Construction of composite piles as foundation structures

Many times two materials can be combined to form a single pile. The most common combination is wood and concrete, and the result is known as a compounded pile.

This type of battery can be used to advantage in various conditions. One is if the permanent water head is no more than 70 feet (21 m) below ground level. This is approximately the length limit for the top concrete section of the pile. Another is where the use of wood piles would only require 10 feet (3 m) or more of dry excavation or as little as 4 feet (1,200 mm) of very wet excavation. This can be eliminated by using a compound stack. A third situation is when the total length of the pile is so great that it would be economically impractical to obtain or manipulate wood or concrete piles. Cast-in-place concrete piles up to approximately 70 feet (21 m) in length may rest on wood piles of any obtainable length. Wood piles are difficult to manage and expensive when they are longer than 80 feet (24 m).

The wood pile is driven to ground level, and the head of the pile is equipped with a steel casing. Driving continues through a mandrel or core until the required depth is reached. The mandrel is then removed and the shell is filled with concrete.

The casing fits onto the head of the wood pile in several different ways. One method is to set the end of the casing over the slightly tapered end of the stack head. Another method is to make a tenon with a square shoulder at the end of the stack, place a sealing ring over the tenon, and rest the cover on the ring. Another method involves the use of a wedge ring, which is forced onto the flat top of the pile head.

Sometimes, several piles are gathered together in a group, and then covered with a common concrete cover. Several points relating to this particular operation deserve special consideration.

First, it is generally recognized that when a series of piles are brought closer together, the load value of each is reduced. As a result, a number of building codes have established a minimum center-to-center spacing for grouped piles. These range from 2 ft 6 in. At 5 feet (750 to 1,500 mm), and the latter is considered less desirable.