The flood season is still ongoing, so construction sites must remain vigilant at all times. The best way to prevent accidents during foundation pit construction is through thorough monitoring of the entire process. This means preparing in advance, managing the process carefully, and handling emergencies promptly. Let’s explore the correct methods for foundation pit engineering together.
01 Preliminary Preparation
1. On-site Survey and Investigation
- Identify the terrain, landforms, climate, and other natural conditions of the construction site.
- Review the geological survey report to understand the distribution, age, and physical and mechanical properties of soil layers within the pile depth range.
- Assess the groundwater level, quality, and fluctuations at the site.
- Investigate human and natural geological phenomena nearby, such as earthquakes, karst formations, mineral deposits, ancient ponds, hidden banks, and any underground structures or obstacles.
- Examine the location, distance, structural properties, current status, and usage of adjacent buildings.
- Identify the distribution, distance, burial depth, service life, pipe diameter, and condition of underground pipelines near the pile driving area, including gas, water supply, sewage pipes, and cables.
2. Technical Preparation
- Prepare a detailed construction plan specifying piling machinery, methods, construction sequence, and protective measures for neighboring buildings and underground pipelines.
- Develop a construction schedule aligned with the overall project timeline, including labor, materials, and equipment requirements.
- Establish quality assurance, safety, and civilized construction measures.
- Conduct process pile testing before construction to determine reasonable parameters and select appropriate mechanical equipment based on pile type and soil conditions.
3. On-site Preparation
- Remove any high-altitude or underground obstacles that could hinder construction, such as poles, electrical wires, foundations of old buildings, or other underground structures, ensuring smooth pile formation.
- Level the site thoroughly before pile driving begins, especially for high-rise building foundations to maintain the verticality of pile drivers and ensure their stable movement.
02 Whole-Process Management
1. Preparation of Safety Special Construction Plan
Ensure that the preparation, review, and expert evaluation of safety construction plans for hazardous projects comply with regulations. Confirm that the plan content is complete and effective, and that all relevant parties have signed and stamped as required.
2. Prefabrication and Assembly According to Safety Plan
Verify that the construction unit has properly conducted technical disclosures before hazardous projects begin. Confirm that construction strictly follows the plan, with a designated supervisor monitoring its implementation. Ensure relevant personnel participate in the acceptance procedures as per regulations.
3. Certification of Employees
Check that project leaders and full-time safety management personnel hold valid safety assessment certificates. Confirm that special operators—such as crane drivers, installation and dismantling workers, riggers, and scaffolders—are certified for their roles.
4. Responsibilities of Construction and Supervision Units
Confirm that the construction unit provides a list of hazardous projects for safety supervision. Ensure the supervision unit includes hazardous engineering in its supervision plan and reviews the special plans accordingly. Verify on-site supervision of plan implementation and participation in project acceptance.
03 Emergency Handling
1. Emergency Prevention Measures for Foundation Pit Accidents
Excessive Displacement and Collapse of Foundation Pit System
Follow the special construction plan for foundation pit support and excavation rigorously. Excavation should adhere to the “spatiotemporal effects” theory, conducted in layers, segments, symmetrically, balanced, and within time limits. Each soil layer should not exceed 2.5 meters in thickness. Use multiple excavation machines working from all sides toward the center to align with the foundation pit enclosure design and maintain stress conditions. Additionally, strengthen open drainage around the pit and prevent surface water from entering.
Bottom Uplift and Soil Gushing
When excavation reaches the pit bottom, install settlement observation points every 20 meters. Workers should monitor these points twice daily. The warning threshold for uplift settlement is 10mm per reading. If this value is exceeded three times consecutively, report immediately to the project department and take appropriate measures.
Defects in Enclosure Structure
Ensure high-quality waterproof curtain construction by controlling cement mixing pile standards. Overlaps must meet design requirements and cold joints should be avoided. Maintain cement content and water-cement ratio (1:1.5) with a cement content of 22%. Use well-stirred cement slurry and high-pressure jet grouting to ensure thorough mixing before initial setting, guaranteeing consistent cement content across piles.
Cracking and Uneven Settlement of Surrounding Buildings
Conduct construction strictly as per expert-approved excavation and support plans. Strengthen foundation pit monitoring to avoid exceeding warning values: 1% tilt, 5mm/day settlement speed, and 30mm total cumulative settlement.
2. Emergency Technical Measures for Foundation Pit
Excessive Displacement and Collapse
- If displacement approaches or reaches warning levels, halt excavation on the affected side, adjust excavation to the opposite side. If displacement worsens, stop excavation entirely and backfill using sandbags or on-site soil.
- In case of collapse, immediately provide temporary support using #14 channel steel piles spaced at 200mm and spray 80mm thick C20 concrete around piles to prevent water leakage.
Bottom Bulging and Surrounding Soil Surging
- Stop construction immediately if bulging occurs. Backfill and load the area with sandbags and soil, reinforce foundation soil as needed, then resume excavation to design elevation. Add settlement monitoring points outside the pit.
- If soil surges around the pit, install sheet piles along the perimeter to prevent soil intrusion. Increase retaining structure depth and perform dewatering at the pit bottom to reduce dynamic water pressure and prevent further surges.
Enclosure Structure Defects
- For minor leaks above excavation surface, apply double fast cement pumping and polyurethane injection to seal.
- For significant leaks on excavation surface, drive drainage pipes into leak points, seal areas with steel pipes and fast cement, then close valves after sealing materials cure.
- For leaks extending below excavation surface, inject double liquid slurry near leak points outside the pit, controlling grouting pressure to a maximum of 0.3MPa.
- For large flow leaks below excavation surface, perform local backfilling inside the pit until flow decreases, then inject polyurethane near leak points.
- If leak points are unknown and water flows from below excavation surface, stop excavation immediately and backfill locally until leakage stops. Follow with double liquid slurry injection outside the pit.
- If leakage water is turbid and prolonged, suspect severe soil loss and voids nearby. Prohibit heavy machinery and use vibration tube grouting to fill voids immediately.
Cracking and Uneven Settlement of Surrounding Buildings
- Immediately stop construction and consult experts to analyze causes. Develop and implement a building protection plan before resuming work.
- Halt excavation, reinforce foundation pit support, and strengthen surrounding ground by grouting within a 5-meter radius. Use pure cement slurry at 0.5-1.0MPa pressure to reinforce soil up to 8 meters deep.
Article source: Architectural Technology Magazine
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