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THE COLLIERY GUARDIAN.

August 21, 1914

SUBSTITUTES FOR WOOD TIMBERING IN
MINE OPERATIONS/

By Howard I. Smith.

Until recent years no successful attempt has been
made in the United States to introduce mine supports
other than wood, as this has been the cheapest material
available. The increase in the cost oh mining, due to
the ever increasing cost of timber, is a problem that
has necessitated a great deal of study.

The reports of the U.S. Department of Agriculture
set forth that 45 per cent, of timber taken into the
mines decays. This statement has been given wide
publicity in various periodicals. To reduce this large
percentage of loss several methods have been adopted.
The simplest expedients are to peel the timber before
use, and to use judgment as to when it should be cut.
Timber cut during the summer months decays much
more rapidly than that cut during the winter. Lob-
lolly pines cut during the summer have been observed
to show the first evidence of decay within six to eight
weeks. Timber after reaching the mine has been given
the brush treatment with whitewash, cheap paint,
creosote, or carbolineum with effective and econo-
mical results. Timber has also been treated with
creosote and zinc chloride at an additional cost of 20
and 12 per cent, respectively, with the result that its
life has already been increased 300 per cent., and it is
still in good condition.

Several large coal mining companies have found it
necessary to instal plants for the treatment of timber
for use in mines. Treated timber would come into
more general use if it could be more readily bought in
the open market. The preserved timber, on first
thought, could be used to advantage only where per-
manent timbers are desired, but this is not necessarily
true. Timber which is to be kept in the mine for one
year would be of more value if treated, for this may
be taken as the life of mine timber in any except the
best of atmospheric conditions. This time limit may
seem rather low, but one would seldom re-use a timber
after it had been underground for a year; it may still
look strong, but it has lost a great deal of its vitality.

The use of mine timber preserved with creosote or
other oils has frequently been condemned on the ground
that they increase the inflammability of the wood.
Again, in case of fire, the irritating effect of the smoke
produced is great. I -am advised by the superintendent
of a preservation plant of one of the railroads that after
treated timber has stood for two or three months it
becomes very difficult to ignite it.

Timber economy may be attempted in two ways : by
the use of a portable sawmill to utilise broken or other-
wise worthless timber, and by dressing the timber to
most economical proportions and shapes. Portable
sawmills have been installed by a number of mining
companies for the purpose of sawing up broken timbers
and odd lengths into sprags, caps, lagging, and even
into lumber.

Decrease in the first cost of timber may be accom-
plished by reducing the freight bills. This may be
done :—(a) By removing the bark as soon as the timber
is cut, and allowing the water to dry out. At the same
time this gives the timber a better opportunity to
season, which adds strength and retards decay; (5) by
slabbing the timbers where cut, thus eliminating the
transportation of a large tonnage. The Department
of Agriculture reports that slabbing a prop tends to
increase its unit strength, probably the effect of
improving its straightness. Conclusive results could
be obtained only by comparing materials cut in the
same area. The unit strength of a round post is much
stronger than the same post split; likewise, the unit
strength of round timbers used for beams is much less
than when the beam is sawed in the correct proportions
of height to breadth to give the best unit strength
design.

Under favourable conditions of roof and floor, props
may be made more serviceable, and at a considerable
saving, by sharpening one end in the form of a wedge,
or to a point, so that the prop will yield somewhat
before the elastic limit has been reached. The results
of some tests published by Emil Stens in Gluckauf,
April 29, 1911, are given in the following table. The
props consisted of straight pine 10 cm. (3-94 in.) in
diameter and 1-4 m. (55-12 in.) long. Five props
were used for each test :—

Pres- Com-
sure. pression.
Tons. Per cent.

1.	Props fastened at upper and lower end 15T ... 0‘31

2.	Prop with a 4 in. wedge-shaped cap on

top .............................. 16*3	... 5’2

3.	Props with a 4 in. wedge on both ends 14T ... 8’7

4.	Prop with a 4 in. wedge-shaped cap on

top and bevelled at bottom ; length

of bevel 1’5 times the diameter .. 16’8	... 17’5

5.	Same with bevel three times the

diameter ......................... 15’3	.. 18’2

The results of these tests show the yielding capacity of
props can be increased from 0-3 per cent, to 18-2 per
cent, without decreasing the carrying capacity of the
timber. On account of the varying grain in the wood,
it is always best to toe the bottom of the prop into the
ground one or two inches. The best results are
obtained by using a cap piece with some sap wood;
this, being soft, allows the prop to adjust itself to more
even pressures.

The use of pointed props for gob timbers is well
worth serious consideration where the pillars are under
considerable pressure and the floor is medium hard.
This same effect is produced in a minor way by placing
the wooden prop on a pile of soft dirt, but this does not
seem so practicable since the compression at first is

* From a paper read before the Coal Mining Institute of
America.

very rapid at low pressure, and then quickly changes
to a very slow rate of compression.

