THE DESIGN OF SEE OUTFALLS.
The detail design of a sea outfall will depend upon the level
of the conduit with reference to present surface of the shore,
whether the beach is being eroded or made up, and, if any part
of the structure is to be constructed above the level of the
shore, whether it is likely to be subject to serious attack by
waves in times of heavy gales. If there is probability of the
direction of currents being affected by the construction of a
solid structure or of any serious scour being caused, the
design must be prepared accordingly.
While there are examples of outfalls constructed of glazed
stoneware socketed pipes surrounded with concrete, as shown in
Fig. 21, cast iron pipes are used in the majority of cases.
There is considerable variation in the design of the joints for
the latter class of pipes, some of which are shown in Figs. 22,
23, and 24. Spigot and socket joints (Fig. 22), with lead run
in, or even with rod lead or any of the patent forms caulked in
cold, are unsuitable for use below high-water mark on account
of the water which will most probably be found in the trench.
Pipes having plain turned and bored joints are liable to be
displaced if exposed to the action of the waves, but if such
joints are also flanged, as Fig. 24, or provided with lugs, as
Fig. 23, great rigidity is obtained when they are bolted up; in
addition to which the joints are easily made watertight. When a
flange is formed all round the joint, it is necessary, in order
that its thickness may be kept within reasonable limits, to
provide bolts at frequent intervals. A gusset piece to stiffen
the flange should be formed between each hole and the next, and
the bolt holes should be arranged so that when the pipes are
laid there will not be a hole at the bottom on the vertical
axis of the pipe, as when the pipes are laid in a trench below
water level it is not only difficult to insert the bolt, but
almost impracticable to tighten up the nut afterwards. The
pipes should be laid so that the two lowest bolt holes are
placed equidistant on each side of the centre line, as shown in
the end views of Figs. Nos. 23 and 24.
[Illustration: Fig. 2l.-Stoneware Pipe and Concrete Sea Outfall.]
With lug pipes, fewer bolts are used, and the lugs are made
specially strong to withstand the strain put upon them in
bolting up the pipes. These pipes are easier and quicker to
joint under water than are the flanged pipes, so that their use
is a distinct advantage when the hours of working are limited.
In some cases gun-metal bolts are used, as they resist the
action of sea water better than steel, but they add
considerably to the cost of the outfall sewer, and the
principal advantage appears to be that they are possibly easier
to remove than iron or steel ones would be if at any time it
was required to take out any pipe which may have been
accidentally broken. On the other hand, there is a liability of
severe corrosion of the metal taking place by reason of
galvanic action between the gun-metal and the iron, set up by
the sea water in which they are immersed. If the pipes are not
to be covered with concrete, and are thus exposed to the action
of the sea water, particular care should be taken to see that
the coating by Dr. Angus Smith's process is perfectly applied
to them.
[Illustration: Fig. 22.--Spigot and Socket Joint for Cast Iron Pipes.]
[Illustration: Fig. 23.--Lug Joint for Cast Iron Pipes.]
[Illustration: Fig. 24.--Turned, Board, and Flanged Joint for Cast Iron Pipes.]
Steel pipes are, on the whole, not so suitable as cast iron.
They are, of course, obtainable in long lengths and are easily
jointed, but their lightness compared with cast iron pipes,
which is their great advantage in transport, is a disadvantage
in a sea outfall, where the weight of the structure adds to its
stability. The extra length of steel pipes necessitates a
greater extent of trench being excavated at one time, which
must be well timbered to prevent the sides falling in On the
other hand, cast iron pipes are more liable to fracture by
heavy stones being thrown upon them by the waves, but this is a
contingency which does not frequently occur in practice.
According to Trautwine, the cast iron for pipes to resist sea
water should be close-grained, hard, white metal. In such metal
the small quantity of contained carbon is chemically combined
with the iron, but in the darker or mottled metals it is
mechanically combined, and such iron soon becomes soft, like
plumbago, under the influence of sea water. Hard white iron has
been proved to resist sea water for forty years without
deterioration, whether it is continually under water or
alternately wet and dry.
