CHAPTER II.

FOUNDATIONS.

THE following chapter on Foundations is intended to furnish the reader with only a general knowledge of the subject, and to enable him to be sure that he is within the limits of safety if he follows what is here given. For foundations of large works, or buildings upon soil of questionable firmness, the compressibility of the soil should be determined by experiments.

The term "foundation" is used to designate all that portion of any structure which serves only as a basis on which to erect the superstructure.

This term is sometimes applied to that portion of the solid material of the earth upon which the structure rests, and also to the artificial arrangements which may be made to support the base.

In the following pages these will be designated by the term "foundation-bed."

Object of Foundations.—The object to be obtained in the construction of any foundation is to form such a solid base for the superstructure that no movement shall take place after its erection. But all structures built of coarse masonry, whether of stone, or brick, will settle to a certain extent; and, with a few exceptions, all soils will become compressed under the weight of almost any building.

Our main object, therefore, is not to prevent settlement entirely, but to insure that it shall be uniform; so that, after the structure is finished, it will have no cracks or flaws, however irregularly it may be disposed over the area of its site.

Foundations Classed.-Foundations may be divided into two classes:

CLASS I.Foundations constructed in situations where the natural soil is sufficiently firm to bear the weight of the intended structure.

CLASS II. - Foundations in situations where an artificial bearing-stratum must be formed, in consequence of the softness or looseness of the soil.

Each of these two great classes may be subdivided into two divisions:

a. Foundations in situations where water offers no impediment to the execution of the work.

b. Foundations under water.

It is seldom that architects design buildings whose foundations are under water; and, as this division of the subject enters rather deeply into the science of engineering, we shall not discuss it here.

Boring. Before we can decide what kind of foundation it will be necessary to build, we must know the nature of the subsoil. If not already known, this is determined, ordinarily, by digging a trench, or making a pit, close to the site of the proposed works, to a depth sufficient to allow the different strata to be seen.

For important structures, the nature of the subsoil is often determined by boring with the tools usually employed for this purpose. When this method is employed, the different kinds and thickness of the strata are determined by examining the specimens brought up by the auger used in boring.

Foundations of the First Class.-The foundations included under this class may be divided into two cases, according to the different kinds of soil on which the foundation is to be built:CASE I. Foundations on soil composed of materials whose stability is not affected by saturation with water, and which are firm enough to support the weight of the structure.

Under this case belong,

Foundations on Rock. To prepare a rock foundation for being built upon, all that is generally required is to cut away the loose and decayed portions of the rock, and to dress the rock to a plane surface as nearly perpendicular to the direction of the pressure as is practicable; or, if the rock forms an inclined plane, to cut a series of plane surfaces, like those of steps, for the wall to rest on. If there are any fissures in the rock, they should be filled with conerete or rubble masonry. Concrete is better for this purpose, as, when once set, it is nearly incompressible under any thing short of a crushing-force; so that it forms a base almost as solid as the rock itself, while the compression of the mortar joints of the masonry is certain to cause some irregular settlement.

If it is unavoidably necessary that some parts of the foundation shall start from a lower level than others, care should be taken to keep the mortar joints as close as possible, or to execute the lower portions of the work in cement, or some hard-setting mortar: otherwise the foundations will settle unequally, and thus cause much injury to the superstructure. The load placed on the rock should at no time exceed one-eighth of that necessary to crush it. Pro

fessor Rankine gives the following examples of the actual intensity of the pressure per square foot on some existing rock foundations:

Average of ordinary cases, the rock being at least as strong

as the strongest red bricks.

20000

Pressures at the base of St. Rollox chimney (450 feet below

the summit)

On a layer of strong concrete or beton, 6 feet deep

On sandstone below the beton, so soft that it crumbles in the hand

4000

The last example shows the pressure which is safely borne in practice by one of the weakest substances to which the name of rock can be applied.

M. Jules Gaudard, C.E., states, that, on a rocky ground, the Roquefavour aqueduct exerts a pressure of 26,800 pounds to the square foot. A bed of solid rock is unyielding, and appears at first sight to offer all the advantages of a secure foundation. It is generally found in practice, however, that, in large buildings, part of the foundations will not rest on the rock, but on the adjacent soil; and as the soil, of whatever material it may be composed, is sure to be compressed somewhat, irregular settlement will almost invariably take place, and give much trouble. The only remedy in such a case is to make the bed for the foundation resting on the soil as firm as possible, and lay the wall, to the level of the rock, in cement or hard-setting mortar.

Foundation on Compact Stony Earths, such as Gravel or Sand. Strong gravel may be considered as one of the best soils to build upon; as it is almost incompressible, is not affected by exposure to the atmosphere, and is easily levelled.

Sand is also almost incompressible, and forms an excellent foundation as long as it can be kept from escaping; but as it has no cohesion, and acts like a fluid when exposed to running water, it should be treated with great caution.

