NOVEL METHOD OF CONSTRUCTING DAMS.

Grey River Argus, Volume IX, Issue 740, 15 October 1870, Page 4

NOVEL METHOD OF CONSTRUCTING DAMS.

The Queenslander extracts the following i from the plans of Mr M'lvor for construct" j ing dams : — lt is not here intended to give working plans or details of construction. The object is merely to explain the general principles and practicability of the new system of forming earth works by means of water or silting. In the construction of all dams, made with a view to future irrigation, it is indispensable to secure means of drawing the water off at the lowest level of the earth inside. This must also be observed in the construction of canals for traffic and irrigation, as this arrangement enables us, at a trifling expense, to clear both reservoirs and canals of the sediment which is certain to accumulate in them by degrees with the passage of fresh water, and which, unless these precautions are taken in the construction, gradually, by "silting up," renders these works comparatively useless, as is now shown in the great Ganges canal. By the silting process above described, we may carry a dam across a hollow of a few yards in width, to form a small pond for driving machinery, or for domestic purposes. By this means we may also bound across valleys of miles in width, or join mountain ranges together to form fresh water lakes to irrigate thousands of square miles of country. The rivers of our mountain ranges have, as a rule, burst or cut their way through narrow gorges or gaps of only a few hundred yards in width. To silt up these gaps would be a work of comparative ease, and in doing so would, in many instances, form lakes of hundreds of miles in length, at a comparatively trifling expense. For example, dams thrown across the exits of the rivers through the front range of the Himalayas would form splendid lakes over the lower broken grounds between the first, second, arid third ranges. These lakes would afford great facilities for internal navigation, and would give unbroken water transport to all hill produce for many hundreds of miles between the ranges, and in all cases to the front range i overhanging the plains. At the same time they would form inexhaustible supplies of water for irrigation, rendering the vast fertile campaign stretching along the foot of the whole line independent of rainfall, by the unfailing supply of water from rain in the Himalayas, and from the eternal melting of the glaciers on its third and. loftiest range of towering snow-clad peaks Prior, to the construction of a dam it is necessary to lay across the site of the embankment, in the bottom of the valley, in the bed of the river, or on the surface of the river banks, on one or both sides, as many lines of pipes as are necessary to allow the ordinary flow of the stream or river to pass through them. These pipes carry off the water of the river during the process of silting, and are subsequently used to regulate the overflow, for the purposes of irrigation or navigation. Prior to the work of silting being begun, the water of the river is raised by impediments of trees, brushwood, or any convenient material being thrown across its bed above where the dam is to be made, till the water is raised slightly above the river bank, when it flows through the pipes. The water of the river or stream being thus diverted into the pipes, and the bed laid quite dry, the work is begun hy excavating across the bed ot the stream, and across the entire width of the valley, a deep and broad trench down to a solid foundation, to prevent the future lake water from percolating through or under the dam, by filling this trench down to the foundation with impermeable silt. The next work is to run a narrow silt wall across the entire width of the valley, along the lower side of the site of the dam. This wall being constructed by a I small stream of water diverted from the . main silting stream, when finished '

another is thrown across the valley above the upper side of the site in the same J manner. In large bunds these walls should be from iive to six feet in height, and nine to ten feet wide at the base, and and formed exactly in the same manner as a canal embankment. In beginning the work it is necessary to take great care to see that the silt is solid and securely deposited around the pipes. The water is first directed into the centre of the dam, and here a silt wall is thus raised along the centre of the site across nearly the entire width of the valley, in the same manner as the silt walls on the outside, only leaving at the far end a passage for the water to flow round. This wall being finished, the stream is then directed into the right half of the embankment, when the solid silt immediately begins to deposit at the entrance of the stream into the still water on the right side of the embankment. As the dam rises at this point by the accumulation of silt, the stream begins to flow more rapidly, causing it to carry forward the solid silt round the corner on the left hand side of the embankment. The still water, while standing on the left half of the embankment deposits all the fine sediment, and finally flows off clear. The stream is Uie'n again directed into the right half of the embaukment, and a new layer of silt formed over this portion as at first, and over the sediment just before deposited there. This operation is repeated toties quoties, thus raising the dam layer by layer of alternate silt and sediment to any desired height. It is of great importance that the solid silt be carried with each layer entirely over the whole length and surface of the dam. If this is not dene, and if the soft sediment is allowed to deposit iv one spot; unaccompanied by alternate layers of solid silt, the side of the dam is apt to give way. This must, therefore, be carefully avoided, and each successive layer of firm silt must be made to double round the far end of the dam alternately from right to left and from left to right. If attention is paid to this point, the greatest possible solidity is secured at once, without time being required for the dam to settle, its composition settling it at first far more densely than a dam made by hand labor ever settles by lapse of time This is shown by the fact that the heaviest flow of water may be brought over the dam, even when in course of construction, without washing away the embankment, or even disturbing the solid silt. The water will of course immediately carry away any of the soft sediment which has not yet been secured by an overlying layer of solid silt. As an evidence of the great solidity of earthwork when performed by this process, it may be observed that our ordinary soils, when silted, are reduced in bulk from one-fifth to one-third of that which they occupied in their natural condition. It thus requires five cubic yards of excavation from our most solid disintegrated rocky soil to form four cubic yards of silted embankment, and three yards of open loam to form two yards when silted. This arises from the earth being run together in a semi-fluid state, in which every cavity between the larger particles is filled np with fine sediment, thus forming a mass greatly exceodingin solidity the natural soil. ,