IRON PILES.
Arrangement--Driving--Sinking by Water-jet.
"Tell me what you have learned about iron pile fixing, same as you have
promised me you will about timber piles."
"Very well. Here goes, then; first a word as to iron piles generally.
"Although a group of piles when properly strutted, tied, and braced
have plenty of stiffness, if you have to deal with them singly they are
never stiff, but they can be made steadier when getting them down by
having two large pieces of wood with a half hole in them, something
like the shape of the old village stocks, and by putting or lowering it
at low water until it is bedded in the ground. It must be weighted
though, so as to prevent it floating. It acts like a waling, and is
useful when the ground is treacherous, and provided it is level.
"From watching the behaviour of piles when doing repairs and at other
times, I think it wants a lot of careful arrangement to be sure the
load is acting equally on the whole group, or, as may be intended, on
say a few piles, and straight down the centre of each pile, for it
makes a lot of difference to the strain on them, and it is not easy to
make them all take the load at once as wished. It wants a good deal of
attention, and the piles are not unlike a pair of horses that are not
matched and don't work together properly--kind of now me, now you
business. Before finishing reference to driving and screwing, let me
say all the parts should be properly fitted together at the works and
numbered so that the putting up on the ground is easier and in order to
be certain all the bolt-holes agree; and it is well to have the lengths
interchangeable and all the same, except the making-up pieces, and all
bolt-holes as well as the flanges should fit in every respect.
"When columns rest on a masonry, brickwork, or concrete base the piles
ought to have a ring or base-plate right round them to hold them
tightly together. It lessens the pier being shaken, and saves the side
pushing of the holding-down bolts. I heard an engineer say the weight
of the pier above their ends should be not less than about four times
any force that might tend to lift them. The anchor-plates should be
well bedded upon a solid mass or the strain upon the pier may go in one
direction, and that the one not wanted. Don't be afraid of bracing and
strutting piles, the more of it the better. I don't think much of a
single turn of a screw blade a few feet below the ground for taking a
load, although some good for steadying purposes generally, because the
bed may become scoured out below the blade and then the screw is no
use. Therefore the depth of possible scour ought to be positively known
before relying upon the blade for permanent support. A lot can be said
as to the grouping of piles, whether in triangles or in rows. In a
triangle, although the load upon the foundations is spread over a
larger area, it does not give as much lateral strength as when the
piles are placed in one row, and taking everything into consideration I
think if I had six piles to put down I should not place one at the top
of a triangle, two lower down, and three at the base, but have two
parallel rows of three piles; besides it lessens the length of the
struts and the bracing, and that is something, but, of course, each
case requires to be treated in a special way, and I have noticed when
doing repairs that if there are six piles fixed thus, [Illustration] in
a triangle, the wind and other force acts principally upon the bracing
between the parallel rows, and the pile at the point does not do much
towards keeping the others in the right place; anyhow the bracing there
does not seem to hold as tightly as it does between the parallel rows,
and I have had to watch groups of them in storms, and when the sea has
been high, and that is my opinion."
"Now, as to fixing iron piles."
"When the ends have to be placed in rock, which has sometimes to be
done in shore pieces, 'jumping' the holes in more than about 2 feet of
water is to be avoided, for if the water is not still the holes become
filled with sand and drift, and you must not take the jumper out but
keep on continuously making the hole. It is ticklish business, because
sometimes the rock grinds the jumper, and then the wings and point wear
away. Occasionally they have to be worked inside a cylinder by ropes,
rods, and gearing fixed in it, the cylinder being movable and held from
the end of the part of the pier that is finished, but where the water
is deep the ends must be put in the rock in Portland cement by divers.
