Topography and Soil Effects in the MS 5.9 Parnitha (Athens) Earthquake: The Case of Adámes

Large concentration of damage to residential and industrial buildings occurred in regions near the banks of the Kifisos river canyon during the 7-September-1999 Parnitha (Athens) Earthquake. One such region, which experienced unexpectedly heavy damage, was the small community of Adámes, which borders the canyon near its deepest point. To explore whether in addition to structural factors the particular topographic relief and/or the actual soil profile contributed to the observed concentration and non-uniform distribution of damage within a 300 m zone from the edge of the canyon cliff, wave propagation analyses are conducted in one and two dimensions. Finite-element and spectral-element formulations are used to this end. To avoid spurious wave reflections at the artificial boundaries, ourtwo-dimensional (2-D) finite-element analyses utilize Bielak's Effective Seismic Excitation method. Soil layering and stiffnesses are determined from 10 SPT-boreholes and 4 crosshole tests. Ricker wavelets and six realistic accelerograms are used as excitation; two of the latter are selected from the literature and four are obtained on the basis of the four strongest motions of the earthquake, recorded in central Athens.The results show that the 2-D topography effects are substantial only within 50 meters from the canyon ridge. These effects materialize only in the presence of the relatively soft soil layers that exist in the profile at a shallow depth. The so-called Topographic Aggravation Factor (TAF), defined as the 2-D over 1-D Fourier spectral ratio, varies around 1.4 over a broad frequency band which covers the significant excitation frequencies. At the location of four collapsed buildings, about 250 m from the edge, 2-D (topography) effects are negligible, but the specific soil profiles amplify one-dimensionally all six ground base excitations to spectral acceleration levels that correlate well with the observed intensity of damage, at least in a qualitative sense.     

Identification and imaging of soil and soil-pile deformation in the presence of liquefaction

1 Introduction Soil and soil-structure systems exhibit complex response patterns during earthquake-induced liquefaction. These patterns depend on geotechnical properties, in-situ stress conditions and interaction with subsurface structural elements. Seismic records of full- scale systems during case histories provide a valuable source of information on the associated response mechanisms. However, the response of these systems is commonly monitored at sparsely distributed locations only, mostly…     

Physical modeling and visualization of soil liquefaction under high confining stress

The mechanisms of seismically-induced liquefaction of granular soils under high confining stresses are still not fully understood. Evaluation of these mechanisms is generally based on extrapolation of observed behavior at shallow depths. Three centrifuge model tests were conducted at RPI‘s experimental facility to investigate the effects of confining stresses on the dynamic response of a deep horizontal deposit of saturated sand. Liquefaction was observed at high confining stresses in each of the tests. A system identification procedure was used to estimate the associated shear strain and stress time histories. These histories revealed a response marked by shear strength degradation and dilative patterns. The recorded accelerations and pore pressures were employed to generate visual animations of the models. These visualizations revealed a liquefaction front traveling downward and leading to large shear strains and isolation of upper soil layers.