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11 inches in greatest height, but chamfered off at their rear and lower corner so that their vertical rear-face is only 3 inches. These run horizontally around the dock, in steps 8 inches high and 10 inches wide, and constitute the altars. These altars are bolted to side timbers that rise at the same slope of 39° from the edge of the flooring. Where the altar crosses the side timber, two bolts are driven, one through the tread of the altar vertically, and a second one through the riser of the altar diagonally, so as to enter the brace-timber normally. Four piles support each bracetimber at equidistant points of its length, and its lower end abuts on the flooring of the dock against square longitudinal timbers bolted thereto and representing the bottom altar. A mass of concrete, varying from 2 to 5 feet in thickness, rises under the altars for a height of 6 feet from the bottom of the dock; above this the backing of the altars is of puddled clay. Behind the coping, tongued and grooved sheet-piling is driven to a distance well below the floor-level, so as to completely surround the dock. This gives a total enclosed width of 182 feet 4 inches. Four rows of piling with cross caps are driven within the area between the sheet piling and the coping, and diagonal braces are carried from the centre of these caps to the centre of the cross-brace timbers that carry the altars.

The dock is closed by a floating caisson. This is an iron vessel with sloping stern-pieces, exactly fitting the dock-entrance. Heavy rubber packing is carried around the entrance-sills and abutments, against which the caisson bears. No grooves are used, the working of the dock being thus greatly facilitated. Two sills are provided, an outer and an inner one. This is to enable the inner or main sill, which is generally used, to be repaired. The caisson can close the dock from either sill.

The machinery for emptying the dock consists of two centrifugal pumps, 42 inches in diameter, driven by two vertical engines having cylinders 28 inches in diameter, with a stroke-length of 24 inches. The pumps have a capacity of 80,000 gallons a minute, and can empty the dock in ninety minutes when no vessel is in it. The caisson is raised or lowered by pumping out or admitting water-ballast, a small engine, boiler, and rotary-pump being contained within it. The same engine works a capstan on the deck of the caisson.

The dock, which was begun on the 16th of December, 1867, is expected to be finished by the 1st of February, 1890.

F. G. D.

Measures adopted for the Safety and Service of Reservoir-Dams. By Dr. P. KRESNIK.

(Wochenschrift des Österreichischen Ingenieur- und Architekten-Vereines, 1889, p. 313.)

This Paper refers mainly to the numerous examples of dams constructed across valleys in France (including Algeria) and Spain. The object of such dams is to provide for an increased demand for water by the formation of storage-reservoirs which may serve any or all of the following purposes, viz., irrigation of the land, water-supply of districts or towns, driving water-wheels or other motors, feeding navigable canals; and it is stated that where the local conditions are such that a dam of moderate height (up to 66 feet) would suffice, there would be little difficulty in providing for its safety; but the case is otherwise where dams 200 feet high have to be built; here the greatest care has to be taken with every constructive detail of the work, and it is the object of the Paper to point out what precautions are necessary, and to show what injury has resulted from a neglect of such precautions.

(1.) Nature of the Foundations.-In the case of dams of masonry (a material equally good for low as for the highest dams), it is stated that this should be so bedded in and incorporated with the foundation-soil as to form a homogeneous whole, which can only be perfectly attained if that foundation be rock, and as examples are given the Almansa and Alicante dams in Spain, built three hundred years ago. These are 68 feet and 134 feet high respectively, are founded upon rock, and stand to this day; while the Puentes dam (164 feet high), which was built about one hundred years ago on alluvial soil, was destroyed in 1802, after ten years' existence only, by a flood which never rose higher than 10 feet below the crown of the dam. When this work was reconstructed in 1881-86 the foundation was considerably widened, and carried down nearly 80 feet below the valley bottom in order to reach rock. The Grands-Cheurfas dam in Algeria is 98 feet high, and founded on limestone-rock, but on the mountain-slope into which one end of the dam was built a cleft full of sand was found, through which the water passed when the reservoir was first filled (January 1885), and this brought about a breach nearly 33 feet wide a month after, through which the water rushed and overturned the Sig dam lower down the valley. The Gros-Bois dam (France) was built on clay soil, which became moistened as the reservoir filled and produced a sliding and bulging of the lower slope, which, however, partially recovered as the reservoir was emptied; but this phenomenon was more striking in the case of the Bouzey dam (72 feet high), which bulged out about 15 inches on the lower side for a length of nearly 150 feet.

