GUIDELINES FOR ROOF BOLT DESIGN

Although it was not possible to develop a universal design equation with this statistical study of
roof falls , some other relationships were established and found to be of value. It was not surprising
that the geology, represented by the CMRR, was the most important variable. Although
Statistically, the effect of the depth of cover were relatively weak, with all else equal, deeper mines
were more likely to have high roof fall rates. Horizontal stress could not be measured directly, but
since it is known that the intensity of horizontal stress tends to increase with depth, the inference is
that the depth of cover is a surrogate for the stress level. When the data were separated into a
shallow cover group (<120 m) and a deeper cover group (>120 m), bolt design equations were
determined for each. Based on these alternative relationships, a set of preliminary guidelines and

limited design equations were developed from this investigation.

Following are step-by-step guidelines:

1. Evaluate the geology. The CMRR should be determined either through underground
observation or from exploratory drill core. Zones of markedly different CMRR should be
delineated. If the thickness of individual beds varies within the bolted horizon, this effect should be
noted. Special features, such as faults or major geologic transition zones, should be treated

Separately.

2. Evaluate the stress level. It is unusual for stress measurements to be available, so the design
procedures use the depth of cover as a rough estimator. However, horizontal stress can sometimes
be intensified by stream valleys or by driving in an unfavorable orientation. Roof support may need

to be increased in these areas.

3. Evaluate mining-induced stress. Vertical, and sometimes horizontal, stresses may also be
intensified by retreat mining or multiple seam interactions. These areas are likely to require

supplemental support.


9 T T T T T T T T T T

8 o. xn. Room and pillar BL
7h 1 a th _-f--n |
ao-O.
ra “df
Y= -0.0445x +7.31
5} R’= 0.0892 4

3b oa

INCIDENT RATE, Number! Million Tons
>

ak Y= -0.1748 +3.22 SO 4
R’= 0.7711 iy emery
th Longwall , 4
0 . i 1 1 1 L lL L 1 a
1988 1989. 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999
YEAR

Figure 2: Incident rate for reportable roof falls based on production from 1988 to 1999 for
both longwall and room and pillar operations, including a regression analysis.

For room and pillar mining, the roof fall rate trend from 1989 to 1998 can be evaluated by fitting a linear
regression to the data. The results indicate that there has been no change in the roof fall rate for the last
10 years in room and pillar mines: For 1975 and 1976, the roof fall rate per million tons was 5.7 and 6.8,
respectively.

For longwall mining, there was a decrease of about 1.6 roof falls per million tons or a reduction of about
50 pct in the roof fall rate between 1988 and 1998. However, other factors such as increased face width,
increased seam height and a decrease in the number of gateroad entries have caused much of this change
by reducing the amount of development mining [3], (9]. In 1990 about 26 pct of the production from a
longwall mine was from development [1]. In 1999, an estimated 15 to 18 pct of production came from
development. Essentially, any change the roof fall rate can be accounted by the changes in these

parameters.


REFERENCES

10.
11.
12.

BHATT, S.K., 1994: Longwall and Room-and-Pillar Productivity: A Review of U.S. Coal
Mines, IC 9375, 6 pp. -

DOLINAR, D.R., BHATT, S.K., 2000: Trends in Roof Bolt Application, Proceedings of the
New Technology for Coal Mine Roof Support, IC 9453, pp. 43-51.

FISCOR, S., 1999: U.S. Longwall Census ‘99, Coal Age, February, pp. 30-35.

KARABIN, G. J., DEBEVEC, W. J., 1976: Comparative evaluation of Conventional and
Resin Bolting Systems, MSHA, IR 1033, 22 pp.

MARK, C., 2000: Design of Roof Bolt Systems, Proceeding of the New Technology in Coal
Mine Roof Support, IC 9453, pp. 111-131.

