Fig. 126.
261. However proper it may be to give to piers the proper form to cause as little contraction as possible to the water, it is no less necessary to give them strength to oppose the shocks to which they are subject from floating ice, timber and shipping. The best method of breaking up ice, when it comes in large masses, is by inclining the front of the pier, as shown in fig. 126. The angle of the front being inclined from 30° to 50°. The ice running up this slope breaks by its own weight, and falls off on either side.
The foundations of piers may be protected by sheet piling, (see chap. XII.,) or the bottom, if soft, may be dredged out for a few feet and filled in with loose rock.
The form of the down-stream end is not of so much importance as of the upper one, but deserves consideration; as when the water is swift or the bottom soft and yielding, the eddies caused by sharp angles wear upon the soil in a dangerous manner.
CHAPTER XI.
MASONRY.
STONES.
262. The varieties of this material most commonly used in engineering operations are granites, limestones, sandstones, slates, brick, and artificial stones; the latter being made by compounding clays, limes, and cements.
Rock taken from the surface, which has been exposed to the atmosphere, is of an inferior quality to that found at a depth where it has been exposed to a strong pressure; and is consequently denser. Therefore, in opening a quarry it is advisable to excavate upon a hill-side and come at once to the sound stone. Rock is generally found in beds, divided by joints or seams, at which the natural adhesion is broken and the layers are easily separated. When the quarry shows no natural line of separation, one may be produced by drilling a line of holes at equal distances from each other, into which conical steel pins are driven, and the stone splits; the pins being placed in the plane of the required seam.
263. Stone is used almost entirely to resist a compressive strain; as in the voussoirs of an arch, or in the courses of a pier. The resistance of stone to crushing, is as follows:—