SCREW PILES.
Details.
"Now for some details.
"Solid piles are usually from 4 to 8 inches in diameter, and hollow
cast-iron from 10 to 30 inches, and generally 10 to 20 inches. Avoid
any cast-iron screw piles that are less than half an inch in thickness.
When they are from 1/12th to 1/18th of the diameter is perhaps the
best, according as their length is little or great; but of course they
have to be of a thickness that will stand the load, and what is the
best foundry practice should not be forgotten.
"Now as to the blade of the screw. If of wrought-iron, which seems to
me the wrong material for that purpose, it should not be less than half
an inch in thickness; if of cast-iron, as usually is the case, the
thickness of the blade of the screw at the pile shaft should be about
1.25 to 1.50 that of the column, and at the edge not less than half an
inch, and it should taper equally on both sides, and care be taken that
the metal is the very best and so cast as to ensure uniformity and
strength.
"All sizes of screws from twice to six times the diameter of the pile
when hollow I have screwed, but the best are from 2 to 1 to 3 to 1, and
when they are more than 4 to 1 it is to be feared they will break
before they can be made to penetrate far enough to say nothing about.
Solid piles with screws four to seven times the diameter of pile I have
also fixed, and 5 to 1 to 6 to 1 is quite large enough; but the kind of
ground and the depth to which they must be got down should govern the
size and the pitch. The greatest depth, apart from imagination for
measurement, to which I have ever screwed a pile is about 25 feet.
Without special tackle I have made a 2 feet in diameter screw penetrate
hard clay, dense sand, and other hard soil from 8 to as much as 17
feet; but then 10 to 15 feet is deep enough, for there is such a thing
as overscrewing. A 3 to 4 feet in diameter screw I have fixed all
depths from 10 to 20 feet in ordinary sand, clay, and sandy gravel. A 4
feet to as large as a 5 feet screw, which great size should only be
used for soft soils, from 15 to 25 feet, and the most usual depth is
about 15 feet, and hardly ever above 20 feet.
"A 9 feet 6 inches screw blade has been used on a 7 feet in diameter
cylinder, but that is the largest I have heard of, but then it only
projected 2 feet 6 inches beyond the column. Five feet is usually about
the largest, and is only used for very soft soils. When more than that
size they are unwieldy and very liable to be broken, and if the screws
are fixed to a shaft and have to be shipped they are awkward things,
and the freight becomes expensive. For hard soil, and that which will
not compress nicely, about 2 to 3 feet is large enough for the diameter
of the screw, and 3 to 5 feet for soft soils. The pitch of the screw is
generally from one-third to one-seventh of the outside diameter of the
blade. It varies according to the hardness and softness of the ground
and is steeper as it becomes harder. When the pitch is increased the
effect of the power applied to screw it is reduced, therefore the
steeper or greater the pitch the harder the screwing.
"Piles can be screwed with a small pitch when sufficient power cannot
be obtained to make a steep-pitched screw penetrate. Piles with a
single turn of the screw, it seems to me, are the best, although the
double-threaded screw may be right in soft marshy ground; but the
usefulness of a double thread is doubtful, for I believe it breaks up
the ground for no good, although some state that the screw threads work
in parallel lines, and that a double-threaded screw is steadier; for
they say a single-threaded pile is always likely to turn on the outside
edge of the blade, and that the double-threaded is not, as it has a lip
on both sides.
"Generally the screw has rather more than one entire turn round the
pile, and when it is below the ground each side of the blade steadies
the other, for the turns range from one to about two. Sometimes the
edge of the blade is notched like a saw; but it is a question whether
the saw-edge blade will screw into ground that an ordinary blade will
not, and until it is proved by experiment it can only be a matter of
opinion; but there is one thing to consider, a saw-cut edge blade may
to some extent wedge the soil between the teeth; still, I have used
them, and they penetrated thin limestone, chalk, and compact gravel
seams. Instead of double threads, double points are the thing, and all
screw piles should have a point of some kind. For soft ground, a single
gimlet, and a double for hard soils, and I have noticed what I call a
double gimlet point is best for keeping a pile in the required
position, as each point prevents the other departing from a correct
line. By points I mean the ends are spread out about 3 to 4 inches on
each side of the axis of pile like spiral cutters.