MARK C., MUCHO, T. P., 1994: Design of Longwall Gate Entry Systems Using Roof
Classification, U.S. Department of the Interior, Bureau of Mines, Technology Transfer
Seminar, Special Publication 01-94, pp. 53-76.

MOLINDA G, MARK C, DOLINAR D [2000] Assessing Coal Mine Roof Stability Through
Roof Fall Analysis, Proceedings of the New Technology for Coal Mine Roof Support, IC 9453,
pp. 53-71. ;

MOLINDA, G.M., MARK, C., 1994: Coal Mine Roof Rating (CMRR): A Practical Rock
Mass Classification for Coal Mines, U.S. Department of the Interior, Bureau of Mines, IC
9387.

MERRITT, P. C., 1991: As the Time Changes So Do Longwalls, Coal, February, Pp. 40-49,
MSHA: 1991: Roof Bolt Survey.

SCOTT, J. J., 1989: Roof Bolting: A Sophisticated Art. Coal, August.

UNAL, E., 1984: Empirical Approach to Calculate Rock Loads in Coal Mine Roadways,
Proceedings of the 5" Ground Control Conference in Mining, pp. 232-246.


Geotechnical variable CMRR Bolt length Tension Grout ‘Capacity, Bolts Row
selected, ft index index kips . per spacing,

row ft

CMRR 1.000 -.329 -.020 ss -,081 166 037.147
Bolt length selected, ft '.329 1.000 251 -.059 134-116 038
Tension index 020.251 1.000.811 326-161 070
Grout index 081-059 811 1.000 196 = 131 012
Capacity, kips 166.134.1326 196 = 1.000 -.178—-.084
Bolts per row 037-116-162 131 -178 1.000.000
Row spacing, ft -147 038 = 070s -012——s« 084 = 000 1.000
Entry width, ft 089.215 107,13 086 = 167.041
Density --134 161 21-169 1907-076 -.375
PRSUP, -282 738-356-176 686-132 -.232
Intersection span, ft "233-067-204. 124 043.042 -.089
Overburden index 2249 -.039 -.188 127 "234-097 -.248
4-ways rate -257 091 217. -.192 442.079 _—_—182
Geotechnical variable Entry Density PRSUP,, Intersection Overburden 4-ways Roof fall

width, ft span, ft index fate _rate
CMRR 089-134-282, "233 "249 = -.257— -.215
Bolt length selected, ft -.215 161 738-067 -039 091.105
Tension index 107.271 356-204 -188 217.187
Grout index 123 -.169 -.176 124 127 -192  -.044
Capacity, kips 086 '907 686 043 "234442322
Bolts per row 167-076 = -.132 042. -097 079031
Row spacing, ft 041 +375 -.232 -.089 -.248 = 182.075
Entry width, ft 1.000 -.172)-.245 1359 124.063 -.011
Density -172 1.000 = .767_— «016 2248 341.294
PRSUP,, -245 767, ‘1.000 049 154 4.273.266
Intersection span, ft 359-016-049 1.000» 1600 = .060-.053
Overburden index "1247248154 600 1.000 089.004 -
4-ways rate 063 341 273 060 089 = 1.000 ‘661
Roof fall rate -215 105-187-044 322031075

"Correlation is significant at the 0.01 level (2-tailed).
Correlation is significant at the 0.05 level (2-tailed).

Table 2: Correlation between geotechnical variables in the study


41 Mines
2,500 km of Drivage

Figure 4: Location on mines in study (41 mines and 2,500 km of drivage).

In the study, geotechnical parameters that were evaluated and quantified included the immediate roof
geology and the depth of cover. The roof geology was quantified by using the Coal Mine Roof Rating
(CMRR). [8]. Mine design parameters that were collected included the entry width and the intersection

Span. The intersection span was calculated as the sum of the two intersection diagonals.

