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.
The strength of such an arrangement, as shown in Fig. 62, depends in great measure upon the strength of the skewbacks. If these are made too small there is a tendency for the piers to slip past the arches, as shown by the broken line.
A good practice is to form the skewbacks of a single block of granite, but where it is not possible to obtain the stone in one piece two pieces may be used jointed in the manner shown in Fig. 63. It adds considerably to the strength of piers to build large horizontal slabs of stone into them at intervals.
When the bearing capacity of a soil is very small, concrete becomes an unsuitable material for foundations on account of its comparatively small power to resist a transverse stress. In such a case planks of oak, beech, pitchpine, creosoted memel or more commonly elm, laid at right angles to the length of the wall, are used as a foundation.
Such foundations are usually formed of two or more layers, the lower one being said to break joint to the full width required, and so to property distribute the pressure, while the upper layer is laid in the direction of the wall and is made a little wider than the footings, the whole being securely spiked together as in Fig. 64.
Pank foundations are calculated in a similar manner to concrete foundations, as the following example will show: A wall 18 inches thick carries a load of 7 tons per foot run and rests upon an elm plank foundation, the safe bearing capacity of the soil being 0.8 tons per square foot. What must be the width and thickness of the plank foundation.
The total load upon the foundation-bed is 7 tons + 4 cwts. (the weight of the footings and planks).
Width of foundation
= Total load in tons / Safe resistance of soil in tons per sq. ft.
= 7.2 / .8
= 9 feet.
The Thickness of the Planks is found from the formula d =
W = 5x.8x20x81 / 2x12 cwts l = 81 / 2 inches b = 12 inches
C for elm = 109 cwts.
. d =
= 7 inches approximately.
It is a good plan to load plank foundations to the amount of the proposed superstructure, to prevent it from sinking when the latter is erected.
Piles are long timbers, varying, for ordinary purposes, from about 10 to 40 feet in length, and shaped as shown in Fig. 13. They may be round or square in section, varying from 9 to 15 inches in diameter. Creosoted memel fir is generally used for piles on account of its suitability and cheapness, but oak, beech, elm, and pitchpine are often preferred.
The piles are usually pointed and shod with iron, as shown in Fig. 65, to prevent them from being split by stones or other hard substances with which they may come in contact when being driven.
The head if square should be neatly trimmed off to a circular shape, while if circular they should be trimmed off to a slightly smaller diameter, and a ring of good malleable iron, varying from about 3/4 to 1 inch in thickness and from 2 to 3 inches deep, is placed on the head of each pile.
Piles are driven by means of a heavy block of iron, called a ram or monkey, varying in weight from 200 to 3300 lbs., which is alternately raised and allowed to fall upon the head of the pile. The pile is supported against the upright member or guide of a pile-driving machine, while a rope is fastened to the monkey by means of a slip-hook and is pulled to a height of from 4 to 12 feet, when the slip-hook is released and the monkey falls upon the head of the pile. As the amount of penetration of the pile lessens under the blows of the ram the fall should be shortened, otherwise the fibres of the timber become strained.
(a) Resistance or Friction Piles is the term applied to piles which are used to support a building by virtue of the resistance of their sides against the earth into which they are driven. They are used when a building is to be erected upon a layer of very soft earth of considerable depth, and vary in length from 15 to 40 feet according to the character of the soil.
The Bearing Power of Friction Piles is usually calculated by one of the numerous formulae which have been devised for this purpose, of which the most popular - probably on account of its simplicity - is that invented by Major Sanders: -
Safe load in cwts. upon each pile = Wh/8d
Where W = weight of ram, in cwts.
,, h = height through which ram falls, in inches at last blow. ,, d= distance through which the pile is driven at the last blow, in inches. This formula is satisfactory if the pile is driven an appreciable distance at the last blow of the ram, but if the amount of penetration is small it gives absurdly high values for the so-called safe resistance, until, when the pile refuses to sink under the blows of the ram, the bearing capacity as given by this formula is infinity, which of course is absurd.