When piles are sunk with the aid of jets of water, the work is accomplished much more simply and rapidly than by driving. A pipe two or three inches in diameter is attached to the side of the pile and connected to a pump. The pile is first covered with pitch and then the water is forced through the pipe under the bottom of the pile, so that the sand is converted to a degree of fluidity that allows the pile to descend rapidly to the desired depth. When one pile has been put down home, the feed pipe is detached and spiked onto another and the operation is repeated. The accuracy and cer tainty of this method is so great, that it is a common practice to make the holes for the bolts in the piles before they are brought into position. After the pile is down and a reason able length of time has been allowed for the churned up sand to subside, it recovers its so lidity and grips the pile so tightly, that it is almost impossible to start it again.
When hollow iron piles are used, the water is conveyed through the centre of the tube un der hydraulic pressure, disturbing the sand under the piles and allowing them to sink by their own weight to the proper depth. Upon withdrawing the pressure and stopping the flow of water, the sand returns to its former con sistency and holds the pile stationary.
Concrete piles have been used during a com paratively recent period, but their success has been so great that their development in the future promises to bring them into general use in the place of timber and iron piles. These piles, as previously mentioned, are either formed in place, or they are molded above ground. A great many methods have been suggested for forming the hole to contain the concrete when the pile is formed in place. One method consists in driving a double shell of metal into the ground and then withdrawing the in ner shell, leaving the outer shell as a mold for the con crete. Another method, shown by Fig. 1, employs a single shell equip ped with a concrete or a steel point. The shell is first driven to the desired depth and then withdrawn slowly and the space it occupied filled with concrete, the surface being kept at a sufficient depth below the end of the tube to maintain the head required to resist the pressure of the ground. Con crete piles used for under-pinning are sometimes sunk with the aid of the water jet to a depth suf ficient to reach firm strata and the bot tom of the excava tion is enlarged by means of a spiral expanding device so as to form a base, as shown in Fig. 2.
Piles made in place may be reinforced, if desired, but usually reinforced concrete piles are formed above ground. They are designed like columns, the reinforcement consisting of vertical bars of steel connected at intervals with horizontal wire rods or plates. They are
made usually in square and triangular cross section, circular molding being too expensive and the arrangement- of the required reinforce ment being too complex. The general char acter of these piles is illustrated by Fig. 3.
The supporting power of depends upon their action as a column resting on a hard base, or upon the friction against their sides devel oped by the gripping action of the material through which they have been, driven. Very often the supporting power is a combination of both actions. The amount of this power may be calculated in various ways, but at their best the results are of necessity only approxima tions. The supporting power of a pile driven to bed-rock is determined by the crushing strength of the material of which it consists, but it it is supported wholly or in part by side friction, as is almost always the case, its sup porting power is calculated by a formula based upon factors' obtained by experiment, or upon the distance penetrated by the pile under the blow of the driving hammer.
The construction of the pile foundations for the Chicago Public Library building afforded valuable information relative to the supporting power of piles as determined under actual con ditions. Piles of Norway pine were driven with a steam hammer having a total weight of 8,300 pounds, the hammer alone weighing 4,500 pounds and delivering 54 blows per minute, with a stroke of 42 inches. The last 20 feet of driving was accomplished by means of a fol lower. The piles were placed about two and one-half feet centre to centre, and the sup porting power of four piles was tested by building a platform on top of them and loading it with pig-iron. Levels were carefully taken on each pile. They stood four days with a loading of six tons on each pile, eight days with a loading of 37 tons per pile, and 10 days with a load of 50 tons per pile. The The settlement did not exceed 0.01 of a foot. These tests indicate that if 250 pounds per square inch be assumed for point resistance, the average frictional resistance will amount to about 3.2 pounds per square inch of side face of pile, or about 432 pounds per square foot. In the case of an ordinary pile, seven inches through the top and 14 inches at the butt, driven in a fairly solid soil, the point of resistance might be 6,000 pounds and the frictional resistance 59,000 pounds, a total earth 'resistance of 65.000 pounds, equivalent to the supporting power of the pile considered as a column and lowing a safe support for 25,000 pounds.