Masonry check dams = بندهای سنگی
Masonry check dams constructed with cement mortar and non-disintegrating stones are generally used in torrent control. The main objective of the dam is to hold fine and coarse material carried by flowing water in the gully or torrent.
From a technical and economical point of view, it is not necessary to build masonry check dams to control channel erosion in gullies. Every kind of gully that is about 1 000 m long and covers an area of 20 ha or less can be stabilized easily by boulder check dams. In a series of boulder check dams, the first dam can be built with cement masonry as well as those above and below a road at the junction points where the gully crosses it.
Calculating the dimensions of masonry check dams
1.For dams less than two m high
If a masonry check dam is less than two m high, its crest thickness should be 0.4 m. The base thickness is calculated according to the height and inclination of the dam's downstream face.
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2.For dams two to six m
If the height of a masonry check dam is two to six m, the base thickness can be computed by using the following "Hoffman" formula:
d = 0.462 H
d : thickness of the base
H : total height of the dam
The crest thickness can be computed by taking into account the inclination of the downstream face, and the height of the dam.
3.For dams six to eight m high I€ the height of a masonry check dam is six to eight m, the crest thickness can be calculated by using the following empirical formula:
| K = | 1 + H 10 |
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10 |
k : thickness of the crest
H : total height of the dam
The base thickness can be gauged by taking into account the inclination of the downstream face, and the height of the dam.
Other specifications and construction procedures for masonry check dams
-The dams must not be constructed on points where there is mass movement of soil blocks. They should be built on a gully bed or torrent channel's stable points just below the sliding area to hold debris and material as well as to stop the movement of soil blocks.
-The foundation of the first dam must be dug to a durable layer below, such as solid rock. If there is no solid layer, the foundation must be dug at least one m deep, and a reinforced, concrete layer at least 20 cm thick must be constructed. On this concrete layer, the body of the first masonry check dam must be built with sharp-edged and non-disintegrating stones, and Portland cement mortar (250 kg cement per cubic meter of sand. The ratio by weight is 1:4 or 1:5).
-The foundation of the other dams must be at least one meter deep, if they are not constructed on solid rock. The inclination of the base's upstream face is minus 20 percent.
-The wings should enter at least one meter into the sides of the gully.
-The foundation is longer than the spillway.
-An aqueduct (diameter 20 to 50 cm) must be built on ground level and drainage holes (diameter of each hole at least 10 cm) must be made during the construction of the dam. The vertical and horizontal distances between the holes are one and two m respectively. The gradient of the aqueduct and drainage holes is five percent (Fig. 21).
-The upstream face of the dam is vertical, whereas its downstream face inclination is 20 percent (1:1/5 ratio).
-The stones must be piled behind the mouth of the aqueduct. If possible, the space behind the dam should be filled to the spillway with soil excavated for the foundation and from the gully bed.
-Wing walls should be built behind the wings of the dam to protect them against flash water. The angle between the wing and wing wall is 30 to 45 degrees. The space behind the wings should be filled with soil. The height of the wing walls is equal to the depth of the spillway. The construction of wing walls is dry rock work.
-The stones used in constructing masonry check dmas must be hard enough to withstand abrasion, non-disintegrating, and resistant to weathering.
-A counter-dam must be constructed in front of the first masonry check dam.It also is built as cement masonry work.