For the primary support, six bolt variables were quantified that included the bolt capacity (yield),
bolt length, applied tension, grout column length, number of bolts per row and row spacing. A summary

support variable was calculated from these parameters as a measure of the roof bolt density:

* *
prsup. -Lb"Nb*C a
" Sb*We )
where Lb = Length of the bolt (m),
Nb = Number of bolts per row,
C = Capacity (kN),

Sb = Spacing between rows of bolts (m),
We = Entry width (m).


NATIONAL ROOF FALL DATABASE

To evaluate the performance of the primary roof support systems a national roof fall database was created
from data obtained from visits to a number of U.S. coal mines [7]. Study mines were selected by
computing the roof fall rate from the MSHA accident database. Drivage was estimated by converting
annual production (excluding longwall production) into linear feet of advance, assuming an average seam
height. Reportable roof falls were then divided by drivage to arrive at the roof fall rate (figure 3). Mines
were then selected for study from this distribution to represent the entire range of roof stability from high,

medium, to low roof fall rates. Mines were also selected to represent a wide range of roof geologies.

Through extensive interviews with mine operators and underground reconnaissance, NIOSH collected
and quantified information on the number of roof falls, roof geology, depth, primary support parameters
and opening geometry during each mine visit. Ultimately 41 mines in 10 states were visited, representing

over 2,500 km of drivage in most of the major coal basins where underground mining occurs (figure 4).

At each of the mines, one or more “case histories” was collected. A case history was a portion of the
mine that could be defined by a number of descriptive parameters and an outcome parameter. The
database ultimately included information from 37 of the 41 mines, but actually contained 109 “‘cases.”
The outcome parameter was based on the number of reportable roof falls that occurred above the

anchorage of the primary support in that portion of the mine.

600.

500

5

200

Number of Mines
$

100

0 03 0.6 09 12 P12
Roof Fall Rate (Falls/ 3,000 m)

Figure 3: Distribution of roof fall rates in U. S. coal Mines.


bolt capacity shows the strongest correlation to the roof fall rate. However, the relationship between bolt
capacity and the roof fall rate is positive where the bolt capacity went up with the roof fall rate. This may
be the result of higher capacity bolts being used when roof conditions deteriorate (increased roof fall
rates) but the relationship is not useful in a design equation. The CMRR shows the highest correlation
with the correct expected relationship (negative) to the roof fall rate. The lack of significant correlation
between the roof fall rates and a number of the other factors results in a low overall fit to the data.

As expected there is correlation between several bolt parameters such as tension and grout indices,
capacity and density, and bolt tension and capacity (table 2). Intersection span and entry width are
naturally related. The CMRR and the bolt length are also related. As the roof gets stronger (higher
CMRR), operators install shorter bolts. These types of intercorrelations of variables further confound the

overall effect of any one variable on the roof fall rate.

Statistically, in the database, the correlation between the depth and fall rate is relatively weak though
positive. However, it seems likely that in the data, depth of cover is an indirect measure, or surrogate, for
horizontal stress level. Other studies show significant evidence of increasing horizontal stress with depth
[6]. Horizontal stresses are seldom measured directly because of the difficulty and expense. Using this
assumption, the case histories were divided into two groupings by depth of cover. The relationship
between CMRR and PRSUP,, is shown in figure 6a for shallow depth (less than 120 m) and in figure 6b
for deeper cover (greater than 120 m). The original discriminate line that can be used to classify the data
as well as a design equation line are shown on both figures. The discriminate line mis-classifies about 45
pet of the high fall rates for the shallow cover and about 30 pct of the high fall rates for the deeper cover.
With the moré conservative design equation line, the misclassification of the high fall rates is greatly

reduced. The equation for this design line is presented later in the paper.


