GNGTS 2014 - Atti del 33° Convegno Nazionale

(2.0-4.5 Hz). The higher resonance modes, in terms of both frequency value and FFT-spectral intensity, are related to the different seismo-stratigraphic conditions of each soil columns and, in particular, to the presence and stratigraphic setting of the layers that are responsible for the highest impedance contrast. The obtained A(f) distribution have to be regarded as influenced by the 1D stratigraphic layering only while it cannot take into account nor the lateral heterogeneity of the alluvial fill neither the shape of the valley. Since it is reasonable to think that the local seismic response and the seismically-induced effects in a heterogeneous geological settings significantly conditioned by 2D effects, in terms of both amplification and shear strain distribution, as demonstrated by G iacomi , 2013 for the Tiber Rivel valley in the historical centre of Rome, the here obtained results can be considered as a starting point for quantifying properly 2D seismic effects along he analyzed section. Calibration of the absorbing boundary conditions for 2D fully numerical modeling. Aproper 2D numerical modeling is planned to be performed through numerical codes implemented by institute of Paris, IFSTTAR, and these codes are based on different numerical solutions, the Finite Element Method (FEM) and the Boundary Element Method (BEM). As reported in Semblat et al., 2011, the numerical analysis of elastic wave propagation in unbounded media can be difficult due to spurious waves reflected at the model artificial boundaries; this point is particular critical for the analysis of wave propagation in heterogeneous or layered systems as in the present study. In this regard, Semblat et al. (2011) proposed an absorbing layer solution, based on Rayleigh/Caughey damping formulation that considers both homogeneous and heterogeneous damping in the absorbing layers. The efficiency of the method was tested through 1D and 2D FEM simulations, and the best results were obtained considering a damping variation up to Q min -1 ≈ 2 (ξ =1.0) defined by a linear function in the heterogeneous case (five layers with piecewise constant damping) and linear as well as square root function in the continuous case. This theoretical study was performed considering a simple model composed by a homogeneous elastic medium and an absorbing lateral layered boundary. Such an approach was not yet tested for heterogeneous deposits, i.e characterized by vertical and lateral contacts among layers with different mechanical and dynamical properties. For this reason, the efficiency of the absorbing layers in case of highly heterogeneous deposits needs to be checked by additional numerical tests. A new numerical model was designed according to the geometry used by Semblat et al., 2011 but introducing two horizontal and homogeneous sub-layers (Fig 3a-3c). The results of the model were analyzed in order to choice the most efficient features (i.e. thickness and damping) of the absorbing layer system, and considering impedance contrasts from 1.4 up to 12.5 between the 2 horizontal sub-layers representing the physical domain of interest. This parametric analysis was performed to evaluate the reduction of efficiency of the adsorbing layer in relation with the longest wave lengths propagated in the model, these latter functions of the maximum wave velocity of the 2 considered sub-layers. Some preliminary results of the performed modeling are here presented for the case in which the two modelled sub-layers, are characterized: i) by the same velocity values for the P and S seismic waves respectively (i.e. 231 m/s and 400 m/s) and ii) by a different density (1800 – 2500 kg/m 3 respectively); the so resulting impedance contrast is of about 1.4. The modeling was performed by the FEM CESAR-LCPC code, by applying a synthetic input characterized by a predominant frequency of 10 Hz according to Semblat et al. (2011). Two different typologies of absorbing layer system were considered, the first one corresponding to a homogenous absorbing layer (HOL) characterized by homogenous damping value equal to Q min -1 ≈ 0.5, or ξ =0.25 (Rayleigh/Caughey damping), and the second one to a heterogeneous layer (HEL) constituted of 5 sub-layers that are characterized by a damping linearly varying from Q min -1 ≈ 0.2, or ξ = 0.1 (in the inner part of the absorbing layer system), to Q min -1 ≈ 2, or ξ =1.0 at the boundary of the numerical model. In agreement with the results obtained by Semblat et al. (2011), the here obtained preliminary 298 GNGTS 2014 S essione 2.2