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Numerical analysis of a highway embankment's response due to rupture of underlying active fault (Doctoral thesis)

Πεταλά, Ελένη/ Petala, Eleni

Seismic design of a highway embankment is a significant issue of Geotechnical Earthquake Engineering, because its potential failure can cause severe social and financial consequences. There are two types of earthquake loading. The first one is transient dynamic oscillations due to propagation of seismic waves away from the seismic source. The second one is a permanent quasi static ground deformation caused by fault rupture propagation through the overlying soil. In the past, occurrence of earthquake events has shown the great impact of fault rupture propagation on the integrity of constructions and technical projects .Based on the above, PhD dissertation investigated the response of a typical highway embankment founded on a single-layered soil due to normal and reverse fault rupture diffusion. The main goal of this work was to assess the adverse effects of disruption spread to the functionality and safety of the embankment. For this purpose, an important number of quasi-static parametric numerical analyses were performed by using the 2D finite difference software, FLAC. However, before performing the analyses, it was necessary to confirm constitutive model’s capability to reliably simulate soil’s response. To accomplish this, numerical outputs were tested with data obtained from centrifuge tests and published numerical analyses results. The PhD dissertation was divided into two parts. In the first part of this research, the effect of overlying embankment on fault outcropping path and on soil surface deformation was studied. To achieve this, parametric analyses were carried out for both single-layered soil and embankment’s presence on the top of single-layered soil. Numerical outputs were compared on the basis of vertical displacement of ground surface and formation of shear bands on the soil layer. In the second part, focus was paid on the assessment of fault rupture effects on an overlying highway embankment. Firstly, fault rupture path through embankment, alteration of embankment’s geometry and vertical displacements of its external surface were examined. However, the main goal of this work was to quantify and specify the spatial variation of embankment’s damage due to rupture diffusion. For this purpose, two criteria were suggested and subsequently adopted. The first criterion relied on the relative compaction (RC) of the embankment. It is widely accepted that RC is a robust criterion of embankment's functionality. According to output of numerical analyses, it was observed that along the main and the secondary rupture or shear zones developed into embankment's body, values of RC decreased less than 94%. Based on this observation, it was regarded that an indicator of quantification of potential damage could be the percentage of embankment’s area where RC<94% (RC94). The second criterion was based on the ratio of volume modification over the original volume of the embankment (ΔV/V0). The combination of the above criteria proved to be a reliable tool that enabled mapping and quantification of damage caused by fault rupture propagation on a highway embankment. Assessment of analyses’ results showed that the existence of the embankment affects both rupture’s path and deformations of the soil’s free surface. In addition, regardless of the type of fault, rupture spread through the embankment causes a significant deformation on its external surface and a significant reduction of its relative compaction. Finally, it was found that for most of the analyses, the disruption of a normal rupture tends to cause a greater drop of embankment’s relative compaction and more pronounced change in its initial volume. The aforementioned results contribute significantly to an effort to assess highway or motorway embankment’s damage caused by both normal and / or reverse fault rupture propagation. The use of the proposed criteria is a first attempt to assess the impact of rupture propagation on safety and functionality of unreinforced highway embankments. They form a reliable tool based on which, quantification and mapping of damage within embankment’s body can be achieved.