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Dikes failures and limit states of the HYD in EC 7. Ivan Vaníček Czech Technical University in Prague Faculty of Civil Engineering Geotechnical Department. vaniceki@fsv.cvut.cz. Earth structures in Water Engineering. Large dams Small dams Anti flood protection systems (Dikes)
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vaniceki@fsv.cvut.cz Dikes failures and limit states of the HYD in EC 7 Ivan Vaníček Czech Technical University in Prague Faculty of Civil Engineering Geotechnical Department vaniceki@fsv.cvut.cz
vaniceki@fsv.cvut.cz Earth structures in Water Engineering • Large dams • Small dams • Antiflood protection systems (Dikes) • Canals (e.g. Panama canal) • Reservoirs (e.g. of pumped storage hydro plans) • all now fully under EC 7 • both 2 positions • Soil and rock as construction material • Interaction with subsoil (ground)
vaniceki@fsv.cvut.cz 3 Geotechnical Categories • 1.GC 2. GC 3.GC • Negligible risk No exceptional risk Very high risk • Limit states • Ultimate limit statesServiceability limit state • GEO Ed ≤ Cd • STR Ed -design value of the effect of actions • EQU Cd limiting design value of the effect of an action • UPL design values for limiting movements and deformations • HYD
vaniceki@fsv.cvut.cz Large Dams • Total failure Defects (accidents) • 1-1,5% ÷5% • Main problems • Higher flow through dam body by preferential paths • = tension cracks • = differential settlement • = internal erosion • Smaller problems • Overflowing • Slope stability
vaniceki@fsv.cvut.cz Specificity of 2002 year floods Character of high-flood-water wave • Red – Vltava river – České Budějovice • Blue – Malše river – Roudné Orlík dam – normal state Character of high flood- water wave for dam Orlík • Red – inflow to the reservoir (3900 m3/s) ≥ Q1000 • Blue – outflow • Green – water level Orlík dam – flood in 2002
vaniceki@fsv.cvut.cz Floods 2002 • Blatná
vaniceki@fsv.cvut.cz Floods 2002
vaniceki@fsv.cvut.cz Historical earth dams • in Middle Ages around 1500 – 1700 A.D. - about 75 000 of small dams, • now about 25 000 • homogeneous from local material – granite elluvium • for bigger ones the outlets were reconstructed - old wooden ones were substituted by concrete ones for 100 year flood • Main problem –for 2002 floods Qmax much higher than Q100 • Result of which – overflowing • Domino effect!
vaniceki@fsv.cvut.cz Floods 2002 • Pond Metelský – two dam failures
vaniceki@fsv.cvut.cz Floods 2002 • Pond Veský – downstream slope erosion
vaniceki@fsv.cvut.cz Floods 2002 • Road E 49 • Pond Zámeckýdownstream slope erosion
vaniceki@fsv.cvut.cz Floods 2002 • Pond Podhajč – dam failure
vaniceki@fsv.cvut.cz Floods 2002 • Pond Dolejší – two dam failures
vaniceki@fsv.cvut.cz Basic limit states causing failures • Internal erosion Metly Podhajský b) Surface erosion Zámecký Dolejší
vaniceki@fsv.cvut.cz • Hydraulic failure HYD • The provisions of the Section apply to four modes of ground failure induced by pore-water pressure or pore-water seepage, which shall be checked, as relevant: • failure by uplift (buoyancy) • failure by heave • failure by internal erosion • failure by piping
vaniceki@fsv.cvut.cz • Example of conditions that may cause piping • 1 free water table • 2 piezometric level in the permeable subsoil • 3 low permeability soil • 4 permeable subsoil • 5 possible well; starting point for pipe • 6 possible pipe
vaniceki@fsv.cvut.cz • Suitable structural measures are: • application of berms on the land side of a retaining embankment, thus displacing the possible starting point of piping farther away from the structure and decreasing the hydraulic gradient at this point; • application of impermeable screens below the base of the hydraulic structure by which the ground-water flow is either blocked or the seepage path is increased, thereby decreasing the hydraulic gradient to a safe value.
vaniceki@fsv.cvut.cz Internal erosion • Failure by internal erosion is produced by the transport of soil particles within a soil stratum, at the interface of soil strata, or at the interface between the soil and the structure. This may finally result in regressive erosion, leading to collapse of the soil structure.
vaniceki@fsv.cvut.cz • The measures most commonly adopted to reduce erosion or to avoid hydraulic failure are: • lengthening the seepage path by screens or shoulders; • modifications of the project in order to resist the pressures or gradients; • seepage control; • protective filters; • avoidance of dispersive clays without adequate filters; • slope revetments; • inverted filters; • relief wells; • reduction of hydraulic gradient
vaniceki@fsv.cvut.cz Summary • Limit states which are playing most important role for earth structures in water engineering are: • Internal erosion (including piping) • Surface erosion EC 7: Internal erosion is specified only in very broad way – also the way by which this limit state should be verified - can we use the adoption of prescriptive measures or experimental models? • Surface erosion is not specified at all (although was mentioned in early versions)
vaniceki@fsv.cvut.cz Internal erosion • Questions? • What is critical gradient? • How this gradient can be predicted – especially for unsaturated conditions ? • Which filtration criteria can be recommended for non-cohesive and cohesive soils? • Must we select filtration criteria for soils with cracks? • How recommended filtration criteria have to be verified? • What to take as representative size of grain (or opening for geotextiles filters) for protected soil and for filter? • How to evaluate a potential danger of filter size separation during filling? • How important is the role of the soils susceptibility to the erosion? Have special conditions to be specified for dispersive soils? • Have we to leave all these questions on National Annexes to EC 7 or have we to try to find more common and acceptable solution in Europe?
vaniceki@fsv.cvut.cz Surface erosion • In the Czech Republic we are using the experimental models and prescriptive measures: • Where it is possible to select the place where overflowing can start (causing minimum problems) - there the crest of dam (dike) is decreased (e.g. 0.3 m) and reinforced. The example for the reconstruction of old earth dam, where additional spillway reinforced by concrete was applied
vaniceki@fsv.cvut.cz • Application of anti erosion geo mattresses with vegetation (grass)
vaniceki@fsv.cvut.cz • Dikes reinforced with fibre-brick-concrete
vaniceki@fsv.cvut.cz Thank you for your attention Acknowledgement: The work presented in this paper was carried out with funding from the MSM CR grant number 6840770005 Sustainable construction.
