Types of Dams

TYPES OF DAMS Dams may be divided into two classes, masonry or embank ments; and the selection of the particular type will depend upon the nature of the materials on which they will rest, and which are available for construction.

Where good sound rock exists at no great depth from the sur face, a masonry dam is to be preferred, but where the rock can )nly be reached at a considerable depth, the cost is prohibitive.

Masonry Dams.

Masonry dams should be arched in plan ;oncave to the water face where the length of the dam is not oo great, as such a form adds to the stability, and the pressure >f the water tends to close temperature or contraction cracks tt right angles to the axis of the dam. It is desirable to slope he foundations towards the water face, especially where the lepth below the surface is moderate, as this reduces the tendency o slide on the foundations and the possibility of overturning lue to the uplifting pressure of water penetrating between the nasonry and the rock. The design should avoid the development )f tension in the masonry, tending to rupture the dam on a lorizontal plane ; and the maximum pressure at any point in he masonry should be limited to i o-15 tons per sq.f t., depend ng on the materials used for construction and the nature of the underlying rock. Prof. Rankine pointed out the importance of avoiding tension, and evolved the theory on which most modern lams have been designed, viz.: that the resultant pressure due :o the weight of the masonry and the water thrust must fall within he inner third of the dam if tension is to be avoided. Any fissure Developing at the water face due to tension tends to increase Dwing to the water pressure, and may ultimately lead to the failure of the dam (see Prof. Unwin, Proc. Inst. C.E., vol. 126, and E. P. Hill, vol. 129).

The masonry of a dam is not isotropic as horizontal planes of weakness, where new work is superimposed upon that which has set, are difficult to avoid. It is advisable therefore to step the masonry at the water face so as to avoid the construction of a horizontal joint between old and new work.

The arched form of dam is economical when the radius of curvature is comparatively small, as the sectional area can be decreased by designing the dam as a horizontal arch transmitting the water thrust to its abutments. For reasons of economy modi fications of the simple type of masonry dam have been introduced; these dams are of ferroconcrete construction, the pressure of the water being transmitted to buttresses by means of steel rein forced slabs or arches.

Earth Embankments.

The profile of the embankment re quires careful study of the materials of which it will be composed, and slips have frequently occurred, leading to the complete or partial failure of banks, due to lack of local study and the adop tion of a design which was unstable. Light sandy soils will stand at a high angle of repose, but clays or plastic materials require flatter slopes, the inclination decreasing as the base of the embank ment is approached. On fig. 2 the inner line profile would apply to good banking material, and the outer line to clay or plastic material.

Care must be exercised to prevent the saturation of the outer slope of the bank, and when possible it should be composed of freely draining material. When such materials are not available, the outer portion should rest on a layer of stone terminating in a stone toe, vertical drains of dry stone being carried up through the bank at intervals.

Cut-off Trench.—In order to prevent percolation below the bottom of the embankment, a trench is first excavated across the valley bottom, carried down—if possible—to an impermeable substratum and continued into the hillsides, so as to cut off any percolation below top water level. This is filled with impermeable material, preferably concrete, so as to form a barrier to per colation below the bank; and an impermeable core wall is brought up in continuity with this barrier to prevent percolation through the bank.