As regards the foundation of earthen dams, it should be on an earthy soil free from vegetable matter, so as to secure thorough

incorporation of the material of the dam with that of the soil beneath; but if this soil be not thoroughly impermeable it is usual to build a puddle-wall, or core, in the body of the dam, as in England, a good example of which is quoted in the Jarrow dam of the Liverpool Waterworks scheme, which is over 85 feet high. An exception to the above rule is that of the Marengo dam in Algeria an earthen dam 101 feet high, built on basalt rock; but the greatest possible care was taken in the construction of it to prevent percolation of water at the base of the dam, and it has stood well to this day.

A curved plan of foundation is recommended, and is adopted in the Spanish and some French dams; but in Algeria the straight plan obtains, and three dams of this type as above shown, viz., the Habra, Grands-Cheurfas, and Sig, have been breached.

(2.) Cross-section. In masonry dams this varies very much, most of the old Spanish dams having an extravagant breadth, while those of France are, on the contrary, too slight, the section having been computed on the assumption that the highest water is more or less below the crown of the dam, and to this error the breach of the Habra dam (Algeria) has been attributed; for the flood of December 1881 exceeded the highest normal water-level in the reservoir by 7 feet, and stood at 2 feet above the crown. As regards earthen dams, the case is different, as the top is usually very wide and the slopes very flat; what is to be avoided is a rush of water over the crown, and the calculations for stability must be made on the assumption that in high floods the water stands at a level with the crown.

(3.) Precautions in Construction.-In the case of masonry dams the stones should be dressed and squared and laid in the best hydraulic cement. Sometimes, for the sake of economy, the best mortar is used only at and near the faces of the dam, and inferior mortar in the centre, as in the Sig dam in Algeria, which, as before mentioned, was breached; it is evident, therefore, that there is a limit to such economy.

In earthen dams the material should, it is stated, be a mixture of two-thirds clay and one-third sand, and should be tipped in thin layers inclined upwards, and made as compact and watertight as possible by ramming and rolling, and it is recommended to water the surface of each layer before tipping the next above it. The English method of building a core of puddled clay, it is said, has this disadvantage-that the parts of the dam in contact with the core are apt to crack unless the greatest possible care is taken. The combination or conjunction of masonry with earthen dams is to be avoided, and masonry culverts under high earthen dams are to be regarded as dangerous, and as an instance the bursting of the dam of the Sheffield reservoir is quoted.

(4.) Limit of highest Water-level in a Reservoir.-This should always be below the crown, especially in the case of earthen dams, and can be arranged in various ways, viz., by discharge-channels with sluice at the head, which can be made to work auto

matically, and by free overfalls or weirs, the length of which can be determined by prescribed formulas, dependent on the area of the catchment-basin. Examples are given of the Almansa masonry dam, where the sill of the overfall is 6 feet below the crown, and is only 39 feet wide, the corresponding catchment-area being 77 square miles. The Habra dam has a catchment-area of 91 square miles, with an overfall 49 feet wide on the right side, and another 124 feet wide on the left side of the dam, the sill of each being about 10 feet below the crown. In rare cases only would it be necessary to make a tunnel or culvert through the mountain-side for the discharge of flood-waters; but an example is met with in the Fureno reservoir, where the sill of the tunnel is 24 feet below the crown of the dam. Occasionally the under- or scouring-sluices are used for discharging surplus floods, and the Habra dam is quoted as an example, but this led to the destruction of the dam when the sluice shutters failed to act.

