This section is from the book "Safe Building", by Louis De Coppet Berg. Also available from Amazon: Code Check: An Illustrated Guide to Building a Safe House.
Clay is a good foundation, if in horizontal layers and of sufficient thickness to bear the superimposed weight. It is, however, a very treacherous material, and apt to swell and break up with water and frost. Clay in inclined or vertical layers cannot be trusted for important buildings, neither can loamy earth, made ground or marsh. If the base-course cannot be sufficiently spread to reduce the load to a minimum, pile-driving has to be resorted to. This is done in many different ways. If there is a layer of hard soil not far down, short piles are driven to reach down to same. These should be of sufficient diameter not to bend under their load; they should be calculated the same as columns. The tops should be well tied together and braced, to keep them from wobbling or spreading.
Georgia-pine piles of 16" diameter are driven through a lager of soft soil 15' deep, until they rest on hard bottom. What will each pile safely carry?
The pile evidently is a circular column 15' long, of 16" diameter, solid, and we should say with rounded ends, as, of course, its bearSand, gravel and clay.
ings cannot be perfect. From Formula (3) we find, then, that the pile will safely carry a load.
w = a. (c/f)/1+l2.n/p2, now from Table I, Section No. 7, and fifth column, we have a =22/7.x2 = 22.82/7 = 201
From the same table we find, for Section No. 7, last column, From Table IV we find for Georgia pine, along fibres, (c/f ) = 750.
And from Table II, for wood with rounded ends, n = 0,00067, therefore: w = 201.750 = 150750 = 63958,
1+1802.0,000067 2,357 or say 30 tons to each pile.
Sometimes large holes are bored to the hard soil and filled with sand, making "sand piles." This, of course, can only be done where the intermediate ground is sufficiently firm to keep the sand from escaping laterally. Sometimes holes are dug down and filled in with concrete, or brick piers are built down; or large iron cylinders are sunk down and the space inside of them driven full of piles, or else excavated and filled in with concrete or other masonry, or even sand, well soaked and packed. If filled with sand, there should first be a layer of concrete, to keep the sand from possibly escaping at the bottom.
Where no hard soil can be struck, piles are driven over a large area, and numerous enough to consolidate the ground; they should not be closer than two feet in the clear each way, or they will cut up the ground too much. The danger here is that they may press the ground out laterally, or cause it to rise where not weighted. Sometimes, by sheath-piling each side, the ground can be sufficiently compressed between the piles, thereby being kept from escaping laterally.
But by far the most usual way of driving piles is where they resist the load by means of the friction of their sides against the ground. In such cases it is usual to drive experimental piles, to ascertain just how much the pile descends at the last blow of the hammer or ram; also the amount of fall and weight of ram, and then to compute the load the pile is capable of resisting: one-tenth of this might be considered safe. The formula then is: w = r.f/10.s (46)
Where w = the safe load on each pile, in lbs.
" r = the weight of ram used, in lbs.
" f= the distance the ram falls, in inches.
" s = the set, in inches, or distance the pile is driven at the last blow.
Where there is the least doubt about the stability of the pile, use three-fourths w, and if the piles drive very unevenly, use only one-half w.
Some engineers prefer to assume a fixed rule for all piles. Professor Rankine allows 200 lbs. per square inch of area of head of pile. French engineers allow a pile to carry 50000 lbs., provided it does not sink perceptibly under a ram falling 4' and weighing 1350 lbs., or does not sink half an inch under thirty blows. There are many other such rules, but the writer would recommend the use of the above formula, as it is based on each individual experiment, and is therefore manifestly safer.
An experimental pile is found to sink one-half inch under the last blow of a ram weighing 1500 lbs., and falling 12'. What will each pile safely carry?
According to formula (46), the safe load w would be w = r.f/10.s we have, r=1500 lbs.
f= 12.12 = 144", and s = 1/2", therefore iv =1500.144/10. 1/2 = 43200 lbs.
