9 Common Causes of Foundation Pit Collapse: Case Studies and Prevention Measures

Foundation pit collapses are among the most critical risks in construction projects. Understanding the failure mechanisms can help engineers and contractors implement proper safety measures. This article analyzes 9 types of foundation pit failures, supported by real-world case studies, to improve construction safety and compliance with Google SEO ranking guidelines for construction-related searches.

1. Overall Instability (Global Failure)

Overall instability occurs when a sliding surface forms in the soil, causing the retaining structure and surrounding soil to collapse. Typical signs include:

• Retaining wall tilting outward at the top
• Wall base shifting inward
• Heaving at the pit bottom
• Ground subsidence outside the pit

Case Study: Longtan Sky Garden Collapse

• A 30 m-wide section showed cracks and settlement at the slope crest.
• Heavy rain accelerated the failure due to:
  • Over-excavation without timely support
  • Poor drainage, leading to water accumulation
  • Leaking underground pipes weaken soil strength

2. Bottom Heave (Basal Uplift)

Bottom heave happens due to:

• Elastic rebound from deep soil unloading
• Pressure differences causing plastic soil flow

Consequences:

• Ground settlement outside the pit
• Damage to adjacent structures

Solution:

• Soil reinforcement in passive zones to improve stability

3. Retaining Structure Overturning

Common in gravity or cantilever retaining walls, where soil pressure causes the wall to rotate inward.

Case Study: Wuhan Torch Building

• A 10 m-deep excavation with embedded rock-socketed piles failed due to:
  • Weakened clay from water infiltration
  • Pile fractures from uneven stress distribution
• Emergency backfilling and additional anchors stabilized the structure.

4. Retaining Structure Sliding Failure

Caused by insufficient foundation bearing capacity, leading to wall displacement.

Case Study: Tianheng Mansion

• A 5 m-deep pit in soft clay failed due to:
  • Poor-quality cement-soil mixing piles
  • Premature excavation before curing
  • Heavy rainfall and improper surcharge loading
• Solution: Additional piles and grouting reinforcement.

5. “Kick Failure” (Toe Failure)

Occurs in single-braced pits where the wall rotates around the support point.

Case Study: Guangzhou Haizhu City Plaza

• A 104 m-long collapse killed 3 people due to:
  • Delayed constructionis  weakening the supports
  • Unaddressed water seepage in weak layers
  • Excessive surcharge loads

6. Structural Failure of Retaining Walls

Caused by cracking, bending, or crushing due to:

• Insufficient reinforcement
• Poor joint connections

Case Study: Hangzhou Metro Collapse

• A 75 m-long collapse killed 21 workers due to:
  • Weak steel brace connections
  • Progressive failure triggers a domino effect

7. Bracing/Anchor System Failure

• Anchor failure: Pull-out, rupture, or prestress loss
• Bracing failure: Often a systemic collapse due to weak nodes

8. Waterproof Curtain Failure

Leads to leakage, piping, or sand boiling, causing:

• External ground settlement
• Adjacent building damage

Main Causes:

• Poor construction quality (cracks/holes)
• Insufficient curtain depth

Summary of 9 Failure Modes

1. Overall instability
2. Bottom heave
3. Retaining wall overturning
4. Retaining wall sliding
5. Foundation bearing failure
6. “Kick” failure
7. Structural failure
8. Bracing/anchor failure
9. Waterproof curtain failure