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thirty feet high, and shows no cracks or other evidences of weakness.

M. Pallu, the founder, certifies that "during the two years consumed by M. Coignet in the building of this church, the béton aggloméré, in all its stages, was exposed to rain and frost, and that it has perfectly resisted all variations of temperature.

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The entire floor of the church is paved with the same material, in a variety of beautiful designs, and with an agreeable contrast of colors.

410. In constructing the municipal barracks of Notre Dame, Paris, the arched ceilings of the cellars were made of this béton, each arch being a single mass. The spans varied from twenty-two to twenty-five feet, the rise, in in all cases, being one-tenth the span, and the thickness at the crown 8.66 inches. In the same building the arched ceilings of the three stories of galleries, one above the other, facing the interior, and all the subterranean drainage, comprising nearly six hundred yards of sewers, are also monoliths of béton.

411. Over thirty-one miles of the Paris sewers had been laid in this material prior to June, 1869, at a saving of 20 per cent., on the lowest estimated cost, in any other kind of

masonry.

The composition of the béton was as follows:

Sand, 5 measures.

Hydraulic lime, 1 measure.

Paris cement (said to be as good as Portland cement),

measure.

412. The works above referred to were visited by the writer in the month of February, 1870, and these statements are based upon close observation and personal knowledge.

Many other interesting applications of this material were examined, of which it is not deemed necessary to make any special mention, except that in combined stability, strength, beauty, and cheapness they far surpass the best results that could have been achieved by the use of any other materials, whether stone, brick, or wood.

In the numerous and varied applications which have been made of it in France, it has received the most emphatic commendations from the government engineers and architects.

413. Its superiority to Rosendale concrete for common work, such as foundations, the backing and hearting of walls, magazine walls, and generally for all masonry protected by earth, and therefore not necessarily required to be of first

quality, lies in its possessing greater strength and hardness at the same cost, and consequently in its being proportionately cheaper when reduced to the same strength by increasing the proportion of sand.

414. Sea-water is nearly as good as fresh water for mixing Portland cements, but injures the Rosendale and all argillo-magnesian cements very considerably.

415. It is of great importance that the incorporation of the lime with the cement should be very thorough, in order to insure a perfectly homogeneous mixture, and this can be obtained with greater certainty by triturating the two together into a thick, viscous paste before the sand is added. In conducting extensive operations the use of two mills of different sizes would perhaps be advantageous, the smaller one being employed exclusively in the preparation of the matrix.

The following proportions may be relied upon to give Coignet bétons of good average quality :—

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416. For foundations and other plain massive work not exposed to view, or where a smooth surface is not specially desired, a liberal amount of gravel and pebbles, or broken stone, may be added to all of the bétons of the above table.

The following proportions will answer for such purposes:

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V.

FOUNDATIONS OF STRUCTURES ON LAND.

417. The term foundation is used indifferently either for the lower courses of a structure of masonry, or for the artificial arrangement, of whatever character it may be, on which these courses rest. For more perspicuity, the term bed of the foundation will be used in this work when the latter is designated.

418. The strength and durability of structures of masonry depend essentially upon the bed of the foundation. In arranging this, regard must be had not only to the permanent efforts which the bed may have to support, but to those of an accidental character. It should, in all cases, be placed so far below the surface of the soil on which it rests, that it will not be liable to be uncovered, or exposed; and its surface should not only be normal to the resultant of the efforts which it sustains, but this resultant should intersect the base of the bed so far within it, that the portion of the soil between this point of intersection and the outward edge of the base shall be broad enough to prevent its yielding from the pressure thrown on it.

419. The first preparatory step to be taken, in determining the kind of bed required, is to ascertain the nature of the subsoil on which the structure is to be raised. This may be done, in ordinary cases, by sinking a pit; but where the subsoil is composed of various strata, and the structure demands extraordinary precaution, borings must be made with the tools. usually employed for this purpose.

420. Classification of Soils. With respect to foundations, soils are usually divided into three classes:

The 1st class consists of soils which are incompressible, or, at least, so slightly compressible, as not to affect the stability of the heaviest masses laid upon them, and which, at the same time, do not yield in a lateral direction. Solid rock, some tufas, compact stony soils, hard clay which yields only to the pick or to blasting, belong to this class.

