1 Introduction
-
only 1D shear wave propagation tests are performed for a more straightforward interpretation of numerical results;
-
discretization effects have been illustrated in both time and frequency domains, and then quantified via modern misfit criteria formulated in the full time-frequency domain;
-
since discretization effects depend in general on the numerical algorithm adopted, a widespread FE approximation scheme has been here adopted;
-
the role of constitutive non-linearity (plasticity) is discussed;
2 FE modeling of 1D seismic wave propagation
2.1 Space discretization and time marching
2.2 Material modeling
2.2.1 Linear elastic model
2.2.2 Elastic-plastic: von Mises kinematic hardening (VMKH) model
-
two elastic parameters—E and \(\nu\);
-
one yielding parameter—k—proportional to the initial size of the cylindrical yield locus in the stress space;
-
one hardening parameter—h—governing the post-yielding (elastic-plastic) stiffness.
2.2.3 Elastic-plastic: Pisanò bounding surface (PBS) model
-
two elastic parameters—E and \(\nu\)—to characterize the material behavior at vanishing strains;
-
one shear strength parameter—M—directly related to the material frictional angle;
-
two parameters—\(k_d\) and \(\xi\)—governing the development of plastic volumetric strains during shearing;
-
two hardening parameters—h and m—to be identified on the basis of stiffness degradation and damping cyclic curves.
2.3 Initial/boundary conditions and input motion
2.4 Misfit criteria
3 Linear elastic wave simulations
3.1 Standard rules for space/time discretization
3.2 Model parameters
-
input 2: \(f_1\) = 0.1 Hz, \(f_2\) = 1 Hz, \(f_3\) = 45 Hz, \(f_4\) = 50 Hz;
-
the amplitude parameter A has been always set to produce at the bottom of the layer a maximum displacement of 1 mm.
3.3 Discussion of numerical results
Case # |
\(f_\mathrm{max}\) (Hz) |
\(\Delta x_\mathrm{std}\) (m) |
\(\Delta t_\mathrm{std}\) (s) |
\(\Delta x\) (m) |
\(\Delta t\) (s) | Brick type |
---|---|---|---|---|---|---|
EL1 | 20 | 5 | 0.005 | 2, 5, 10 | 0.005 | 8-node |
EL2 | 20 | 5 | 0.005 | 2, 5, 10 | 0.002 | 8-node |
EL3 | 50 | 2 | 0.002 | 0.8, 2, 4 | 0.002 | 8-node |
EL4 | 50 | 2 | 0.002 | 0.8, 2, 4 | 0.001 | 8-node |
EL5 | 20 | 5 | 0.005 | 2, 5, 10 | 0.002 | 27-node |
EL6 | 20 | 5 | 0.005 | 5 | 0.002, 0.005, 0.01 | 8-node |
EL7 | 20 | 5 | 0.005 | 2 | 0.001, 0.002, 0.005 | 8-node |
EL8 | 50 | 2 | 0.002 | 2 | 0.001, 0.002, 0.005 | 8-node |
EL9 | 50 | 2 | 0.002 | 0.8 | 0.0005, 0.001, 0.002 | 8-node |
EL10 | 20 | 5 | 0.005 | 5 | 0.002, 0.005, 0.01 | 27-node |
3.3.1 Influence of grid spacing
-
even though \(\Delta x_\mathrm{std}\) is set on the basis of the maximum frequency \(f_\mathrm{max}\), its suitability is not uniform over the input spectrum. Indeed, increasing inaccuracies in the frequency domain are clearly visible as \(f_\mathrm{max}\) is approached (check for instance the Fourier amplitudes compared in Figs. 3c and 4, 5, 6b). Grid spacing affects output Fourier spectra both in amplitude and phase;
-
reducing \(\Delta x\) below \(\Delta x_\mathrm{std}\) is beneficial only if \(\Delta t\) is also lower than \(\Delta t_\mathrm{std}\). This is apparent in Fig. 3e, where an increase in EM and PM is observed as \(\Delta x\) gets lower than \(\Delta x_\mathrm{std}\). Conversely, monotonic EM/PM trends are shown in Figs. 4, 5d;
-
at given grid spacing \(\Delta x\), reducing the time-step improves the numerical solution mostly in terms of Fourier phase, not amplitude (compares Figs. 3c–d, 4b–c). It may be generally stated that, when \(\Delta x\) is not appropriate, reducing the time-step size does not produce substantial improvements;
-
based on these initial examples, a grid spacing \(\Delta x\) in the order of \(V_\mathrm{s}/20f_\mathrm{max}=\Delta x_\mathrm{std}/2\) ensures high accuracy (EM and PM < 10 % ) in combination with \(\Delta t=\Delta x/2V_\mathrm{s}=\Delta t_\mathrm{std}/2\). These enhanced discretization rules hold for low-order FEs (8-node brick elements) but are not affected by the frequency bandwidth of the input signal. In the latter respect, Figs. 4, 5d show quantitatively similar EM-PM trends for \(f_\mathrm{max}\) equal to 20 Hz and 50 Hz. Also, minimum misfits are attained in the EL2 case (Fig. 4d), where a smaller \(\Delta t/\Delta t_\mathrm{std}\) ratio has been purposely set.
