THE COLLIERY GUARDIAN

AND

JOURNAL OF THE COAL AND IRON TRADES.

Vol. CXII.

FRIDAY, JULY 7, 1916.

No. 2897.

Mine Ventilation Stoppings.*

By R. Y. WILLIAMS.

The data given below were obtained through an inves-
tigation conducted under a co-operative agreement
between the United States Bureau of Mines, the Illinois
State Geological Survey, and the department of mining
engineering of the University of Illinois.

Method of Presenting Cost Data.

The method adopted for presenting cost data in this
report is as follows :—All the installation and mainten-
ance charges, including first cost, maintenance,
renewals, and emergency repairs, have been reduced to
an annuity basis so as to show the amount of money
that must be set aside annually to cover the upkeep,
and to provide for the amortisation of capital at the
current rate of interest for a given term of years.





■i-Z"

■ ■ *• I

Fig. 1.—Stopping Faced with Slate and
Roof Coal.

A great many types of stoppings are in use in the
coal mines of Illinois, the types varying from efficient
concrete stoppings to inefficient stoppings constructed of
powder cans and coal dust. The following nine types
were considered:—(1) Board stopping, rough lumber;
(2) board stopping, tongue and groove; (3) board stop-
ping, planed on two sides and painted; (4) brick
stopping, 4 in. wall; (5) brick stopping, 8 in. wall; (6)
concrete monolith, 8 in. wall; (7) concrete blocks, 8 in.
wall; (8) slate or gob, dry wall; (9) slate or gob, faced
with cement. The main purpose of the investigation
was to suggest a way in which a superintendent or mine
manager may solve mine stopping problems by substi-
tuting particular values for those given in the tables.

Sizes of Stoppings.

The dimensions of stoppings vary, as follow :—
Height, 4 ft. 6 in. to 9 ft.; width, 8 ft. to 25 ft.; thick-
ness, 1 in. to 20 ft.; average surface area, 79 sq. ft. The
variations in height are due to differences both in the
thickness of the coal and in the physical properties of
the beds. In some of the beds a strong top coal is left
as the roof, and in others a “ draw slate ” above the coal
is removed in mining. Otherwise the stopping height is
usually the same as the thickness of the coal bed.

The width of stoppings varies between the wide limits
mentioned above because of the attempt at some mines
to reduce the yardage in cross-cuts by driving them full
width. The objections to that method are that wide
areas of roof are left unsupported, and that wide stop-
pings increase the air leakage. Except in gob stoppings,
the width of stopping is made 12 to 36 in. wider than
the cross-cut, in order that the stopping may be “ set
in ” a groove or channel which is cut in the rib to make
a tight joint. The thickness of the stopping varies
with the material of which it is constructed, and, in
the case of gob stoppings, with the amount of waste
rock made in entry driving.

As cross-cuts in Illinois mines should have an area of
not less than 36 sq. ft. when used for the passage of
air, the smallest stopping will have a surface area,
including the groove, of about 50 sq. ft. The higher
limit should not be over 100 sq. ft. In the following
tables, all data relating to stoppings are reduced to a
common basis, that for a stopping having an area of
80 sq.ft., which is approximately the average size of
Illinois stoppings.

Number of Stoppings Required.

In the usual room-and-pillar mines in Illinois, where
the coal beds average 5| ft. in thickness, and cross-cuts

* From Bulletin 99, U.S.A. Bureau of Mines.

are driven on 70 ft. centres, the building of two entry
stoppings for every acre of coal bed developed is
required. In that State in 1913 there were 114 indi-
vidual mines, with an output of more than 200,000 tons
of coal. This output required at each mine the develop-
ment of about 50 acres and the construction of 100 new
entry stoppings, of which 25 per cent, were on main
entries and 75 per cent, on cross entries.

Cost of Erecting and Maintaining Stoppings.

Where a coal bed must be developed by expensive
shafts, it is considered necessary to have at least 2,000
acres tributary to each hoisting shaft. This acreage will
require 4,000 stoppings, having a first cost of 20,000 to
60,000 dols., a fact that shows how heavy an item in the
development of a coal mine the cost of stoppings may
be.