The foundation bed in soils of this kind is prepared by digging a trench from four to six feet deep, so that the foundation may be started below the reach of the disintegrating effects of frost.

The bottom of the trench is levelled; and, if parts of it are required to be at different levels, it is broken into steps.

Care should be taken to keep the surface-water from running into the trench; and, if necessary, drains should be made at the bottom to carry away the water.

The weight resting on the bottom of the trench should be proportional to the resistance of the material forming the bed.

Mr. Gaudard says that a load of 16,500 to 18,300 pounds per square foot has been put upon close sand in the foundations of Gorai Bridge, and on gravel in the Lock Ken Viaduct at Bordeaux.

In the bridge at Nantes, there is a load of 15,200 pounds to the square foot on sand; but some settlement has already taken place. Rankine gives the greatest intensity of pressure on foundations in firm earth at from 2500 to 3500 pounds per square foot.

In order to distribute the pressure arising from the weight of the structure over a greater surface, it is usual to give additional breadth to the foundation courses: this increase of breadth is called the spread. In compact, strong earth, the spread is made one and a half times the thickness of the wall, and, in ordinary earth or sand, twice that thickness.

CASE II.

Foundations on soils firm enough to support the weight of the structure, but whose stability is affected by water. The principal soil under this class, with which we have to do, is a clay soil.

In this soil the bed is prepared by digging a trench, as in rocky soils; and the foundation must be sure to start below the frost-line, for the effect of frost in clay soils is very great.

The soil is also much affected by the action of water; and hence the ground should be well drained before the work is begun, and the trenches so arranged that the water shall not remain in them. And, in general, the less a soil of this kind is exposed to the air and weather, and the sooner it is protected from exposure, the better for the work. In building on a clay bank, great caution should be used to secure thorough drainage, that the clay may not have a tendency to slide during wet weather.

The safe load for stiff clay and marl is given by Mr. Gaudard at from 5500 to 11,000 pounds per square foot. Under the cylindrical piers of the Szegedin Bridge in Hungary, the soil, consisting of clay intermixed with fine sand, bears a load of 13,300 pounds to the square foot; but it was deemed expedient to increase its supporting power by driving some piles in the interior of the cylinder, and also to protect the cylinder by sheeting outside.

Mr. McAlpine, M. Inst. C.E., in building a high wall at Albany, N.Y., succeeded in safely loading a wet clay soil with two tons to the square foot, but with a settlement depending on the depth of the excavation. In order to prevent a great influx of water, and consequent softening of the soil, he surrounded the excavation with a puddle trench ten feet high and four feet wide, and he also spread a layer of coarse gravel on the bottom.

Foundations in Soft Earths. There are three materials in general use for forming an artificial bearing-stratum in soft soils.

Whichever material is employed, the bed is first prepared by excavating a trench sufficiently deep to place the foundation-courses below the action of frost and rain. Great caution should be used in cases of this kind to prevent unequal settling.

The bottom of the trench is made level, and covered with a bed of stones, sand, or concrete.

Stones. When stone is used, the bottom of the trench should be paved with rubble or cobble stones, well settled in place by ramming. On this paving, a bed of concrete is then laid.

Sand. In all situations where the ground, although soft, is of sufficient consistency to confine the sand, this material may be used with many advantages as regards both the cost and the stability of the work. The quality which sand possesses, of distributing the pressure put upon it, in both a horizontal and vertical direction, makes it especially valuable for a foundation bed in this kind of soil; as the lateral pressure exerted against the sides of the foundation pit greatly relieves the bottom.

There are two methods of using sand; viz., in layers and as piles. In forming a stratum of sand, it is spread in layers of about nine inches in thickness, and each layer well rammed before the next one is spread. The total depth of sand used should be sufficient to admit of the pressure on the upper surface of the sand being distributed over the entire bottom of the trench.

Sand-piling is a very economical and efficient method of forming a foundation under some circumstances. It would not, however, be effective in very loose, wet soils; as the sand would work into the surrounding ground.

Sand-piling is executed by making holes in the soil, or in the bottom of the trench, about six or seven inches in diameter, and about six feet deep, and filling them with damp sand, well rammed so as to force it into every cavity.

In situations where the stability of piles arises from the pressure of the ground around them, sand-piles are found of more service than timber ones, for the reason that the timber-pile transmits pressure only in a vertical direction, while the sand-pile transmits it over the whole surface of the hole it fills, thus acting on a large area of bearing-surface. The ground above the piles should be covered with planking, concrete, or masonry, to prevent its being forced up by the lateral pressure exerted by the piles; and, on the stratum thus formed, the foundation walls may be built in the usual

manner.

Foundations on Piles.- Where the soil upon which we wish to build is not firm enough to support the foundation, one of the most common methods of forming a solid foundation bed is