3.3.2 Influence of time-step size
-
in combination with \(\Delta x=V_\mathrm{s}/20f_\mathrm{max}=\Delta x_\mathrm{std}/2\), \(\Delta t=\Delta t_\mathrm{std}\) may still result in some high-frequency phase difference with the respect to the analytical solution, (Figs. 9, 10, 11, 12c). As found by investigating grid spacing effects, \(\Delta t={\Delta x}/{2V_\mathrm{s}}=\Delta t_\mathrm{std}/2\) yields sufficient accuracy (EM-PM lower than 10 %) to most practical purposes (see Figs. 9, 10, 11, 12d);
-
when 27-node bricks are used, the use of \(\Delta x=\Delta x_\mathrm{std}\) and \(\Delta t\le \Delta t_\mathrm{std}/2\) is still an appropriate option, giving rise to EM and PM lower than 5 % (Fig. 12). Even in this case, discretization errors are still governed by phase differences, while excellent performance in terms of Fourier amplitude is observed;
-
Figs. 13 and 14 show that the above findings apply qualitative to acceleration time histories as well. However, EM and PM values are quite high (significantly larger than 10 %) when \(\Delta t\ge \Delta t_\mathrm{std}\), regardless of the grid spacing ratio. Accuracy is quickly regained when \(\Delta t\) is reduced and \(\Delta x<\Delta x_\mathrm{std}/2\).
4 Non-linear elastic-plastic wave simulations
-
the non-linear problem under consideration cannot be solved analytically. Therefore, the quality of discretization settings may only be assessed by evaluating the converging behavior of numerical solutions upon \(\Delta x\)–\(\Delta t\) refinement;
-
the accuracy of non-linear computations is highly affected by the input amplitude. This governs the amount of non-linearity mobilized by wave motion and, as a consequence, the accuracy of numerical solutions at varying discretization.
4.1 VMKH model
4.1.1 Model parameters and parametric analysis
-
mass density and elastic properties: \(\rho =\) 2000 kg/m3, \(E=\) 5.2 GPa and \(\nu =\) 0.3, whence the elastic shear wave velocity \(V_\mathrm{s}=\) 1000 m/s results (same elastic parameters employed for both the elastic and the VMKH sub-layers);
-
yielding parameter (radius of the von Mises cylinder): \(k=\) 10.4 kPa;
-
different h values (hardening parameter) have been set: \(h=\) 0.5E, 0.05E, 0.01E.
Case # |
\(\Delta x_\mathrm{std}\) (m) |
\(\Delta t_\mathrm{std}\) (s) |
\(\Delta x\) (m) |
\(\Delta t\) (s) |
h
|
A (mm) |
---|---|---|---|---|---|---|
VMKH1 | 5 | 0.0005 | 1, 5 | 0.0001 | 0.5E
| 0.1 |
VMKH2 | 5 | 0.0005 | 1, 5 | 0.0001 | 0.05E
| 0.1 |
VMKH3 | 5 | 0.0005 | 5 | 0.0002, 0.0005, 0.001 | 0.5E
| 0.1 |
VMKH4 | 5 | 0.0005 | 5 | 0.0002, 0.0005, 0.001 | 0.05E
| 0.1 |
VMKH5 | 5 | 0.0005 | 5 | 0.0002, 0.0005, 0.001 | 0.01E
| 0.1 |
VMKH6 | 5 | 0.0005 | 1, 5 | 0.0001 | 0.5E
| 1 |
VMKH7 | 5 | 0.0005 | 1, 5 | 0.0001 | 0.05E
| 1 |
VMKH8 | 5 | 0.0005 | 5 | 0.0002, 0.0005, 0.001 | 0.5E
| 1 |
VMKH9 | 5 | 0.0005 | 5 | 0.0002, 0.0005, 0.001 | 0.05E
| 1 |
VMKH10 | 5 | 0.0005 | 5 | 0.0002, 0.0005, 0.001 | 0.01E
| 1 |
4.1.2 Influence of grid spacing and time-step size
-
propagation through a dissipative elastic-plastic material alters significantly the shape of the input signal. All plots display significant wave attenuation/distortion, while final unrecoverable displacements are produced by soil plastifications (Figs. 16, 17a). Steady irreversible deformations are associated with prominent static components (at nil frequency) in the Fourier amplitude spectrum (Figs. 16, 17c), not present in the input Ormsby wavelet (Fig. 2b);
-
the numerical representation of wavelengths is dominated by soil plasticity, producing more deviation from the input waveform than variations in grid spacing. For this reason, only two \(\Delta x\) values have been used in this subsection for illustrative purposes, whereas EM/PM plots have been deemed not necessary;
-
the influence of \(\Delta x\) seems slightly magnified when lower h values, and thus lower elastic-plastic stiffness, are set (see Fig. 17). It is indeed not surprising that wave propagation in softer media may be more affected by space discretization, as in linear problems. However, it should be noted that \(\Delta x\) mainly influences the final irreversible displacement (Fig. 17b, c), which leads to presume substantial interplay of grid effects and constitutive time integration;
-
since the effects of \(\Delta x\) reduction are quite small in both time and frequency domains (for a given \(\Delta t\)), there is no strong motivation to suggest \(\Delta x=V_\mathrm{s}/20f_\mathrm{max}\). \(\Delta x=V_\mathrm{s}/10f_\mathrm{max}=\Delta x_\mathrm{std}\) should be actually appropriate in common practical situations, as long as no soil failure mechanisms are triggered – as for example in seismic slope stability problems [17]. The occurrence of soil failure may introduce additional discretization requirements for an accurate representation of the collapse mechanism.
