and materials.

CONSTRUCTION WITH MONOLITHIC BLOCKS.

209

Operations were carried on at a maximum depth of 44 feet. With two gangs of six men, each working alternately in 4-hour shifts, at a cutting 4 feet deep, in stiff clay, the preparation of the foundation for one block occupied about 62 hours.

The masonry of which the blocks were composed consisted of a bulk of irregularly bonded rubble, in pieces not exceeding 2 tons weight, set in cement, with a facing of calp limestone, squared and jointed, the mortar being composed of 4 parts sand to 1 of Portland cement. The blocks were built in wooden frames at a wharf some distance away, and, when ready for depositing, were lifted by a pair of floating sheers. For the purposes of lifting, four wrought-iron suspension bars, 5 inches diameter, having 1-shaped extremities, passed through vertical rectangular holes in each. block, at the foot of which were circular* cast-iron washers, 2 feet 2 inches diameter (figs. 144 and 145), to distribute the pressure. By turning them through an angle of 90° the bars could be engaged or released.

Conveyance was usually made with the block submerged to half its height, thus relieving the strain on the lifting tackle by some 80 to 100 tons. Arrived at the site, no difficulty was found in bringing the block rapidly into its assigned position. Ranging was performed while it was about 3 or 4 inches off the ground, by means of short timber uprights wedged into the dowel grooves at each side of the block. These were brought into line against a horizontal balk, extending from and attached to the blocks already set. Close contact of adjoining blocks was achieved by the use of a small tackle, the average joint in a length of 300 feet being only inch. The dowel grooves were then filled with concrete and the operation was concluded.

The upper portion of the wall was built by tide work to a height of 18 feet 10 inches above equinoctial low water, giving a total height to the This, however, is a later improvement; the earliest type of washer was girder

*

shaped.

wall of 42 feet 10 inches. The coping is of granite in blocks of from 2 to 4 tons weight. The profile of the wall is shown in fig. 146.

The cost of a quay wall constructed in this way and to these dimensions came to £40 per foot run, including 7 per cent. interest on a sum of £33,800 for plant. The rate of construction was 400 lineal feet per annum. On the same principle, but with blocks of smaller dimensions, a quay wall (fig. 147) some 500 yards in length, was constructed at Cork about the year 1877.* * The submerged portion of the wall consisted of three rows of blocks, rectangular in plan, weighing from 35 to 49 tons each. As in the case of the Dublin blocks, they were constructed at a wharf some distance away and transported to their respective positions by a floating sheers. The composition of the monoliths, however, was different, in that they were made entirely of concrete in the proportion of 7 to 1-viz., 5 parts river ballast, 2 parts broken limestone passed through a 3-inch ring, and 1 part Portland cement.

The foundation consisted mainly of fine compact gravel and sand. After being dredged to within 2 feet of the required depth the remaining material was removed by divers. A rectangular frame of angle iron slightly larger than the block was then laid on the ground and adjusted by soundings from above. The surface inequalities within the enclosure were levelled by an iron straight edge.

The blocks (figs. 148 and 149) were suspended by four stirrup-rods passing down vertical grooves, 10 inches by 5 inches, in the sides of the blocks, Barry on "Deep Water Quays at the Port of Cork," Min. Proc. Inst. C. E., vol. c.

*

CONSTRUCTION WITH MONOLITHIC BLOCKS.

21I

which were afterwards used for the reception of 10-inch square stone dowels, 3 to 4 feet long, to connect adjoining lengths. The ends of two small wrought-iron girders in recesses, at or near the bottom of each block, rested in the stirrup-rods, and all were withdrawn together at the close of the setting operations.

The superstructure consisted of a facing of regularly coursed limestone ashlar, backed by 6 to 1 concrete, with a coping of Cornish granite.

