder is

turned off if there are changes in coal properties, such as the

angle of repose of the coal. (A safety gate may be installed

to prevent such flow. However, an additional condition may

be set up if a safety gate is installed but is open.)
Figure 2. Fault tree analysis
A contradiction is faced in the question “How can

surface flow not occur when subsurface flow occurs?” A

basic understanding of flow in a surge pile is important for

understanding why surface flow does not take place even

when there is subsurface flow. The “funnel flow” concept

(Jenike et al. 1959) describes the condition in a pile when

coal is drawn from an underlying feeder. A typical opening

dimension of 5 by 5 ft at the base of a pile will only allow

vertical movement in a column of coal having the same

dimensions. As the flow column reaches the surface, a void

or hole will form. The upper sides of the hole will fail at the

angle of repose as the column is drawn down. However, if

the upper layer of coal is held together by a cohesive force

acting between coal particles, then the strength of this layer

may prevent surface flow, and a void will form below the

surface to a depth comparable to the amount of coal drawn

from the feeder. 

In the fault tree analysis, two cohesive conditions were

proposed: simple compaction and binding of coal particles

by freezing water. In order for freezing water to bind the

coal together, a water source is needed. Because a coal

surge pile is open to the elements, rain and/or snow will provide that source. In clean coal piles, moisture will also be

left over from the cleaning operation. During fall, winter,

and spring, low temperatures may result in freezing water

binding the coal particles so no surface flow can take place,

even though above-freezing temperatures are present in the

coal below the surface. Time is also a factor, and an inactive pile will be more prone to surface freezing.

Compaction of coal near the surface of the pile can also

prevent surface flow. Compaction requires that a force be

applied over an area of the coal. The degree of compaction

will vary depending on water content. If no moisture is

present, then the likelihood of cohesive strength and therefore compaction diminishes. As discussed above, moisture

may come from rain or snow or from the cleaning plant.

The force applied to the coal at the surface over the feeder

is most likely the result of the weight and vibration of the

dozer. A less likely source is coal overburden pressure.

According to the MSHA accident report for the fatality in

November of 1998, compacted layers were observed overhanging less-compacted layers below in the void (Harding,

1998). Here again, the dozer must be positioned over the

feeder to compact the coal.

Quantification of the Fault Tree

The qualitative construction of the fault tree shows the interdependence of events. It does not, however, depict the

amount of influence the basic events have on the top fault

event. A quantified fault tree does show the influence of a

basic event on the top fault event and ranks the basic events

in terms of this influence. The practicality of a fault tree

approach becomes apparent in such a construction. A quantified fault tree is a strategy, a plan of action, for it shows

which events have the most influence on the occurrence of

the top fault event and therefore which events should be

addressed first in any type of efficient and effective remedial

action. A quantified fault tree analysis can show where to

act first to generate the most results for the least amount of

work.

The first step in quantifying a fault tree is to assign initial probabilities to the basic events. This step was taken by

gathering information from a focus group familiar with coal

surge piles. The group was given the graphic of the fault

tree and then asked to assign qualitative ratings for the probabilities of occurrence of the basic events using their experience and best judgment. Table 1 was used as a guide for

collecting this qualitative evaluation. 

Once the focus group agreed on the qualitative evaluation of the probability of a basic event (for example, a "low"

probability), the corresponding numerical value of 10 from -4

Table 1 was assigned to “low” in the fault tree. This

approach was chosen over a detailed questionnaire given to

plant operators because there was little time to construct,

send out, and evaluate a comprehensive survey. What is important here is not the absolute probability values, but the

relative values. If a consistency in the assignment of values

can be maintained, then all that is needed to evaluate the

fault tree was to identify the events that have the most influence on the occurrence of the top fault event relative to

the other events in the fault tree.

