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SEISMIC ANALYSIS

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# SEISMIC ANALYSIS - PowerPoint PPT Presentation

Stability of a slope can be affected by seismicity in two ways: earthquake and blasting. These seismic motions are capable of inducing large destabilizing inertial forces.

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Stability of a slope can be affected by seismicity in two ways: earthquake and blasting. These seismic motions are capable of inducing large destabilizing inertial forces.

• Pseudostatic Method: The earthquake’s inertial forces are simulated by the inclusion of static horizontal and vertical forces in limit equilibrium analysis.
• Newmark’sDiaplacement Method: This method is based on the concept that the actual slope accelerations may exceed the static yield acceleration at the expense of generating permanent displacements (Newmark, 1965).
• Dynamic Finite Element Analysis: This is a coupled two or three dimensional analyses using appropriate constitutive material model that will provide details of concerning stresses, strains, and permanent displacement.

SEISMIC ANALYSIS

The main seismic wave types are

• Compression (P)
• Shear (S)
• Rayleigh (R)
• Love (L)
• P and S waves are known as body waves, because they propagate outward in all directions from source (such as an earthquake) and travel through the interior of the earth. Love and Rayleigh waves are surface waves and propagate approximately parallel to the earth’s surface.

Types of seismic wave

Typical seismogram ( www.geo.mtu.edu)

definition of earthquake terms (www.culcanhammer.net)

Dynamic Soil Properties

Important elements in a seismic response analysis are:

input motions,

site profile,

static soil properties,

dynamic soil properties,

constitutive models of soil response to loading and

methods of analysis using computer programs.

there are two basic approaches to incorporate the seismic effect on slope stability

Inertia slope stability analysis

weakening slope stability analysis

Simulation of seismic effect

In pseudo-static methods, the cyclic earthquake motion is replaced with a constant horizontal acceleration equal to kc (g), where kc is the seismic coefficient, and g is the acceleration of gravity. A force is applied to the soil mass equal to the product of the acceleration and the weight of the soil mass.

Pseudo-static approach

Newmark’s method assumes: existence of a well-defined slip surface, a rigid, perfectly plastic slide material, negligible loss of shear strength during shaking, and that permanent strains occur if the dynamic stress exceeds the shear resistance. Also, the slope is only presumed to deform in the downslope direction, thus implying infinite dynamic shear resistance in the upslope direction. The procedure requires that the value of a yield acceleration or critical seismic coefficient, ky, be determined for the potential failure surface using conventional limit equilibrium methods.

Inertia Slope Stability – Newmark Method