Brochure - Seismic design of sheet piles: economic benefits of advanced design methods

To further enhance design and project efficiencies with sheet piling solutions in areas of high seismic risk, the brochure ‘Seismic design of sheet piles: economic benefits of advanced design methods’ presents innovative design methods for extreme dynamic loading conditions in ports and waterways and other infrastructure domains.

Feel free to contact ArcelorMittal Sheet Piling for more detailed information or for assistance with the dynamic design of sheet piles using FEM.

Economical and safe design approaches for steel sheet piling structures in highly seismic zones

Widely used for the construction of a variety of structures such as quay walls and breakwaters in harbours, bank reinforcements on rivers and canals, underpasses, as well as global hazard protection schemes, sheet piles have proven their performance in seismic areas in many countries around the globe.

Chile, the country that has suffered the strongest earthquakes in recorded history, provides an excellent example: Whereas its concrete-based ports have been severely damaged, the Port of Mejillones, constructed in 2003 using an HZ®/AZ® combined wall for the quay wall and AS 500 straight web sheet piles for the breakwater, has not suffered any damage throughout many heavy earthquakes with magnitudes of up to 7.7. This is the perfect example of the effectiveness of flexible sheet pile structures under extreme seismic conditions.

Nevertheless, a certain reluctance to use sheet piles in seismic areas remains common among some designers. This concern may come from their experience of conventional design methods which do not favour flexible walls in seismic areas. These design methods are usually comprised of pseudo-static calculations using the Mononobe-Okabe theory (1931).

Numerical studies and physical experiments (centrifuge testing) have shown that these conventional methods of design are overestimating the loads on retaining walls - especially in the case of flexible walls. Although EN 1998-5 allows for a reduction of the seismic action depending on the acceptable displacements (reduction factor “r”), this only applies to gravity walls and not to anchored walls such as sheet pile walls, despite their inherent ductility.

Today, powerful design tools using Finite Element Modelling (FEM) allow for dynamic calculations that can accurately predict the behaviour of the retaining walls undergoing different seismic loadings, including internal forces, deformations, increases in pore water pressures, and expected modes of failure.

Material cost savings thanks to dynamic design methods

A research project led by ArcelorMittal R&D and a study carried out by world-leading maritime consultant engineering company SENER demonstrated significant optimisation potential. In the study, a wide spectrum of cases was analysed (four water depths; four seismic accelerations; two soil conditions) that compared the conventional pseudo-static method based on EN 1998-5 using elasto-plastic subgrade reaction software and the fully dynamic advanced method using FEM software.

All of the cases studied showed substantial optimisation potential when using FEM design. The bending moments in pseudo-static design are 40% to 126% higher than those of FEM design. When considering the respective sheet pile sections, this could result in up to 50% of material cost savings using the advanced seismic design methods.

Hydrodynamic loads properly analysed contribute to additional material savings

Hydrodynamic loads are commonly considered pseudo-static and calculated according to EN 1998-5 using the Westergaard formula as a permanent load from the water during the entire duration of the earthquake. SENER used FEM and Computational Fluid Dynamics (CFD) to calculate the impact of hydrodynamic loads on a sheet pile wall during seismic action, considering soil-fluid interactions under dynamic analysis.

Using the traditional Westergaard formula in EN 1998-5 matches the hydrodynamic loads obtained by CFD at a certain point of time during the earthquake. However, as Westergaard considers these to be permanent loads during the entire earthquake, an overestimation of its effect is evident.

The FEM calculations carried out using Plaxis 2D compared the results obtained when using the traditional Westergaard load with those obtained when using a realistic time-dependent variable load (through a dynamic load or added masses). The results showed an increase of 24.5% in the bending moment (with respect to the effects from the purely seismic action) when using the traditional Westergaard load compared to 4% when using the instantaneous load. In this case study, considering a realistic hydrodynamic load translated into a material cost saving of 14% when considering the corresponding sheet pile sections.

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