Notes on EARTH DAMS

EARTH DAMS
1 Geometry

1.1 The downstream slope of earth dams without seepage control measures should be no steeper than 1 vertical on 3 horizontal. If seepage control measures are provided, the downstream slope should be no steeper than 1 vertical on 2 horizontal.

1.2 The upstream slope of earth dams should be no steeper than 1 vertical on 3 horizontal.

1.3 The side slopes of homogenous earth dams may have to be made flatter based on the results of design analyses or if the embankment material consists of fine grained plastic soils such as CL, MH or CH soils as described by the Unified Soil Classification System.

1.4 The minimum allowable top width (W) of the embankment shall be the greater dimension of 10 feet or W, as calculated by the following
formula:
W = 0.2H + 7; where H is the height of the embankment (in feet)

1.5 The top of the dam should be sloped to promote drainage and minimize surface infiltrations and should be cambered so that the design freeboard is maintained after post-construction settlement takes place.

2 Slope Stability

Where warranted and especially for new Hazard Class "C" dams, the department may require that slope stability analyses be provided for review. The method of analyses and appropriate factors of safety for the applicable loading condition. Earth dams, in general, should have seepage control measures, such as interior drainage trenches, downstream pervious zones, or drainage blankets in order to keep the line of seepage from emerging on the downstream slope, and to control foundation seepage. Hazard Class "A" dams less than 20 feet in height and Hazard Class "B" dams less than 10 feet in height, if constructed on and of erosion-resistant materials, do not require special measures to control seepage. In zoned embankments, consideration should be given to the relative permeability and gradation of embankment materials. No particle greater in size than six inches in maximum dimension should be allowed to be placed in the impervious zone of the dam.

3 Compaction Control and Specifications

Before compaction begins, the embankment material should be spread in lifts or layers having a thickness appropriate to the type of compaction equipment used. The maximum permissible layer thickness should be specified in the plans or specifications.
Specifications should require that the ground surface under the proposed dam be stripped of all vegetation, organic and otherwise objectionable materials. After stripping, the earth foundation should be moistened, if dry, and be compacted before placement of the first layer of embankment material. Inclusion of vegetation, organic material, or frozen soil in the embankment, as well as placing of embankment material on a frozen surface is prohibited and should be so stated in the specifications. For all dams, compaction shall be accomplished by appropriate equipment designed specifically for compaction. The type of compaction equipment should be specified in the plans or specifications. The degree of compaction should be specified either as a minimum number of complete coverage’s of each layer by the compaction equipment or, in the case of higher or more critical dams, based on standard test methods.
When the degree of compaction is specified as a number of complete coverage’s or passes, the final number of passes required shall be determined by the engineer during construction. In order to insure that the embankment material is compacted at appropriate moisture content, a method of moisture content control should be specified. For Hazard Class "A" dams less than 20 feet high, the moisture content may be controlled visually by a qualified inspector. Hand tamping should be permitted only in bedding pipes passing through the dam. All other compaction adjacent to structures should be accomplished by means of manually directed power tampers.
Backfill around conduits should be placed in layers not thicker than 4 inches before compaction with particle size limited to 3 inches in greatest dimension and compacted to a density equal to that of the adjacent portion of the dam embankment regardless of compaction equipment used. Care should be exercised in placing and compacting fill adjacent to structures to allow the structures to assume the loads from the fill gradually and uniformly. Fill adjacent to structures shall be increased at approximately the same rate on all sides of the structures. The engineer in charge of construction is required to provide thorough and continuous testing to insure that the specified density is achieved.










4 VEGETATION CONTROL - TREES AND BRUSH

4.1 Trees and Brush

Trees and brush are not permitted on earth dams because:
a. Extensive root systems can provide seepage paths for water.
b. 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.
c. Brush obscures the surface limiting visual inspection, provides a haven for burrowing animals and retards growth for grass vegetation. 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 6 inches below the ground surface. The cavity should be filled with well compacted soil and grass vegetation established.

4.2 Grass Vegetation

Grass vegetation is an effective and inexpensive way to prevent erosion of embankment surfaces. It also enhances the appearance of the dam and provides a surface that can be easily inspected.

Erosion and Sediment Control actions (I)

Check Dam


When
· To stabilize constructed and existing flow corridors when flow is anticipated to exceed the erosive velocity.
· To control sediment in a stream in conjunction with a sediment sump.

Why
· To reduce water velocity minimizing erosion in flow corridors and channels.
· To temporarily protect vegetation during early stages of growth or permanently to reduce flow velocities.