The timbering problem is so closely related to mining
methods that this phase must be considered sooner or
later. The methods which undoubtedly give the least
trouble from falls of roof are the strippings of Kansas
or Eastern Pennsylvania. Beginning with this method,
timber consumption increases until we consider the
caving methods, which are undoubtedly among the
greatest consumers of timber. Among other methods
we can compare the room-and-pillar system, where so
often no attempt is made to recover timber, and with
the longwall method, where a large part of the timber
is recovered. In a class by itself is the system of
hydraulic filling.

After the adoption of such of the preceding methods
as may be practical, the mining industry may still be
in need of more permanent, substantial, or more
economical mine props, especially as our mines become
deeper and extend over more area, and where a large
percentage of the coal is to be extracted. With this in
view, we may consider what has already been done in
securing substitutes for wood.

First, hydraulic filling; this has been of the greatest
economic value from the standpoint of recovery of coal
and saving of wood.

Second, hand stowing of waste materials from the
mine either along the main passages or in the working
place or as pack walls in longwall work.

Third, the building of piers or columns with mine
rock without the use of cementing material, as shown
in an accompanying illustration. Piers of this type
when compressed about 30 per cent, will carry a load
about equal to what coal will stand in the pillar.

Fourth, the use of well constructed concrete, brick,
or stone piers or walls, either alone or in conjunction
with steel or wood beams. Supports of this construc-
tion must be considered as perfectly rigid, and are used
to prevent the roof from bending and subsequently fall-
ing. After weight has been supplied to a support of
this type greater than its compression strength it is of
no more value. It is unlike a support of similar
material which has been built without mortar, the
latter having a greater supporting value with the
increase in the compression. The use of concrete or
masonry arches and walls with steel beams are gradu-
ally eliminating the old-time prop with brush lagging,
and the later use of large and more permanent timber-
ing for mine portals, shaft bottoms, underground
stables, pump houses, and other similar places where
it is very important that there will be no delays due to
a fall of roof. A concrete or masonry wall built along
the haulage way to support the cross beams not only
acts as lagging, but should a trip of cars become badly
wrecked, there is no danger of the end supports of the
crossbars becoming dislodged and allowing the roof to
fall. Where concrete or brick piers for end supports of
beams are to be built, advantage should be taken of this
practice and the wall built up solid to the top of the
car. The pier for the beams may then be built on top
of the wall.

Fifth, steel sets are frequently used in place of wood
on important haulage or airways as a permanent mine
roof support. Among the more important installations
of*steel timber sets, the Carnegie Steel Company gives
the names of 50 companies that are using steel in gang-
ways and entries, pump houses, underground stables,
etc. A complete list would probably include nearly all
the large companies of the United States.

The first cost of steel timber, erected, is usually two
to three times that of wood, but this is usually war-
ranted by the increased life and the large expense some-
times necessary to clean up a fall. Another very
important consideration is the reduced output caused
by blocking a haulage way.

Sixth, the use of steel or iron props at the face. This
practice is possible only where all the props can be
recovered, or in that portion of the mine where it is
possible to recover the props before they have become
overstrained to such an extent that they must be
straightened, repaired, or sold for scrap.

Props are made to serve either one or two general
purposes. The first type is the rigid prop, which fails
upon a compression of a very small per cent. Such
props are used to keep the roof in place and prevent it
from starting, for instance, in the case of a tender shale
above a flexible roof coal, the use of the rigid prop
being to hold the roof coal up against the shale. The
second type of prop is used to support the immediate
roof, where its movement is practically irresistible.
This type of steel prop can be made yielding only in
combination with wood or by allowing a part of the
prop to rest in a cavity filled with peat or other loose
materials. The yielding steel prop is probably used in
Germany more than any other country.

In either case the type of steel prop giving the best
results from a structural standpoint is the pipe. This
has been tried in the anthracite region where great
supporting power was necessary. The pipe was cut to
length and a flange placed in both ends to increase the
bearing surface. Scrap pipes from old culm lines have
been used in stables and such places after filling the
pipe with concrete.

The most economical design of steel to withstand
compression is the “ H ” section beam, where the
radius of gyration is nearly equal about both axes.
This design has been used in England since about 1890,
and is now quite common. It has also found a market
in the anthracite district to a limited extent. The
chief drawbacks in both the pipe and “ H ” section is
the inability to readily change the length to suit con-
ditions. This has been overcome in several different
way. A most ingenious design of a roof support is in
the form of a pipe with a screw jack head; this was
used in a bituminous mine of Pennsylvania about 1906.
These props were made to protect a coal conveyor in
longwall work, but they would also be of advantage in

many mines as temporary props under drawslate, or
for protection in taking down bad roof.

