There are three general types of foundations.
(1) Spread foundations.
(2) Foundations to bed rock by piers or caissons.
(3) Pile foundations.
The form of foundation used depends largely on the charac ter of underlying soil, and the amount and arrangement of the loads and the depths which can be allowed for foundation.
Spread Foundations. This general division covers all forms of construction in which the foundations are spread out suffi ciently, either by offsets of masonry or by steel beam grill to distribute the load with out exceeding the safe-bearing capacity of the soil. Fig. 150 shows a masonry footing and Fig. 151 a grillage footing. Bearing capacity of soils vary consider ably and there are no rigid limits fixing the allowable bear ing values of different kinds of soil. Table XIX. represents good general practice.
In some localities, notably Chicago, footings, if they are to be spread, require the use of beams because of the relatively thin bearing stratum, the low allowable bearing value, and the magni tilde of the loads to be supported. To offset by successive layers of masonry would require too great a depth for the thin layer of hard clay; it thus necessitates the use of grillage beams. In other places either masonry offsets or grillage could be used.
In Boston the usual soil encountered is a stiff blue or yellow clay, 15 feet or more thick and underlaid with a boulder clay of varying depth, but generally of from 15 to 75 feet. Under these conditions footings for isolated columns are very commonly made by offsetting the masonry until the required area is gained. In some cases the water level and a combination of footings may make it desirable to spread by means of beams.
Caisson Foundations. In a yielding soil, or where the area available for spread footings is not sufficient, or where these foot ings would be excessive in size, foundations are often carried to bed rock.
The most common method is by the use of compressed air. Generally steel caissons, of the size of the pier, are used. These
caissons have their edges extending below an air-tight floor, thus forming what is called the working chamber. Compressed air is forced into this chamber which keeps out water and soft material and enables workmen to excavate. The workmen gain access through air-tight shafts with double sets of doors forming an air lock between the pressure below and above; they of course work under the pressure of the compressed air. The material exca vated is hoisted up through shafts and the caisson is sunk by building up the masonry foundation in the caisson at the same time the excavation is going on and this weight sinks it down. When the caisson has reached the grade at which it is to rest, the working chamber is filled with concrete making a solid founda tion.
Pile Foundations. Piles support their load both because of the friction between their surface and the surrounding soil and because of resting on solid stratum at the bottom. In some cases probably the greatest support is from the friction on the surface of the piles. They should be driven into a solid stratum far enough to resist any tendency to side deflection. In some instances, notably in old wharf construction, the piles have been driven through a soft mud perhaps fifteen or twenty feet, and only a few feet into the hard clay below. In such cases the piles have deflected under heavy loads, and have assumed an inclined position, their tops having moved laterally ten or twelve feet. This of course causes failure.
Piles should be driven with care so as to be kept in line, and the blows should not be so heavy as to cause brooming either of the head or point. A number of rules are given for driving piles and for determining the load they will support. Two mules in common use are the following: In determining this last penetration it should be observed that the pile must be driven continuously, as, if allowed to stand some time between blows the soil becomes settled around the pile and the friction thus makes the penetration much less.