Retaining Walls Damaged in the Chi-Chi Earthquake

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Retaining Walls Damaged in the Chi-Chi Earthquake

• Y.S. Fang, Y.C. Yang, and T.J. Chen
• Presented by Amanda Luksetich
• Spring 2005

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Introduction

• September 21, 1999 at 147 am
• Earthquake of magnitude 7.3 on the Richter Scale
• Near the town Chi-Chi in Taiwan, Chi-Chi
  Earthquake
• 49,542 dwellings completely destroyed
• 42,746 dwellings partially damaged
• 2,432 people killed, 46 still missing

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Chi-Chi Earthquake
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Chi-Chi Earthquake

• Shaking frequency 1.0-4.0 Hz
• Focal depth 8 km
• Rupture along Che-Lung Pu fault was 105 km

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Seismic Behavior of Gravity Walls

• 3 retaining wall failures investigated
• Base of wall lacks resistance to dynamic thrust
  active wall movement occurs
• Wall gradually moves away from backfill
• Soil wedge slides along failure plane
• Causes tension cracks and settlement in backfill

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Site 1 - Introduction

• Taiwan Provincial Highway 14
• Built to retain steep excavation for highway
  construction
• Concrete placed in 5 pours
• No special treatment provided at construction
  joints

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Site 1 Failure Analysis

• Top layers moved away from backfill along
  construction joints
• Top 2 blocks fell to side ditch
• 20 m damage zone
• Suspect frictional resistance not adequate at top
  of wall

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Site 1 Failure Analysis

• FS overturning gt FS sliding
• Sliding failure occurred along construction joint
• FS decreases with increasing horizontal ground
  acceleration
• Static FS acceptable
• FS lt 1 when Kh 0.24g
• Peak Kh observed to be 0.38g and 0.47g
• Wall may have moved some with each effective
  cycle of ground shaking

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Site 1 Failure Analysis

• Soil properties influence analysis results
• FS increases with increasing friction angle
• 20 m failure segment
• Locally poor compaction
• Weaker soil present
• Design Kh only 0.23g
• Kh measured up to 0.47g and 0.79g

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Site 1 Recommendations

• Wall repaired in 2 months
• Shearing reinforcements at construction joints
• H-piles to resist potential movement of steep
  slope
• Shearing key to prevent sliding failure along
  joint (recommended)

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Site 2 Introduction

• Provincial Hwy 21 near Sun-Moon Lake
• Retain steep slope for highway construction
• Close to epicenter
• Lateral movement observed
• Active soil wedge collapsed, but most of body
  wall undamaged
• 40 km damage zone

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Site 2 Failure Analysis
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Site 2

• Unexpected strong ground motion (close to
  epicenter)
• FS against sliding is 1.15 (static), 1.5 required
• Short, thick wall should resist overturning well
• Bearing capacity failure may occur under seismic
  load
• Settlement will occur if qmax exceeds qult,
  result in overturning

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Site 2 Failure Analysis

• Kh recorded as 0.59g at nearby station
• FS bearing capacity slightly less than FS sliding
• Overturning wall most likely result of
  insufficient bearing capacity under the toe

Recommendation Do not neglect stability check
for bearing capacity failure
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Site 3 Introduction

• Taiwan Cinema Culture Town
• Constructed above the Che-Lung Pu fault line
• Vertical and horizontal displacements of 2.0 m
  and 1.3 m, respectively
• Body of wall unbroken
• Buildings on top of backfill severely damaged
• Deformation
• Strong ground shaking

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Site 3 Failure Analysis
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Site 3 Failure Analysis

• Vertical displacement uplifted backfill, caused
  overturning
• Horizontal displacement caused sliding failure
  and heave in front of toe
• Kh recorded as 0.79g
• FS acceptable for static conditions

Recommendation Do not build on or near any
active fault lines!!!
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Questions?