A Treatise On The Principles And Practice Of Harbour Engineering

HARBOUR ENGINEERING. 164 Tituber piles driven to a hard bottom will support a load of 10 cwts. per square inch of cross-sectional area ; if dependent entirely upon the frictional resistance of the ground against its sides, and not upon basal support, the bearing power will vary with the perimeter of the pile; but in any ease not more than 2 cwts. per square inch of sectional area should be imposed. The loads actually due to the substance of a breakwater may be computed from the following table, which gives the weight in Ibs. per cubic foot of various minerals. When completely immersed in salt water, they lose 64 Ibs. of the weight given; but there are circumstances under which the deduction is not justifiable, at anyrate for purposes of calculation. Approximate Weight per Cubic Foot of Mineral Substances. Ibs. Ibs. Ibs. Basalt . . 187 Limestone— contd. Sandstone —contd. Briclr . 115 to 135 Purbeck . 150 Talacre 150 Granite— Chilmark. . 155 York 157 Cornish . 164 Kentish rag 166 Dundee . 159 Aberdeen . 166 Marble 170 Monmouth 168 Guernsey . 187 Magnesian 175 Slate— Limestone— Masonry . 116 to 144 Cornwall. 157 Bath . 120 Sandstone—• Westmoreland . 173 Portland . 130 Red.. 130 Welsh . 180 Chalk . 145 Craigleith 141 Trap rock 170 Fig. 133.—Moulds for Concrete Foundation. into the rock, or by slabs of stone. Surface Treatment. —The surface of a reliable natural foundation generally requires some treatment before it is ready to receive the first course of wall structure. In rock there are always numerous cracks, crevices, and fissures, and a general unevenness of surface. Cavities and pockets containing soft material should be cleaned out and filled with concrete prior to extending the concrete over the entire site. To prevent lateral escape of the concrete while soft, it should be flanked on each side, temporarily, with bags of sand, planking on edge secured to iron pins driven Small irregular apertures may be staunched by packing with clay, or by covering with strips of jute or canvas. Where the top of the rock, however, is not very hard, it may be found preferable to dress it down to a level surface, or to a series of benched beds of sufficient area to receive one or more blocks. Dips should likewise, where possible, be benched out to prevent any tendency of the wall to slide over the sloping surface (fig. 135). The levelling of the surface is, of course, only absolutely essential for blockwork. For walls built of concrete in mass, though benching is desirable,