"I have driven a good many iron piles with a ram, but you have to be
careful, no matter whether the soil is sand, gravel, clay, or silt. I
like a copper ring on the head of the iron pile and a good long timber
'dolly,' not less than 4 or 5 feet in length, and then the ram does not
burst the top. When the ground is hard the best way is to make a hole
by jumpers of about 3 inches less diameter than the pile to be fixed,
and in chalk soil it is doubtful whether they will go down right unless
that is done; perhaps they won't drive at all, or a lot of them will be
broken. I have used a ram weighing from 1 to 1-1/2 ton for an 8 to a 10
inch pile and about a 3-feet fall, and never more than 4 feet, unless
you want to deal with some old metal merchant that will give a good
price for the scrap, and it does not matter how many get broken, or it
is a positive advantage to break a certain quantity out of every lot,
so as to have a big price for such difficult driving, and get 'extras'
that way."
"I understand, no breakages deducted."
"That's it. I have driven them at the rate of fully 6 inches a minute
for a few feet. They often rebound, so I had a boy with a lever, the
end of it being clinched to the pile. Directly the ram fell, he gave
the pile from quarter to half a turn for the first 4 or 5 feet of
driving, and they scarcely rebounded at all; and he earned his wages,
for I considered fully one pile extra was got down out of about every
ten by the turning movement. The points require to be regulated
according to the ground. From 1-1/2 to twice the diameter or width for
the length of the point is about right, but if it is made too sharp it
may break. Iron piles that have to be driven are seldom more than 12
inches in width, and the thickness of the metal is generally from
one-ninth to one-twelfth of the diameter. I heard an engineer say, I
think it was Mr. Cubitt, experiments showed that a T-shaped cast-iron
pile about 30 feet in length, should have the top of the T two and a
quarter times the length of the upright part, and the thickness a
twelfth of the top. Of course, the length of the pile must be
considered. I doubt if you can get equally sound metal throughout when
the thickness is much more than 2-1/2 inches. From 3/4 to 1-1/2 inch is
best, and piles I have broken up always seemed more even throughout
about those thicknesses; but there, I suppose it is all a question of
care in casting and proper machinery.
"One thing, don't drive any piles from a floating stage on the sea if
you can help it, it will make you pay for the privilege; besides I have
known some places where the sea was always so disturbed it could not be
done, even if the moorings were as tight as you dare make them. Driven
iron piles are not much seen now, and Portland cement concrete seems
the fashion, and no doubt it is better. Still, iron piles can be driven
in deep water without much trouble from it, and one might combine the
two nicely--the iron to act as a shield to the concrete while
depositing it, and give it time to set without disturbance and preserve
the face."
"Have you sunk any disc piles?"
"Yes, they are all right for fine sand and silt, but you must be
careful the discs are the same in form and dimensions upon all sides,
or a pile will almost certainly tilt and sink crookedly. I was busy on
the Lancashire coast once, and heard that Mr., now Sir James Brunlees,
tried a lot of different kinds of hollow disc piles, and that the best
was one with a plain flange base three times the diameter of the pile,
and circular, with the bottom nearly closed, it only having a hole in
it in the centre of the base 3 inches in diameter. Some ribs and
cutters were cast on the bottom of the disc to break the ground up if
it was hard. This is what I know about disc piles and have been put up
to.
"When piles have to be sunk by water pressure, rotate the pile, and
don't let it be still long, so as to lessen or prevent surface friction
on the pile shaft and the sand settling round it. Always have circular
discs and not too large, not above 3 feet in diameter, for they do not
sink nearly so easily as the size of the disc is increased. About 2
feet discs are my choice as they go down much quicker than 2 feet 6
inches or 3 feet.
"Don't try to sink them in sand to a greater depth than 18 to 20 feet,
and remember that although they may sink easily for about 12 or 15
feet, afterwards they will want some labour. When you have finished
sinking piles with the water-jet, it is best to drive them down an inch
or two further by a heavy ram and a very small fall, or heavily weight
them as soon as possible after having done with the jet; then the disc
has a bearing on firm and undisturbed ground, and if you are afraid of
a blow on the pile you can have a heavy weight placed on it to help it
into position and the sand to become solid. Obtain considerable
pressure of water, and always cause the pile to rotate when sinking.
