THE COLLIERY GUARDIAN

AND

JOURNAL OF THE COAL AND IRON TRADES.

Vol. CXVI.

FRIDAY, NOVEMBER 15, 1918.

No. 3020.

Sinking Shafts through Thick Beds of Mud and Sand.*

By T. BORLAND.

foregoing method. The sinking difficulty had in-
creased enormously with the depth, and whereas
progress at the beginning could be counted by yards
per day, less than as many inches could be got towards
the end.

Among the many difficult problems with which a
colliery manager has to contend, the sinking of shafts
through a thick surface of loose sand and mud is
perhaps one of the most trying. The venture bristles
with potential disaster, and emergencies continually
crop up which call for instant decision and action if
the undertaking is to proceed expeditiously to a safe
and successful termination. Throughout the Scottish
coal fields this treacherous surface on so large a scale
is rarely met with, and since few colliery managers,
as a result, have had the opportunity of experiencing
such an undertaking, the author submits his expe1-
rience in the sinking of two circular shafts through a
thick deposit of loose sand and mud of over 100 ft.,
some 50 or 60 ft. of which contained a moderate
quantity of water.

Shape of Shaft.—Before proceeding to sink, it is
first of all important to consider the best form of shaft
to be adopted. • Even at the present day it is difficult
to break down the prejudice existing among some
Scottish colliery managers and contractors regarding
wood-lined rectangular shafts, and it is still to be
found in almost universal evidence. It is certainly
not without its advantages, but it seems probably best
suited where the duration of the colliery is. not likely
to exceed the life of the wood lining of the shafts.

The elliptical brick-lined shaft has also consider-
able and important advantages over other shapes in
certain circumstances, but when we come to deal with
thick loose surface deposits as above stated, then in
my opinion no other form can compare favourably
with the circular shaft.

Instances are on record of rectangular shafts having
been sunk through 120 ft., and even 200 ft., of loose
material. For such a purpose nothing beats the
circular form of shaft from the point of view of safety,
expediency, efficiency and cost, and these embrace all
the important considerations. Rectangular shafts in
loose soil may be started and kept plumb and true
in shape, but where the deposits are thick they will
surely, before any great distance is reached, have
lost their truly vertical and correct form. On the
other hand, with ordinary careful management cir-
cular shafts can be sunk considerable distances
without deviation or alteration of shape to any appre-
ciable extent. Another important point is that it can
be sunk with much less risk of accident, and when
finished it is much stronger than any other form of
shaft, and is unlikely ever to give cause for a moment’s
anxiety.

With the relative merits of the different shafts
before me, and knowing the existing circumstances,
the author had little hesitation in deciding upon the
circular form.

Size of Shafts.—After the form of shaft the sizes
necessary have to be determined. After due considera-
tion it was decided that a pair of shafts, one 12 ft.
in diameter and the other 16 ft. diameter inside the
lining would meet the requirements of the colliery,
and these were proceeded with.

Mode of Procedure.—In the first place some arrange-
ment was necessary to deal with the debris from the
shafts. Owing to the nature of the ground, anything
in the shape of an ordinary pithead frame and
temporary winding engines was out of the question,
as, in addition to unsuitable foundation, there always
existed the danger of surface movement when sinking
operations were proceeding. As both shafts were to
be sunk simultaneously, two steam derrick cranes
were erected, being placed midway between the shafts,
and in such a position that each of the cranes could
deal with the debris from one or other of the shafts
as occasion necessitated. (See fig. 1.) Even the
derrick cranes were not immune from movement of
the surface, and they had to be frequently levelled up
during the sinking operation. This, however, was
easily accomplished, and at the most only entailed the
labour of a few hours. The first work of the cranes
was to build the sinking cribs or shoes necessary for
cutting through the loose ground. These were built
on the surface, on the site of each shaft, before the
ground was cut. They consisted of twelve C.I. seg-
ments bolted together to complete the circle. To form
a cutting edge the segments were V-shaped, the outer
edge being vertical and the inner one inclined. (See
fig. 2.) The inside of the V was filled with concrete
in order to strengthen the crib and at the same time
form a foundation on which to build the cylindrical
brickwork forming the shaft lining—in this case 2 ft.
in thickness. After careful plumbing and levelling of
the crib, excavation commenced inside, the soil and
mud being dug out and filled into kettles. These
in turn were raised by the derrick cranes and swung
round clear of the shafts and deposited into bogies
suitable for side and end tipping. The process of
excavating allows the crib to sink, and this was con-
tinued with until the crib had sunk to about its own
height—5 ft. or thereabout. With a view to re-
inforcing the brick lining, and at the same time

* Paper read before the Scottish Mine Managers’ Asso-
ciation.