In order to show the annual expense necessary to
bring about the amortisation of the first cost at the end
of the period of years during which service is required,
the following formula may be used :—

. _ C r

X '	(1 + r)" - 1

where C = first cost in dollars for construction; n =
number of years of service required of stopping; r =
current.rate of interest; n = number of dollairs that must
be put away at r interest which will yield C dollars in n
years; and xr = annual expense necessary to yield C
dollars in n years.



—___

Fig. 2.—Stopping Faced with ( lay.

Most of the main-entry stoppings and many of those
on the cross entries must be kept in good repair in order
that the ventilation for any given development may be
maintained in accordance with the State mining law. In
carelessly constructed stoppings of lumber or gob material
it has frequently been found that the maintenance cost
amounted to 1 dol. per stopping per year—a figure that
is larger than the annual expense due to the first cost of
a substantial stopping with amortisation in a 15-year
period. The annual maintenance charges, therefore,
assume importance in mine economy.

When the length of service required of a stopping is
longer than the life of the materials used in construction,
renewals will be necessary. The average main-entry
stopping will have to give 15 years of service, and the
cross-entry stopping three years. The choice of the type
of stopping, therefore, should follow a comparison of the
probable length of service of the stopping with the life
of the materials to be used in its construction.

There are, in addition, extraordinary agencies that
destroy stoppings, some of these agencies partly or
wholly wrecking certain types of stoppings, and yet in no
way injuring other types. Such agencies are—fires,
windy shots, blown-out shots, explosions, local roof falls,
general mine squeezes, and water.

In order to obtain the annual expense for emergency
repairs on the different kinds of mine stoppings, informa-
tion was obtained from coal operators regarding the
number of stoppings and the cost of repairs each year
because of these extraordinary agencies. The quotient
obtained by dividing the total cost by the number of
stoppings in use at the particular mine was charged to

each stopping as the annual cost of emergency repairs.
For example, at one mine the records showed that 18
stoppings were destroyed by windy or blown-out shots;
the cost of each repair averaged 6-67 dols.; the total cost
was 120-06dols., and, as 1,000 stoppings were in use, the
expense to be distributed over each stopping was 12 c.
per annum.

Cost of Air Leaks Through Stoppings.

The sole purpose of a stopping is to prevent the air
of the ventilating current from passing through the
cross-cut in which the stopping stands. The Illinois
mining law specifies the minimum amount of air that
must be delivered to each man per minute. With a
given number of men, therefore, a definite quantity of
air per minute should pass through the last cross-cut of
each split. If the stoppings are absolutely tight, the
horse-power required by the fan to furnish this air may
be obtained by the formula

tt	k sv3

Horse-power =	-

where k = 0'0000000217*; s = rubbing surface in
square feet; and v = velocity in feet per minute.

If the stoppings are not tight, air will leak through
each cross-cut, the amount of leakage depending on the
area of the hole or crack in the stopping, and on the
difference between the air pressures on the two sides.
It is obvious that if air is lost before it reaches the
working faces, additional air must be delivered by the
fan in order to have the desired volume reach the last
cross-cut. Inasmuch as the horse-power required to
move the air varies as the cube of the velocity, a serious
leakage will require a great increase of horse-power to
move the larger volume of air at the higher velocity.
At Illinois coal mines a horse-power costs about 40 dols.
per year, including all engine and transmission losses.
Therefore, if the quantity of air lost be known, it is
possible to figure accurately the cost of the leaks
through the mine stoppings. This cost of stopping
inefficiency should be charged against the cost of
stoppings quite as much as first cost, maintenance
charges, etc.

In obtaining the data and in making the computations
for determining the cost of horse-power lost by the
leakage of air through mine stoppings the following
method was used : In the entry or part of an entry chosen
the stoppings were all of the same type of construction
and the travel of the air was confined to the entry. The
stoppings were examined, and their number, type, cost,
condition, etc., were recorded. The volumes of air
passing the starting and the finishing points on the entry
were carefully measured.

Fig. 3.—Lumber Stopping Faced with
Wood-fibre Cement.

The difference between the volume at the starting
point and that at the finishing point was taken to repre-
sent air lost through the stoppings. As all the stoppings
were of the same type in each test and as their number
was small, it was assumed that the air leaked equally
through each stopping.

* The value of k is empiric, and is based on investigation
by J. J. Atkinson. Other investigations have given different
values. The friction constant should vary for the different
points of air passage in the mine; but as the result would
complicate without seriously affecting the present problem,
the generally used value of k a« given is assumed to operate
continuously.

391233