"Unless it is certain the ground is easy and uniform, a pile with a
screw having one turn to two turns for bearing purposes, and two,
three, or four solid inclined screw-threads projecting about
three-quarters of an inch with two end spiral cutters as just named, is
my desire, or in addition to the bearing blade a single-turn thread of
about 3 to 4 inches projection and the same kind of point; then unless
it will screw, none will. They are less trouble when cast in one piece
with the pile; but not for transport or shipping, or foreign work
generally, because to be able to detach the screws is an advantage in
many ways, such as packing, defects, breakages, carriage, and I think
the castings are better when the blade is not cast on the pile. It may
also happen that a rocky bed is unexpectedly encountered, then the pile
is useless with the screw, but might be fixed firmly in Portland cement
without the blade in a hole made in the rock. At the top of the screw
blade seat in which a pile has to be fixed there should be a
wrought-iron ring about half to three-quarters of an inch in thickness,
and not less than 2 inches in width, to relieve any strain on the
casting. It may be put on hot, so as to cool sufficiently tight but not
strain the casting. A firm and even bearing for the pile on the socket
seat is important, and it should fit accurately.
"I have heard of screw piles in which the blade was made of two or more
separate segments so as to obtain, it was supposed, equal pressure all
round, and to ease screwing, but rather fancy they might be inclined to
jam the ground, as they would be not unlike a lot of very large round
saw teeth. They may be right, but it has to be proved they will screw
where a plain blade will not, provided the latter pile has double
cutter-points to steady it.
"Give me a screw blade not more than about 2 feet from the points, and
not one with a blade 10 feet or so above the points and say from 5 to 6
feet in the ground, for then, should the screw work at all crooked and
the pile be not exactly upright at the commencement of screwing, it is
no easy task to get it to stand vertically upon applying the power,
because such piles are generally long and slender, and shift about
until the blade is screwed. They want careful and constant guidance. Of
course, the idea of placing the screw a little way down is that when
the ground bears as well at that place as at the point, and there is no
scour, it is no use putting the bearing blade lower. That is right; but
then it always occurs to me to ask what is the use of anything below
the bearing level if the foundation be protected from scour, for a thin
pile by itself has little lateral strength.
"Of course, you are bound to make out a pile requires a lot of screwing
or you will be considered as making too much profit, but always take
care to watch how the first pile screws, and measure the distance every
few minutes. What the ground is can then be judged, and you will be
able to think out things for 'extra' profit. It causes me a lot of
consideration sometimes, but after a struggle I generally manage to
think rightly for my pocket, and work it all serene. What a beautiful
sharpener of one's brain 'extras' are!
"It is not always an experimental pile is screwed so as to judge of
the distance the permanent piles should penetrate, and therefore a
guess has to be made from the experience of screw piles under the
same conditions of screwing and in the same soil. There is a good
deal of chance about it, for although the soil may be of the same
general character it often varies in hardness; and that is where the
bother is, for it makes the 'extras' to be wrong way about for some
time. What I do then is to work the oracle, and try to make out the
screw blades will be broken or injured for certain if I am compelled
to screw them as ordered, and I work on the proverb that equal
support is not to be obtained at a uniform depth when the ground
varies, which is true; and I state that the resistance is different
and offer to screw on, but say am afraid the blade may be broken, and
in that how-kind-I-am-to-consider-your-interests sort of way
generally manage to obtain a bit 'extra,' or save something that
would have been loss, and get the pile measured at once for a
making-up length, and really without damaging any one, for if the
ground is harder at one place than at another there is no occasion to
go so deep, always provided scour is not to be feared. So I am
pleased, and it does not hurt them.
"Now for a hint or two on screwing piles. I shall not refer to the
columns above the ground, but to the bearing piles below, i.e., the
part that has to be screwed into the ground. However, I will just say
that upon the top of some of the columns the usual hinged shoes of
bearing-blocks should be placed to receive the ends of the girders, and
by that means the pressure on the columns will be on the centre of the
pile, and allowance be made for expansion and contraction, and that is
important.
"Fixed staging is far the best from which to screw piles, but the
chances must be considered of its being swept away by floods in a
river, or smashed by the sea, and on any exposed coast there may not be
time to construct it during the working season, so as to give a
sufficient number of days for screwing operations. When a fixed stage
cannot be erected, or the work be done from the end of a finished pier,
pontoons or rafts are then a makeshift, but care must be taken that
they do not break from the moorings. A couple of pontoons well braced
together will do with a space between them to screw the pile, but in a
steady or shallow river, perhaps making a timber stage upon the shore
and floating it out can be done if a centre pile is fixed on the bed of
the river to be certain it is in the right position when grounded. The
staging must be equally weighted to make it sink, and arrangements made
so that it can be floated away at any time if necessary.
