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 pile

driver.

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 a caisson, and the function

built on the crib, either in the open or in 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 Design 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 be built at a much lower cost.

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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

putations for earth pressure assume a theoretical condition, that of perfectly dry sand, and yet this condition is but seldom found, but as it gives safe values, its assumption is justified.

When a bank of such sand has an unrestricted surface its sides will assume a natural slope of about 1 1/2 feet horizontal to I foot vertical. This is referred to as the "Angle of Repose," or "Angle of Friction."

If a wall is placed at the edge of a bank and the space between the back of the wall and the bank filled in, this earth or "backing" will tend to slide along the line of repose, and thus produce a pressure against the wall. The upper half of this prism is considered as producing the maximum pressure effect on the wall and its weight is employed in computing this pressure.

Effect of Earth Pressure.-Mathematical investigations have determined:

I. That the entire effect of this pressure may be considered as concentrated at a point 1/3 the height from the bottom.

II. That this pressure will tend to either slide or push the wall bodily out of place, or to rotate it about its toe and overturn, or both.

III. That since the wall is rigidly constructed and cannot yield, the effect of the external pressure is to induce strains in the material of the wall.

IV. That the material of the wall can resist safely certain specified strains per unit of area of material such as the square inch or square foot, the amount of such safe strains varying with the kind of strain and the material.

V. That the foundation material must not be subjected to unsafe strains. From these assumed conditions the dimensions of the wall are fixed so that the strain in the material will never exceed what it can safely stand. It is thus seen that the following methods of failure are possible.

Methods of Failure.-A retaining wall may fail in one or more of the following ways:

1. By revolving about any horizontal line in the face. This is the most frequent mode of failure, and it is due to the overturning moment, due to the earth backing being greater than the righting moment of the wall itself. A failure of this type indicates too light