vaniceki@fsv.cvut.cz

transcript

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

1vaniceki@fsv.cvut.cz

vaniceki@fsv.cvut.cz 2

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)

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

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

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

Floods 2002

Blatná

Floods 2002

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!

Floods 2002

Pond Metelský – two dam failures

Floods 2002

Pond Veský – downstream slope erosion

Floods 2002

Road E 49 Pond Zámeckýdownstream slope erosion

Floods 2002

Pond Podhajč – dam failure

Floods 2002

Pond Dolejší – two dam failures

Basic limit states causing failures

a) Internal erosion

Metly Podhajský

b) Surface erosion

Zámecký Dolejší

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

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

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.

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.

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

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)

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?

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

Application of anti erosion geo mattresses with vegetation (grass)

Dikes reinforced with fibre-brick-concrete

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

vaniceki@fsv.cvut.cz

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