Springing and Bouncing

"Springing" means that the pile vibrates too much laterally from the blow of the hammer. Springing may occur when a pile is crooked, when the butt has not been squared off properly, or when the pile is not in line with the fall of the hammer. In all pile-driving operations, ensure the fall of the hammer is in line with the pile axis; otherwise, the head of the pile and the hammer may be damaged and much of the energy of the hammer blow is lost.

Excessive bouncing may come from a hammer which is too light. However, it usually occurs when the butt of the pile has been crushed or broomed, when the pile has met an obstruction, or when the pile has penetrated to a solid footing. When a double-acting hammer is being used, bouncing may result from too much steam or air pressure. With a diesel hammer, if the hammer lifts on the upstroke of the ram piston, the throttle setting is probably too high. Back off on the throttle control just enough to avoid this lifting. If the butt of the timber pile has been crushed or broomed more than an inch or so, it should be cut back to sound wood before driving operations continue.

Driving Bearing Piles in Groups

Bearing piles are frequently driven in groups, as in a pile group which will support a column footing for a building or in closely spaced rows, as beneath a wall. When piles must be driven in closely spaced groups, these principles are observed:

1. When a pile is driven into sand or gravel deposits, the soil must be compacted or displaced an amount equal to the volume of the pile. If the deposit is quite loose, the vibration of pile driving frequently results in considerable compaction of the soil. The

surface of the ground between and around the piles then may subside or shrink. This action may result in damage to the foundation of nearby structures. If piles are driven into dense sand and gravel deposits, the ground may heave.

2. Clay soils are hard to compress in pile driving; hence, a volume of soil equal to that of the pile will usually be displaced (fig. 12-68). The ground will heave between and around the piles. Driving a pile alongside those previously driven will frequently cause those already in place to heave upward. If the piles are driven through a clay stratum to firm bearing beneath, the heave may destroy the contact between the tip of the pile and the firm stratum. Such cases maybe detected by taking a level reading on the top of the piles previously placed. Piles which are raised appreciably should be redriven to _{a firm bearing.} Soil displaced by the pile may cause enough lateral force to move previously driven piles out of line.

3. The sequence of driving piles in groups should be as follows: . Driving should progress from an area of high resistance to one of low resistance, toward a stream, or downslope to reduce the shoving of previous driven piles that are out of place when succeeding piles arc driven. l Outer rows in the group should be driven first if the piles derive their main support from friction. Inner rows are driven first if the piles are supported from a point bearing.

Figure 12-68.-Displacement of clay soil caused by driving solid piles.

Obstruction and Refusal

The condition reached when a pile being driven by a hammer has a 1-inch penetration per blow or zero penetration per blow (as when the point of the pile reaches an impenetrable bottom such as rock) or when the effective energy of the hammer is no longer sufficient to cause penetration (hammer is to light or velocity at impact too little), under which circumstances the pile may cease to penetrate before it has reached the desired depth is known as refusal. Further driving after refusal is likely to break or split the pile, as shown in figure 12-69.

When a pile has been driven to a depth where deeper penetration is prevented by friction, the pile has been driven to refusal. A pile supported by skin friction alone is called a friction pile. A pile supported by bedrock or

Figure 12-69.-Pile damage caused by overdriving timber piles.

an extra dense layer of soil at the tip is called an end-bearing pile. A pile supported partly by skin friction and partly by a substratum of extra dense soil at the tip is called a combination end-bearing and friction pile.

It is not always necessary to drive a friction pile to refusal; such a pile needs to be driven only to the depth where friction develops the required load-bearing capacity.