Page:Encyclopædia Britannica, Ninth Edition, v. 4.djvu/498

450 BUILDING [GENERAL PRINCIPLES fig. 1, to make each aid the other, was applied to meet the emergency ; but this is limited to the upper parts of the walls. Abutting struts from opposite walls, occurring at intervals only, leave the intermediate portions of the walls exposed to pressure from behind without support, unless these intermediate portions are so disposed as to communicate the pressure upon them to the struts. Hence a common retaining wall, abutted at intervals, would require these intervals to be more or less distant, in proportion to the strength of the wall between them. Instead, therefore, of a continuous wall on each side of the cutting, buttress walls should be placed at intervals, opposite to one another, and strutted apart at their toes by an inverted arch (as in fig. 2) ; and above, at a height sufficient for whatever traffic the cutting is to accommodate, by a built beam of brickwork, in vertical courses, supported on an arch, and prevented from rising under the pressure by an invert upon it, as in fig. 3. This built beam will then be, as it were, a piece of walling turned down on its vertical transverse section, and will resist any pressure brought upon it through the buttress walls, to the full extent of the power of such a wall built vertically to bear weight laid upon its summit; the pressure would be applied in the line of the greatest power of resistance, and there would be no tendency to yield, except to a crushing force. Let such transverse buttress walls, so strutted apart, with the road between them, be the springing walls of longitudinal counter-arched retaining walls, which, being built vertically and in horizontal courses, but arched in plan, against the ground to be retained, will carry all the force exerted against them to their springing walls, and the springing walls or buttresses will communicate, through the struts, the power of resistance of each side to the other, and thus insure the security of both. materials. FIG. 3. Built abutting Beams. This arrangement may be carried to any extent in height, by repeating the abutting beam or strut at such intervals as the thrust to be resisted and the strength of the buttress springing-walls may require. Quantity of To constructions thus arranged, any requisite power may fc e g{ veii) by altering the quantity of materials in each part, the length of the buttresses transversely of the cutting, the number of struts to each pair of buttresses, or the length of the compartments. The thickness of the buttresses should be in proportion to their height and length, and their length should be in proportion to the flatness and weight of the struts with their arches, and to the space in height between any two of them, as well as to the magnitude of the thrust brought to them by the counter-arched retaining walls. The inverted arch below and the built beam above must, of course, have sufficient substance to enable them to resist, without yielding in any direction, the pressure brought to them through the buttresses ; and the retaining walls themselves must have substance given to them according to their height, to the pressure they are liable to receive from behind, to the length of the compartments, and to the extent of their flexure ; subject, of course, as to all these, to the nature of the materials, workmanship, and mode of structure. The positive strength which such constructions should Strength o possess depends much, of course, upon the nature of the tlie con- soil, and its susceptibility of being affected by external stmctions - influences ; but it depends, even in a greater degree, upon the manner in which the constructions can be applied to the ground they are intended to retain. A very slight power will retain at rest a body which the exertion of great force could not stop if once in motion, and a half -brick counter-arch, set in close contact with undisturbed ground, would hold safely up what three times the substance would not stop if there were space and opportunity for motion between the ground and the brickwork. It is impossible, therefore, to state precisely what is the least strength which the retaining constructions must have, but there can be no question that too much strength is better than too little, and it is generally cheaper to pay in materials than in labour to save materials. FIG. 4. Transverse section through the centre of a Bay. The diagrams, figs. 4 and 5, represent a cutting G5 feet Anexamp deep to the level of the rails. It is assumed that the f a cut - ground at the top may stand for the first 15 feet at less ing than 2 to 1, and that it may, therefore, be cheaper to run out to that depth with slopes, leaving 50 feet from the rails, or about 52 feet in all, to be retained. As the bricklayer may follow up the excavator with bay after bay, his work lying mostly on the side and out of the way of the excavator, the latter would run out the spoil without interruption, his work being benched onwards and shored as he proceeded. As every compartment, with its buttresses, invert, abutting beams, and counter-arches is complete in itself, the ground being backed against the counter-arches as the work rises, the shoring would come out, and be sent on for use on the forward benches. The invert may be turned upon footings in half-brick rings, to get the largest quantity of solid resisting matter in the curved line. At a height from the surface of the rails sufficient for headway assumed at 14 feet 6 inches a 14-inch bonded arch is turned from buttress to buttress, springing from skewbacks on corbelled courses. Upon the back of this arch the abutting beam is built of brick on end and edge, bonded as a wall, with beds vertical and widening over the haunches of the discharging arch and under the similar inverted arch turned upon it ; so that although the beam be in the centre but 21 inches deep, it presents an abutment at each end of three times that depth. The object of the invert over the abutting beam is to stiffen it, and to bring down and distribute the weight and pressure from the buttresses more effectually.

The built beam, and its sustaining and stiffening arches,