Design of primary roof support systems in US coal

mines based on the analysis of roof fall rates

D. Dolinar, C. Mark, G. Molinda,
NIOSH Pittsburgh Research Laboratory, USA

ABSTRACT

Each year, about 2,000 falls of supported roof occur in the more than 800 underground U.S. coal mines.
Therefore, to help improve the design of primary support systems, the National Institute for
Occupational Safety and Health (NIOSH) conducted a nationwide study of roof falls in over 2,500 km of
roadways at 37 coal mines. Information on the roof falls as well as other geotechnical, mining, geologic
and roof bolting factors were collected and quantified. Then a multi variate statistical analysis was
conducted on the data with the roof fall rate being the outcome variable. The results were used to derive
Preliminary design guidelines for predicting the roof bolt length, capacity, and pattern required to
effectively reduce the roof fall rate. The equations are fairly limited by a relatively high statistical
variance in the data. Also reported is a recent survey of U.S. roof bolt manufacturers that shows a
Significant change in bolt types used over the last ten years.

ZUSAMMENFASSUNG

Jedes Jahr erfolgen itber 2000 Steinfille in den mehr als 800 Kohlenbergwerken der USA. Um die
Gestaltung des ersten Ausbausystems zu verbessern, hat daher das National Institute for Occupational
Safety and Health (NIOSH) eine landesweite Untersuchung auf 37 Bergwerken mit mehr als 2500 kan
Strecke zum Steinfall durchgefiihrt. Dabei sind Informationen zu Steinfall, zur Geotechnik, Bergtechnik.
Geologie und Ankertechnikfaktoren gesammelt und quantifiziert worden. Danach wurde eine statistische
Multivarianzanalyse mit diesen Daten durchgefiihrt, die die Steinfallraten als Resultat ergaben. Die
Ergebnisse wurden zur Ableitung vorlaufiger Gestaltungsrichtlinien fur die Bestimmung der Ankerlange,
Tragfahigheit und des erforderlichen bchemas genutzt, um wirksam die Steinfallrate zu senken. Aufgrund
der relativ hohen statistischen Abweichungen in den Daten sind die Formeln nur begrenzt anzuwenden.


true even in the high roof fall rate range. So, through a range of CMRR (30-58), an increase of at
least one foot in bolt length can be expected to result in a decrease in the roof fall rate,

a

w

Roof Fall Rate(Falis/3,000 m)
s

Bolt Length
BB shen
0 Gi
$23 45678 9 0 2B
Case Number

Figure7: Paired cases of long and short bolts showing benefit of long bolts and decreasing
roof fall rates.

The relationship between CMRR and intersection span was also analyzed. The data was partitioned
by 4-way intersection fall rate into low, moderate and high. Additionally, only fully grouted bolts
were used in the analysis. By logistic regression a line (solid) was fitted to the data and presented in
figure 8. There are no cases with high roof fall rates which fall below the regression line. Based on
the data there is also a likelihood that intersection falls will be reduced by a decrease in intersection
span. Also shown on the figure is a less conservative regression line (dashed) that might be an

appropriate first approximation of the intersection span.


A a) © No Roof Falis
Ty ; © Few Roof Fails
65 4 Many Roof Falls
a ; oO
s mg 0
a A Design
Equation
a Op,
87 116 ar? 3

145 . 203
PRSUP, kKN/m

A shallow cover (depth < 120m)
B, deep cover (depth > 120m)
Design equations for selecting bolt pattern and capacity (PRSUP ,») is a solid line and the original

discriminate line used to fit the data is a dashed line.
Figure 6: Relationship between PRSUP ,,, CMRR and roof fall rate.

An alternative to the multivariate statistical analysis was to conduct an analysis using “paired data”

from individual mines. In these cases, only a single variable changes.

The most successful of these analyses was on roof bolt length. From the large data set, 13 pairs of
_ data where two different lengths of roof bolts were used at the same mine were extracted. The roof
bolt lengths differed by at least 0.3 m in the pairs. The data show that in 11 of 13 cases, the roof fall
fate was less with the longer bolt (figure. 7). The roof fall rates for the paired data range from 0.0 to

18.3/3,000 and the decrease in roof fall rate (avg. 65 pct, n=13) with increasing bolt length holds


‘The roof fall rates were calculated as the outcome variable in each case. This rate was calculated based on
the total number of roof falls divided by the total drivage for a given case. A 4 way intersection fall rate
was also calculated based on the number of falls in the 4-way intersections divided by the total number of
4-way intersections. Figure 5 shows the roof fall rate distribution by number of cases for the study.

