DAMS

EARTH AND ROCK FILLED DAMS

Overview of a DAM

• Dam is the most expensive multipurpose civil engineering construction.

• It delivers beneficial results for a long time to mankind

• A dam failure can create unprecedented gargantuan havoc to life and property through lightning floods.

Objectives for a DAM• A Dam is a solid barrier constructed at a suitable location

across a river valley with a view of impounding water flowing through the river.

• Dams are constructed to achieve the following objectives:• Generation of hydropower energy.• Providing water for irrigation facilities• Providing water supply for domestic and industrial uses• Fighting droughts and controlling of floods• Providing navigational and recreational facilities

Earth or Rock Dams• Earth or rock dams are mainly built out of clay, sand,

gravel etc.• Earth or rock dams have smaller height, broad based and

light weight.• The earth dams are trapezoidal in shape. • Earth dams are planned in such places where the

underlying material is too weak to support masonry dams or where suitable competent rocks occur at great depth.

• When most of the material used in the dam is earth, or rock it is called earth fill or rock fill dam respectively,

Earth or Rock Dams

• An earth dam is constructed as a homogeneous construction with a properly compacted core of an impervious material such as clay.

• One of the largest earth dams is the famous Ft Peck, Wyoming dam.

Geotechnical Considerations

• The exact location where the dam should be placed against the river along its longitudinal profile.

• The type of dam that will be most suitable for that particular site.

• The availability, cost and quality of the materials required for the construction of the dam.

Selection of sites

• Topography: The most suitable site should have a narrow gorge or a small valley with enough catchment area available behind it to ensure minimum uprooting of population, loss of cultivable land due to submergence.

• Technically the site should be strong, impermeable and stable.

• Availability of construction materials• Economically realistic and justified.• Environmentally, the site should not cause ecological

disorder.

Geological Characters for investigation

• Geology of the area:• Preliminary geological survey s of the entire catchment

area should include• Main topographic features• Natural drainage patterns • General characters and structures of rock formations

such as their stratification, folding, faulting and igneous intrusions.

• The trend and rate of weathering and erosion in the area.

Geology of the site

• Preliminary Investigation• Lithology: lithology provides the details of rock types occurring in

that area. Varieties of rock types present, their nature and extent of weathering, soil and rock debris should be noted.

• Structure: strike and dip of beds, folds, faults, joints, unconformities and foliation influence on the suitability of a site.

• Topography: information on features like slope stability, occurrence of landslides valleys, hills, the trend of river course. Can determine the seismicity of their area.

• Groundwater conditions: water table position and the scope for leakage from the associated reservoir.

Detailed Investigation• Surface Investigation: • Lithology: The important factor in the lithological studies is to prepare

a geological map of the area by undertaking extensive field work and noting carefully the outcrops, their attitudes, thicknesses of different beds, their succession, intrusion if any, and their arrangement

• Weathering, soil occurrences, their extent and depth, availability of construction materials should be recorded

• Laboratory studies: provide engineering properties such as compressive and tensile strength, porosity, , permeability, and durability

• Petrograhical analysis of thin sections of rocks to determine the mineralogy.

Detailed Investigation

• Structure: Study in detail and mark accurately on the map the various rock types, the attitude of the beds (strike and dips) and structural features such as folds, joints, foliation, rock cleavage and unconformities.

• These details indicate the scope of leakage of water, the stability and the durability of the dam

• Topography: detailed topographical studies help in knowing the stability, economy and safety of the site.

• Ground water conditions: detail investigation of aquifer characteristics.

Detailed studies (structure)• If the joints are too many (i.e., more sets), closely spaced

and are of great magnitude, then such a fractured site will be physically too weak to withstand stresses of dam.

• Saturation with water along with the accompanying decay of rocks will make the site more incompetent for foundation.

• If the joints dip in the downstream direction, they adversely influence water leakage.

• Joints which dip in the upstream direction are less harmful.

Detailed Investigation

• Subsurface Investigation: Planned geological and geophysical investigations help in interpreting the subsurface conditions fairly accurately.

• Suitable trenches, pits, boreholes and coring are made at a few places to get information about the actually existing geological conditions at those places.

Causes of Dam Failures• There are essentially five events that can lead to catastrophic

dam failure:• Overtopping of the dam by a flood wave due to insufficient

spillway capacity• Movement within the rock foundations or abutments on

planes of geological weakness• The development of large uplift pressures on the base of the

dam• Piping at the dam toe• Slope failures on the upstream or downstream face of the dam• Excessive leakage from the reservoir

Case histories of Dam Failures due to Geological Conditions

• St Francis Dam of California• This was a gravity dam having a height of 205 feet and a

length of 700. it was constructed on a composite foundation of schists and soft argillaceous conglomerates. These , in turn, were mutually separated by a distinct fault. This conglomerate also had veins of gypsum. The main loads of the dam exceeded the crushing strength of this poor quality rock leading to the dam failure. Enormous leakage of stored water occurred through the conglomerate and the dam failed by sliding two years after the reservoir was filled in 1926

Malpasset Dam of France

• This 200 feet high arch dam was constructed in 1954 in the narrow gorge of Le Reyran river. It suddenly collapsed on 2 December 1959 following heavy rains.

• The main cause for the failure was found to be geological- a rupture of the rock below the foundation. This had induced substantial displacement particularly of the abutment which led to dam failure.

• The bedrock was a sheared and jointed mica schist with a wedge with a wedge mass overlying a clay-filled seam.

Practical Problems • An earth dam with a 45000 cubic meters is to be built from a

sandy clay soils trucked from a borrow pit. The water content of the sandy clay soil in the borrow pit is 15 % and its void ratio is 0.69. the specification requires the earth dam’s embankment be compacted to a dry unit weight of 18 kN/cu.m. Specific gravity of solid particles =2.7

• Determine a) the weight of sandy clay soil from the borrow pit required to construct the embankment.

• B) the number of 10,0 cubic meter truckloads of sandy clay soil required for the construction

• C) the weight of water per truckload of sandy clay soil.• D) the degree of saturation of the sandy clay soil in situ

Earth dams• Solution:• Step 1. calculate the unit weight for the borrow pit material• gd =Gsgw/(1+e) = 2.7 *9.8/(1+0.69) =15.7kN/cu.m• Step 2 Determine the weight of borrow pit soil required. gd reqd./gd

borrowed =18*45000 = 81000 kN• Step 3. Determine the number of trucks required.• Number of trucks =18*45000/157=5159.24 =5160• Step 4. Determine the weight of water required.• Weight of dry soil in one truckload (Wd) = 10*15.7 =157 KN• Weight of water wWd =0.15*157 = 23.6 kN• Step 5 Determine the degree of saturation s=wGs/e• 0.15*2.7/0.69 =59 %