-
at variance with the previous elastic cases, envelope (EM) and phase (PM) misfits are quantitatively quite different (EM > PM);
-
EM/PM trends do not depend monotonically on the hardening parameter h. For \(\Delta t=\) 0.0002 s, the EM/PM values at \(h=0.05E\) are indeed larger than those obtained for \(h=0.5E\) and \(h=0.01E\).
4.1.3 Influence of input motion amplitude
-
EM/PM values are in general higher at larger input amplitude (Fig. 22d), and experience a slower decrease as \(\Delta t\) is reduced (still depending on the specific h value);
-
the shear stress-strain loops in Fig. 23 show how inaccurate the simulated constitutive response can be when \(\Delta t\) is too large (e.g., \(\Delta t=\) 0.001 s) and substantial plastic degradation of material stiffness takes place (see the case \(h=0.01E\)).
4.2 PBS model
4.2.1 Model parameters and parametric analysis
-
\(\rho =\) 2000 kg/m3, \(E=\) 1.3 GPa and \(\nu =\) 0.3, implying an elastic shear wave velocity \(V_\mathrm{s}=\) 500 m/s ;
-
shear strength parameter: \(M=\) 1.2, corresponding with friction angle equal to 30 deg under triaxial compression;
-
hardening parameters: \(h=\) 300 and \(m=\) 1.
Case# |
\(\Delta x_\mathrm{std}\) (m) |
\(\Delta t_\mathrm{std}\) (s) |
\(\Delta x\) (m) |
\(\Delta t\) (s) |
A (mm) |
---|---|---|---|---|---|
PBS1 | 2.5 | 0.0005 | 0.5, 2.5 | 0.0001 | 1 |
PBS2 | 2.5 | 0.0005 | 0.1, 0.5, 1 | 0.00002 | 1 |
PBS3 | 2.5 | 0.0005 | 2.5 | 0.0002, 0.0005, 0.001 | 1 |
PBS4 | 2.5 | 0.0005 | 2.5 | 0.00001, 0.00002, 0.0001 | 1 |
4.2.2 Influence of grid spacing and time-step size
-
grid spacing turns out to be influential again (Figs. 24, 26), as a consequence of more severe variations (than in VMKH cases) in shear stiffness during cyclic loading. In fact, one would have to follow the stiffness reduction curves arising from the constitutive response, and use minimum stiffness to decide on space discretization;
-
as in VMKH simulations, grid spacing mainly affects residual displacements. This is clearly shown by the EM/PM plots in Fig. 26b, where EM errors larger than 10% arise even when a very small time-step size is used (\(\Delta t =\Delta t_\mathrm{std}/25=\) 0.00002 s); conversely, phase misfits are less affected by residual displacements and thus always quite limited. In presence of high non-linearity, it seems safer to use \(\Delta x\) \(4\div 5\) times smaller than \(\Delta x_\mathrm{std}=V/10f_\mathrm{max}\);
-
the combination of explicit constitutive integration and high non-linearity makes time-stepping effects quite prominent, as is shown by Figs. 27 and 28. Further, Fig. 29 leads to conclude that \(\Delta t=\Delta t_\mathrm{std}/50\) may be needed to obtain EM errors lower than 10 % (Figs. 29, 30). Apparently, analysts have to compromise on accuracy and computational costs in these situations;
-
as expected, the shear stress-strain cycles in Figs. 25 and 28 show that the sensitivity to discretization builds up as increasing non-linearity is mobilized. This is the case for instance at the top of the PBS layer, where cycles are more dissipative than at the bottom due to lower overburden stresses and dynamic amplification.