Where
· Within and across an existing or constructed flow corridor.

Scheduling
· Around Year.

How

1. Configure check dams to site specific conditions. Utilize an engineer as necessary to determine the notched center dimensions and spacing between check dams based on channel slope, flow length, discharge, flow velocity, and soil type. Permanent check dams should be designed to pass, at a minimum, a 10‑year, 24‑hour storm at non-erosive velocity.
2. Permanent check dams should be constructed of clean rock placed on geo-textile fabric which has been toed in a minimum of 3 inches. Ninety percent of the rock should range between 2 to 4 inches for slopes less than 2 percent and 3 to 12 inches for steeper grades. The rock size should be large enough to stay in place during anticipated flows. When larger rock is used, place smaller aggregate immediately upstream to filter sediment and improve efficiency.
3. Temporary check dams that will experience low flow conditions can utilize pea-stone or gravel filled bags instead of rock over geotextile fabric. New commercially available technologies include prefabricated check dams that are effective and sometimes reusable.
4. When not engineered but used in series, the toe of the upstream check dam should be set at the same elevation as lowest point in the top of the downstream check dam.
5. The side slopes of the check dam should be 2 horizontal to 1 vertical or flatter or equivalent to the existing streambank slopes.
6. The middle of the dam should be a minimum of 9 inches lower than the outer edges, allowing flow to go over the depression in the center as opposed to around the sides where it could erode the banks.
7. The outer edges should be keyed into adjacent banks and extend to an elevation above the anticipated flow depth to prevent washouts.
8. Sediment sumps should be used upstream of check dams when working in sandy soils when excessive amounts of sediment is expected to accumulate.
9. Riprap should be placed immediately below the check dam to help dissipate the energy of the water flowing over the dam. In areas of higher velocities energy dissipation may be needed downstream of the check dam to prevent undercutting.
10. Temporary check dams should be constructed to handle the anticipated flow and sediment load until the site is stabilized. Aggregate filled bags are easier to remove than a rock check dam and the aggregate can usually be spread along the channel bottom when the check dam is removed. Aggregate meeting the gradation requirements of 6A is recommended; use nothing finer than pea-stone.




Maintenance
· Inspect check dams following each runoff event to ensure there is no piping under the structure or around the banks until the flow corridor has been stabilized.
· Initiate identified repair needs as soon as possible following inspection.
· Remove and properly dispose of sediment when it accumulates to 1/2 the check dam height. Spread sediment in an upland area and seed immediately.
· In some instances clogged stone must be cleaned to remain effective.
· Inspect downstream structures to ensure they have not been damaged or clogged with displaced rock or stone.
· After flow corridor or channel has stabilized remove accumulated sediment from behind the check dam. If check dam is temporary, remove check dam and then stabilize the area.



Limitations
· Check dams greater than two feet in depth at the center may seriously impact the flow characteristics of the flow corridor or channel and should not be used.
· Removal of rock check dams is labor intensive and expensive.
· Does not remove suspended clay and silt, therefore polymers may be needed.

Indian standard (IS) for Design of Dam

Indian standard (IS) for Design of Dam

1. Criteria for Design of Solid Gravity Dams- I.S. 6512-1984.
2. Guidelines for fixing spillway capacity –I.S. 11223-1985.
3. Criteria for Earthquake resistant design of structures I.S. 1893-1984.
4. Code of practice for stability analysis for earth dams- I.S. 7894-1975.

The Indian Standard IS: 11223-1985 “ Guidelines for fixing spillway capacity” gives the criteria for inflow design flood as under:
The seismic zone together with appropriate coefficients for use in such analysis is given in IS: 1983-84 “Criteria for Earthquake Resistant Design of Structures”.

The various design condition of analysis along with the minimum values of factors of safety to be aimed at and use of type of shear strength for each condition of analysis is given in I.S.:7894-1975 Code of Practice for stability analysis of earth dams .
IS:1893-1984. Criteria for earthquake resistant design of structures (3rd rev.).
Till specific reliable procedures become available for assessment of ice pressure, it may be provided for at the rate of 250 kpa applied to the face of the dam (I.S..6512-1984).
I.S. 1893-1984 “Criteria for Earthquake resistant design of structures” or similar Method.
As per Indian Standard IS:6512-1984 the compressive strength of concrete and masonry
The IS : 6512-1984 states that the factor of safety against sliding may be calculated on the basis of partial factor of safety in respect of friction (FQ) and partial factor of safety in respect of cohesion (Fc)

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