The 2d class consists of soils which are incompressible, but require to be confined laterally, to prevent them from spreading out. Pure gravel and sand belong to this class.

The 3d class consists of all the varieties of compressible soils; under which head may be arranged ordinary clay, the

common earths, and marshy soils. Some of this class are found in a more or less compact state, and are compressible only to a certain extent, as most of the varieties of clay and common earth; others are found in an almost fluid state, and yield, with facility, in every direction.

421. Foundations on Rock.-To prepare the bed for a foundation on rock, the thickness of the stratum of rock should first be ascertained, if there are any doubts respecting it and if there is any reason to suppose that the stratum has not sufficient strength to bear the weight of the structure, it should be tested by a trial weight, at least twice as great as the one it will have to bear permanently. The rock is next properly prepared to receive the foundation courses by levelling its surface, which is effected by breaking down all projecting points, and filling up cavities, either with rubble masonry or with béton; and by carefully removing any portions of the upper stratum which present indications of having been injured by the weather. The surface, prepared in this manner, should, moreover, be perpendicular to the direction of the pressure; if this is vertical, the surface should be horizontal, and so for any other direction of the pressure. Should there, however, be any difficulty in so arranging the surface as to have it normal to the resultant of the pressure, it may receive a position such that one component of the resultant shall be perpendicular to it, and the other parallel; the latter being counteracted by the friction and adhesion between the base of the bed and the surface of the rock. If, owing to a great declivity of the surface, the whole cannot be brought to the same level, the rock must be broken into steps, in order that the bottom courses of the foundation throughout, may rest on a surface perpendicular to the direction of the pressure. If fissures or cavities are met with, of so great an extent as to render the filling them with masonry too expensive, an arch must then be formed, resting on the two sides of the fissure, to support that part of the structure above it.

The slaty rocks require most care in preparing them to receive a foundation, as their top stratum will generally be found injured to a greater or less depth by the action of frost.

422. Foundations in Stony Ground.-In stony earths and hard clay, the bed is prepared by digging a trench wide enough to receive the foundation, and deep enough to reach the compact soil which has not been injured by the action of frost; a trench from 4 to 6 feet will generally be deep enough for this purpose.

423. In compact gravel and sand, where there is no lia

bility to lateral yielding, either from the action of rain or any other cause, the bed may be prepared as in the case of stony earths. If there is danger from lateral yielding, the part on which the foundation is to rest must be secured by confining it laterally by means of sheeting piles, or in any other way that will offer sufficient security.

424. Foundations on Sand.-In laying foundations on firm sand, a further precaution is sometimes resorted to, of placing a platform on the bottom of the trench, for the purpose of distributing the whole weight more uniformly over it. This, however, seems to be unnecessary; for if the bottom courses of the masonry are well settled in their bed, there is no good reason to apprehend any unequal settling from the effect of the superincumbent weight: whereas, if the wood of the platform should, by any accident, give way, it would leave a part of the foundation without any support.

When the sand under the bed is liable to injury from springs they must be cut off, and a platform, or, still better, an area of béton, should compose the bed, and this should be confined on all sides between walls of stone, or béton sunk below the bottom of the bed.

425. Precautions against Water. If, in opening a trench in sand, water is found at a slight depth, and in such quantity as to impede the labors of the workmen, and the trench cannot be kept dry by the use of pumps or scoops, a row of sheeting piles must be driven on each side of the space occupied by it, somewhat below the bottom of the bed, the sand on the outside of the sheeting piles be thrown out, and its place filled with a puddling of clay, to form a water-tight enclosure round the trench. The excavation for the bed is then commenced; but if it be found that the water still makes rapidly at the bottom, only a small portion of the trench must be opened, and after the lower courses are laid in this portion, the excavation will be gradually effected, as fast as the workmen can execute the work, without difficulty from the

water.

426. Foundations in Compressible Soils. The beds of foundations in compressible soils require peculiar care, particularly when the soil is not homogeneous, presenting more resistance to pressure in one point than in another; for, in that case, it will be very difficult to guard against unequal settling.

427. In ordinary clay, or earth, a trench is dug of the proper width, and deep enough to reach a stratum beyond the action of frost; the bottom of the trench is then levelled off

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