In the "Raymond" pile, a collapsible core, the size and shape of the pile desired is enclosed in a thick, closely fitting, steel shell, and driven with a pile driver in the usual manner. After driving, the core is withdrawn, leaving the shell in the ground. This shell is then filled with concrete and should reinforcement be used, which is seldom with this pile, it is placed before the filling commences. The shell of these piles must be of sufficient strength to hold its shape after the withdrawal of the core.

The "Simplex" system consists in driving an extra heavy iron pipe into the ground with a special point to exclude the dirt; when this pipe is driven to the proper bearing, a drop-bottom bucket, filled with concrete, is lowered to the bottom of the pipe and dumped. The bucket is then removed, a heavy weight lowered into the pipe, and the pipe raised nearly to the top of the concrete, the weight being repeatedly dropped on the concrete, thus forcing it out of the end of the pipe. Another bucket of concrete is then placed in the pipe and the operation repeated until the pile is formed.

Two kinds of points are used, depending upon the character of soil through which the pipe is driven. If in soft soil, a castiron point closes the end of the pipe, while in stiff clay a pair of jaws are used. These jaws are attached by hinges to the bottom of the pipe, and automatically open to permit the concrete to flow through them as the pipe is raised. The pipes used are extra heavy steel, banded where necessary, and are made up in sections of varying lengths to suit the length of pile required.

Relative Advantages and Disadvantages.-The chief objection to the cast-and-driven pile is that it may possibly be injured in driving. Careful driving will, however, prevent this. The advantages of piles of this type are that the piles may be of any length, and can be thoroughly inspected during manufacture.

The chief objections to the cast-in-place pile, where a shell remains, are: First, the danger of the light shell collapsing; second, the dropping of the concrete through such a height may cause separation. The first objection is overcome by using sufficiently heavy shells and inspecting with an electric light before filling begins. As the concrete is usually a very wet mixture and deposited in very small quantities, the second objection is more fancied than real.

The disadvantage of a cast-in-place pile unprotected by a shell is the impossibility of any sort of inspection. A wet soil may carry off some of the cement, and a dry soil may absorb the water necessary for the proper setting, in either case resulting in a pile of decreased efficiency. The rough surface of a pile of this sort greatly increases its bearing capacity.

Compressed Pillar.-The compressed system, controlled by the Hennebique Co., consists of making a hole by dropping a twoton perforator. In very soft soil, clay, cinders, and broken stone are dumped into the hole from time to time and compressed by the

FIG. 76.—Concrete Pile with Enlarged End Showing Progressive Stages in Driving.

perforator against the sides of the hole, thus forming an almost water-tight lining. The hole is thus carried down a suitable depth, concrete is then placed in it, and is rammed with a drop tamper. This results in a pillar of large diameter, in which the concrete is forced into the surrounding soil, thus greatly increasing its bearing power. Both perforator and tamper are operated by a piledriver.

When soft material overlies strata of firm material, the compressed system is particularly advantageous, as by its means a large pier, resting on the firm soil and extending through the soft strata, results.

Caissons.-Where all other methods of securing a satisfactory foundation fail, caissons, either open or pneumatic, carried down to bed rock or hard pan, are used.

An open caisson is a strong, water-tight, bottomless box, usually constructed of steel or timber. It is sunk by excavating the material inside of it, and if necessary by adding additional weight at its top.

Open caissons of reinforced concrete have been used in many instances, notably in the Cockle Creek Bridge, New South Wales, and in the Catskill Aqueduct, in New York State.

In the Cockle Creek Bridge, two open cylindrical reinforced concrete caissons were driven through a depth of 36 ft. of silt, sand, and gravel to hard clay. When finally seated in this clay, they were filled with concrete and used as piers for the bridge.

In the Catskill Aqueduct, three open reinforced-concrete caissons were sunk in constructing the Rondout Siphon. These caissons were sunk to rock, by excavating, under ordinary air pressure, the material within, and allowing the caissons to sink of their own. weight. When the aqueduct is completed two of the caissons will serve as part of the permanent lining.

Pneumatic Caissons.-A pneumatic caisson is a strong, watertight box, open at the bottom and closed at the top. This forms a working or air chamber. Usually the sides of the caisson are continued above the top, thus forming a second box closed at the bottom but open at the top. This is called the cofferdam. The pier is built within this cofferdam and on top of the caisson as the sinking progresses. The working chamber is supplied with compressed air which serves the double purpose of forcing out all water, and supplying the men with the necessary fresh air.

Pneumatic caissons are usually constructed of steel or timber, though a few have been made of reinforced concrete.

Reinforced concrete was recently used in the construction of the large tunnel caisson on the Jersey shore connecting the tunnels of the Hudson Co., crossing the Hudson River.

Caissons are mostly used in constructing the foundations of bridges and high buildings. When the work is under water or in water-bearing soil, the pneumatic caisson is usually used, although at times an open caisson and a bucket dredge are substituted.

Cribs. A crib is usually a timber grillage, which instead of being built in place, is first constructed, then floated to its final resting-place and sunk in a single mass. The superstructure is then built on the crib, either in the open or in a caisson, and the function of this crib is to distribute the load carried by the superstructure over the foundation bed.

While cribs are usually constructed of timber there is no reason why reinforced concrete could not be used with economy.

CHAPTER XXII

CONCRETE RETAINING WALLS, ABUTMENTS, AND BULKHEADS

Design of Walls in General.-Methods of Failure.---Kinds of Retaining Walls.-Design of Gravity Walls.-Reinforced-Concrete Walls.-Details of Construction.— Foundations.-Abutments.-Bulkheads.-Appearance of Walls.-Tables for De

sign of Walls.

UNTIL the advent of concrete, retaining walls for the support of embankments and cuts as well as reservoir walls, bulkheads, etc., were constructed of rubble or ashlar masonry laid with or without mortar as the importance of the problem demanded. Concrete, especially when reinforced, has supplied a material which gives a far greater power of resistance, occupies a minimum of space and may be built at a much lower cost.

The design of concrete retaining walls follows the same general methods that are employed for ordinary masonry, the design being based upon the action of the wall when the load caused by earth, water, or other material from behind, comes upon it. Certain conditions of failure deduced from observation, experience, and mathematical reasoning are assumed to be possible and the wall so proportioned that it will be safe against any and all such possible failures. Thus it is assumed that:

Assumptions Made in Design.-A wall holding up a bank of earth, or water will be subjected to a pressure: the amount of which will depend upon the depth of the wall below the surface and upon the weight and mobility of the material pressing against it. The question as to how much pressure is produced by banks of earth resting against walls has given rise to much discussion, and even to-day there is no general agreement among engineers as to what this pressure is. The difficulty arises from the fact that earths vary so much, their weight, consistency, and cohesive power are so constantly changing with change of the contained water, that no general pressure rule can be applied. It is thus that most com