1534

THE COLLIERY GUARDIAN.

June 20, 1913.

be an admirable material of construction in these cases,
though of course economical considerations may often
decide that other materials have to be used instead.

If settlement is likely to have to be provided for,
reinforced concrete is the only satisfactory material.
A few years ago a building erected at Tunis tilted over,
owing to the failure of the foundations, till it was
inclined at 25 degs. to the vertical. By excavating
under the higher side and loading the building it was
brought to a level bearing, and though now it is several
feet lower than originally designed, it remains an
example of the bad treatment reinforced concrete may
receive without failing. In the great earthquake at San
Francisco, in every case structures of reinforced concrete
resisted the effects of the earrhquake better than those
built in any other material. The capability of resisting
shock is also shown of its use as sleepers, piles, and
various railway structures.

2.	Shafts.— While plain concrete would in some cases
be a very suitable material for use in lining shafts
owing to its great crushing strength and to the ease
with which it may be put in position, still it is often
quite impossible to use it in a satisfactory way. A
considerable time is required for concrete to develop a
reasonable strength, and therefore it is necessary that
the expensive timbering or steel centres which support
the green concrete should remain in place till this
strength is attained. But it is quite feasible to use
reinforced concrete in a way exactly similar to that in
which a cast iron curb is used. Reinforced concrete
seems a suitable material for making a shaft lining by
building it in cylindrical sections keyed and grouted
together, the cylindrical sections being formed either by
keying together several curved plates of reinforced
concrete or by being cast in complete rings. Cylindrical
tubes 7 feet in diameter have recently been successfully
made in this material, and then lowered into trenches to
form a main sewer.

3.	Permanent Roads.—For these works it is safe to
say that reinforced concrete is the best material. Its
great strength, resistance to shock, and the effects of
settlement, and fire, and its durability combine to make
it superior to any other material. While, when
properly designed, it will be found economical. Already
it is used in several important mines in the country. In
some cases it has been claimed to be too expensive.
Many engineers, however, use reinforced concrete like
plain concrete—that is, the steel work is put in merely
to ornament the drawings and make them appear up to
date, there being ample concrete in place to carry all the
load, and so naturally the method of construction
appears costly.

The author then called attention to one of the early
tests on the strength of reinforced concrete on the
Monier system. This method of reinforcement consists
of embedding in the concrete a net of steel wires about
3 in. centre to centre, the diameter of the wires in the
arch in question being about 0’34 in. The dimensions
of the arch shown were 32 8 ft. span, the rise being
one-tenth of span, and the width was 13 ft., the depth at
crown was 6 in. The following test loads were applied:—
First, a locomotive weighing 40 tons was run across the
arch. Then a locomotive weighing 53 tons was placed
on the half span. Having stood these loads satisfac-
torily a deadlock of 110 tons was piled on the half span,
this being about 1,000 lb. per square foot. The deflection
under this load was only 0’12 in. When the load was
increased to 2,000 lb. per square foot the arch failed
owing to the abutments giving way.

Recently at the University of Birmingham some
experiments have been carried out on the strength of
arches on a reasonable though smaller scale than that
of the above experiment.

Arches 10 ft. span and 1 ft. rise are constructed, the
width being 3 ft. A uniform load is applied by means
of a pump and hydraulic jacks. Several arches in brick
concrete and reinforced concrete have already been
tested, and the great strength of concrete and reinforced
concrete arches has been fully demonstrated. The depth
at the crown of the concrete arches was 6 in., and the
reinforced concrete arch tapered from 6 in. at the
springing to 3 in. at the crown.

The total uniformly distributed loads supported by
these arches when tested three months after making
were 53 tons and 42 tons for the concrete and reinforced
concrete arches respectively, this giving crushing loads
of 4,000 lb. and 3,200 lb. per square foot of arch. It is |
hoped to carry out exactly similar tests on timber I
beams, but if a maximum fibre stress for a round 10 in. j

diameter beam of larch be assumed to be 4,000 lb. for concrete might be used, and as they frequently were
average conditions, then the breaking load of a solid called upon to pass buildings which were subse-
™lhese l0in‘ l,arf on 10ft'span would be about t) to he undermined, the details given were

It is evident from these experiments that there should Particularly interesting. For lining shafts and perma-
be no difficulty in designing a light section in reinforced nent roads a large field was open to mining engineers,
concrete with side walls and invert which might be Indeed he believed that a good deal of reinforced
depended on to support a main road in any ground, i concrete was now being used in that direction. From
This has been done at several mines in Englmd.	. . , « . , k j+ii.	k./i

