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Site Response Analysis of Submarine Slopes
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The geological profile of continental shelf slopes is mostly composed of parallel layers of normally consolidated
to lightly overconsolidated clayey soils. When only gravity loads are acting, a generic soil element is subjected
to a stress in the direction normal to the slope, represented by the effective normal stress and a stress in the
plane of the slope, parallel to the dip, represented by the consolidation shear stress. For simplicity of the formulation,
the earthquake motion is assumed to consist of shear waves propagating perpendicular to the slope, therefore an
additional cyclic shear stress will act on the plane of the slope in a direction oriented at some angle with that
of the consolidation shear stress (i.e., multidirectional shaking). In order to fall back into the one-dimensional
case, most analyses choose the critical direction to be parallel to the dip of the slope (i.e., direction of shaking
and consolidation shear stress coincide) even if the seismic shear stress changes direction instantaneously. The
stress state in this case is the same as that developed in simple shear tests.
Most of these codes for seismic site response analysis can only consider level ground conditions. However, the
slope has significant effects on the behavior of the soil both during monotonic and cyclic loading and it is not
possible to ignore these effects, especially in view of the fact that many submarine failures attributed to seismic
excitation have been observed on slopes of only a few degrees, sometimes less than 5°. AMPLE2000 solves the
one-dimensional shear wave propagation in a horizontally or sloping layered soil profile. Several constitutive
laws are available, including SIMPLE DSS. The program allows dividing the soil profile into any number of layers,
each with separate characteristics, including height, material model parameters and preconsolidation pressure.
The input requires a profile of small strain shear modulus (Gmax) or shear wave velocity. The output includes acceleration,
stress and strain time histories at user specified depths, maximum and end-of-shaking profiles of shear stress
and strain, displacement, excess pore pressure and spectral accelerations for 5% damping. |
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| Predictions of displacements and stains in a simplified slope subjected to earthquake shaking. |
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