(5.) There should be telegraphic communication between the dam and the village lying below the reservoir, in case of a breach, and it is stated that, for want of such warning when the old Puentes dam was breached in 1802, six hundred and eighty inhabitants of the village of Lorca (about 7 miles down the valley) were drowned, but this defect was remedied on the reconstruction of the dam in 1885.

Arrangements for Service. These are, first, the appliances for drawing off the water; and, secondly, those for cleansing the reservoir from the mud, sand, &c., brought down by floods. For the first purpose the Spanish system consists of a well, built just within the upper face of the dam and extending to the bottom, and communicating with the water in the reservoir by numerous small openings furnished with shutters, and is drawn off on the lower side through a culvert or pipe passing under the dam. In the French system the openings are fewer and larger, and in the Gros-Bois dam there are only two, the sill of the upper being 18 feet and that of the lower 60 feet below the crown. The culvert leading from the bottom of the well is sometimes divided by a short partition wall, in order to make the shutters more easy to work; for instance, in the Vingeanne dam (France) the two branch channels are 2 feet 7 inches wide and 3 feet 3 inches high, while the main culvert is 6 feet 3 inches wide, and a similar arrangement obtains in the Bouzey reservoir.

Scouring- or under-sluices are constructed for the second purpose above mentioned. The amount of silt deposited in reservoirs depends on the catchment-area, the rainfall, and the geological character of the soil. In the Sig, Tlelat, Djidionia, and Habra reservoirs, in Algeria, the yearly deposit is from 0.16 to 1.6 cubic yards per acre of catchment-basin. It is stated that the most simple and effectual method of cleansing the bed of a reservoir is by the sudden opening of a large under-sluice, which produces a violent rush of water which carries off the silt with it; but this involves a great waste of water. The Alicante reservoir was

cleansed in this way after an accumulation of ten years' deposit; the cost was £400, and 2,600,000 cubic yards were removed. But reservoirs should be cleaned out annually or every three or four years, as the silt is then looser and easier to carry off.

For closing the sluice-openings, either beams working in grooves and gates or shutters are employed; the former is the system in the older Spanish dams, and also in the Habra dam in Algeria; it possesses the advantage of simplicity of action, and can be adapted to large openings, but it has this disadvantage, that once opened, the sluice can only be closed again after the reservoir has been emptied.

In the Habra dam four men are employed to open the sluices, and it takes them five hours and a half to open them completely, that is, to a height of 6 feet, hence much water is wasted in the process; but in the Puentes dam steam-power is used. In the earthen dam of Marengo the sluice is provided with a door, which can be kept closed by a bar or rod hinged on the door at one end, and the other end secured by a pin attached to a draw-bar worked from the top. By lifting out the pin the bar is released, and the pressure of water on the door forces it open; this method, however, has the disadvantage of not being able to close the door again till the reservoir has been emptied.

In order to remedy the defects of the methods hitherto employed, Calmels and Jandin have invented special appliances. Calmels uses compressed air, which is blown on the deposit of silt through a tube and so loosens and stirs it about that it can be carried away by the rush of water through the under-sluice. Jandin uses a pipe or tube, one end of which lies in the deposit of silt, and at the other end is attached a suction-pump; this method is efficient, but costly.

A large Plate accompanies the Paper, containing cross-sections and other details of the dams referred to.

W. H. E.

Statistical Review of the Results of the Irrigation Works of India in 1887-88.1 By R. B. B.

These works are divided, for statistics, into two main classes, namely, Major Works and Minor Works: the former are works of greater magnitude, mostly constructed by the British Government; the latter are, in many cases, modifications or improvements of the old native systems. The Major Works are subdivided into (A), works carried out with borrowed money, and (B), with money from the Indian revenues; and the Minor Works (A), of which

1 This pamphlet, entitled "The Irrigation Works of India: a Statistical Review of the Financial and Agricultural Results obtained from them in 1887-88," Calcutta, 1889, is in the Library of the Institution.

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