Am	rt j Ti. • • x	that point of view he hoped that members who had had

4.	Temporary Roads.—It is m temporary roads, how-1 F	F	.

ever, and as a substitute for ordinary timbering that experience would add to the discussion. Those who had
an extended use of reinforced concrete in mines is not ventured to use reinforced concrete would no doubt

render the posts portable and secure against shock.
These posts and beams were only preliminary attempts
to determine the strength of the material under similar
conditions to those in which timber is used. It would
be easy to suggest other shapes as being probably more
economical in mines. For example, it may be shown
that a triangular shape may be best for both beams and
pumps, and a series of experiments were carried out on a
simple triangular section which gave excellent results.
Also, the ends of postsand beams might be modified for
the sake of getting better bearings, and also for
increasing the strength of the posts. The successful
economical manufacture of the structures will depend to
a very large extent on the design of the forms and
moulds, and there is no doubt that in a short time it will
be possible to order these small structures for delivery
just like timber or steel.

The advantages of these posts and beams when used
will, of course, strike everyone. Besides their consider-
able strength, the following points may be noticed.
They will be durable. It has been proved conclusively
that steel props, if embedded in concrete, are not
deteriorated by time under the most severe conditions.
The strength increases with age to a maximum, and
then does not decrease. The strength is not to any
great extent affected by moisture, the material in this
respect differing so much from timber. It may be
roughly stated that timber when wet has only 50 or 60
per cent, of the strength which it would have when dry,
and, as is well known, in a dry heading the timber rots
quickly. All tests (outside mines it is true) show that
of all materials of construction properly constructed
reinforced concrete is the best for resisting destruction
by fire. A further advantage that the use of these
posts and beams presents is that the method of con-
struction, while being as rapid as the ordinary method
of timbering, could be at slight expense conveyed to
permanent construction. This would be done by fitting
reinforced slabs between the settings, or by ramming
plain concrete between them. Modifications of this
design would always allow this to be done.

Discussion.

Mr. Alexander Smith said that with regard to
what Prof. Dixon had brought forward as to its suita-
bility in cases of settlement, no doubt it was all right
where the settlement was of a steady and gradual
character, but the kind of settlement met with in the
thick coal mines was not at all a steady settlement.
They were very liable to severe bumps and virtual earth-
quakes, because of the weight that came on the top of
the strata. He remembered making a sketch of a bar
at Ham stead Colliery after a bump. It was 18 in. in
diameter, yet it was absolutely split in pieces, and he
did not think that the thickest part of it, which happened
to be in the centre, was thicker than a match. When
they got the pressure like that, they wanted a cushion
to take the shock, because the action was very like a
blow from a steam hammer. In all similar cases they
provided timber to serve as a cushion for sustaining the
strain. He questioned very much whether reinforced
concrete would answer for these purposes as well as
timber.

The President said he knew there were some gentle-
men who had had experience in the use of concrete
underground, and they would be glad to have that kind
of first-hand knowledge. Not only was concrete a
material, as Prof. Dixon had shown, of abundant
strength, but it was a material which he regarded as
eminently suited for mining purposes, and one which
had a good deal to do with the solution of mining
problems from different points of view* In particular
it did not readily oxidise, and therefore would be
invaluable for the lining of roads in which the high rate
of absorption of oxygen produces an unworkable tem-
perature in the mine. That was a subject which young
mining engineers would have to tackle in the future
working of pits. With regard to the use of reinforced
concrete as a material to resist settlement, Prof. Dixon
had shown them some notable examples in which

sure that the author had rendered the institution a
valuable service in bringing the subject forward in such
an interesting way. There were many reasons why
reinforced concrete should be employed in mines other
than the question of its strength as a material. For
example, it was uninflammable ; it was not subject to rot
or decay. It did not oxidise and produce heat in the
same way as coal and timber. Indeed, concrete was a
material which seemed to be designed for mining pur-
poses. and there was little doubt that it would be used
very largely in future. There was some doubt, how-
ever, whether concrete would be used for face work
where it was not desirable to support a roof permanently,
and where general subsidences were taking place which
it was imperative should be permitted to occur slowly.
In such cases it was a question whether props manu-
factured of concrete could ever be used to replace timber
for such purpose. He was sure he would be expressing
their wishes by voicing their indebtedness to Prof. Dixon
for that interesting and instructive paper. They might
also take advantage of the presence of Prof. Dixon to
elicit answers on questions bearing upon his experience
of the use of that material.

