Khyati Sonpal

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Comparison of different methods of Seismic Design
for the Earth Retaining Structures

• Khyati Sonpal
• On 5th November 2009
• Rowan University

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Overview

• Background
• Forces on Retaining Walls
• Retaining Wall failure
• Design Conditions for Earthquake Prone zone
• Seismic Design (Static and Dynamic)
• Mononobe-Okabe Seismic Method
• Pseudo Dynamic Method

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Background

• Retaining walls are extensively used worldwide
• Need to design the for seismic forces on
  retaining wall

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Forces on Retaining walls

• Static forces
• Body forces (mass of the wall)
• Soil pressure
• External forces
• Earthquake
• Inertial forces

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Retaining Wall Failure

• Sliding
• Overturning
• Gross Instability

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Design Earthquake-prone zone

• Correct estimation of seismic active earth
  pressure
• Its distribution along the depth of the wall

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Seismic Design

• Seismic behavior total lateral earth
  pressure (Earthquake)

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Mononobe-Okabe Seismic Coefficient

• Soil behind the wall behaves as a rigid body
  therefore acceleration uniform thru mass

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Mononobe and Okabe

• Total pressure acts at the height of H/3 above
  the base.
• Earth pressure coefficients for the cohesionless
  backfill computed from the Mononobe-Okabe
  analysis are in reasonably good with values
  developed in small scale (model) structures

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Drawback of Mononobe Okabe

• The mobilization of active earth pressure depends
  on mode and amount of wall movement both under
  static and seismic conditions.
• Though for active case very small movement is
  required for full mobilization, for some cases
  even this small amount of movement does not
  occur.
• In those cases, the actual value of seismic
  active earth pressure is in between fully
  mobilized active pressure and earth pressure at
  rest.

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Mode of wall movement

• a) Rotation _at_ Top b)Rotation
  _at_Bottom

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Assumptions

• At the bottom when db da, the mobilized soil
  friction angle Fm will be zero at the top full at
  the bottom
• db lt da, Fm lt F at the bottom and Fm0 at the
  top

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• The mobilized seismic active earth pressure
  coefficient Kaem at any depth is calculated as

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Discussion

• Mononobe-Okabes theory shows a linear seismic
  active earth pressure distribution with height.
• RT and RB showed non-linear distribution with
  height

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Pseudo Dynamic Method

• A pseudo-dynamic analysis incorporates finite
  shear wave velocity based on the assumption that
  the shear modulus is constant with depth
• Acceleration is varying

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Pseudo Dynamic Method

• Able to fully reflect dynamic properties of soil
  when compared with Pseudo Static method
• Two extra dynamic parameters to analyze the
  problem (shear wave velocity of soil
  predominant frequency of probable earthquake)

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