Design of Earth and Rockfill Dam_Final

Design of Rockfill Dams

What are dams ? A barrier built across the river.

How dam works? Create a permanent reservoir of water to be used at some later date.

History of Dam Construction

First Thought ???????

BEAVER

History of Dam Construction As per Archaeological evidences 3000 to 5000 yrs. For Irrigation purpose Built out of logs, buttressed with twigs and branches and sealed with mud and stone. Called as Beaver dams.

Classification Of Dams According to Material of Construction Earth dams Rockfill dams Solid Masonry gravity dams Hollow masonry gravity dams (RCC deck and arches) Timber dams Steel dams Roller compacted Concrete dams

Classification of Dams (Contd.) (Contd.) According to Resisting Action Gravity Dam Arch dam Buttress Dam

CLASSIFICATION OF DAMS (contd.) According to Hydraulic DesignOverflow dam designed to pass the surplus water over the crest, often called spillways Non overflow dam Not designed to be overtopped.

Design of & Earth & Rockfill Dams

Definition of Rockfill Dam A dam that relies on rock, either dumped in lifts or compacted in layers, as a major structural element. An impervious membrane is used as a water barrier and can be placed either within or on the u/s slope of the embankment. An impervious membrane can be either of earth or concrete or asphaltic concrete or Geomembrane

Design of Earth & Rockfill Dams

Choice of Type of Dam Shape of Valley - Wide Foundation conditions - Weak rock, overburden Construction Material Use of naturally occurring material Material obtained from excavation for other components of the projects like spillway, powerhouse may be used as construction material Preferable in high seismic zones flexible structure Availability of space for flood diversion


Design of Earth Rockfill Dams

Kol Dam HEPP Main Dam and U/s Cofferdam


AdvantagesCan be constructed on any given foundation condition Soil/rock materials locally available are used with negligible processing. Use of costly manufactured items like cement and steel is eliminated and thus saving in cost. Embankment dam is more resistant to seismic forces and are preferred in areas of high seismicity. Embankment dam can be constructed in stages and the dam height can be increased later on easily, if needed. With modern earth moving machineries, the dam can be completed in less time compared to a rigid dam. Embankment dams are generally much cheaper.

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Conditions Deciding Economy in Construction of Rockfill Dams Availability of large quantity of rock available at site or made available during project excavation works Excessively wet climatic conditions/ monsoon season Tight construction schedule -Round the year construction Earth fill materials require extensive processing Dam is to be raised at a later date Grouting operations required during embankment construction. High cost of diversion structure overtopping of dam is not allowed.

Design of Earth & Rockfill Dams Failure Categories Two failure categories are given by ICOLD Type 1 (F1): A major failure involving the complete abandonment of the dam Type 2 (F2): A failure which at the time may have been severe, but yet has permitted extent of damage to be successfully repaired and the dam brought into use again.

Design of Earth & Rockfill Dams Failure Modes Overtopping of embankment Piping through embankment Piping through Foundation Slope failure Erosion of slopes due to water waves or storm water

Design of Earth & Rockfill Dams Failure of Large Embankments during Operation Stage (Foster et al. 1998, 2000) Overtopping 34.2% Piping through embankment Piping through embankment into foundation Piping through Foundation Slope failure Other 32.5% 1.7%

15.4% 6% 12.8%

Design of Earth & Rockfill Dams Failure of Large Embankments Overtopping Seepage effect, Piping and Sloughing Slope Slides Conduit Leakage Damage to slope Paving Miscellaneous Unknown 30% 25% 15% 13% 5% 7% 5%

Design of Earth and Rockfill Dams Cracks in Embankments

Reasons for cracking Unsuitable/poorly compacted fill Different compressibility and stress- strain characteristics of fill material Variation in thickness of fill


General Criteria for Design No risk for overtopping sufficient free board shall be provided No possibility of piping through embankment Adequate filter and transition zones shall be provided. No possibility of cracking of core due to differential settlement Foundation shear strength shall be greater than shearing stresses on foundations Factor of safety against Slope failure for all possible critical conditions.


Alignment Excessive skewness of dam axis with respect to valley alignment shall be avoided. Divergence of abutment contours with respect to dam axis shall be avoided. In case undesirable contour orientation, abutment shall be excavated in core contact and transitions to get a maximum 90 with the axis.


Foundation Entire foundation area has to stripped of top soil, roots etc. In shell portion removal of overburden may be necessary, if overburden material is so week that it can slide of cause excessive settlement of shell which may result in cracking of core. Overburden shall be removed down to the material which in physical properties is equal to or better than shell material


Slope modification to reduce differential settlement & cracking of core For core foundation: to allow compaction of earthfill; to allow maintenance of positive pressure of the earthfill on the abutment; to limit cracking of the earth core due to differential settlement over large discontinuities in the abutments.


Typical Core Abutment Excavation Detail

Design of Earth and Rockfill Dams


Kol Dam HEPP Main Dam and U/s Cofferdam Excavation


Free Board Vertical Distance between the crest of embankment (excluding camber) and still reservoir water surface. Factors to be considered for freeboard Wave height and wave length Height of wind set-up above still water level Slope of dam and roughness of pitching Analysis to determine extra free board to take care the effect of earthquake, settlement of dam and foundation


Top WidthFixed as per requirement of working space. Shall not be less than 6m W=3.6H1/3-3(m)

Height of Dam (m) Top Width (m) 30 50 100 200 8 10 13 18


Settlement AllowanceSettlement in the Rock fill dam depends upon many factors: - Foundation characteristics - Core and shell material characteristics - layer thickness, - rate of construction, - methodology for construction, - dam height - quality control exercised during execution.


