Sand Soil In hard stable sand or gravel, or in compact dry clay above water level, the construction of strong, perma nent foundations require comparatively simple methods. If the location is in cold countries, the preliminary excavation is carried well be low the frost line, and the bottom is carefully leveled off to receive the concrete bed, or the broad footing formed by the lower courses of the masonry.
In the case of compressible soils, artificial means in the form of piles or of beds of con crete are first employed, and upon the plat forms thus obtained the foundation proper is constructed. If foundations have to be con structed upon running sand, a satisfactory method is to make a good concrete floor the entire width of the trench, putting it in quickly, sealing up the sides and then pumping out the water when it reaches the level of the top of the concrete.
Clay Soil The construction of foundations in dry clay calls for the appli cation of the simplest methods; but, in clay strata impermeated with water, and having a tendency to slide, especially in the case of hill side excavations, the problem is difficult and complex. As a rule, such soils do not slide piecemeal, but, where they do move, will slide as a mass with an almost irresistible energy capable of rupturing the strongest timbers as if they were toothpicks. The only way to pro ceed in such cases is to disturb as small a sec tion as possible at a time so that any forward impulse may not be communicated to the entire mass, and to provide strong cross walls in the basement to act as buttresses. The foundations should be carried down to or below the ulti mate drainage level so as to prevent shrinkage by any subsequent draining of the subsoil.
Pile These consist of groups of piles of timber, iron or concrete,plain or reinforced, sunk into the substrata and capped with platforms of timber or concrete upon which the superstructure is finally built.
Timber piles are tree trunks of varying diameter and length, with the bark removed and the knots and lateral branch stems cut off. Iron piles are of two kinds: the screw pile, consisting of a shaft of iron or wood, equipped at the foot with an iron casting in the form of two screw blades ranging in size from one to five feet in diameter; and the disc piles, con sisting of hollow tubes carrying discs at their lower ends instead of screw blades. Concrete piles consist of tapering cylinders of concrete formed in place, and of columns of reinforced concrete of rectangular cross-section made above ground and subsequently driven into place by means of a pile-driver.
Various methods are employed to sink or drive piles into place. Timber piles are driven
by the use of hammers which are raised and dropped by some form of engine. They have also been sunk by the aid ofjets of water under pressure, as suggested by Sir James Brunlees as early as 1850, and first employed by him in connection with the sinking, of iron piles in the construction of the foundations to carry a rail way across the treacherous sands of Morecambe Bay. Some of the most notable deep pile foun dations constructed by the driving method are found in Chicago, where the soil is soft, as those under the Public Library building, and the Illinois Central Railway passenger station. The latter is a structure 180 by 220 feet in plan, 9 stories high, with a tower of 13 stories and a station 3 stories high connected with a train shed 680 feet long. • Borings taken on the site showed the substrata to consist of 10 to 20 feet of rubbish accumulated by dump ing, and below that, several irregular layers of stiff blue clay and quicksand down to bed-rock, more than 60 feet below the surface. These conditions led to the adoption of a deep pile foundation, and about 1,700 piles, arranged, in groups or clusters, were driven under the col umns. These piles ranged in size from 40 to 60 feet in length, and from 11 to 16 inches in diameter at the butt. Thirty-two per cent were black gum, 22 per cent pine, 21 per cent oak, 7 per cent basswood, and 15 per cent hickory, with a few maple and elm. They were driven with drop hammers weighing 2,800, 3,200, and 3,800 pounds respectively, the fall ranging from 35 to 50 feet. A cast-iron cap was fitted over the heads of the piles to prevent them from being crushed or split, but in spite of this pro tection over 8 per cent of the heads suffered serious injuries of that character. The piles were all driven in groups until the tops of all were below the leads, then the driving was completed by the use of a follower. Water was kept running continuously around the pile at the surface during the driving operations and was found to materially aid in the sinking of the piles. After the piles had been driven home, the tops were sawed off to a uniform height of three feet below datum, thus placing all the timber below low water level. As this was at least 10 feet below the surface, the trenches had to be sheathed, and were kept drained by continual pumping. The earth was excavated to the depth of 18 inches below the top of the piles and carefully mixed concrete was tamped in flush with the tops. Finally, oak caps 12 inches square were drift-bolted to the centre of each pile, and the space between the timbers was filled with concrete.