Don't let the pressure get much below 40 lbs. per square inch, and use
about 60 lbs. if you can get it. I have worked up to 100 lbs. per
square inch but not beyond, and fancy there is then too much pressure,
and that more sand is disturbed than is necessary. All that is wanted
is to make the sand boil and remove itself from the underside of the
pile and disc, but always have a few ribs or cutters on the underside
of the disc as they loosen the sand as the pile is rotated--besides,
should there be a strip of harder soil, it may be impossible to sink
the pile without them. A rather large tube and a moderate pressure are
best, and a tube not less than about 2 to 4 inches in diameter
according to the size of the pile, and it is better from 3 to 6 inches,
of course, if the pressure is high a larger size jet can be used, but
if it is less than 2 inches it will only make a small hole, and too
much below the disc, and not enough water passes through it. Try to
ascertain what pressure of water makes the piles sink the easiest.
Sometimes they will go down at the start as much as 3 feet in a minute,
and often 2 feet, and from that to 1 foot they should do for about the
first 6 or 8 feet in sand, but then the rate quickly decreases. The
nozzle should be properly shaped so that the jet is whole. I mean the
shape of the pipe at the place where it touches the sand. What is
wanted is to get just enough force to cause the sand to separate and
boil and to push it away from the disc and no further, or some of the
water power is wasted, therefore a good volume of water is as necessary
as a high pressure. You understand?"
"Yes."
"The tube should project about 6 inches below the bottom of the disc. A
toothed tube can be fixed round it so as to help to disturb the ground
and strengthen the pipe. The water supply may perhaps be obtained at a
sufficient pressure from the local water-works company, then, probably,
a force pump will not be required, but the pressure that can always be
relied upon should be known.
"In sand, and when the water power can be easily obtained, I prefer
disc piles to screw piles, because there is hardly any chance of
breaking or injuring the disc; you always know where the disc is, but
cannot positively say where the screw is--it may be sound and may not
be; in addition, the disc is stronger than a screw blade, as it can be
strengthened by ribs almost as much as one likes, and the disc in
sinking is hardly strained at all compared with a screw pile. They can
be sunk quicker, and do not require nearly as much plant to do it, for
when you have a force pump, a guide frame--something like an ordinary
pile-driving machine 25 or 30 feet in height, with a grooved pulley at
top in which the chain or rope runs so that one end can be attached to
the pile flange either by jaws or temporary bolts, and the other to a
crab winch, which, with the guide frame, is used for lowering and
keeping the pile in position, and stay the top of the guide frame by
ropes to short piles driven into the ground--and a hose and two levers,
with a collar to grasp the pile so as to rotate it, you have about all
the _special_ plant that is wanted.
"Of course, piles can be sunk by water pressure from a floating stage
such as a barge, pontoon, or raft, so long as the pile is kept
vertical, but there are the same objections to that method as with
other piles. Piles are, however, got down much quicker and easier by
the water jet than by screwing or driving, but the ground must be loose
granular soil, such as ordinary sand.
"There is not much 'extra' to be got out of iron piles. You can only
dodge a bit with a length short now and then when you have the right
parties to work with, and the inspector is cross-eyed or a star-gazer,
but you may get something 'extra' out of the filling them in. As usual
I draw the line somewhere. Everything on earth has a boundary line.
This is where I draw it. Listen!
"After as much water as possible--possible is a nice elastic word--is
got out of the pile, and it is as clear of deposit as convenient--another
nice easy word--and before commencing the filling, I put inside the
pile everything I can get hold of that is dry, for just then I have but
one way of looking at anything, and that is to consider it Portland
cement concrete, unless it costs me more to use it; but when the
filling is concrete, I make that as dry as mixing will allow, and
sometimes hardly that. The inside of the pile is sure to be wet, and
that will help the mixing. I never ram the concrete, but gently cast it
in. It is only a sort of anti-rust covering, and is put in for that and
to keep water out, and no weight comes upon it--it is not like the
hearting of cylinder bridge piers. Ramming the concrete is not far from
being a mistake, because the pile should have a chance of contracting
without straining, and may be it will crack; and it is just as well to
remember that although by ramming tightly you may get more solid
filling and better protect the inside from rust, the pile may be
strained, and it is a choice of evils, possible rust, or strain."