"Piles can also be fixed in a medium depth of water by ordinary
gantries, but if they are in the sea the road on the staging should be
kept from 12 to 15 feet above high water on an open sea coast or the
inclined struts and ties and rail tops as well are very likely to be
destroyed, and it is also advisable to construct the flooring of the
stage so that it can be easily taken away in case of storms. The stage
piles also require to be well stiffened by struts, transoms, diagonals,
and capping sills. I have screwed piles from a floor that has been
suspended from staging by chains and ropes to the height wanted, and
when lowered it was fixed temporarily and as many guides as possible
were made for the piles. Perhaps as good a way as any is to fix, say
four guide piles having a space between them a shade larger than the
outside dimensions of the screw blade and braced to the rest of the
stage, and after the screw is in position and ready for screwing in the
ground, place, say a couple of frames, one at top and one as low as
possible between the guide piles, about an eighth of an inch more than
the outside dimension of the pile shaft, for then the pile is kept in
its right position as it is screwed. The guide frames should be at
about every 10 or 15 feet of the height above the ground, and at some
point between the capstan level and the ground. Should it be a tidal
river, fix guide booms if a properly made iron frame cannot be placed,
and remember the more a pile is guided the easier it is to screw, and
especially so at the start.
"The size and strength of the staging must be regulated according to
the power available for screwing the piles, but the length of the lever
arms and the capstan bars require a space in which to revolve, from,
say, 35 to 60 feet square. No timber stage is immovable, for the wood
yields. It is well to have two floors in a stage if it does not cost
too much, and there is plenty of tackle and a lot of screwing to do;
say, one fixed above high-water level and the other about half tide in
order to obtain double power, and sufficient power to screw the piles
cannot sometimes be otherwise secured. A word about floating stages.
With them it is not easy to make a pile screw vertically unless the
ground is uniform, and should a pile meet a boulder it will most
probably be forced out of position. According to the power
required--which really means the nature of the ground, as the harder
the soil the harder the screwing--the form of the pile and the depth to
which it has to be screwed, so must be the size and strength of the
raft, pontoon, or lighter, and the moorings must hold it tightly. In
some places a screw cannot be fixed from a floating stage, for the
water may nearly always be too disturbed, and the pontoons may sway too
much, for in all cases men, horses, or bullocks must have a steady
footing, and screwing machinery also requires a firm base. Unless the
moorings are very secure the platform will be unsteady. Its level
should be as little above the water as practicable for work, so as to
keep the point of resistance and that at which the screwing power is
applied as near together as possible, and the lower the pontoon the
less it rolls. It does not matter much what craft is used so long as it
is broad and steady and not high, as a platform or deck must be made
upon it in any case. To do any good with floating stages the power
required should be little, and the ground soft and uniform, for
sufficient force to screw may not be obtainable from a floating body,
and in hard soil it may only be possible to screw piles a little way
down and not to a sufficient depth for the load they will have to bear.
"Of course, vertical pile screwing is the easiest, and to try to screw
them at a greater angle than 63 deg., or about 1/2 to 1, is unadvisable,
and may not succeed, and even if they do it is too steep to be nice. 1
in 10 to 1 in 20 for raking piles is enough; for if they have to carry
girder ends, the more the batter the greater the strain on the pile,
and the same during screwing.
"Sometimes in loose soil it is difficult to start screwing, and then a
good plan is to cast some clay or solid earth round the pitch; it
steadies the pile and will probably make it bite properly, or a heavy
weight placed on the pile may make it catch hold of the ground; if not,
a few blows from a ram may do it. As a hollow pile penetrates, the core
requires to be removed, so as to help it to descend. If it is not large
enough for boys to get inside, scoops and tackle can be used. Water
forced down makes sand boil round the screw blade, and when the pile is
empty the unbalanced head of water outside relieves the pile and the
screw blade from some of the surface friction. If water pressure cannot
be used, the water inside the pile should be removed either by pumps or
buckets so as to help to loosen the ground.
