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Engineering assessments of ground failure potential commonly express seismic demands in the form of shear stresses or normalizations thereof. In soil materials underlying foundations, referred to as ‘foundation soils’, dynamic stresses result from wave propagation associated with site response in combination with demands associated with soil-structure interaction (SSI). Engineers presently lack a simplified procedure for analysis of SSI-related stress demands. To provide insight into the physics of the SSI-induced stresses, and to lay the ground work for an eventual simplified procedure, this paper presents preliminary solutions for the problem of a dynamic vertical point load on the surface of an elastic half-space (Lamb’s problem) and the resulting stresses within foundation soils. Results are presented in dimensionless graphical form and indicate the amplitude and phase of dynamic stresses.

Foundation damping incorporates the combined effects of energy loss due to waves propagating away from the vibrating foundation (radiation damping), as well as hysteretic action in the soil (material damping). Two closed-form solutions for foundation damping of a flexible-based single degree-of-freedom oscillator were derived from first principles where the inherent (structural) damping ratio, radiation damping and the soil hysteretic damping ratio appear as variables. Since both formulations are theoretically defensible, yet provide numerically distinct results, we look to case histories for validation. In addition to previously established case histories we evaluated data derived from forced vibration testing of a field test structure installed at two sites in California. Parametric system identification was employed to evaluate the fixed-base and flexible-base first mode period and damping ratio during tests performed across a wide frequency range and multiple load amplitudes. Foundation damping derived from these results is shown to favor one of the theoretical models over the other.

During earthquake ground shaking earth pressures on retaining structures can cyclically increase and decrease as a result of inertial forces applied to the walls and kinematic interactions between the stiff wall elements and surrounding soil. Limit equilibrium analysis imposes a pseudo-static inertial force to a soil wedge behind the wall (the mechanism behind the widely-used Mononobe-Okabe method), which is a poor analogy for either inertial or kinematic wall-soil interaction. Many basement walls and retaining structures are dominated by kinematic soil-structure interaction (SSI) effects arising from differences in displacement between the wall and the free-field soil. Kinematic SSI solutions are often formulated for uniform soil conditions, but the shear modulus of most soils is known to increase with mean effective stress, and therefore with depth. We examine the influence of vertical heterogeneity of shear modulus on kinematic SSI for rigid walls. An existing free-field displacement solution is presented first, followed by analysis of earth pressure increments using a Winkler assumption. Vertical heterogeneity is shown to reduce seismic earth pressures compared with a uniform soil case (for a given frequency and peak ground surface displacement) because free-field displacements are largest near the surface, where the soil is softest and Winkler stiffness is lowest. The proposed Winkler solution is then compared with an exact analytical solution for vertically heterogeneous soil over a rigid base and retained between two opposing rigid walls. The agreement is imperfect, but reasonable, with differences likely due to assumptions regarding the dynamic Winkler stiffness intensity.

On October 30, 2020 14:51 (UTC), a moment magnitude (M) 7.0 (USGS, EMSC) earthquake occurred in the Aegean Sea. This paper presents the reconnaissance findings regarding the site effects on recorded strong ground motion intensities and duration, along with the resulting induced-structural damage in Izmir Bay and Samos Island, respectively. In all rock records, relatively high intensity long period rock spectral accelerations were observed in the mid to long period range of 0.5–1.5 s, which are attributed to the source, more specifically, to the slower rupture-mechanism of the event. These rich spectral intensities were further amplified by soil site effects and soil-superstructure resonance, leading to two to six times amplified overall responses and prolonged seismic shaking durations, more pronounced in Bayrakli and other Izmir Bay sites in Turkey. However, these amplified and prolonged excitations are still below design basis earthquake levels, which addresses the lack of proper structural design and construction deficiencies, as the underlying causes for the collapse to heavy damage performance of 795 buildings. On the other hand, although located only about 10 km from the rupture (22 km from the epicenter) and within the near fault zone, the town of Vathy on Samos Island (Greece) was rather lightly affected by the earthquake, with relatively few collapsed or heavily damaged buildings, partially attributed to the low height/low weight of structures in the area. However, a concentration of damage in low-rise buildings in Ano Vathy hill is considered indicative of a combination of coupled valley and topography effects on the strong motion. This event once again addressed the need to develop region-specific zonation and provisions, when more general code practices are proven to be inadequate to assess these extreme site effects.

On October 30, 2020 14:51 (UTC), a moment magnitude (Mw) of 7.0 (USGS, EMSC) earthquake occurred in the Aegean Sea north of the island of Samos, Greece. Turkish and Hellenic geotechnical reconnaissance teams were deployed immediately after the event and their findings are documented herein. The predominantly observed failure mechanism was that of earthquake-induced liquefaction and its associated impacts. Such failures are presented and discussed together with a preliminary assessment of the performance of building foundations, slopes and deep excavations, retaining structures and quay walls. On the Anatolian side (Turkey), and with the exception of the Izmir-Bayrakli region where significant site effects were observed, no major geotechnical effects were observed in the form of foundation failures, surface manifestation of liquefaction and lateral soil spreading, rock falls/landslides, failures of deep excavations, retaining structures, quay walls, and subway tunnels. In Samos (Greece), evidence of liquefaction, lateral spreading and damage to quay walls in ports were observed on the northern side of the island. Despite the proximity to the fault (about 10 km), the amplitude and the duration of shaking, the associated liquefaction phenomena were not pervasive. It is further unclear whether the damage to quay walls was due to liquefaction of the underlying soil, or merely due to the inertia of those structures, in conjunction with the presence of soft (yet not necessarily liquefied) foundation soil. A number of rockfalls/landslides were observed but the relevant phenomena were not particularly severe. Similar to the Anatolian side, no failures of engineered retaining structures and major infrastructure such as dams, bridges, viaducts, tunnels were observed in the island of Samos which can be mostly attributed to the lack of such infrastructure.