Workshop of the ISSMGE TC 32, 24-25 September 2010, Budapest
Dikes failures and limit states of the HYD
in EC 7
Ivan Vaníček
Czech Technical University in Prague
Faculty of Civil Engineering
Geotechnical Department
Workshop of the ISSMGE TC 302, 24-25 September 2010, Budapest
Earth structures in Water Engineering
Large dams
Small dams
Anti flood 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)
Workshop of the ISSMGE TC 32, 24-25 September 2010, Budapest
3 Geotechnical Categories
1.GC 2. GC 3.GC
Negligible risk No exceptional risk Very high risk
Limit states
Ultimate limit states Serviceability 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
Workshop of the ISSMGE TC 302, 24-25 September 2010, Budapest
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
Workshop of the ISSMGE TC 32, 24-25 September 2010, Budapest
Specificity of 2002 year floods
Character of high-flood-water waveRed – Vltava river – České Budějovice
Blue – Malše river – Roudné
Orlík dam – normal state
Character of high flood- water wave for dam OrlíkRed – inflow to the reservoir (3900 m3/s) ≥ Q1000
Blue – outflow
Green – water level
Orlík dam – flood in 2002
Workshop of the ISSMGE TC 32, 24-25 September 2010, Budapest
Floods 2002
Blatná
Workshop of the ISSMGE TC 302, 24-25 September 2010, Budapest
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!
Workshop of the ISSMGE TC 32, 24-25 September 2010, Budapest
Floods 2002
Pond Metelský – two dam failures
Workshop of the ISSMGE TC 32, 24-25 September 2010, Budapest
Floods 2002
Pond Veský – downstream slope erosion
Workshop of the ISSMGE TC 32, 24-25 September 2010, Budapest
Floods 2002
Road E 49 Pond Zámeckýdownstream slope erosion
Workshop of the ISSMGE TC 302, 24-25 September 2010, Budapest
Floods 2002
Pond Podhajč – dam failure
Workshop of the ISSMGE TC 302, 24-25 September 2010, Budapest
Floods 2002
Pond Dolejší – two dam failures
Workshop of the ISSMGE TC 302, 24-25 September 2010, Budapest
Basic limit states causing failures
a) Internal erosion
Metly Podhajský
b) Surface erosion
Zámecký Dolejší
Workshop of the ISSMGE TC 302, 24-25 September 2010, Budapest
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
Workshop of the ISSMGE TC 302, 24-25 September 2010, Budapest
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
Workshop of the ISSMGE TC 32, 24-25 September 2010, Budapest
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.
Workshop of the ISSMGE TC 302, 24-25 September 2010, Budapest
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.
Workshop of the ISSMGE TC 302, 24-25 September 2010, Budapest
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
Workshop of the ISSMGE TC 302, 24-25 September 2010, Budapest
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)
Workshop of the ISSMGE TC 302, 24-25 September 2010, Budapest
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?
Workshop of the ISSMGE TC 302, 24-25 September 2010, Budapest
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
Workshop of the ISSMGE TC 302, 24-25 September 2010, Budapest
Application of anti erosion geo mattresses with vegetation (grass)
Workshop of the ISSMGE TC 302, 24-25 September 2010, Budapest
Dikes reinforced with fibre-brick-concrete
Workshop of the ISSMGE TC 302, 24-25 September 2010, Budapest
Acknowledgement: The work presented in this paper was carried out with funding from the MSM CR grant number 6840770005 Sustainable construction.
Thank you for your attention