"Piles do not generally screw to the full pitch, but when a pile
descends _more_ than the pitch at the last turn, it can be considered
the weight of the pile is too great for the ground. The slip usually
increases according to the yielding or plastic nature of the soil, and
the depth to which the pile is screwed. When water reaches such soils
the slip is increased, but not perceptibly in sand and loose grained
soils. Suppose the full pitch is 9 inches. The slip may be anything
from about 1 inch to as much as 4 inches. By watching the way in which
the screw penetrates, and whether it descends about the same distance
_each_ turn, or regularly decreases, it can be judged whether the bite
of the screw is right. Some slip will generally take place, therefore
note at first how much it is, and consider whether it will not churn up
the ground, for if the screw blade turns on nearly the same lines, the
bite will be gradually destroyed, and then it may be very difficult to
obtain a fresh hold of the ground, and the pile will most probably not
screw vertically, and the screw blade is liable to be injured and may
become worn away considerably.
"Piles can be screwed by means of men, horses, oxen, and machines.
Man-power can be used anywhere, machines in most places, but horses and
oxen only on land when the piles are screwed on a foreshore or between
tides; of course all live power works at the end of the capstan bars.
Once I had the option of screwing by horses or oxen, and chose oxen.
Another man had horses. I made more profit than he did, and the piles
screwed easier than his. I did not let him come near me when screwing;
but if you have the choice, use oxen in preference to horses. Of
course, I am speaking of those countries where they are used to the
yoke."
"Why?"
"Because they do not stop at any time or back like horses, not even
when the resistance of the pile becomes too great without more power,
but continue to pull, and therefore backward motion of the pile is
prevented. The oxen were yoked to two cross-arms attached to the end of
the lever.
"There are several machines for screwing piles worked by steam or other
power, and when the ground is not easy to penetrate, and a large number
of piles have to be screwed, their cost will be saved in the
regularity, quickness, and ease in screwing, and in stiff soil by
machine power I have known them screwed at the rate of 4 to 6 inches
per minute. Of course, it is a special machine, and not easily sold
when not further wanted except at a much less price than has been paid
for it, and that has to be considered. There are several different
methods of screwing piles from a fixed stage; for instance, suppose a
pile of sufficient length and with the screw attached is brought to the
site by barge or otherwise, the capstan head is then fixed, and the
pile swung vertically over the pitch by sling-chains fastened to
temporary eye-bolts passing through the bolt-holes in the flanges or
otherwise, and is moved either by a jib crane, a derrick upon a raft,
or some such hoisting apparatus; it is lowered into its place between
the guide-piles or steadied by sling-chains or other means, then the
capstan bars are put into the sockets of the capstan head, which should
be at equal distances apart, and the pile is ready for screwing after
it is known that it is vertical.
"Where circumstances did not allow of room for capstan bars of
sufficient length for men to walk round, I have screwed piles by ropes,
but it will only do when the soil is easy to penetrate. The way we
worked was something like this, we had two endless ropes passing round
the ends of short capstan bars and round two double purchase crabs
placed upon opposite sides of the pile, about six or eight men worked
at each crab, four or five winding, and two or three hauling in the
slack, one rope being passed through a sufficiently deep upper slot in
the capstan bar end so that it did not slip, also one in the lower slot
same end. Both the taut and the slack ends of the lower and upper ropes
were attached each to its own crab. A man must be stationed at the end
of the capstan bars to put the slack ends of the taut and slack ropes
into the slots. One rope gives the capstan half a turn when it is taut,
and then it falls out of its slot and is slack, and so with the other
rope, but it is not easy to keep the two ends of the rope equally
tight, and the power obtained is not great and may not be sufficient.
It is a kind of makeshift."
"How do you fix the capstan head to the pile shaft?"
"In many different ways. Sometimes it is keyed on or clamped tightly to
the top of the pile length by steel wedges, also placed upon the pile
length and fixed by temporary bolts passing through the top flanges of
the pile length, and also by fixing a temporary ribbed pile into the
capstan head, and by connecting it with the permanent pile by bolts or
slots, and so wedging is not wanted and it can be raised and lowered.
Another way is, two of the internal sides of the pile at top are cast
flat for a foot or so down into which the capstan head fits, and the
inside diameter is lessened for an inch or two to prevent the capstan
head slipping down, but it generally can't do that, even without the
narrowing of the pile for that object.
