«1.0 WATER HARVESTING AND STORAGE IN VALLEYS USING SMALL EARTH DAMS Training notes by Prof. Bancy Mati 1.1 What is a small earth dam? A dam is a structure ...»
1.5.3 Steps in construction Stripping This really means land clearing. The area covered by the base of the dam must be stripped of all vegetation and organic soil. The organic soil can be stockpiled and used on the downstream slope of the fill. All slopes steeper than 1.5:1 on sides of draw should be flattened to minimum of 2:1.
Key trench A key trench (cutoff trench) is excavated below the base of the fill upstream of the centerline of the fill. The key trench is incorporated in the design for two reasons: to anchor the dam to the base material and to prevent piping (seepage under the fill). The key trench should be a minimum of three feet deep for a dam the height of about 4 m. It should extend the full length of the dam and reach one third to one half of the way up the side slope of the draw.
Fill construction The earth dam is normally constructed using impervious clay or clay-based material (Figure 1.10).
A simple field test is used to determine the suitability of the material for compaction requires adding a small amount of moisture to a handful of soil then rolling it between the palms and hands.
The material having good compaction characteristics is the one which can be rolled to the diameter of a pencil, approximately six inches long, then bent into a loop without breaking.
Construction material taken from the surrounding hillsides or an excavation in the reservoir area must be placed close to horizontal in the fill in six inch layers and compacted. If the material is dry, moisture will have to be added, and suitable compaction equipment such as a sheeps foot packer used to obtain the proper compaction.
A simple test to evaluate proper compaction is to place the edge of the heel of a hard-soled boot on the fill and push down hard with all your weight. If only a mark is left, compaction is satisfactory. If the heel sinks in, compaction is poor. No rocks over 15 cm in diameter should be placed in the fill.
Figure 1.10 (a) Earth-fill dam under (b) Earth dam construction nearing construction (photos by Bancy Mati) completion – front is lined with masonry
1.6 Safety features Dams can fail if the structure is breached or suffers significantly damage. Dams may also fail slowly through siltation of the reservoir or loss of water through seepage. If a dam fails due to structural weakness, it can cause extensive damage including fatalities and this should be avoided at all costs.
It is therefore necessary to monitor signs of weakness such as cracks, submergence or seepage around the structure. Most dams are designed with mechanisms to permit the reservoir to be lowered or even drained in the event of such problem. Cracks and other fissures can be remedied through rock grouting – which involves pressure pumping of concrete mix into weak fractured rock. Small earth dams should be fenced and the catchment area protected from damage so as to reduce siltation damage. Animals and people should not access water directly from the dam, but the design should incorporate water off-take structures to minimize human traffic and trampling.
Communities must be trained on the acre, utilization and management of the dam.
Vegetation control Trees and bushes are not permitted on earth dams because: (i) Extensive root systems can provide seepage paths for water, (ii) Trees that blow down or fall over can leave large holes in the embankment surface that will weaken the embankment and can lead to increased erosion, and (iii) trees and bushes obscures the surface limiting visual inspection, provides a habitat for burrowing animals and retards growth for grass vegetation. The stumps of cut trees should be removed so grass vegetation can be established and the surface mowed. Stumps should be removed either by pulling or with machines that grind them down. All woody material should be removed to about 15 cm below the ground surface. The cavity should be filled with well compacted soil and grass vegetation established.
Grass vegetation is usually planted on dam embankments as it is an effective and inexpensive way to prevent erosion and stabilize the surface. Grass also enhances the appearance of the dam and provides a surface that can be easily inspected.
1.7 Maintenance of earth dams Earth fill dams require regular inspection and maintenance. An inspection before spring runoff is critical to ensure the spillway is not blocked with snow or other material. All blockages must be removed to prevent overtopping and the dam washing out. During runoff, additional inspections should be carried out to watch for signs of erosion, spillway blockages (ice or debris) or overtopping of the dam. After the dam is free of snow, a visual inspection can be completed to assess the slopes for erosion, rodent damage, seepage or slumping. Burrowing rodents such as beavers, muskrats and gophers should be removed from the dam immediately. All potential problems must be repaired as soon as possible to safeguard the dam. Side slopes should be cleared