70

60

Number of Cases
8 8 8 8

=
o

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32
Roof Fall Rate (Falls/3,000)

Figure 5: —_ Distribution of roof fall rates in the study sample

STATISTICAL ANALYSIS

One of the goals of the study was to determine if there was a universal design equation which would
utilize all or some of the geotechnical variables to predict the roof fall rate. A multivariate linear
Tegression was performed which included all the significant geotechnical variables, including
overburden, bolt length, grout length, density, entry width, CMRR, intersection span, tension, and bolt
capacity (yield). The resultant regression equations at best could explain only 29.9 pct of the variation of
the 4-way intersection fall rate and much less for the overall roof fall rate. In part this is caused by the
uncontrolled nature of the experiments or cases where for each case a number of factors were varied.

Another issue is the correlation between individual parameters.

Table 2 (Pearson correlation) is a test of the codependence of the geotechnical variables. There is only a
significant correlation to roof fall rates for the CMRR, capacity (PRSUP,,, density) and grout index. The


In 1999, approximately 100 million bolts were used in the U.S. mining industry. Fully grouted resin
rebar (headed) comprise about 80 pct of these bolts. For the grouted rebar, 80 pet were 1.6-cm diameter,
#5 rebar , and nearly all the remaining were a 1.9-cm diameter #6 rebar. The #5 rebar are usually
installed in a 2.5 cm diameter hole. Mechanical anchor bolts comprise only about 8 pet of the supports,

Surveys on bolt usage were also conducted in 1988 and 1991 representing all mine types (10}, [11].
Table 1 shows a comparison between the percentage of each bolt type for these years and 1999. An
estimate of the distribution of bolt types for 1976 is also given in the table [4]. For the periods from
1976 to 1991 and again from 1991 to 1999, there has been a substantial shift away from mechanical
anchor bolts to the fully grouted resin rebar.

Of interest is what if any changes in roof stability has occurred with this shift in the type of roof
support over. the last 10 or even 25 years. The number of reportable roof falls that occur each year
can be used to assist in evaluating the overall affects of the change that may have occurred in roof
stability with the changing bolt trend. However, to compare the data for each year, the roof fall rate
based on production must be used. Figure 2 shows the roof fall rate per million tons of coal for the

years from 1989 to 1998 for both longwall and room and pillar mining.

Percentage
Bolt Type 1976 1988 1991 1999
Mechanical Anchor 80 35.3 34.1 77
Fully Grouted 20 40 48.2 83.1
Torque Tension 3.5 4.6 5.6
Point Anchor 14.1 11.51 23
Combination © 3.5 1.3
Friction Stabilizer 2.4
Other 1.2 15
Total 100 100 100 100

Tincludes both torque tension and combination bolts. The two systems were classified as point
anchor tension rebar in the survey.

Table 1: Bolt usage by type in US mines given in percent


4. Determine the intersection span. An equation was derived from the data which suggests that
the appropriate diagonal intersection span (I,) which is the average of the two diagonals is
approximately:

95+(0.2*CMRR))

L-—-———— (2)
2

If the CMRR > 65, it should be set equal to 65 in equation 2. The intersection span can also be

estimated from the entry width using table 3 where the typical spans are based on the field data. As

table 3 shows, the field data indicated that for the same entry width, spans at deep cover (depth >

130 m) exceeded the shallow cover spans by an average of 0.6 m due to pillar sloughing.