Another instance of monolithic construction, with yet smaller blocks of concrete, is to be found at Kurrachee (fig. 150). The dimensions of the blocks were 12 feet by 8 feet by 4 feet, and their weight 27 tons each. Lifting and setting were performed entirely by land carriage with the aid of a Titan, which travelled over the sections of work already executed and deposited the blocks in front of it. The depth of the foundation bed was 15 feet below the surface level of the water, and the blocks were laid in three horizontal tiers or courses to a total height of 24 feet 6 inches. The blocks were not set vertically, but with a slight backward inclination as shown in fig. 150. The sea bottom was sandy at a depth of 25 to 30 feet, and was surmounted with a rubble foundation, levelled by divers, and upon which the blocks were laid.

A similar method was adopted for building the quay walls at Suez. The blocks, which were about the same size as those at Kurrachee, were conveyed to their destination in barges.

Other examples may be quoted from ports in the Mediterranean, at Marseilles and elsewhere. The French were, in fact, the pioneers of the system, when they inaugurated it at Algiers as far back as the year 1840. It is still being practised for harbour work in Algeria at the present time, and the following particulars, furnished by the courtesy of the Engineer in charge, M. Georges Boisnier, relate to a quay wall at the port of Bougie, now under construction (see fig. 151).

The sea bottom is mud to a considerable depth, and in order to obtain a sufficiently broad area for the pressure, a foundation of rubble stone, 111 feet in depth, is deposited within a trench dredged to a bottom width of 55 feet. The wall consists of five tiers of masonry blocks of varying size, only one of which is above the surface of the water. The blocks are constructed on a neighbouring quay with limestone from a local quarry. Those in the

two lowermost tiers weigh about 35 tons each, the upper tiers average 5 tons less. An interval of from three to four months is allowed to elapse

between making and using, when the mortar is composed of hydraulic lime, but only three weeks, when of cement. The blocks are set by a float

ing crane with the assistance of a diver. When the four submerged courses have been constructed, the wall is weighted with a temporary surcharge of two tiers of blocks, which causes the structure to settle bodily to the extent of about 3 feet in a period of two months, at the end of which time the rate of settlement is found to be insignificant, the surcharge is removed and a coping course substituted. The backing behind the wall is of rubble with a covering layer, 3 feet thick, of quarry spalls, above which is discharged the mud dredged from the foundations.

The cost of this type of wall works out to rather more than £14 per foot

run, made up, approximately, as follows:

Experience has shown inadequate stability in a portion of the wall, as constructed above, and several important modifications are being introduced into another section of the same undertaking. The dredged mud is no

8' 3"

་་་་

FAILURES.

213 longer used for any part of the backing, its place being taken by dry quarry rubbish. The blocks are made to larger dimensions, but, in order to facilitate setting operations, they are rendered temporarily lighter than they would otherwise be by the arrangement of voids or pockets in their interiors, as shown by the plan in fig. 152. The lowermost blocks weigh some 50 tons prior to the filling of the pockets with concrete, an operation which is performed when they are in position. The former face batter of 1 in 10, found to be unsuitable for vessels with vertical sides, is now reduced to 1 in 20.

The profile thus adopted may be compared with that of a quay wall at the neighbouring port of Sfax* in Tunis, similarly constructed, but with the face receding in a series of offsets as shown in fig. 153.

The difficulty caused by excessive settlement in walls of this class is well illustrated by the case of a wall at Smyrna, where no less than six or seven tiers of blocks had to be superimposed, instead of four, as originally intended, while the front of the wall had to be supported by a rubble mound carried up to within 7 feet of mean sea-level.

Failures.

Failures of dock walls are by no means scarce, and they often present interesting and instructive features, but, in nearly every case, the cause can be traced to a bad foundation. Movement to a greater or less degree is to be expected, and has been experienced in all walls founded upon any other stratum than hard rock. It is stated as the experience of Voisin Bey, the Engineer-in-Chief of the Suez Canal, that he had never found a long line of quay wall which, on close inspection, proved to be perfectly straight in line and free from indications of movement.

* Baron de Rochemont on "Quelques Ports de la Mediterranée," Int. Nav. Cong., Paris, 1900.