Table 1. Classes for probability of occurrence (Kirsten

1999)

Qualitative evaluation Quantitative evaluation

Certain Every time 1.0

Very high 1 in ten 10-1

High 1 in a hundred 10-2

Moderate 1 in a thousand 10-3

Low 1 in ten thousand 10-4

Very low 1 in a hundred thousand 10-5

Extremely low 1 in a million 10-6

Practically zero 1 in ten million 10-7

These initial probabilities were mathematically propagated through the fault tree. For comparison, two fault tree

analysis computer programs, SAPHIRE (Russel, 1997) and

Faltrese (Wilcox, 1996), were each used to generate the

probabilities for each cut set and the top fault event.

Mathematical measures of importance and sensitivity for

basic events, probability, and importance for cut sets were

also computed.

The importance measure was computed for both basic

events and for cut sets. The importance of an event E is the

ratio of the sum of the probabilities of the cut sets containing

E to the probability of the top fault event T. Intuitively, this

means how much influence the event has in affecting the
occurrence of the top fault event. The formula is—

Importance of event E = (3 probability of cut sets

containing event E)/Probability of top fault event

T).

The measure of importance for cut sets is the ratio of

the probability of occurrence of the cut set to the probability

of occurrence of the top fault event. The formula is—

Importance of cut set C = (Probability of cut set

C)/(Probability of top fault event T).

The sensitivity measure is used for only initial events

and measures the amount of change in the top fault event

given a set amount of change in the basic event. This measure is the percentage of change in the top fault event divided

by a given percentage of change in the basic event of interest. In these computations, a change in the assigned probability of 10% to the basic probability was used. Whatever

number is used, however, is arbitrary, since the results are

relative comparisons. The formula is—

S = E T T E E (Î(P ) / P ) / (Î(P ) / P ),

wh Eere S = sensitivity of event E,

PE = probability of event E,

T

and P = probability of top fault event T.

Table 2 lists the 28 identified basic events along with

their assigned probabilities of occurrence, their computed

importance, and their computed sensitivity measures. Table

3 lists the top 36 cut sets ordered according to their probability of occurrence, the percentage of their contribution to

the top fault event, and cumulative percentage. Fourhundred-eighty cut sets were determined from the fault tree.

Of these cuts sets, 408 composed five basic events, and 72

composed six basic events. The overall probability of the

main event was 4.0E-08 or a practically zero chance of

happening at one surge pile over a period of 1 year If the

number of surge piles (337) in the United States is considered and a sum of probabilities is produced for all surge

piles, the value is 1.3E-05, or very low.

COMPARISON OF FAULT TREE ANALYSIS TO

CURRENT RESEARCH

The applicability of using fault tree analysis can be determined further by comparing current research on surge piles

to results from the fault tree analysis. Solutions to the surge

pile problem are being worked on by NIOSH, MSHA, and

industry, as mentioned earlier. Although most of the important cut sets determined by fault tree analysis are not being

investigated, the use of GPS is one exception. GPS fits well

into fault tree analysis and is being studied as a means of

preventing operators from positioning dozers over a potential void. Using GPS positions with respect to feeders provides warnings in real time, as well as position data for

safety analysis and training. Other topics that might be

investigated using fault tree analysis are improved feeder

markers, prevention of coal compaction, and dozer operating

procedures when pushing coal.

CONCLUSIONS

Fault tree analysis is a useful technique to define root causes

of a fault event such as a dozer falling into a void on a coal

surge pile. The quantitative analysis made possible the

determination of the most influential root causes of this

event. Even though the assigned probabilities for each root

cause may be somewhat subjective, the outcome provides

insight into which cut sets or combination of events are

more likely to cause the top fault event. Comparison of

current research to the results of a fault tree analysis indicates new topics for research that are of high importance. 

REFERENCES

Fredland, John. 1999. Personal communication.

Harding, Benjamin S. 1998. Accident Investigation

Report Surface Area-Underground Coal Mine, Fatal Machinery, Buchanan Mine No. 1, Consolidation Coal Company, Mavisdale, Buchanan County, Virginia, November

22, 1998. ID No. 44-04856. 6 pp.

Jenike, A. W., P. J. Elsey, and R. H. Woolley. 1958.