Entry width, m Ideal Typical diagonal intersection spans
span, m
Shallow cover, m Deep cover, m
49 710 8.9 95
55 78 95 10.1
6.2 87 98 10.5

NOTE: The "ideal span" is determined by applying the Pythagorean theorem (a”
+b?=c’). "Typical" spans are based on actual measurements (7).

Table 3: Diagonal intersection spans (I,)

5. Determine the bolt length. Where the roof geology is such that the suspension mode is
appropriate, the bolt length should be selected to give adequate anchorage in the strong rock. For
the beam building mode, a bolt length formula was derived by modifying the Unal [12] rock load
height equation. The intersection span was substituted for the entry width, a depth factor was
added, and then the constant was adjusted to fit the data:

(3)

LyA 0.12, log (.25m)| 100 CEMRR |

“where l,= — diagonal intersection span (m)

H= depth of cover (m).


CONCLUSIONS

With the significant change in the type of bolts used in U.S. coal mines, there has been little change
in the roof fall rate that can be linked to this trend for either the room and pillar or longwall mining.
However, there are many other aspects to the design of a roof support system other than the support
type while in many situations the roof falls may have been prevented only with the addition of
supplemental support. Also, this is a general analysis where a number of factors may have changed
including the number of mines, roof condition and accuracy of reporting roof falls though the affects
of the roof support type did not overcome these factors.

Collecting and analyzing the national roof fall database that included information and cases from 37
mines did not lead to the development of a universal design equation for primary roof support
systems in U.S. coal mines. There were a number of factors that confounded the results including
the lack of experimental control where between most cases more than one factor was varied while
only a few of the parameters investigated had a significant impact on the roof fall rate. However
useful relationships were developed regarding the influence of specific factors such as bolt length,
intersection span and mining depth. The mining depth was used as a surrogate for the horizontal

stress.

Based on these specific relationships, preliminary guidelines for the design of primary support
systems were developed. Formulas were presented that may be used to select appropriate
intersection spans, bolt lengths and bolt capacity/patterns. The formulas require a determination of ~
the roof quality (CMRR) and the stress level (depth of cover). The equations though, should be used
with caution because the data used to derive them were highly scattered. However, these
preliminary guidelines provide a start to the development of a more rational approach to roof
support design out of the results of trial and error methods and of mining experience for roof

Support design that is still within the constraints of the U.S. mining industry.


Statistical analysis used in evaluating that database as well as the development of preliminary design

guidelines for primary roof support based on the database.

Another aspect of this investigation was the determination of the support types and usage in the U.S. coal

mines. This information was obtained from a survey U.S. roof bolt manufacturers and is also presented in

this paper.

TRENDS IN U.S. ROOF BOLT USAGE AND ROOF FALL RATES

To evaluate the roof bolt usage and trends in U.S. coal mines, NIOSH recently collected information
from U.S. roof bolt manufacturers [2]. Figure 1 shows the results of this study for 1999. However, the
data is not complete in every instance and the data are for all bolt usage, not just coal mines. Therefore
the results shown in figure 1 should be considered estimates. In the figure, 5 bolt types are indicated:
mechanical anchor, point anchor or speciality, torque tension, combination and the full grouted resin
rebar bolts. Although this figure does not break down the bolt usage by commodity, the coal industry
uses an estimated 80 pct to 85 pct of the reinforcement. Therefore, the percentage of the different types

of bolts used are probably very representative of the distribution of the type of bolts used in coal mines.

Ress Assoc
Methanca:
Aronar
5%

Combnaticn

T aC
loreus Terson 1%

5%
Mach artea: Anchor

8%

Fully Grouted

Figure 1: Percentage of a bolt type used in underground coal mines,
1999


1
Der Vortrag gibt ebenfalls einen Uberblick iiber amerikanische Antkerhersteller, der einen wesentlichen
Wandel der benutzten Ankertypen in den letzten zehn Jahren aufzeigt.

