This section is from the book "Modern Buildings, Their Planning, Construction And Equipment Vol1", by G. A. T. Middleton. Also available from Amazon: Modern Buildings.
Some soils may be consolidated into homogeneous masses by driving in a number of piles all over a site. Such piles are called Consolidating Piles, and should not be driven closer than 2 feet 6 inches, centre to centre, otherwise they tend to loosen the soil rather than consolidate it. These piles are often used in connection with sheet piling to consolidate the sheathed-up earth.
In soft but dry earths, holes about 6 inches in diameter and 6 feet deep are often bored with an auger and filled with well-rammed sand. The columns of sand thus formed are used as friction piles. These are often preferable to timber piles, which transmit the pressure brought upon them in the direction of their length, while sand acts more or less as a liquid and transmits the pressure to the sides as well as to the bottom of the boring. When the jarring of driving wood piles would endanger adjacent buildings this system of piling is usefully employed.
A method of forming foundation which sometimes proves very economical is that shown in Fig. 71. The piles, which may either be bearing or friction piles, are clustered together to form isolated supports. These are usually capped by large blocks of stone or concrete, forming skewbacks for arches which span the openings between the group of piles. Strong tie bolts are used to tie in the end abutment. Steels girders are sometimes used instead of arches, and it is often difficult to determine which is the more economical or sounder method to adopt.
It sometimes happens that piles become damaged in driving, and have to be drawn. To do this, long arms of timber are bolted to the sides of the head, and are used as levers. Sometimes a temporary wooden structure is erected over the pile, from which hangs strong pulley tackle. The tackle is fixed to a strong screw eye fixed in the head of the pile, which is then drawn by tightening up the tackle with a winch.
A few smart taps on the side of the head of a pile will usually enable it to be drawn with comparative ease.
When a hard substratum underlies a soft soil at no considerable depth, pits are excavated down to this substratum, and in these pits piers of brick, masonry, or concrete are built. The spaces between the piers are spanned by means of arches or steel girders, and upon this foundation the superstructure is raised.
If the soil is sufficiently firm the holes for the piers are excavated in the ordinary manner, the sides of the excavations being timbered as shown in Chapter II (Timbering To Excavations). These excavations are then fitted up with concrete, brickwork, or masonry, the timbering being extracted and the earth filled in where necessary as the work proceeds.
When the soil is of a soft nature, circular shafts are sunk as deep as possible without causing the earth to fall in, and a strong wood curb is placed round the bottom, as shown in Fig. 72. Upon this curb a cylinder of brickwork is built up to the surface of the ground, care being taken to pack the earth well behind the brickwork as the work proceeds. A hole is now dug in the centre of the bottom of this brick cylinder, and a stout piece of timber, called a Sole Piece, is bedded at the bottom. Stout struts are then fixed to this, and placed so as to support the wood curb. The shaft is now excavated to the level of the sole piece, and a second curb is placed round the bottom, upon which more brickwork is built up until the first curb is reached. This operation is repeated until the desired depth is reached.
The following is another method of sinking brick shafts used in very soft soil: A strong curb with a sharp iron cutting edge is bedded on the surface of the ground, and brickwork is built upon it. The soil is excavated from beneath the curb, so that the whole brick cylinder sinks by its own weight. As the sinking continues so fresh courses of brickwork are added until the cylinder has been sunk to the required depth. Great care should be taken to cause the cylinder to sink vertically, and if there is any tendency for it to do otherwise, the part which offers greatest resistance to sinking should be more heavily loaded than the rest.
Sometimes the friction of the earth on the outside of the cylinder prevents it from sinking altogether before the desired depth has been reached, in which case a smaller cylinder is sunk within the first cylinder.
Iron Cylinders or Caissons (see Fig. 73) are more convenient than brick cylinders, being composed of plates which are readily fixed together to form one mass, thus obviating all chance of splitting or of sinking obliquely. These are sunk in a similar manner to brick cylinders, fresh plates being bolted or riveted on as the sinking continues. When the cylinders of brick or iron have been sunk to the desired depth they are filled with concrete so as to form solid piers. These are spanned by means of arches or girders, upon which the superstructure is built.
Fig. 73. Section.