THE COLLIERY GUARDIAN

AND

JOURNAL OF THE COAL AND IRON TRADES.

Vol. CVIII.	FRIDAY JULY 31, 1914,.	No. 2796.

ARCHING IN COLLIERIES.*

By ROBERT G. CLARK, A.M.I.C.E., M.C.I.

Though it must be admitted that the loads on the
supports or arching as the result of a squeeze are
enormous, yet it may be of interest to endeavour to find
some of the causes which contribute to their failure to
support the load. This investigation will cover the
design and materials used in the construction and also
the abnormal conditions that have to be contended with,
and it may point to some weak parts which, when
remedied, will add to the stability of the arches.

The designer of arching has to face at least three
unknowns, namely, the intensity of the load, its point
of application, and its direction. It is obvious that with
three unknown factors to deal with, any refinement in
calculations is impossible, and the successful solution
of this problem will rely more on comparative strengths,
guided largely by practical experience.

With this fact in view, it may be permissible to
enquire how masonry and brick arches may prove
unstable under varying conditions, and in fig. 1 is shown
a segmental arch constructed of block-stone. All the
members in the arch are in a state of compression, and
represent the ideal and economic arch for materials such
as stone or bricks under these conditions. If, however,
one or both of the abutments should move under the
application of the load, a new condition of affairs is
presented; and in fig. 2 is shown the left-hand abutment
having yielded. The arch then becomes crippled more
or less on the line shown, and is no longer subjected to
compressive stresses only. The flattened portion is
subjected to tensile stresses on one side and compres-
sive stresses on the reverse, so that it acts in the nature
of a beam. This is noticeable by the opening joints,
and when this happens the load-carrying capacity of
the arch becomes seriously reduced, even if the load is
distributed evenly over the arch.

Fig. 1.

Fig. 2.

Going further, even if it is allowed that the abut-
ments are immovable, yet if considerable loads are
concentrated on one half or haunch of the arch, as
shown in fig. 3 as Wx, W2 and W3, the effect would be
for these loads to cause the arch to deflect immediately
under the loads, with a corresponding rise of the reverse
side. In other words, the arch is distorted and tensile
stresses are set up as well as compressive stresses. The
effect of this distortion would be to crush some of the
stones or bricks, and others again would open at the
joints. In the case of brickwork with concentric rings,
there may be evidence of peeling, and this, no doubt,
is a contributory cause to the failures referred to; and
it is clear that although some parts of the arch show
signs of crushing, it is not due so much to excessive
pressure as to the effect of distortion. The safe load
that can be carried on arches loaded in this manner is
determined by the rise, span and thickness of the arch.

The result is this, that stone or brick arches are
admirably fitted for such conditions as will insure the
various members being in a state of compression, but
any considerable tensional stresses render the arch
unsuitable. In other words, the curve of equilibrium
must fall within the middle third. This is denoted by
the hatched portion in fig. 4. If the curve of equili-
brium comes outside this, then the arch is unstable.
As is well known, it is very difficult to accurately
determine where the curve of equilibrium will run in a

* From the Proceedings of the South Wales Institute of
Engineers.

stone or brick arch, as all depends on the perfect con-
struction and the perfect filling up of the joints. It
will be realised from this that much material in a stone
or brick arch is wasted.

In order to overcome the effects of distortion in stone
or brick arches for railway or road bridges, it is
customary to fill in over the haunches with concrete or
hard filling; but the great difference in bridge arches
and arching in mines is that in the former the live load
to be carried is very little compared with the weight of
the structure itself, termed the dead-load, whereas in
mine arching the reverse is the case, as the loads to be
supported are enormous compared with the weight of
the arch; so much so that for all practical purposes
the load due to the squeeze is the only load to be con-
sidered.

Quite apart from the question of distortion, there has
to be considered the thrust of the arch. In flat arches,

Fig. 3.

Fig. 4.

Fig. 5.

Fig. 6.

i.e., arches where the ratio of span to the rise is small,
the resultant thrust is nearly in a horizontal line, but
as the rise increases the line of thrust assumes a more
vertical position, as shown in fig. 6. Railway com-
panies rarely make segmental arches with the rise H
less than one-fifth of the span S (see fig. 5). This
not only affects the cost of the arch, but in a greater
degree the abutments, as the more vertical the thrust
the less expensive to construct the abutment.

Leaving for the moment the general design, it is
proposed to deal with the materials used in the con-
struction. In masonry arches each stone should have
chisel-dressed joints, so that the jointing material is as
thin as possible. The joints should be normal to the
arch, and the materials for the joint should be cement
and sand, rather than lime and ashes. Great care
should be exercised in making the joint, and as little
mortar as possible used, so long as the adjacent stones
are properly bedded. This also applies to brick arches,
and if anything to a greater extent, as there are in
proportion many more joints to deal with.

If the arch is constructed in rubble masonry, the
result is more or less a trial and error problem, the
stones being so irregular that to bed them at all large
quantities of mortar are required. But as the crushing
value of this mortar is so much less than the crushing
value of the stone, it is evident that to all intents and
purposes the strength of the arch is governed by the
strength of the mortar. This is a very important point
which should not be overlooked, as by increasing the
strength of the mortar, the weaker component is
rendered more capable of taking higher stresses, both in
compression and tension.

In order to get over this matter of joints, trials have
been made of moulding concrete blocks, and they are
an excellent substitute for masonry or brick. It is true
that the crushing value of concrete is not equal to that
of stone, being about equal to the best brick; but as
the joints can be made correctly and economically,
there is no doubt that for equal thicknesses concrete
blocks made of suitable materials would be much
stronger, as the joints are correctly and well made, and
they can be made less in number, which would be a very
great advantage.

Fig. 7.

Fig. 8.

The mottled portion represents ash packing.

Briefly reviewing the foregoing, it appears that
masonry and brick arches should have immovable abut-
ments, and that even then their capacity for supporting
uneven loads is very limited; and last, but not least,
that all joints should be made with great care and with
strong materials.

Reverting to the matter of the loads or squeeze : If
it is to be assumed that the worst conditions have to
be faced, inasmuch as the squeeze may come on from
any direction, it is obvious that the support must be
continuous, or in other words, a closed figure. The
circle at once commends itself, or some other curved
figure such as a two-, three- or four-centre arch, some-
thing after the shape shown in fig. 8. Combinations
of the curve, such as an elliptical roof and a straight
invert or three parts of a circle for the upper portion
and a segmental invert, might meet the case also.

In the case where the arcliing is made up of two
or more segments of circles, great care should be
exercised in constructing the junctions to the right
angle. The junctions act as skew-backs or abutments,