If several other piles should give about the same result, we would take the average of all, or else allow say 20 tons on each pile. If, however, some piles were found to sink considerably more than others, it would be better to allow but 10 tons or 15 tons, according to the amount of irregularity of the soil.
All cases of pile-driving require experience, judgment, and more or less experiment; in fact all foundations do.
All piles should be straight, solid timbers, free from projecting branches or large knots. They can be of hemlock, spruce or white pine, but preferably, of course, of yellow pine or oak.
There is danger, where they are near the seashore, of their being destroyed by worms. To guard against this, the bark is sometimes shrunken on; that is, the tree is girdled (the bark cut all around near the root) before the tree is felled, and the sap ceasing to flow, the bark shrinks on very tightly.
Others prefer piles without bark, and char the piles, coat them with asphalt, or fill the pores with creosote. Copper sheets are the best (and the most expensive) covering.
Piles should be of sufficient size not to break in driving, and should, as a rule, be about 30' long, and say 15" to 18" diameter at the top. They should not be driven closer than about 2' 6" in the clear, or they will be apt to break the ground all up. The feet should be shod with wrought-iron shoes, pointed, and the heads protected with wrought-iron bands, to keep them from splitting under the blows of the ram.
In sheath-piling it is usual to take boards (hemlock, spruce, white pine, yellow pine or oak) from 2" to 6" thick. Guide-piles are driven and cross pieces bolted to the insides of them. The intermediate piles are then driven between the guide piles, making a solid wooden wall each side, from 2" to 6" thick. Sometimes the sheath-piles are tongued and grooved. The feet of the piles are cut to a point, so as to drive more easily. The tops are covered with wrought-iron caps, which slip over them and are removed after the piles are driven.
Piles are sometimes made of iron; cast-iron being preferable, as it will stand longer under water. Screw-piles are made of iron, with large, screw-shaped flanges attached to the foot, and they are screwed down into the ground like a gimlet.
Sheath-piling is sometimes made of cast-iron plates with vertical strengthening ribs.
Where piles are driven under water, great care must be taken that they are entirely immersed, and at all times so. They should be cut off to a uniform level, below the lowest low-water mark. If they are alternately wet and dry, they will soon be destroyed by decay.
After the piles are cut to a level, tenons are often cut on their tops, and these are made to fit mortises in heavy wooden girders which go over them, and on which the superstructure
The sizes given on this page are for heavy buildings; for very light work use piles of 8" to 0" diameter at tops, about 20 feet long and 16" apartrests. This is usual for docks, ferry-houses, etc. For other buildings we frequently see concrete packed between and over their tops; this, however, is a very had practice, as the concrete surrounding the tops is apt to decay them. It is better to cover the piles with 3"x 12" or similar planks (welllag-screwed to piles, where it is necessary to steady the latter) and then to build the concrete base course on these planks.1
Better yet, and the best method, is to get large-sized building-stones, with levelled beds, and to rest these directly on the piles. In this case care must be taken that piles come at least under each corner of the stone, or oftener, to keep it from tipping, and that the stone has a full bearing on each pile-head. On top of stone build the usual base-courses.
Piles should be as nearly uniform as possible (particularly in the case of short piles resting on hard ground), for otherwise their respective powers of resistance will vary very much.
It is well to connect all very heavy parts of buildings (such as towers, chimneys, etc.) by vertical slip-joints with rest of building. The slip-joint should be carried through the foundation-walls and base-courses, as well as above.
Where there are very high chimneys or towers, or unbraced walls, the foundation must be spread sufficiently to overcome the leverage produced by wind. These points will be more fully explained in the chapter on "Walls and Piers."
All base-courses should be carried low enough to be below frost, which will penetrate from three to five feet deep in our latitudes. The reason of this is that the frost tends to swell or expand the ground (on account of its dampness) in all directions, and does it with so much force that it would be apt to lift the base-course bodily, causing cracks and possible failure above.