This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1851 edition. Excerpt:...at this part that the earth was insecure; beneath the clay bed, which was not thick, it appeared there was one of sand, and the cutting away a portion of the bed for the concrete completed its weakness. In some cases the ground is rocky, and the engineer is much assisted in his work; but where movable sands or muddy beds are presented for the foundations, great skill and labour is required to form them. Where bridges are thrown across roads or other localities where they are not afFected by currents of water, a layer of concrete, which forms a hard and firm bed, is sufficient to trust the piers upon; but in large rivers all resources are required. Where the bed calls for strengthening, piles of 8 or 9 inches square are driven in until they are almost immovable after several blows of a ram or monkey; these piles are placed 3 or 4 feet apart, and extend over the area intended for the foundation. Another method, called by the French encaissement, is to encompass the area by a series of piles, filling up between them with sheeting piles, so as to form an enclosed space, which is dug out for the depth of a foot, and the hole so made filled with concrete. Such a method is serviceable in a quicksand or movable sand bed, as the sand of itself is firm, and when protected by the encaissement and concrete from the drifting power of the tide, has been found to form a sufficiently strong base for the masonry; unprotected, however, sand has proved very treacherous to the engineer. In the Hexham Bridge, built under Smeaton's direction, the gravel bed, owing to the floods, which he thought were properly provided for, was removed from under the foundation of a pier, and the bridge fell down. A fine bridge at Plymouth, over the Lary, was found to be in imminent...