measures, decrease, settlements
There are some measures to decrease the settlements of the buildings. The removal of earth for basements is an effective measure, because no heave and therefore no weakening of the under lying layers lakes place. But the time during which an excavation pit is open should be reduced to a minimum, or the underlying clay may expand by absorption of water. This will increase its compressibility and decrease its shearing strength. When the excavation process is over, a working mat should be placed on its surface at once to prevent the displacement of the clay by moving men and equipment. The second type of a measure for the decrease of the settlements consists in the increase of the unit pressures on the soil at the outer periphery of the building, especially at its corners. A third type of a measure consists in the design of walls for buildings of several storeys, as solid reinforced-concrete girders. If loads are high compared with the allowable soil pressure, the total area of footings can constitute a large fraction of the base area of the structure. Then all the footings can be combined into one foundation known as a mat or a raft. If foundation material is usually compressible, greater settling may take place, if some part or all of the weight of the structure is not compensated by excavating a necessary weight of the material and providing one or more basements. The substructure, consisting of floors, cross walls and columns, can serve as a raft. b) Piles If foundation materials at normal footing or raft level are too weak or compressible, but become more suitable with increasing depth, the structure may be supported on piles. Pre-historic people built pile foundations with stakes driven into the ground. The Romans drove timber piles of great length with drop hammers. Piles transmit foundation loads to deeper soil layers and may be made of timber, concrete, or of steel. They may decay above ground-water level and are often treated with preservation coatings. Piles are driven with drop hammers or more rapidly with the single-acting or double-acting hammers powered by steam, compressed air or diesel engines. Water-jetting or the drilling of a hole may help a better penetration, but final positioning must always be made by impact. The load capacity of a pile may be determined on the basis of past practice in the construction area by test-loading or with formulas based on the penetration of the pile under the last few hammer blows. The piles penetrate to a depth sufficient to provide good support even when materials on the bed surface are temporarily removed. Piles are also used to support bridge piers where the bed of a stream may be much lowered by dredging. The engineers should inspect the piles before driving to see if they satisfy the specifications. The proportions of the load carried down by the piles and by the soil depend on the nature of the soil — stiff soils have greater carrying capacity. c) Piers Piers usually serve the same purposes as piles, but are not installed by hammers. Piers are usually cylindrical shafts of concrete cast in holes excavated by hand or machine, or cast in the form of caissons which are sunk into position. The lower portion of a pier may be widened to distribute pressure. Before hand-dug shafts were widely used where ground water was not a problem. Most of the tall structures in Chicago are founded on concrete piers cast in such shahs. Hand excavation has been largely replaced by rotary or vibratory machine drilling. Under favorable subsoil conditions, drilled piers may be more economical than footings. In materials too hard or too full of boulders for machine drilling, a heavy steel casing is driven or rotated into the ground and the material within is broken up with chopping tools.
