Failures of Earth Retaining Structures and Lessons Learned

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Failures of Earth Retaining Structures and
Lessons Learned

• Presented By
• Ashish Wadkar

5th November, 2009
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Importance of Retaining Stuctures

• Varying topography of earth makes retaining
  structures vital
• Resist the thrust of earth bank
• Provide soil stability

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Failures

• Failures of earth retaining structures can be
  quite catastrophic
• It is important to understand the assumptions
  made in the design philosophy
• Design of walls backfilled with clay is based on
  the analysis that applies to granular soils only
• Only savior margin would be overgenerous factor
  of safety assumed in the design

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Failure Modes
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Common reasons for failures

• Often times, granular material is specified for
  the backfill by the designer which is not always
  available at site
• To avoid additional cost, the material available
  at site is used for backfilling
• Lack of awareness for internal and external
  stability

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Importance of engineering design
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Lack of External Stability

• A cantilever retaining wall collapsed due to
  lack of external stability

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Poor Design and Workmanship

• A cantilever retaining wall collapsed due to poor
  design and workmanship
• The active pressure was calculated considering a
  flat backfill while at site a sloping backfill
  was constructed
• Design engineers assume a drained backfill with
  granular material for full height
• Clayey soils available at site was used for
  backfilling
• Damage occurred in the form of cracking and
  tilting

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Imbibing waters

• Rainfall is the triggering factor for many cases
  of retaining wall failures
• Geogrid reinforced retaining wall collapsed in
  Kentucky shortly after construction

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SRW failure

• Investigation revealed that backfill was not
  adequately compacted
• Triaxial tests suggested that loss of strength
  occurred as the moisture content became dry of
  optimum
• Steeper backslope was constructed due to space
  limitations
• Length of geogrid reinforcement was not adequate

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Segmental retaining wall
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SRW failure

• A hybrid SRW failed in New England due to
  improper design
• Field reports indicated no deviations from
  construction drawings and proper compaction was
  achieved
• Backfill became saturated with water due to
  rainfall

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Construction sequence
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Additional failure reasons

• Wall was designed for dry backfill however the
  as-built stability FOS were found to be safe
• However FOS for connection of geogrid with wall
  units was under the required limit
• Wet backfill was not only the reason of failure

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Backfill Material

• The most important single consideration in wall
  design is insuring good drainage. There are two
  approaches
• - Remove water from backfill
• - Keep water out of backfill
• The best backfill is rigid, free draining and
  with a high angle of internal friction
• Lightweight artificial material such as expanded
  shale and crushed slag often make good backfill.

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Summary of Lessons Learned

• Conservatism may lead to the original designer
  ignoring the design of the connections
• Failure of retaining walls takes place when
  as-built conditions varied considerably from the
  design conditions
• Lack of co-ordination between the structural
  engineer and the contractor is the primary reason
  for such deviations

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Summary of Lessons Learned

• The properties of soil available at site should
  be identified at the beginning in order to
  incorporate necessary changes in the design of
  walls
• Walls shall be laterally supported by provision
  of shear key and earthquake loads shall be
  considered in design of walls for a seismic zone
• Backfill material should be compacted adequately
  without development of excessive compaction
  pressures

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THANK YOU !
William John Macquorn Rankine (1820-1872)