Mr. Bernard Clark said he thought the last
speaker had hit the nail on the head when he said that
the reason why concrete had not been used in mines to
any great extent was because the mining engineer was
usually ignorant of the use of this material. Whilst it
was attractive from the theoretical point of view, it was
difficult to find mining engineers who had had experience
of the use of reinforced concrete. If they went to a
civil engineer, while that gentleman was an expert in his
own line, he did not understand the exact circumstances
existing in mines. Naturally this had the effect of
retarding the use of this material. With regard to its
surface employment, he had seen some concrete used for
enginehouses, but he did not think it would be described
by Prof. Dixon as reinforced concrete. It consisted of
concrete reinforced with an occasional girder and a few
odd pieces of wire. He believed that reinforced
concrete was largely used in Yorkshire collieries in
connection with enginehouses, and in some cases for the
side walls of main roads. It seemed to him that if they
used it for the tops of arches, and the arch gave way,
there was bound to be more difficulty in replacing it.
A brickwork arch, on the contrary, could be replaced
exactly and with ease, and, in fact, could be put in or
taken out in a night shift, but it would be impossible to
take out an arch or arch the top of a working with
reinforced concrete without some stoppage of part of
the pit. With regard to structures on the surface, it
was interesting to note that several collieries in the
north, instead of em ploying reinforced concrete where
subsidences were expected, employed complete steel
structures as, for example, large enginehouses and
other buildings in preference to using reinforced con-
crete. Possibly Prof. Dixon could tell them something
about such cases. In regard to buildings on the surface
generally, he believed a very great deal of work was
being done with reinforced concrete, and that was
bound to increase, because in such cases they had no
mining difficulties to face, and they were able to follow
the lead of the civil and mechanical and mechanical
engineer, and just in the same way as they had improved
the mechanical plant in recent years, so they would be
able to improve the structural plants.

Mr. W. Charlton said that, in response to the appeal
of the chairman for instances of the use of concrete in
mining work, he might remind them that a considerable
number of years ago the late Mr. Bennett Brough
contributed a rather striking paper on the use of
concrete blocks for shaft linings. It was probable that
not sufficient time had yet elapsed to express any verdict
as to the suitability of the material for securing main
roads. He might also mention the Bentley Colliery, in
the Doncaster district of Yorkshire, where, he believed,
the shafts were commenced practically in quicksands
where 100 ft. of extremely bad ground was met with at
the beginning of their sinking. He believed that the
whole of their surface structure was erected practically
on beds of concrete. There was also another extremely
interesting case in progress at present which might be
suggested. In a paper before the Concrete Institute this seriously consider its use after the valuable information worth watching, and that was the sinking on the east
year the author gavei results of anumber of’experiments ; placed in such an interesti ng manner Wore them by ’	* ’	~	'	......... ”	"	”

on reinforced concrete beams and posts 8 ft. long. Since L* _ _.	T_ r

reading that paper he found that such work is coming Pr°f- Dixon. He must confess that he was somewhat
into use in the United States and Canada. In this surprised to hear from Prof. Dixon that concrete as used
method posts and beams suitably reinforced are made for arching was capable of standing almost double the
and stored ready for use. In many metal mines ’ pressure endured by larch timber. It appeared to him
admirable material for the greater bulk of the concrete	X1 , • . k	■>

-the broken stone-is being constantly mined, and so 'that olw 8 ldeaS On the S“bjeCt Were baSed Up?U uns0"ud
the sand and cement only needs to be brought into the foundations, and so they hesitated to introduce such a
mines, the beams and props being made and stored material because they were afraid of the consequences,
below ground.	I anj dislik ?d to initiate experiments

For use in mines, the beams should be reinforced on I f which th knew g0 httle p;„:1
the upper side as with the lower, to save accidents in	_ , ,	.	.	„	. . .	„

transport. For posts, the reinforcement should be of volume of data and experience, some of which Prof,
the simplest kind, the object aimed at being chiefly to Dixon had very clearly placed before them, and he felt

side of the Doncaster Colliery, known as the Hatfield ;
that was being carried through by means of the
interesting process of injecting cement, following these
with concrete blocks and backed with reinforced
concrete. Both these cases would be worth watching.
He bad no doubt it had come as a surprise to most of
them to learn of the use of reinforced concrete in the
shape of props or posts. That was not exactly new to
himself, because only a week ago he saw concrete piles
being driven down in one of the big London docks.

Mr. G-. M. Cockin, seconding the vote of thanks,
said that one very important feature had not been

s on a material
of which they knew so little. Civil engineers had a