Another form of adjustable prop consists of a channel
bar with a T-beam telescoping into the channel; the
lower end of the beam is slightly wedge-shaped. The
beam contains horizontal slots, and it is raised to the
proper height, then forced against the roof by iron
wedges driven into slots, and resting on the band on
top of the channel bar. A wood wedge is then driven
in between a movable ring and the wedge-shaped end
of the T-beam. The iron wedges through the beam are
then removed and used at the next post. An oval-
shaped bolt is provided for use on the back side of the
channel. When it is desired to remove the prop this
bolt is given a one-quarter turn, which brings it to the
flat side of the bolt and loosens the wood wedge on the
front of the channel and allows the prop to collapse.
The compression of the wedge allows the props to yield
at a load of 15 tons, which is regulated somewhat by
the kind of wood used for the wedge. This prop has
found use in both Germany and Belgium.

The Mannesmann weldless steel prop consists of two
pipes, one telescoping into the other, so that they can
be adjusted to the proper height. They are held in
position by a clanp which is attached on the outside of
the prop in such a manner that it cannot slide down.
It is claimed that the clamp can be screwed to a resist-
ing force of 15 tons, after which time the telescoping
will proceed as that weight is being applied. Good
results have been obtained by using these at gate ends
and to the first waste conjointly with tapered props.
These props, as the ones above, can be easily removed
from a distance.

The investigation of mine roof supports has been
started by the Bureau of Mines for the purpose of
determining what are the best materials to use for sup-
porting the roof and how these different materials and
appliances act under load. Forty-eight tests were made
in co-operation with the Pennsylvania State Anthracite
Mine Cave Commission during January 1913. These
tests and others made later, supplementing that work,
are as follow :—

Four tests were made to determine the bearing value
of props placed in a vertical position.

Four tests were made on dry rubble masonry to
determine the unit bearing value at different percentages
of compression.

Five tests were made to determine the unit bearing
value at various percentages of compression on loading
yellow pine, oak and spruce cogs, unfilled and filled,
with loose mine rock.

Two tests were made to determine the unit bearing
value at different percentages of compression for square
mine rock cogs laid up in cement mortar, the interior
hydraulically filled with culm, sand and ashes.

Seven tests wrere made to determine the unit bear-
ing value at various percentages of compression of sand
and anthracite ashes equal parts, broken mine rock
and breaker refuse passing through in. screen. Placed
in steel cylinders.

Twenty-one tests on concrete of various mixtures of
sand, sand and gravel, anthracite ashes, sand and
breaker refuse, sand and broken mine rock and culm to
determine the unit bearing value. The first set of
seven was tested at the age of seven days, second set at
34 days and the third set at 96 days.

Five tests consisting of loading reinforced concrete
columns and brick piers to determine the unit bearing
value at maximum load. These tests were constructed
and tested by the U.S. Bureau of Standards and the
results given to the Anthracite Mine Cave Commission.

One test was made on a support designed by George
S. Rice. It consisted of a concrete cylinder filled with
mine rock and sand. The object was to determine the
advisability of using such a design around gas or oil
wells. This type may be used to great advantage for
a pack wall along haulage ways in longwall work, in
arresting a squeeze or in supporting valuable improved
property on the surface.

Three tests were made on concrete blocks to deter-
mine the advisability of their use under valuable
surface improvements.

Twenty-one tests wrere made on loosely packed
materials from the anthracite region to obtain a com-
parison between the different materials when subjected
to loads.

Eighty-four tests were made to determine the com-
pressibility of different materials from the anthracite
region that are available for dry and hydraulic filling.

Fifteen tests were made to determine the effect of
settlement of the above materials due to shaking,
spading, and tamping.

Seventeen tests were made on bituminous coals from
the experimental mine.

All of these tests were made to determine what
different uses materials around the mines may be
economically put to for the purpose of supporting the
roof while getting out all the coal, and at the same time
causing as little damage to surface property as possible.
Some of these tests were made on wood for the purpose
of getting a comparison between the materials used
and some material with which we are already familiar.

In Bulletin No. 25 of the U.S. Bureau of Mines are
the results of 15 tests conducted under the direction of
Messrs. Griffith and Conner, and in the appendix are
the results of 416 tests on anthracite coal made for the
Scranton Engineers’ Club in 1900. With the excep-
tion of the latter, which is very complete, these tests
must be considered only as a preliminary investigation,
and each branch must be continued to duplicate the
tests conducted before the results can be accepted as
authentic.

In furthering these tests a large amount of work has
been outlined, including tests on concrete and steel
props, not so much to determine their strength as to
determine their behaviour, and what is the best type
that can be adapted to mining practices.