INTRODUCTION

In a typical year, there are approximately 2,000 roof falls reportable to the Mine Safety and Health
Administration (MSHA) that are distributed through about 800 operating underground U.S. coal mines.
Over 95 pct of these falls are reported because they go above the depth of the primary roof bolt
anchorage. These falls impact mining operations but more importantly they create a hazard or a potential
hazard to the underground miners.

This large number of roof falls highlight the present state of roof support systems and the state of the art
in roof support system design that is used in the U.S. In general, there is a lack of an adequate and
accepted support system design for primary roof support systems. The design of primary support systems
is based on trial and error and on previous inining experience. To an extent this has worked but this
method does not provide for a rational approach for improved design and the elimination or reduction of
roof falls. Also, the primary support systems used in the U.S. coal mines can be considered relatively
light compared to that used in some other countries [5]'. This may reflect the generally better ground
control conditions seen in a majority of U.S. mines. However, when more difficult ground conditions are
encountered problems in the form of increased roof instability and roof falls result. Further, changes in
support system design and approaches to the roof support requiring more extensive primary support are
often not acceptable because of the cost and impact on development mining.

Because of the large number of roof falls that occur each year, the National Institute for Occupational
Safety and Health (NISOH ) researchers decided that an empirical approach to the design of primary roof
support systems could be developed for U. S coal mines where the roof falls could be considered as tests
of the roof support system. This empirical approach would be based on a statistical analysis of parameters
that affect the number of roof falls that occur. Because of the wide range of conditions and the large
number of U.S. coal mines, this approach to be successful required a sufficient database as well as the
identification and quantification of parameters relevant to the roof falls. Therefore, a national database on

roof falls and relevant geotechnical parameters was developed. This paper discusses that database and the

“The number in square brackets are related to the reference list at the end of the text.


CMRR VS Sum of Intersection Diagonals

18.24

Sum of Intevection Diagoanls, m

a Ww

Figure 8: Relationship between CMRR, sum of intersection diagonals and roof fall rate.

Intersection diagonals are usually related to the entry width. The data were studied to determine t
typical intersection spans that are encountered underground. Figure 9 shows the mean for t
database of the sum-of-the-diagonals for 4.9, 5.5, and 6.1 m entries. It also shows that in dee}
mines, the sum-of-the-diagonals were 0.9-1.2 m wider than in the shallow mines, with the sa

entry width probably because of greater rib sloughage.

22
=n
S
&
&
5
F 19
Bomb
=
3 17
&
ad
4

4s

49 ss 61
Entry Width (m)

[_ Shallow Cover<120m _guDeeper Cover >120 m )

Figure9: — The effect of overburden on sum of intersection diagonals


6. Determine bolt pattern and capacity: As has already been stated, the data could not
determine which bolt parameter was most important. Therefore, the design variable is PRSUP,,
which includes both, plus the bolt length is given by equation 1.

The suggested value of PRSUP,, (metric) for shallow cover is determined as:

PRSUP,, = 225 - 3.33 CMRR (4a)
and for deeper cover:

PRSUP,, = 258 - 3.33 CMRR (4b)

Figure 6 shows these equations, together with the field data from which they were derived. The
design equations are slightly more conservative than the discriminant equations on which they are
based. The field data also indicated that in very weak roof, it may be difficult to eliminate roof falls
using typical U.S. roof bolt patterns. When the CMRR was <40 at shallow cover and <45-50 at
deeper cover, high roof fall rates could be encountered, even with high roof bolt densities. It should
also be noted that these equations have been derived to reduce the risk of roof falls in intersections.
In some circumstances, it may be possible to reduce the level of support between intersections.”

Finally, the minimum recommended PRSUP,, is approximately 43.5.

7. Select skin support: Plates, header, mats, or mesh should be specified to ensure that loose
rock between the bolts does not pose a hazard.

?
/
é

8. Monitoring: The installation of telltales or other simple extensometers should be considered
for critical intersections so that, if it becomes necessary, supplemental support can be installed in a
timely fashion.