This section is from the book "A Treatise On Architecture And Building Construction Vol2: Masonry. Carpentry. Joinery", by The Colliery Engineer Co. Also available from Amazon: A Treatise On Architecture And Building Construction.
Protection Of Piles. When timber foundations have to be constructed, and the piles are exposed to sea-water, they are likely to be attacked by various wood-boring worms that will penetrate the piles and destroy ordinary limber in from three to five years; but they do not often bore through the bark of piles, which seems to kill them before they can penetrate it.
To prevent the attack of these destroyers, the piles are often treated with creosote, or the heavy oil of tar. The sap and moisture are exhausted from the wood by creating a partial vacuum in an air-tight vessel or tank, in which the piles have been placed, and then forcing the creosote into the pores of the timber under a heavy pressure. By this means, the depredations of the teredo navalis, or sea worms, are almost completely checked.
81. Iron screw piles are often used for building piers and lighthouse foundations. These piles vary from 6 to 16 inches in diameter, and have a screw disk at one end, similar to one turn of a wood auger. They are screwed into the soil, sand, soft rock, coral reef, etc.
82. Sand piles are sometimes used in place of timber piles, especially where the soil is of a soft alluvial clay. The method usually adopted is to drive a pile from 10 to 14 inches in diameter into the ground 6 to 8 feet, and then pull it out; or this hole may be bored by means of a large screw auger. The holes thus formed are filled with damp sand well rammed down. This is done at intervals of 2 or 3 feet all around the walls of the structure to be supported. Owing to the great mobility of the sand grains, they press equally in all directions at any given depth, and hence transmit the pressure to the sides of the hole as well as to the bottom.
Timber Footings On Piles. For footing courses on pile foundations several methods are practised. Fig. 24 shows a timber footing course, or capping, laid below the water level to prevent rotting. The piles a are cut off to an even height, and on these the heavy timbers shown at b are spiked longitudinally, and on top of the longitudinal timbers, the timbers c are laid transversely, and are secured to the first course on the piles. By this method the load is distributed evenly over the top of the piles.
Stone Footings On Piles. Fig-. 25 illustrates footings made of large-sized building stone with level beds. At a is shown the piles and at b the building-stone footings. These stones must in every case rest directly on the piles. Great care must also be taken that one pile comes under each corner of the stone, to keep it from tipping, and that the stone has a full bearing on each pile head. To insure this, the piles must be sawed off perfectly level and all the same height, as no pieces of wood or small bits of stone should be placed under the stones to give them bearing on the piles. Wooden chips crush under a load, and pieces of stone are likely to be broken or dislodged, leaving the block in a state of dangerous instability.
85. In many cases concrete filling is used between the piles, as shown in Fig. 26. After the piles are cut off at the water level, which is shown at a e, the earth is excavated to bf, usually about 2 feet, and the space thus obtained is filled in with the concrete c, well rammed around the sides of the piles d, and leveled off at the top to carry the foundation walls. This method is best adapted to situations where the soil is constantly wet, as then the piles will not become dry and rot.
In some instances the piles are planked over with 3-inch planking-, laid transversely on top of the concrete, and on this the foundation walls are built.