keeping the different lengths bound one to the other,
eight steel permanent rails were built vertically on
the top of the crib at equal intervals round the circle,
and in the centre of the brick lining. In order to
keep the rails plumb and in position, they were bound
together at intervals with light bar iron 3 in. by | in.
section. (See fig. 3.) A length of about 6*ft. of brick-
work was then built up on the top of the crib, the
outside of the brickwork being dressed smooth with
a coating of cement in order to reduce skin friction
and allow the crib sinking easier. This length of
brickwork completed, sinking was again resumed, care
being taken to see that the crib and brickwork sank
uniformly all round. Should, by any means, one sector
tend to sink faster than its opposite sector, the whole
brickwork tilts off the plumb. When this happens,
as it frequently does, the cure is obvious. The lagging
side must be encouraged to keep pace with the side
which tends to sink the faster. This is usually an
easy matter, but if it so happens that one side is held
or retarded by some obstruction, such as a boulder,
and the shaft goes off the plumb, sinking must be

Derrick Crane No. 1.

Derrick Crane No. 2.

Fig. 1.—Plan showing Arrangement of Derrick Cranes.

confined to the lagging side until the obstruction is
removed, and until the crib again becomes level and
the shaft restored back to the plumb. So long as the
crib and the brickwork continue to sink by this
method, it is easy to keep it plumb, or to get it back
to the plumb if deviated by means of a boulder or
other obstruction.

This method of sinking the crib and building on
the brickwork was continued so long as the weight of
the brickwork, etc., was sufficient to force it down at
a moderate rate of speed. However, as the depth
increased, the resistance or forces acting on the out-
side surface of the lining became so great that progress
gradually diminished until a point was reached when
assistance had to be acquired. It was found that
the foregoing method was sufficient in the case of
the smaller shaft to a depth of 70 ft., whereas in the
case of the larger shaft little over 45 ft. was attained,
when extra weight of brickwork had to be added
on top in order to increase the speed of sinking. At
this depth the top brickwork had to be kept built
up at least 12 ft. above the surface level. As the
depth increased, so did the height of the top brick-
work, until at a depth of about 70 ft. the brickwork
was 24 ft. above the surface level. It was now found
that something more than weight of brickwork was
required. At this depth a number of H steel joists
were laid across on top of the shaft brickwork, and on
the joists pig iron bars were built, the weight being
distributed equally all round the shaft, leaving a space
in the centre for the kettle to pass through. By this
means sinking was continued for another 17 ft., but
at a constantly reducing rate of speed, until it finally
came to a standstill at a depth of 87 ft., despite the
fact that the weight on the top had now become
almost 1,000 tons. It was then deemed inadvisable to
add more weight, and the shaft had therefore reached
the maximum depth it was possible to reach by the

The above remarks refer entirely to the larger of
the two shafts, and the foregoing method was success-
ful in sinking the smaller shaft to the rock head.
Here again, however, the rate of sinking when nearing
completion was anything but satisfactory; indeed, it
had almost entirely ceased when it touched the rock
head. It was quite evident that had it even only a
few more feet to go something else than weight would
have been required to get it to sink.

It is easy to account for the disparity in the depth
attained in the two shafts when we compare the
relative resistance or forces acting on each shaft
separately. The nature of the ground being the same
in both cases, a greater total amount of resistance has
to be overcome by the larger shaft compared with
the smaller. The resistance due to- friction varies
directly as the rubbing surface; consequently, the
resistance encountered for a given depth by both
shafts will vary directly in the proportion of their
outside diameters—i.e., as 20:16, or as 5:4.

With a pressure of mud and sand at 50 lb. per
sq. in. at the bottom, or a mean pressure of 25 lb.

per sq. in. throughout the total cylindrical pressure at
a depth of 87 ft. would be

20 x 31416 x 87 x 144 x 25 Q .

--------------—------------- = 8.785’26 tons
2,240

for the larger shaft and

16 x 31416 x 87 x 144 x 25 _ 7 028-20- tons
2,240

for the smaller shaft, a ratio of 5 is to 4, both shafts
being the same depth.

At half the depth the total pressure is only one
quarter, because then we have only half depth by half
mean pressure. It is this quadrupling of the total
pressure when the depth is double that causes the rate
of sinking to be rapidly reduced.

Otherwise, down to the depth of 87 ft. no great
difficulties had to be contended with. The quantity of
water which had to be pumped was not‘more than
80 gals, per minute. The boulders, which were
numerous, were composed of sandstone of the new
red sandstone formation. These were generally easily
dislodged by the crib itself, and only occasionally
had explosives to be used to shatter them from under
the crib. As a matter of fact, the only trouble expe-
rienced was the constantly reducing rate of progress,
which after 60 ft. had been reached became extremely
tedious.

Attempts to Bestart the Crib Sinking.—All sorts of
expedients were tried to get the brickwork to sink
faster, but without avail. In one case the water from
the pump was delivered all round on the outside of
the brickwork, but did not appear to take any effect.
After numerous theories had been advanced as to the
cause of stoppage, it was decided to relieve the bottom
of the crib all round in order to make sure that no
boulder was impeding its progress. This was a highly
dangerous undertaking, and it proved a costly one,