Abstract
The possibility is here explored to use an ‘equivalent’ homogeneous configuration to simulate 1D seismic response of heterogeneous engineering-geological bodies when relatively weak seismic impedance contrasts internal to the bodies (it was assumed a shear wave velocity variation between the alternating layers equal to 150 m/s) only exist above the seismic bedrock. This equivalent configuration is obtained by considering an equivalent Vs value the harmonic average of the actual Vs values and a linear combination of G/G0 and D curves relative to the lithotechnical components present in the actual configuration. To evaluate feasibility of this approach, a wide set of numerical simulations was carried out by randomly generating subsoil layering including sequences of alternating thin layers of geotechnical units (e.g. sands and clays) each characterized by a characteristic nonlinear curve. Outcomes of these simulations are compared with those provided by considering a single homogeneous layer characterized by equivalent nonlinear curves obtained as a weighted average of the original curves. By comparing the heterogeneous and the homogeneous columns seismic response in terms of amplification factors and fundamental period, the results confirm the possibility to model a 1D column characterized by a generic lithostratigraphic succession with an equivalent one without introducing significative errors that, at least for the studied cases, do not exceed the 6%. This conclusion is substantially confirmed by extending the comparison to a real case, i.e., the 113 m-thick heterogeneous soil profile at Mirandola site (Northern Italy), presented in the last part.
Highlights
Accounting for nonlinear behavior of soil under dynamic loads is necessary to provide a consistent modelling of local seismic response during strong earthquakes (Kramer, 1996)
For Seismic Microzonation (SM) purposes, an attractive simplification is to replace the heterogeneous subsoil model with a homogeneous equivalent one characterized by a very close seismic response at surface. This operation requires the definition of an equivalent shear wave velocity, Vs,eq, and equivalent variation curves of the shear modulus Geq/G0-g and the damping ratio Deq-g
It can be observed a good match between the seismic responses of heterogeneous and the equivalent columns, the differences are in the same order of the uncertainties associated to the analysis method, at least for the considered signals: the equivalent linear approach slightly overestimates the Amplification Factor (AF) with respect to the nonlinear approach, at low and medium periods, in any case less than 5%, while the difference between the heterogeneous and the equivalent column is contained within 6%
Summary
Accounting for nonlinear behavior of soil under dynamic loads is necessary to provide a consistent modelling of local seismic response during strong earthquakes (Kramer, 1996). The above problems are enhanced when extensive seismic response studies are of concern and detailed geotechnical soil characterization cannot be performed This is the case of site response analyses for SM, where subsoil investigations are carried out at urban scale and a detailed characterization of stratigraphic profile as well as mechanical proprieties is not available in the numerical simulations. In most cases (in particular in the Italian practice), soil structure is inferred by geological considerations and indirect geophysical investigations (e.g., SM-WG, 2008; Albarello et al, 2015; Albarello, 2017; Moscatelli et al, 2020) This implies that soil characterization is performed in terms of engineering-geological categories, (TCSM, 2020, 2021; Amanti et al, 2020), by grouping the different rock-units into two main categories: “Cover terrains” and “Geological bedrock” units, on the base of their lithological feature and stratigraphic position.
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