Several types of sea outfalls are shown in Figs. 25 to 31.[1]
In the example shown in Fig. 25 a solid rock bed occurred a
short distance below the sand, which was excavated so as to
allow the outfall to be constructed on the rock. Anchor bolts
with clevis heads were fixed into the rock, and then, after a
portion of the concrete was laid, iron bands, passing around
the cast iron pipes, were fastened to the anchors. This
construction would not be suitable below low-water mark. Fig.
26 represents the Aberdeen sea outfall, consisting of cast iron
pipes 7 ft in diameter, which are embedded in a heavy concrete
breakwater 24 ft in width, except at the extreme end, where it
is 30 ft wide. The 4 in wrought iron rods are only used to the
last few pipes, which were in 6 ft lengths instead of 9 ft, as
were the remainder. Fig. 27 shows an inexpensive method of
carrying small pipes, the slotted holes in the head of the pile
allowing the pipes to be laid in a straight line, even if the
pile is not driven quite true, and if the level of the latter
is not correct it can be adjusted by inserting a packing piece
between the cradle and the head.
Great Crosby outfall sewer into the Mersey is illustrated in
Fig. 28. The piles are of greenheart, and were driven to a
solid foundation. The 1 3/4 in sheeting was driven to support
the sides of the excavation, and was left in when the concrete
was laid. Light steel rails were laid under the sewer, in
continuous lengths, on steel sleepers and to 2 ft gauge. The
invert blocks were of concrete, and the pipes were made of the
same material, but were reinforced with steel ribs. The Waterloo
(near Liverpool) sea outfall is shown in Fig. 31.
[Footnote 1: Plate V.]
Piling may be necessary either to support the pipes or to keep
them secure in their proper position, but where there is a
substratum of rock the pipes may be anchored, as shown in Figs.
25 and 26. The nature of the piling to be adopted will vary
according to the character of the beach. Figs. 27, 29, 30, and
31 show various types. With steel piling and bearers, as shown
in Fig. 29, it is generally difficult to drive the piles with
such accuracy that the bearers may be easily bolted up through
the holes provided in the piles, and, if the holes are not
drilled in the piles until after they are driven to their final
position, considerable time is occupied, and perhaps a tide
lost in the attempt to drill them below water. There is also
the difficulty of tightening up the bolts when the sewer is
partly below the surface of the shore, as shown. In both the
types shown in Figs. 29 and 30 it is essential that the piles
and the bearers should abut closely against the pipes;
otherwise the shock of the waves will cause the pipes to move
and hammer against the framing, and thus lead to failure of the
structure.
Piles similar to Fig. 31 can only be fixed in sand, as was the
case at Waterloo, because they must be absolutely true to line
and level, otherwise the pipes cannot be laid in the cradles.
The method of fixing these piles is described by Mr. Ben
Howarth (Minutes of Proceedings of Inst.C.E., Vol. CLXXV.) as
follows:--"The pile was slung vertically into position from a
four-legged derrick, two legs of which were on each side of the
trench; a small winch attached to one pair of the legs lifted
and lowered the pile, through a block and tackle. When the pile
was ready to be sunk, a 2 in iron pipe was let down the centre,
and coupled to a force-pump by means of a hose; a jet of water
was then forced down this pipe, driving the sand and silt away
from below the pile. The pile was then rotated backwards and
forwards about a quarter of a turn, by men pulling on the arms;
the pile, of course, sank by its own weight, the water-jet
driving the sand up through the hollow centre and into the
trench, and it was always kept vertical by the sling from the
derrick. As soon as the pile was down to its final level the ground
was filled in round the arms, and in this running sand the pile
became perfectly fast and immovable a few minutes after the
sinking was completed. The whole process, from the first
slinging of the pile to the final setting, did not take more
than 20 or 25 minutes."
[Illustration: PLATE V.
ROCK BED. Fig. 26--ABERDEEN SEA OUTFALL. Fig. 27--SMALL GREAT
CROSBY SEA OUTFALL. Fig. 29--CAST IRON PIPE ON STEEL CAST AND
BEARERS. Fig. 31--WATERLOO (LIVERPOOL) SEA OUTFALL.]
(_To face page 80_.)
Screw piles may be used if the ground is suitable, but, if it
is boulder clay or similar material, the best results will
probably be obtained by employing rolled steel joists as piles.