CASE STUDY

Behavior of Rockfill Dam under Dynamic Loading

Date:

2023

Industry Area:

Civil

Client:

EDF – CIH

ITASCA Office:

France

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FLAC3D

PFC

Project Background

Earthquakes are natural hazards that occur all over the world due to the movements of the Earth’s crust, leading to the damage or destruction of civil engineering structures like buildings, bridges, and dams. The design of these structures needs to considers the probable seismic loading caused by an earthquake according to building codes.

In this study, the dynamic behavior and resistance of a real-scale rockfill dam, with dry-stone pitching on both faces, under earthquake dynamic loading were studied to understand the resulting mechanical behavior of the dam and effects on each component. The main objectives were to estimate the crest displacements at the end of the earthquake and to quantify its dynamic resistance.

Escoubous dam (Pyrenees, France).

ITASCA’s Role

The dam in this study is located in the Pyrenees mountains in France and was built in the early 1950s. It was modeled using the mixed FDM-DEM approach (continuum-discrete). The dry-stone pitching was modeled discretely in PFC3D while FLAC3D was used to model the dam body and the foundation. The mesh and boundary conditions are shown on a figure below; free-field boundaries at the model sides and a quiet boundary at the foundation base were used to avoid the wave reflections back into the model.

The Mohr-Coulomb model coupled with a hysteretic damping model were used to represent the dam body and foundation. The parameters were estimated from a literature review. The model was run under dynamic loading, applied at the foundation base, for about 20 seconds, and the obtained crest settlements were analyzed. Dynamic resistance of the dam was studied using several methods.

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FLAC3D

PFC

Project Results

Using PFC3D and FLAC3D, ITASCA modeled the mechanical behavior of the dam under earthquake dynamic loading and found:

  • The moraine layer in the foundation and under the dam amplifies the motion by about 25%.
  • The resulting post-earthquake crest settlements are within the range of observed historical results on similar dams.
  • The dynamic load has led to a change in the contact distribution and arrangement at the pitching-dam body interface.
  • The post-seismic static safety factor was found to be 1.3, which is lower than the initial value of 1.4, meaning the seismic motion has slightly weakened the dam.
  • The dam can withstand a dynamic motion up to 0.48 g, but it fails at higher PGAs (with the signal characteristics used in this study).
Sketch of dam. Backfill made of decametric blocks protected by a face composed of tightly hand-placed stones.
Dynamic numerical model and boundary conditions.
Contour of zone shear strain after earthquake dynamic loading.
Failure of dam at PGA = 0.52 g at 12 seconds and 13 seconds of dynamic loading.

Acknowledgments

The study was part of the Ph.D. project of Ali Haidar done in ITASCA France in collaboration with École centrale de Lyon and the client (EDF).

References

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