Cities laying in moderate to high earthquake hazard zones, an accurate prediction of site response is of paramount importance. The 1985 Mexico City earthquake is renowned for the devastation that local basin effects may cause through soil amplification. Moreover, erratic damage patterns and prolonged site and building responses were observed during the earthquake. Understanding the effect of basins along with the clustering effect of buildings on the amplification of ground motions can provide scientific guidelines for the design of buildings and provide a more accurate evaluation of regional scale seismic hazards. Many cities around the world are built on shallow alluvial basins. However, not much attention has been given to the investigation of the 3D effects of these basins o...[ Read more ]

Cities laying in moderate to high earthquake hazard zones, an accurate prediction of site response is of paramount importance. The 1985 Mexico City earthquake is renowned for the devastation that local basin effects may cause through soil amplification. Moreover, erratic damage patterns and prolonged site and building responses were observed during the earthquake. Understanding the effect of basins along with the clustering effect of buildings on the amplification of ground motions can provide scientific guidelines for the design of buildings and provide a more accurate evaluation of regional scale seismic hazards. Many cities around the world are built on shallow alluvial basins. However, not much attention has been given to the investigation of the 3D effects of these basins on ground motion. Moreover, due to vertical expansion in cities, high-rise buildings with deep foundations and large underground chambers are often simultaneously present. The role of these structures and their layouts in Site-City Interaction (SCI) have not been considered nor quantified before.

The work in this thesis aims to isolate and numerically quantify 3D Soil-Underground Structure-Soil Interaction (SUSSI), SCI and shallow basin effects through realistic physics-based earthquake scenario simulations. The models in this study are based on real sites in Hong Kong: the Kowloon Station transportation hub and the Tuen Mun-Yuen Long basin. For our SCI studies high-rise superstructure-foundation systems in various layouts and a metro station are fully integrated into large-scale soil models. While a 3D multi-layered model of the Tuen Mun-Yuen Long basin with equivalent non-linear soils is analyzed for our basin study. Moreover, to isolate SCI effects on site and structural responses, freefield site, and standalone building models were created and analyzed. To isolate 3D basin effects 1D site response analyses were conducted for Tuen Mun-Yuen Long. The results of these analyses were then contrasted to the fully integrated city models and the 3D basin model, respectively. All of our models are based on borehole or structural design data. A 3D Discontinuous Galerkin Spectral Element Method (DGSEM) is adopted to address meshing difficulties related to the complexity of geology and the scale difference present between the fully integrated city models, the soil layers and the bedrock. The DG formulation of SEM allows the use of a non-conforming mesh which consist of individually meshed then combined sub-domains. The sub-domains may have discontinuities between each other, such as sharp changes in mesh size and their shape function’s polynomial degree.

For building clusters surrounding a center plaza we found that the key SCI phenomena are (1) a wavefield propagating outward from the building cluster and (2) surface waves trapped between structures (SUSSI). Focused zones of amplification (up to 150% in PGA) emerge at the unbuilt areas in the center plaza (2) and the immediate outskirt (1) of these clusters. The phenomena always occur in high frequency ranges (above 2 Hz) thus, SCI may significantly increase seismic demand on short (<5 story) structures in the vicinity of high-rise buildings (>40 stories). Our results also showed that closely-spaced high-rise buildings increase each other’s relative story accelerations by up to 150% during earthquakes. It was shown that SUSSI can adversely affect secondary and interim structures at the plaza. Moreover, SCI will significantly increase the maximum relative story accelerations which will increase serviceability demands and building content loss. We found that different building layouts will generally result in a considerably different site response and marginally different structural responses. When the center plaza area was increased, the adverse SUSSI and SCI effects both diminish. The Tuen Mun-Yuen Long city model showed that strong surface waves incoming from the basin edge will intensify SCI however, the spatially variable local site effects will control structural responses.

Within the shallow basin a good correlation between soil depth and spectral amplifications was demonstrated. The spectral amplification vs. soil depth curves presented in this study can aid safe seismic design by informing engineers on suitable locations for structures with different modal frequencies during urban planning. From the detailed analysis of three sub-basins in Tuen Mun-Yuen Long, we conclude that 2D/3D effects, such as the edge and the focusing effect can be substantial in narrow and shallow basins. The focusing effect is particularly accentuated for bowl-shaped symmetrical basins due to their 3D geometry. While in all analyzed sib-basins, the surface waves generated at the edges are further trapped by the top soil layer. We found that this phenomenon can significantly increase the site response near the edges as well as in the center of narrow basins where the waves can constructively interfere. The site along the basin cross-sections is amplified at its 1D fundamental frequency which is slightly shifted as a result of 3D basin effects. The wavefield in the basins showed significantly higher in-plane (e.g. SH) and out-of-plane (e.g. SV) wave amplitudes at the basin edges and basin centers, respectively.

Our study concluded that Site-City interaction of high-rise buildings will increase serviceability demands and adversely affect short structures in their vicinity. Furthermore, a detailed, fully integrated 3D numerical analysis, as presented in this thesis, can realistically predict Site-City Interaction and seismic demand in shallow basins, which might be significantly underestimated by conventional analysis. Finally, based on all the simulations conducted in study, we derived SCI factors to account for the additional demand generated by SCI on buildings in congested urban environments.