Flow of Bulk Solids Progress Report (Rev.). Salt Lake City,

UT: Univ. of Utah, Bull. 96, Vol. 49, No. 24, Dec., 33 pp.

Kirsten, H. 1999. Workshop on Evaluation of Risk as

Decision Making with the Criterion, Denver, CO. 

Lambert, H. E. 1973. Systems Safety Analysis and

Fault Tree Analysis. Livermore, CA: Lawrence Livermore

Laboratory, Univ. of California, UCID-16238.

Mine Safety and Health Administration (MSHA).

2000. Program Information Bulletin 99-01, Attachment,

Http://www.msha.go/regs/complaian/pib/1999/pib991a.htm,

9/22/2000.

____ 1993 (reprint). Stockpiling Safety. Safety

Manual No. 30, 58 pp.

Russel, K. D. 1997. SAPHIRE, Version 6.13, Beta

Version for Windows 95. Idaho Falls, ID: Idaho National

Engineering and Environmental Laboratory.

Wilcox, Gregory C. 1996. Faultrease, Version 2.0.

Cambridge: Arthur D. Little.
Table 2. Identified basic events listed by computed importance along with assigned probabilities of occurrence and computed

sensitivity measures.

Basic event Computed

importance

Assigned

probability

Qualitative

probability

Computed

sensitivity

Dozer weight over feeder 1.16859E+00 1.00E-03 Moderate 9.98990E-01

Feeder on and flow occurs 1.15808E+00 1.00E-01 Very high 9.89990E-01

Processing water 5.84890E-01 1.00E-01 Very high 4.73117E-06

Processing water 2 5.84890E-01 1.00E-01 Very high 4.73680E-01

Precipitation 5.84880E-01 1.00E-01 Very high 4.73680E-01

Conveyor operated earlier for other feeders 3.89920E-01 1.00E-01 Very high 2.98890E-01

Conveyor operating for feeder under void 3.89920E-01 1.00E-01 Very high 2.98890E-01

Conveyor running for other feeders 3.89920E-01 1.00E-01 Very high 2.98890E-01

Cab structure obstructs vision 1.76700E-01 1.00E-03 Moderate 1.50640E-01

Dirty cab windows 1.76700E-01 1.00E-03 Moderate 1.50640E-01

Night time 1.76700E-01 1.00E-03 Moderate 1.50640E-01

Poor weather conditions 1.76700E-01 1.00E-03 Moderate 1.50640E-01

Rising steam 1.76700E-01 1.00E-03 Moderate 1.50640E-01

Sun glare 1.76700E-01 1.00E-03 Moderate 1.50640E-01

Inadequate training 1.76703E-02 1.00E-04 Low 1.50504E-02

Pushing coal away from feeder area 1.76703E-02 1.00E-04 Low 2.98890E-01

Pushing coal over feeder to adjacent feeder 1.76703E-02 1.00E-04 Low 1.50504E-02

Relocating on pile passing over feeder 1.76703E-02 1.00E-04 Low 1.50504E-02

No gate installed 1.15808E-02 1.00E-03 Moderate 8.91884E-03

Distracted operator 1.76703E-03 1.00E-05 Very low 1.50490E-03

Tired operator 1.76703E-03 1.00E-05 Very low 1.50490E-03

Overburden pressure 1.16859E-03 1.00E-06 Practically zero 9.97993E-04

Fatigued operator 1.76703E-03 1.00E-05 Very low 1.50490E-03

Feeder gate open 1.15808E-04 1.00E-05 Very low 8.91001E-05

Feeder gate open 1.15808E-04 1.00E-05 Very low 8.91001E-05

Pile inactivity 1.16859E-05 1.00E-05 Very low 9.98802E-06

Temperature of coal pile <32° 1.16859E-05 1.00E-03 Moderate 9.98802E-06

High precipitation 4.99496E-06 1.00E-01 Very high 4.73117E-06
Table 3. Thirty-six of the highest probability (1.0E-09) cut sets and their cumulative percentages. Percentage of individual

cut sets is 2.5%.

