FOUNDATION. The term designates either the lower courts of a masonry structure, or a specially prepared surface or bed in con tact with the soil or bed-rock upon which a building or other weighty structure of any kind is to be built. In practice there are many cases, however, in which the bed and the lower courses of the masonry structure jointly com prise the foundation proper, and render difficult or unnecessary the drawing of any marked dis tinction between them. The earlier Romans erected their buildings on the most solid foun dations constructed of large blocks of concrete, composed of quarry rubbish, gravel or burnt earth, bonded by an excellent mortar. This material formed under the superstructures homogeneous basements of veritable artificial rocks capable of sustaining the heaviest of buildings without rupture or settlement. Dur ing the later Roman period, however, the foun dations were much neglected, so that the archi tests of the 12th century afforded many ex amples of important edifices fallen on account of bad foundations.
The architectural and engineering works of to-day have passed from the types of com paratively light superstructures to those of mammoth size and enormous weight, requir ing the strongest and most permanent founda tions that modern engineering skill is capable of designing, and the application of reinforced con crete or concrete steel methods to the construc tion of practically all classes of foundations appears to represent the best practice of modern times.
The employment of a particular type of foundations depends upon the character of the soil and thepresence of water, and the widely varying conditions met with in practice have developed several classes of foundation struc tures which may be briefly designated as peat foundations, sand foundations, hard soil foun dations, pile foundations, etc.
Foundation These vary in char acter from hard and solid bed-rock, hard-pan, and firm sand to liquid mud, quicksand and silt. It is clear that hard bedrock, hard-pan, firm sand and various kinds of compact clays are the best materials to sustain foundation struc tures; but practical experience has very satis factorily demonstrated that almost every sub stance in nature is capable of supporting the weight of other substance, no matter how small the sustaining capacity of that material may be, provided the weight to be sustained is distrib uted over a sufficiently large area, and provided the conditions of the soil are permanent.
The sustaining power of soils depends upon their composition, the amount of water which they contain or which may drain through them and the degree to which they are confined. Sound hard bed-rock of ledge formation will support loads up to 36 tons per square foot, but if the rock is seamy, rotten or in slippery inclined layers its sustaining capacity will be materially less. In general, the latest practice assumes the following safe allowable pressures: For hard-pan, eight tons per square foot; com pact sand and clean gravel free from lateral movement, five tons per square foot; dry clay, three tone per square foot; and loam, one ton per square foot. Soft, watery clays, mud, quicksand and silt have very little or no sus taining power, and have to be penetrated until firmer material is reached and require to be compacted by draining, or consolidated by other means. If piles are employed, and they are driven to bed-rock or to refusal, the sustain ing power is determined by the crushing strength of the material of the pile—timber, iron or concrete, as the case may be.
Peat Soil In soils such as peat, it is almost impossible to carry the walls down to a sufficient depth to reach a solid base. In such cases one of three methods has to be adopted—laying a strong concrete floor spread over a sufficient area; planting masses of iron or brickwork; or driving of piles. If a con crete floor is employed, it may cover the entire surface to be occupied by the building, and even extend to some distance beyond the foot ings of the walls in order to prevent cracks and settlements. These injuries are usually caused by heavy walls being placed too close to the edge of the concrete floor, causing it to buckle and crack, and to settle irregularly under the unequal weights of walls of different thick ness. This condition is somewhat obviated un der the French system by forming a lip under the edge of the concrete floor, converting it into a kind of inverted tray which confines the substratum within its limits.