The sand absorbs the impact of the hammer so as to permit the piles to be driven safely, and it raises the hood sufficiently above the top of the pile to permit the reinforcement rods to extend beyond the concrete for connection with the superstructure.
Concrete Piles Formed in Place. Fig. 9 shows this type of pile. The hole is made by driving with an ordinary pile-driving apparatus, a sheet steel tube tapering from 20 inches at the top to 6 inches at the bottom, the tube is driven by means of a col lapsible core which is with drawn. When the desired depth is reached, the tube is then filled with concrete. Fig. 10 shows another method of forming this type of pile. A sheet steel shell is formed by telescopic sec tions, each section is 8 ft. long and has at its upper end projections which en gage with projections on the lower end of the next section. To the bottom sec tion is attached a cast iron point with a jet hole or nozzle, to which is fitted a 21-in. pipe, this pipe is rs and remains in place in the finished pile, strength. The shell is sunk by water jet and Timber piles are driven either point or butt end down; the latter is considered the better method. When piles are directed to be sharpened the points should have a length of from one and a half times to twice the diameter.
To prevent the head of the pile from being broomed or split by the blows of the driving-ram it is bound with a wrought-iron hoop, 2 to 3 inches wide and to 1 inch thick. Instead of the wrought iron band a cast-iron cap is sometimes used. It consists of a block with a tapering recess above and below, the chamfered head of the pile fitting into the one below, and a cushion piece of hard wood upon which the hammer falls fitting into the one above.
When brooming occurs the broomed part should be cut off, because a broomed head cushions the blow and dissipates it without any useful effect. Piles that split or broom excessively or are other wise injured during the driving must be drawn out.
Bouncing of the hammer occurs when the pile refuses to drive further, or it may be caused by the hammer being too light, or its striking velocity being too great, or both. The remedy for bouncing
is to diminish the fall.
Excessive hammering on piles which refuse to move should be avoided, as they are liable to be crippled, split, or broken below the ground. Such injury will pass unnoticed and may be the cause of future failure.
As a general rule, a heavy hammer with a low fall drives more satisfactorily than a light one with a high fall. More blows can be made in the same time with a low fall, and this gives less time for the soil to compact itself around the piles between the blows. At times a pile may resist the hammer after sinking some distance, but start again after a short rest; or it may refuse a heavy hammer and start under a light one. It may drive slowly at first, and more rapidly afterwards, from causes difficult to discover.
The driving of a pile sometimes causes adjacent ones previously driven to spring upwards several feet. The driving of piles in soft ground or mud will generally cause adjacent ones previously driven to lean outwards unless means of prevention be taken.
A pile may rest upon rock and yet be very weak, for if driven through very soft soil all the pressure is borne by the sharp point, and the pile becomes merely a column in a worse condition than a pillar with one rounded end. In such soils the piles need very little sharp ening; indeed, they had better be driven butt end down without any point. Solid metal piles are usually of uniform diameter and are driven with either blunt or sharpened points.
Piles are driven by machines called pile drivers. A pile driver consists essentially of two upright guides or leads, often of great height, erected upon a platform, or on a barge when used in water. These guides serve to hold the pile vertical while being driven, and also hold and guide the hammer used in driving. This is a block of iron called a ram, monkey, or hammer, weighing anywhere from SOO to 4,000 pounds, usually about 2,000 to 3,000 pounds. The accessories are a hoisting engine for raising the hammer and the devices for allowing it to drop freely on the heads of the piles.