Cut no. Cumulative

percentage

Cut sets

1 2.5 Conveyor running feeder, dozer over feeder, feeder on and flow occurs, precipitation2, rising steam

2 5.0 Conveyor running for other, dozer over feeder, feeder on and flow occurs, precipitation2, rising steam

3 7.5 Conveyor running feeder, dozer over feeder, feeder on and flow occurs, processing water2, rising

steam

4 10.0 Conveyor operated earlier, dozer over feeder, feeder on and flow occurs, precipitation2, rising steam

5 12.5 Conveyor running for other, dozer over feeder, feeder on and flow occurs, processing water2, rising

steam

6 15.0 Conveyor running feeder, dozer over feeder, feeder on and flow occurs, night time, precipitation2

7 17.5 Conveyor operated earlier, dozer over feeder, feeder on and flow occurs, processing water2, rising

steam

8 20.0 Conveyor running feeder, dozer over feeder, feeder on and flow occurs, precipitation2, sun glare

9 22.5 Conveyor running for other, dozer over feeder, feeder on and flow occurs, night time, precipitation2

10 25.0 Conveyor running feeder, dirty cab windows, dozer over feeder, feeder on and flow occurs

11 27.4 Cab obstructs vision, conveyor running feeder, dozer over feeder, feeder on and flow occurs

12 29.9 Conveyor running for other, dozer over feeder, feeder on and flow occurs, precipitation2, sun glare

13 32.4 Conveyor running feeder, dozer over feeder, feeder on and flow occurs, night time, processing water2

14 34.9 Conveyor operated earlier, dozer over feeder, feeder on and flow occurs, night time, precipitation2

15 37.4 Conveyor running for other, dirty cab windows, dozer over feeder, feeder on and flow occurs

16 39.9 Cab obstructs vision, conveyor running for other, dozer over feeder, feeder on and flow occurs

17 42.4 Conveyor running feeder, dozer over feeder, feeder on & flow occurs, precipitation2, weather

18 44.9 Conveyor running feeder, dozer over feeder, feeder on & flow occurs, processing water2, sun glare

19 47.4 Conveyor operated earlier, dozer over feeder, feeder on & flow occurs, precipitation2, sun glare

20 49.9 Conveyor running for other, dozer over feeder, feeder on & flow occurs, night time, processing water2

21 52.4 Conveyor running feeder, dirty cab windows, dozer over feeder, feeder on and flow occurs

22 54.9 Cab obstructs vision, conveyor running feeder, dozer over feeder, feeder on and flow occurs

23 57.4 Conveyor operated earlier, dirty cab windows, dozer over feeder, feeder on and flow occurs

24 59.9 Cab obstructs vision, conveyor operated earlier, dozer over feeder, feeder on and flow occurs

25 62.4 Conveyor running for other, dozer over feeder, feeder on and flow occurs, precipitation2, weather

26 64.9 Conveyor running for other, dozer over feeder, feeder on and flow occurs, processing water2, sun glare

27 67.4 Conveyor operated earlier, dozer over feeder, feeder on and flow occurs, night time, processing water2

28 69.9 Conveyor running for other, dirty cab windows, dozer over feeder, feeder on and flow occurs

29 72.4 Cab obstructs vision, conveyor running for other, dozer over feeder, feeder on and flow occurs

30 74.9 Conveyor running feeder, dozer over feeder, feeder on and flow occurs, processing water2, weather

31 77.3 Conveyor operated earlier, dozer over feeder, feeder on and flow occurs, precipitation2, weather

32 79.8 Conveyor operated earlier, dozer over feeder, feeder on and flow occurs, processing water, sun glare

33 82.3 Conveyor operated earlier, dirty cab windows, dozer over feeder, feeder on and flow occurs, processing

water2

34 84.8 Cab obstructs vision, conveyor operated earlier, dozer over feeder, feeder on and flow occurs,

processing water2

35 87.3 Conveyor running for other, dozer over feeder, feeder on and flow occurs, processing water2, weather

36 89.8 Conveyor operated earlier, dozer over feeder, feeder on and flow occurs, precipitation2, weather
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