Page:EB1911 - Volume 10.djvu/762

the band prevents them from splitting (fig. 5). The methods of driving these piles are various. The usual plan is to erect a temporary structure, upon one side of which is a guide path faced with sheet-iron so as to give a smooth face. Up and down this guide path a heavy iron weight, called a monkey, is worked; the monkey is hoisted to the top of the guide path by means of a crab worked by hand or steam, and when released descends with a good force, and so drives the piles into the ground. The monkey usually weighs from 2 cwt. to 10 cwt. and is allowed a drop of 15 to 40 ft.

Fig. 3.

Piles are driven all round under the walls at varying intervals or under piers where the weights of a building are to be concentrated. In the erection of the Chicago public library four Norway pine piles, each with an average diameter of 13 in., were driven to a depth of 521/2 ft. and loaded with a dead load of 50.7 tons per pile for a period of two weeks, and no settlement taking place 30 tons per pile was adopted as a safe load. The following are some examples of loads used in practice: passenger station, Harrison Street, Chicago, piles 50 ft. in length, each carrying 25 tons; elevator, Buffalo, N.Y., piles 20 ft. in length, weight 25 tons; Trinity church, Boston, 2 tons; Schiller building, Chicago, 55 tons per pile, but in this case the building settled considerably. All timber grillage and the tops of all piles should be kept below the lowest water level, and be capped with concrete or stone. In Boston it is obligatory to cap with blocks of granite.

Fig. 4.

Another form of foundation takes the shape of Portland cement concrete blocks, and is used chiefly for bridges and in marshy land, &c. In some cases cylinders of brickwork are built, and the centres are filled with blocks of concrete and grouted in. The Yarmouth destructor cells and chimney shaft Concrete piles. were built in this way; the cylinders were constructed of 9 in. brickwork built in Portland cement, the lower 4 ft. being encased in a wooden drum with cutting edge sunk into the gravel and sand at least 2 ft. The cylinders were sunk by the aid of a grab, the bottom being levelled and the concrete blocks laid by a diver. Use is also made of piles consisting of Portland cement concrete having steel rods embedded in it, and provided with iron shoes and head for driving (fig. 6).

Fig. 5.	Fig. 6.	Fig. 7.	Fig. 8.

Cast iron screw piles (fig. 7) used in very loose sandy soils, consist of large hollow cast iron columns with flat screw blades cast on the lower ends. The projection of this screw from the pile may vary from 9 in. to 18 in. with a pitch of from one-quarter to one-half of the projection, the blade making a little over one turn round the shaft. For most requirements a diameter of screw from 31/2 to 41/2 ft. will be found sufficient, a sandy foundation requiring the largest. The lower end of the tube is generally left open, the edge being bevelled and occasionally provided with teeth to assist in cutting into and penetrating the soil.

Another system of piling known as sheet piling (fig. 8), consists in driving piles into the ground at intervals, and between these, also driven into the ground, are timbers measuring 3 in. by 9 in., which form a wall to keep the soft earth up under the building. In this way the earth is prevented from spreading out and so causing the building to settle unevenly.

Another kind of foundation, known as plank foundation (fig. 9), consists of elm planks, about 9 in. by 3 in. laid across the trench and spiked together; on the top of these are laid similar planks but at right angles to the last, and upon the platform thus formed the wall is built. This method Plank foundations. is used in soft ground.

Caissons are usually employed by engineers for the construction of the foundations of bridge piers, but instances of their use in foundations for buildings are to be found in the American Surety and the Manhattan Life Insurance buildings, New York City. The latter building is 242 ft. high to the parapet, Caissons. and the dome and tower rise 108 ft. higher. The building is carried on 16 solid masonry piers, taken down 54 ft. below the street level to solid rock, and these piers support the 34 cast iron columns upon which the building is erected. The piers to each building were constructed by the pneumatic caisson process (see Caisson).

Fig. 9.

A good plan for foundations when the ground is loose and sandy is to build upon wells of brickwork, a method which has been successfully practised in Madras. The wells are made circular, about 3 ft. in diameter and one brick thick. The first course is laid and cemented together on the Well foundations. surface of the ground when it is dry, and the earth is excavated inside and round about it to allow it to sink. Then another is laid over it and again sunk. The well is thus built downwards. The brickwork is sunk bodily to a depth of 10 ft. or more, according