Foundation pit collapses are among the most critical risks in construction projects, often leading to severe accidents, property damage, and even fatalities. Understanding the causes and failure modes of foundation pit collapses is essential for engineers and construction professionals. This article analyzes nine common failure mechanisms of foundation pits, supported by real-world case studies, and provides insights into prevention strategies.
Definition: Overall instability occurs when a sliding surface forms in the soil, causing the retaining structure and surrounding soil to lose stability. The typical failure mode involves the upper part of the retaining wall tilting outward, the bottom moving inward, soil heaving at the pit bottom, and ground subsidence outside the pit.
Incident: A 30 m-wide section of the pit slope cracked and settled overnight. By morning, heavy rainfall triggered a complete slope failure.
Causes:
Over-excavation on the north side with prolonged exposure.
Failure to drain accumulated water inside the pit.
Weakened soil strength due to water infiltration from a leaking adjacent pipeline.
A 40 m-long slope collapsed after excavation reached 5.2m depth, highlighting the risks of improper slope support.
Definition: Bottom heave is an upward displacement caused by:
Elastic rebound of deep soil layers due to unloading.
Plastic soil flows from pressure differences induced by excavation.
Consequences:
Ground settlement and horizontal displacement outside the pit.
Structural damage to adjacent buildings and utilities.
Solution:
Passive zone reinforcement to increase soil resistance.
Proper dewatering stabilizes soil conditions.
Definition: Common in gravity or cantilever retaining walls, where external soil pressure causes the wall to rotate inward at the top.
A 10 m-deep excavation with rock-socketed piles failed when water infiltration weakened clay layers, increasing lateral pressure.
Result: Multiple piles fractured, endangering a nearby six-story building. Emergency backfilling and anchor reinforcement stabilized the site.
Definition: Occurs when the base resistance of a gravity retaining wall is insufficient, leading to horizontal displacement.
A 5 m-deep pit in soft clay (20m thick) failed due to:
Poor-quality cement-soil mixing piles.
Premature excavation before proper curing.
Excessive surcharge and rainfall.
Outcome: Widespread slope failure, tilted piles, and foundation damage required 6 months of remediation.
Definition: In single-braced pits, the wall rotates around the support, causing the top to tilt outward and the bottom to uplift.
A 104 m-long section collapsed, killing 3 and injuring 8.
Causes:
Prolonged construction weakened supports.
Inclined bedrock with water seepage.
Excessive surcharge and ignored warning cracks.
Definition: Cracking, buckling, or shear failure due to:
Inadequate design (underestimated loads or overestimated material strength).
Poor joint connections (especially in steel bracing systems).
A 75 m-long, 15.5 m-deep collapse killed 21 workers.
Cause: Weak connections between steel braces and walls led to progressive failure.
Failure Modes:
Anchors: Pull-out, fracture, or prestress loss.
Braces: Global buckling due to local joint failure.
Causes:
Poor construction (cracks/holes in the curtain).
Insufficient depth, allowing seepage.
Effects:
Water infiltration → soil erosion → ground subsidence → adjacent structure damage.
To mitigate foundation pit collapses:
Design: Ensure proper load calculations and support systems.
Construction: Avoid premature excavation; ensure curing time.
Monitoring: Detect early signs (cracks, deformations).
Dewatering: Control groundwater to stabilize soil.
By understanding these 9 failure modes, engineers can enhance pit stability and prevent catastrophic accidents.
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