Introduction
In foundation pit construction, selecting an appropriate support scheme based on geological conditions, hydrological environment, and pit characteristics is crucial. Simultaneously, implementing deformation control measures ensures structural stability and safety.
Key Construction Points
1. Dewatering
- Continuous dewatering must be maintained until the main structure meets anti-floating requirements.
- Prevent cable abrasion against well walls or sharp objects.
2. Sheet Pile Installation & Leak Prevention
- Insert wedges (iron or hardwood) into unengaged interlocks to prevent sediment clogging.
- In dense soil, angle the pile tip to leverage counterforce for realignment.
- Leak mitigation: Pour fine coal slag around leaking interlocks via external conduits to seal gaps.
3. Earthwork Safety
- Excavators must stabilize before digging; avoid rotation/movement while the bucket is engaged.
- Use brake systems (not clutch reversal) for rotation control.
- Park on flat, solid ground post-operation, bucket retracted, controls neutralized.
4. Deep Excavation Protocols
- Lower groundwater ≥1m below the pit base using well-point systems.
- Maintain >10m spacing between excavators; dig top-down in layers.
- Keep materials/machinery ≥0.8m from edges (1.5m height limit on stable soil).
5. Backfilling
- Backfill symmetrically in compacted layers to avoid uneven pressure.
6. Rainy Season Measures
- Install perimeter drainage to prevent water ingress.
- Leave 15–30cm of soil above the final grade; excavate post-rain.
7. Monitoring
- Place benchmarks ≥2x excavation depth from the pit; ≥2 reference points beyond impact zones.
- Track horizontal/vertical displacement (rate, direction).
- Establish pre-excavation baselines (≥2 readings). Increase frequency if deformation exceeds thresholds.
Support System Selection
Shallow Pits (<5m Depth)
- Anchor/Diagonal Bracing: Ideal for wide, moderate-depth pits with mechanical excavation.
- Short Pile-Timber Sheeting/Temporary Walls: Suits broad pits or areas with limited slope space.
- Steel Sheet-Timber Sheeting: Best for low-water-table, cohesive/sandy soils.
Deep Pits (≥5m Depth or Complex Conditions)
- Soldier Piles/Diaphragm Walls: For safety grades 1–3.
- Cement-Soil Mix Walls: Grades 2–3.
- Inverse Arch Walls: Grade 3 only (excludes mucky soils).
Accident Response & Mitigation
1. Excessive Displacement/Collapse
- Displacement: Halt excavation on the affected side; backfill (sandbags/soil) if displacement persists.
- Collapse: Install temporary 14# channel steel piles (@200mm) with 80mm C20 shotcrete to block water.
2. Base Heave/Soil Eruption
- Heave: Backfill immediately (sandbags/soil), reinforce base, and add external settlement monitors.
- Eruption: Install perimeter sheet piles, deepen supports, and lower the groundwater to reduce pressure.
3. Support Structure Failures
- Minor Leaks (Above Cut): Seal with rapid-set cement + polyurethane grout.
- Major Leaks (Above Cut): Use drain pipes + cement encapsulation; close valves post-curing.
- Deep Leaks: Inject dual-fluid grout externally (pressure ≤0.3MPa).
- Severe Subsurface Leaks: Partially backfill to reduce flow, then inject polyurethane externally.
- Unidentified Leaks: Halt excavation, backfill, and grout voids via vibration pipes.
- Muddy Leaks: Suspect soil loss/cavities; prohibit heavy machinery and fill voids immediately.
4. Adjacent Building Damage
- Cracks/Settlement: Suspend work, engage experts for analysis, and implement protective measures.
- Stabilization: Strengthen supports + inject cement grout (0.5–1.0MPa, 8m depth) within 5m of the pit.