Settlement AllowanceSettlement of 14 rockfill dams were found to range 0.25% to 1% of dam hieght. (Sowers et.al.) Settlement from a number of rock fill dams, (Lawton et. al, 8th ICOLD) s= 0.001h3/2 (Kol Dam 2.15m.) IS:8826-1978, total settlement equal to 1 to 2% of height. As per Norwegian practice, expected settlement including a certain margin of safety is about 1% of height for well compacted fill founded on rock.


Earth Core Type Upstream Core More stable under the water load, because the downward force of the water produces frictional resistance to sliding The permeable rock embankment develops no uplift The impermeable deck can easily be inspected and repaired if necessary. During construction the height of the dam can be increased by dumping only on the downstream side and extending the membrane upward on the sloping surface.


Earth Core Type Upstream Core The deck is vulnerable to weather and wave attack. If constructed of earth, sudden drawdown greatly reduces its stability and may cause it to slide. Settlement of the rock embankment tends to produce tensile cracks in the membrane.


Earth Core Type Sloping core Less Clay requirement Can be constructed after placement of rockfill. Reduced pore pressures in downstream portion of the dam - Resulting in higher safety of downstream slopes More quality control required less thickness


Earth Core Type Central core Gives higher pressures at foundation contact Reduces the possibility of leakage and piping at foundation contact. Less quality control required Large thickness of core Better performance under earthquakes. Simultaneous construction of rockfill, filters and core


Earth Core Width depends on tolerable seepage loss; minimum width which will allow proper construction; the type of material chosen for the core and shoulders of the dam design of proposed filter layers


Earth Core Width of 30% to 50% of the head of water have proved satisfactory on many dams under diverse conditions. Cores of this width are adequate for any soil type and dam height. Width of 15% to 20% of the head of water are considered thin. However, when adequately designed and constructed filter layers are used, then the core is satisfactory under most circumstances. Widths of less than 10% of the head of water are not used widely and should only be used when a large leak through the core would not lead to failure of the dam.


Earth Core Minimum top width of the core 3m Top level of core should be 1 m higher than MWL Material Clay, Sandy Clay, Silty Clay, Clayey Silt With some Gravels


Earth Core Material Inorganic clay, Gravelly clay, Sandy Clay, Silty Clay, with medium to low plasticity High plasticity clay, Clayey or Silty gravel, poorly graded gravels- sand silt mixture Inorganic silts with low, medium or high plasticity Organic soils and Peat Not Sutable


Earth Core Material Dispersive soils Dispersive soils by nature of mineralogy and the chemistry of water in the soil are susceptible to separation of the individual clay particles and subsequent erosion. Different from erodible soils like silt or sand which erode by physical action


Earth Core Material ICOLD (1990) suggested that dams can be built with dispersive soils also but with Properly designed and constructed filters Proper compaction of soil Lime or Gysum modification of soil Sealing of crack in the abutment and cut-off trenches High turbidity of reservoir water


Rockfill Material Hard, sound and durable Resistance to excessive break down during handling, placement and compaction Un-weathered igneous and metamorphic rocks are most suitable Shales are not suitable. Angular and bulky rocks preferred over flat elongated rocks and rounded boulders


Rockfill Material Well graded gravels or Gravel sand mixtures; with little or no fines. Silty gravel, poorly graded gravels- sand silt mixture Poorly graded gravel or gravel sand mixture; with little or no fines

Design of Rockfill Dams Seepage Control Measures Zoning of Materials Chimney drain Blanket Drain Collector drain Toe drain Grout Curtain Core Trench Cut off Surry Trench Cut-off Upstream Blanket Gallery Pressure relief Wells and Trenches


Intermediate & Surface Grouting

EL. 648 1Spillway PI Dam

11 10 9 4Grout Curtain

2

3 8 5 6 7

12

Projected L-Section (Through Dam Axis) L-

Grout Curtain through DT

Diversion Tunnels


SP- 2

15

Diversion Tunnels 141 3

Plugs

13

2 1

D -2 A

SP- 1

4 3D -1 A

12 5 8 9 10 16 11

6

7


Filter Design Functional Requirements No segregation during processing, handling, placing, compaction. No change in gradation during processing, handling, placing, compaction or wetting and drying during seepage flow. No chance of cementation due to chemical, physical or biological action. Does not have any apparent or real cohesion Internally stable fine particles of filter should not migrate under seepage flow Have sufficient permeability Have ability to control and seal the erosion of base soil


Filter Design Filter material should satisfy the following criteria Shall be more pervious than base material Gradation shall be such that base material do not totally migrate through and clog the filter Category of Base Soil Category 1 2 3 4 % Finer than 75 > 85 40-85 15-39 0.1


Filter Design -

D15(F) 5 D15(B) > 0.1To minimize segregation D10(F) min. (mm)

Design of Earth and Rockfill Dam_Final

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