Practical masonry, bricklaying and plastering, both plain and ornamental : containing a new and complete system of lines for stone-cutting... by Nicholson Peter 1765-1844

Author:Nicholson, Peter, 1765-1844
Language: eng
Format: epub, pdf
Publisher: London : Thomas Kelley, Paternoster Row
Published: 1841-03-25T05:00:00+00:00

Such a communfcation would either be found convenient or necessary, where some intervening object would render the conveyance dangerous or extremely expensive, or even, in some cases, impossible. By means of a tunnel, a stream of water may be conducted from a given entrance on one side of a hill to a given outlet on the other ; or a passage of communication between the opposite shores of a deep river, without interrupting its navigation, may be effected.

If the object of the tunnel be to conduct vessels by means of water, it is evident that the bed of the fluid must be nearly in a horizontal plane, and if it be required to effect the passage in the least time, its direction ought to be in a right line. This mode of communication would also be found necessary in conducting a roadway through a hill, which would otherwise be found insurmountable.

The figure of the section of such an aperture will depend greatly upon the soil through which it is cut; but whatever be the form of the right section through any particular point, the same figure must invariably continue, unless the quality of the substance is found to vary. In this case, the figure of the section must be such as will meet every contingency. The sides of the aperture ought always to be lined with brick or stone, and the top ought to be arched. If the soil be sand, a tunnel cannot be constructed so as to support the superincumbent mass; and therefore, in this case, such a conveyance, though ever so desirable, ought never to be attempted. If the soil be clay or solid earth, the figure of the right section of the tunnel must be of such a form all round that the mutual pressure upon the parts may balance one another ; or by making allowance for the convenience of practical operations, the figure ought to be as near as possible to the curve of equilibrium for the support of such a surrounding mass. If the substance be rock through which a tunnel is to be cut, it will not be necessary to form an arch all round, the bottom may be paved in the usual manner, and the sides which form walls may be built upon the solid rock.

In executing a tunnel, it is evident that no more of the ground should be cut away than is absolutely necessary to effect a part of sufficient length for the operation of walling and arching, and thus the building and the excavation must be carried on alternately at regular intervals. The lengths excavated at a time before the brick-work is built must depend upon the nature of the ground, and may be from 3 or 4, to 7 or 8, feet, in the fine of direction of the tunnel, observing the quality of the ground with the utmost circumspection, so as not to endanger its falling by taking away too much at a time.

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