"As the capstan is subject to great wear and tear and sudden strain, it
should be strong, for if it breaks the work is stopped. Wrought-iron
capstan heads are used, but cast-iron are perhaps better. Sometimes the
capstan sockets are made to fit the ends of rails, if rails instead of
timber are used for the capstan bars, but rail bars are rather heavy
and are not nice to handle. The capstan socket is generally made to
receive from eight to ten or more radial lever arms, and the lengths of
the bars are anything from 5 to 40 feet, but the latter is rather too
long as it is very difficult to control the strain and the bar usually
bends and springs. The best working lengths are from about 8 to 20
feet, if the staging is so large. The best height for the capstan bars
above the floor stage is from 3 feet 6 inches to 4 feet 6 inches. The
capstan bars have to be lifted and again fixed as the pile penetrates,
or a temporary pile of different length has to be fixed in it, unless
the capstan head can be slipped up and down on a ribbed pile, hence you
may want a platform you can raise or lower easily when required. If you
use double-headed rails of the same section top and bottom for the
bars, you can have them bent up a little near the capstan head, and
when you start, the bent end is lowest, and then the bars can be
reversed and so the work proceeds.
"Put the men, horses, or oxen in the most natural position for exerting
their full strength or a loss of power will result, and therefore it
will cost more to screw the piles.
"Should there be gantry staging on the site, the piles can be pitched
from a traverser, or by means of an ordinary crab winch. They can also
be screwed from the permanent structure by means of a projecting stage
temporarily fixed to it, and of a length sufficient to reach the next
span. The pile is run forward upon rollers and placed in the right
position. Then it is screwed on the endless rope system previously
described, or by passing the rope round a deep groove in the capstan
bar ends, and the rope is held tightly by being placed round a smaller
grooved pulley fixed about a hundred feet or so back towards the shore.
The men haul the endless rope and so the screwing is done. The worst of
screwing by endless ropes this way is that the pile very probably may
be pulled over towards the source of power as it comes from _one_
direction, therefore, support is required on the side of the pile to
prevent this tendency. The circumference of the ropes used varied from
4-1/2 to 6 inches, but I have used a 10 inch rope. Small ropes are
generally relatively stronger than large ones. Stretch a rope well
before using, as it yields, especially hemp ropes. The distance between
the point at which the power is applied, and the ground should be as
little as possible. In firm sand, when the power has been more than
about 20 to 25 feet above the ground, it is often very difficult to
screw piles by ordinary means to more than a small depth, as two places
in the pile are wanted from which to apply the screwing force, and both
as low down as convenient; but in screwing from a second stage care
should be taken that the pile shaft is not bent, for it may then be
strained like a girder and not merely as a column, also when much power
to screw is required it is not easy to avoid pulling them out of the
vertical. Always screw them steadily and prevent jerking. Any
obstruction, such as a boulder, tends to displace a pile, and loosens
the ground around it. In soft soils it may be possible to pull piles
upright by pushing aside an obstruction if the pile is given a turn or
two after meeting it and before pulling; but it must be carefully done,
or the pile may be smashed, and it is only safe to pull it over in easy
soils and when much force is not required."
"How much power is generally wanted for screwing?"
"That is not so easily answered as asked. It varies very much, and, of
course, depends upon the kind of soil and the size and pitch of the
screw. Ten men may be sufficient and a single stage, but two stages may
be necessary should the pile be 50 or 60 feet in length, and then not
far from one hundred men. An engineer told me the force generally
required for piles of usual sizes under ordinary screwing circumstances
varies from about 8 to 10 tons to as much as 50 tons, and usually from
about 10 to 25 tons, and, of course, the number of men to screw in
proportion.
"Ordinary piles and screws have gone down 21 feet in sand in eight
hours, and by steam machinery in clay at the rate of 6 inches per
minute, and also, to my loss only about 1 foot in a day--and then it is
time to stop altogether, should many piles hold like that. To compare
what has been done with what has to be done is misleading unless the
conditions are alike, for if they are otherwise the power required,
cost, and rate of screwing will all be different. I have screwed a
6-inch pile with a 2-feet one-turn screw into 20 feet of ordinary sand
with an applied power of 30 tons as calculated by an engineer from
measurements and the force of men applied at the capstan bars. There is
the surface friction on the screw blade and the pile shaft in the
ground, the cutting of the earth by the edge of the blade and the
points, and the loss of power from torsion and that applied compared
with the effective force, slip, friction, &c., to consider; and the
relative surface of the blades, width, and thickness of the cutting
edge and the pitch--for a steep pitch means harder screwing. By using
capstan bars and men at them, instead of ropes at the ends of the arms
worked by crabs, you will find about one-fifth more power is gained, or
rather is not lost. Of course, place the men as near to the end of the
capstan bar as convenient for work. My lecture is finished, and I am
parched."