Seismic design of sheet piles. Flyer | 2021

Innovative Seismic Design Solutions for Sustainable Sheet Piling Infrastructure

Excellent performance of sheet piles under earthquake loading

Steel sheet piles are widely used for the construction of a variety of structures: quay walls and breakwaters in harbours, bank reinforcements on rivers and canals, urban infrastructures such as underpasses, as well as global hazard protection schemes. Sheet piles are also used in seismic areas and have shown their good performance when undergoing an earthquake.

Chile is the country that suffered the biggest earthquakes in recorded history, of which the 8.8 magnitude Maule earthquake that hit the Pacific coast in 2010. Many of the earthquakes that hit Chile in the last decade caused severe damages to the concrete-based ports of the country. Port of Mejillones, that was constructed in 2003 using the HZ/AZ combined wall for the quay wall and AS 500 straight web sheet piles for the breakwater, suffered no damages throughout many heavy earthquakes with magnitude of up to 7.7. All the involved parties in this project (Port authority, consultant, contractor and technical university) agreed that this port is a perfect example of the effectiveness of flexible sheet pile structures under extreme seismic conditions.

 

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A parametric study covering a wide spectrum of cases

Numerical studies and physical experiments (centrifuge testing) have shown that these conventional methods of design are overestimating the loads on retaining walls, and especially in the case of flexible walls. EN 1998-5 allows for a reduction of the seismic action depending on the acceptable displacements, through a reduction factor “r”, but this factor is mainly thought for gravity walls and does not allow any reduction for anchored walls, including sheet pile walls despite their inherent property of ductility.

Today, powerful design tools using Finite Element Modeling (FEM) allow dynamic calculations that can accurately predict the behaviour of the retaining walls undergoing different seismic loadings. 

Advanced seismic design methods allow up to 50% cost savings

The study considered 11 cases crossing different soil conditions, seismic accelerations and water depths (see table), in order to draw clear conclusions on the advantages and disadvantages of each design method. 

The pseudo-static design is performed using the elasto-plastic subgrade reaction software RIDO. The seismic action is considered by modifying the earth pressure coefficients Ka and Kp based on the well-known Mononobe-Okabe formula. This results in an increase of the active pressure behind the wall and a decrease of the passive pressure in front of the wall.

Proper consideration of hydrodynamic loads is necessary for an economical solution

The common practice for taking into account hydrodynamic loads is to consider a pseudo-static load calculated from EN 1998-5 (Westergaard formula). This translates into considering a permanent load from the water, for the whole duration of the earthquake, on the shaking quay wall. SENER carried out FEM and Computational Fluid Dynamics (CFD) calculations to measure the impact of hydrodynamic loads on a sheet pile wall during the seismic motion.

The CFD calculations considered soil-fluid interactions under dynamic analyses. The hydrodynamic load calculated at an instant “t” of the earthquake showed a very good match with the Westergaard load calculated with the seismic acceleration at that same instant. This means that the hydrodynamic load mentioned in EN 1998-5, which uses the Peak Ground Acceleration, represents the higher bound envelope of the hydrodynamic loads. 

Italian standard NTC 2018 rewarding Tanzanian proverb the flexbility of sheet piles 

After pinpointing the overestimation that can result from performing pseudo-static calculations using EN 1998-5, the study analyzed what the recent Italian seismic standard NTC 2018 has to offer in this matter. 

NTC 2018 follows the same philosophy as EN 1998-5 for pseudo-static calculations but introduces many amendments on the parameters defining the seismic action. The main changes concern the seismic reduction coefficient that accounts for the deformability of the structure and the deformability of the soil among other things. In practice, NTC 2018 allows further reduction of the seismic coefficient for more flexible walls. 

SENER carried out the pseudo-static calculations using NTC 2018 as a reference standard for the seismic coefficient. The resulting sheet pile sections were lighter than those obtained with EN 1998-5, and closer to those from dynamic FEM calculations.