Illustrations of the Croton Aqueduct

by F. B. Tower

GENERAL CONSTRUCTION OF THE AQUEDUCT.

[Plate I]. is a section of the Aqueduct showing the form of the masonry used in earth excavations. The foundation is formed with concrete; the side walls of stone; the bottom and sides of the interior being faced with brick, and the top covered with an arch of brick.

In forming the concrete a mortar is made by mixing three parts of sand with one of hydraulic lime, and then mixing about three parts of stone, broken to a size allowing them to pass through a ring an inch and a half in diameter. Having thoroughly mingled the broken stone and mortar, the concrete is placed in its proper position and form, and brought into a compact state by using a pounder; and is then suffered to remain until it set, or become indurated, before any work is commenced upon it. The object should be to mix as many stones or pebbles as will thoroughly bed in the mortar, allowing none of them to come in contact, but all to be enveloped in mortar. This forms a body which becomes indurated and makes a foundation under the whole length of the Aqueduct like one continuous stone. It attains a degree of hardness which gives it the appearance of the conglomerate bearing the name of Pudding-stone, and is an article of the greatest importance in forming foundations for walls of great weight; superseding in many instances, where the soil is soft, the use of piles or other timber foundation.

Though we have evidence that concrete was used by the Ancient Romans in the foundations of some of their structures and even in the formation of their roads—such as the Appian-Way, and though we find it used in the foundations of the feudal castles of the Norman Barons of England, still it has not been introduced into the general practice of architecture until quite a modern date, and even at the present time is not widely appreciated in this country as a material of so much importance in foundations.

The side walls are laid up in a character of workmanship styled “rough-hammered work;” the stone required to be of sound and durable quality and laid in a manner to render the work water-tight. Though attention is given in some degree to insure a proper bond to the wall, yet the point more particularly attended to, is to make it compact and impervious to water. The bonding of the wall is not by any means disregarded, in all situations where it is required, yet the position of the work generally, where it is in excavation below the natural surface of the ground, renders such precaution of less importance than that of making it compact. The mortar used in these side walls is formed by mixing clean sharp sand with hydraulic lime, using the proportions of three parts of the sand to one of the lime; and these are thoroughly mixed and incorporated before they are wet; when this mixture is wet and thoroughly worked, it is used immediately and always kept properly tempered so as to render it plastic, and to prevent any disposition to become hardened before it is in the wall. After the side walls are finished and the concrete between them has received its proper form, a coating of plastering, about three eighths of an inch in thickness, is put on over the surface of the concrete and on the face of the walls before the interior facing of brick is commenced. The proportions of this plastering are two parts of sand to one of the hydraulic lime.

The bricks used in this work are generally of quite a different character from those used in ordinary house-building; being harder burnt and of a superior quality of material. They are required to be burnt to such a degree of hardness that they present a cherry red, or brownish color, and give a clear ringing sound when struck; and when broken, must present a compact and uniform texture. All bricks brought upon the work which are soft and of a pale color, such as are usually denominated salmon brick, are rejected. Those which are used, possess nearly the hardness and durability of ordinary building stone, and are calculated to resist the action of the water, to which they will be exposed.

The advantage of using brick is, that a smooth channel offering little resistance to the flow of water can be formed with less expense than with stone, and greater security can be obtained against any leakage; for besides the coat of plastering which covers the face of the walls and the top of the concrete, there is also a mortar joint between this plastering and the brick work. The bricks being of good form and easily handled, can be more expeditiously and closely laid than the face of a wall of stone, and afford a smooth and uniform face to the wall with less expense. They are required to be bedded full and flush with mortar, so that on lifting one from its position in the work, no imperfections be discovered, but the impress of the brick be found distinct throughout.

The proportions of the mortar for the brick work, are two parts of sand to one of hydraulic lime.

The inverted arch of brick, as well as the brick facing on the sides, is four inches thick, and the roofing arch of brick is eight inches thick.

After the masonry is finished the excavation which was done to receive it, is filled up around it, and over the top of the roofing arch generally to the height of 3 to 4 feet, and in some instances of deep excavation, up to the natural surface. If the natural surface be not of sufficient height for the top of the earth covering, the earth is raised to the requisite height with proper width on the top and slopes on the sides for protection to the Aqueduct masonry.

