"The portions of the north and south walls of the old tidal harbour, extending along the side of the new dock, were retained, but having been founded on clay they were underbuilt to the rock with rubble concrete, to a depth varying between 2 feet and 20 feet. The clay below the walls was excavated back 5 feet from the face of the wall, and the front of the wall was supported from the rock by raking shores. The rubble concrete underbuilding has a uniform thickness of 5 feet, where the depth is less than 10 feet, but for greater depths the underbuilding is 6 feet thick at the top, increasing downwards with the batter of the wall. The excavation was taken out in alternate lengths of about 10 feet, and the clay left between until the blocks on each side were thoroughly set; then the intermediate spaces were excavated and built up. The rubble concrete was built in layers of about 18 inches or 2 feet, until too close to the underside of the old walls for men to go in below; it was then built from the front and the concrete carefully rammed into the back. When the concrete was within 1 inch or 2 inches of the underside of the wall, an additional board, bevelled outwards, was put in the front of the frame; liquid grout of cement and sand was poured in, filling up the small space between the concrete and the wall. This proved most satisfactory, as shown by an examination of the side of each block on excavating the intermediate space. No trouble was experienced in supporting the walls, and no settlement took place during the underbuilding." Equally satisfactory, though attended by more risk, has been the result of underpinning operations as carried out at certain of the Liverpool. docks. Owing to the exigencies of traffic the work had to be done in sections, with the dock full of water, so as to interfere as little as possible with shipping accommodation. Fig. 160 shows a section of the old wall, at the commencement of the underpinning, and fig. 161 is a section of the completed undertaking. It will be observed that the work was carried on under cover of a sheeting dam, described elsewhere (p. 105, ante), strutted and shored to the old wall, at a distance of about 17 feet. Below the level of the dock bottom, an inner trench was excavated between two rows of sheeting piles, one of which was situated at the extreme back of the wall and the other in front of it. Within these limits the underpinning was effected on similar lines to the underbuilding at Ardrossan. The bays were from 10 to 15 feet in length and were dealt with singly, the work being attacked at several points simultaneously. The new work consisted entirely of 6 to 1 Sand Marl Rock Masonry Highest Water Level in Dock Old Deck Sill Datum Rock Former Dock Bottom New Dock Bottom Fig. 161.-Dock Wall as Underpinned. concrete, carefully tongued into the old masonry, the surface of which was well washed and picked rough. When the concrete had been deposited to within 3 feet of the underside of the existing base, the remaining layer was put in, in three sections, advancing from the back towards the front, behind roughly constructed barriers of rubble, the concrete being carefully rammed tight and the whole grouted. Miscellaneous Types of Wall.—It will be as well to conclude the chapter with some miscellaneous examples of the very varied range of types to be found among dock walls. Figs. 162 to 164 are plan and sections of the Albert Dock wall at Hull,* or, rather, the wall as originally designed and only executed for a part of its length, owing to modifications introduced as the work proceeded. This type of wall with an arched front is unusual, and it has obvious inconveniences, though as regards its structural qualities, a broad base with a minimum of masonry was held to counterbalance these drawbacks on a foundation which was incapable of sustaining much pressure. A similar type of wall, consisting of alternate piers and arches, is to be found at Bordeaux. The sections (figs. 165 and 166) of two dock walls at Greenock are selfexplanatory and do not call for any remarks, except that it may be well to add that the quarry refuse filling behind the western tidal harbour *Hawkshaw on "The Albert Dock, Hull," Min. Proc. Inst. C.E., vol. xli. wall was washed in with Portland cement in the proportion of 1 to 15, as high as low-water level. * Fig. 167 is a section of the Alexandra Dock wall at Hull. Originally designed to be constructed with an ashlar stone face and rubble chalk back * Kinipple on "Greenock Harbour," Min. Proc. Inst. C.E., vol. cxxx. TILL Feet 10 5 Figs. 165 and 166.-Dock Walls at Greenock. Fig. 167.-Dock Wall at Hull. Fig. 168.-Quay Wall at Tilbury. 40 TILL 50 Feet ing up to 14 feet below coping, a strike of masons led to the substitution of Portland cement concrete. The upper part of the wall, 14 feet in height, was built as designed with ashlar facing, projecting 6 inches to form a fender, and with granite coping. The weep-holes are at 75 feet intervals.* The section of the tidal basin wall at Tilbury Docks, London, is given in fig. 168. The material used for the bulk of the wall was concrete, composed of 10 parts of ballast to 1 of Portland cement. The concrete work was faced above low-water mark with blue bricks, having a stock brick backing the whole being 9 inches in thickness, with half brick piers, about 4 feet apart, dovetailing into the concrete.† The latest type of Liverpool wall (fig. 169) is built entirely of concrete, with the exception of a granite coping. The hearting is composed of 8 parts of gravel to 1 of Portland cement, with as many burrs or plums of clean The facing, rubble and old masonry as can conveniently be bedded in. 12 inches thick, is of 6 to 1 concrete without burrs. The new wall for the extension of the Manchester Docks is also mainly composed of concrete (fig. 170). It has a blue brick facing above water level, surmounting a limestone fender course. The coping is of granite. * Hurtzig on "The Alexandra Dock, Hull," Min. Proc. Inst. C. E., vol. xcii. |