I

F. B. Tower.

Gimber.

II

F. B. Tower.

Gimber.

III

F. B. Tower.

Gimber.

IV

F. B. Tower.

Gimber.

[Plate II]. is a section of the Aqueduct in open cutting in rock.

After the rock has been excavated to the required depth and width, the bottom is levelled up with concrete to the proper height and form for the inverted arch of brick, which is laid in the manner before described for earth excavation. The side walls of stone and brick are bonded together by headers of brick entering the stone walls as shown in the drawing, and the walls of stone are built closely against the sides of the rock and forming a junction with it. On the exterior of the roofing arch a heavy spandrel of stone masonry (of the same character as the stone walls beneath it) is built, filling the space between the arch and the rock. After the masonry is finished, the rock cut above it is filled with earth to the same height above the roofing arch as mentioned for earth excavation.

[Plate III]. is a section of the Aqueduct in tunnel cutting in rock.

The width of the tunnel excavation in rock is the same as that of open excavation in rock; and the manner of building the masonry to form the channel-way is the same, with the exception that the rock roof of the tunnel serves as the roof of the channel-way, where it is sound, but in cases where the rock is soft and liable to fall, a brick arch is built over the channel-way, and the space between its extrados, or outer surface, and the rock roof is filled with earth closely rammed in. In some instances where the tunnel perforated rock which was at first quite hard, the roofing has by exposure to the air, become soft and insecure, so as to render it necessary to turn an arch for its support. This is attended with inconvenience and some difficulty after the channel-way has been completed and closed through the tunnel.

[Plate IV]. is a section of the Aqueduct in tunnel cutting in earth.

When the earth is dry and compact, the excavation for the bottom and sides is made of a proper form to receive the masonry, which is built closely against it: the top is excavated sufficiently high to give room to turn the arch, and the space above is afterwards filled with earth closely rammed in. Where the earth is wet and there is difficulty in making it stand, the excavation is made larger, and props of timber and plank are used to support the top and sides until the masonry be completed; and the whole space exterior to the masonry is then compactly filled with earth.

[Plate V]. is a section of the Aqueduct showing the manner of constructing it across valleys, or where the natural surface of the ground falls below the plane of grade.

In such cases the Aqueduct is supported upon a foundation wall of stone laid dry, and formed by using large stones laid in positions to give proper bond, and to allow small broken stone to be closely packed in, filling up all the interstices so as to form a compact and uniform mass. The wall is generally allowed to stand some months after it is completed, before the masonry of the Aqueduct is commenced upon it, lest by this weight being placed upon it before it has found its bearing, it should settle and cause cracks in the masonry. That such settlement should in some instances occur, even after the Aqueduct is completed, is not surprising, for passing over so many different elevations, and encountering such numerous transitions from a hard soil, or from rock, to valleys of alluvial deposit, it would be beyond human powers of foresight and vigilance to prevent it.

V

F. B. Tower.

Gimber.

To render the Aqueduct more secure in such positions, the concrete foundation has an increased thickness, and in preparing it a greater proportion of hydraulic lime is used; the proportion being two and a half parts of sand to one of lime. The dimensions of the stone side walls and the spandrel backing of the roofing arch, are also increased; and the proportion of hydraulic lime to the sand in the mortar for these is increased. Another precaution has been taken to render the work secure, by plastering the interior of the Aqueduct over these foundation walls. The embankment adjacent to foundation walls has various slopes according to circumstances, and is generally protected with a dry stone wall on the face, and is carried up of sufficient width to insure the requisite covering over the Aqueduct masonry.

Along side hills an excavation is made for the Aqueduct into the hill, and a protection wall of stone built on the lower side so as to support a covering of earth over the masonry; great care being taken to obtain a deep and firm footing for this wall in order to render the work secure. In such a position the Aqueduct is perhaps less secure than in those before described. Where the soil is wet from springs, and the formation clay, there is danger of slides; and in rainy seasons there is danger from the torrents which gather on the hill sides and come down with destructive force: the earth covering is liable to be carried away, and the Aqueduct itself to be undermined. Great care has, however, been used in such cases to form strong paved channels for the passage of the water over the top of the Aqueduct, or by culverts to pass it underneath.