The flood season is not over yet, and construction sites must remain vigilant at all times. During foundation pit construction, the best way to ensure no accidents occur is to implement full-process monitoring, which includes thorough preliminary preparations, comprehensive management throughout the process, and proper handling of emergencies. Let's take a look at the correct practices for foundation pit engineering.
01. Preliminary Preparation
I. Site Survey and Investigation
(1) Identify the terrain, landforms, climate, and other natural conditions of the construction site.
(2) Review the geological survey report to understand the distribution of soil layers within the pile depth range, their formation ages, and the physical and mechanical properties of each layer.
(3) Understand the groundwater level, water quality, and their variations at the construction site.
(4) Understand man-made and natural geological phenomena in the construction area, such as earthquakes, karst, mineral rocks, ancient ponds, hidden streams, and underground structures and obstacles.
(5) Understand the location, distance, structural properties, current status, and usage of adjacent buildings.
(6) Understand the distribution, distance, burial depth, service life, pipe diameter, and structural conditions of underground pipelines (gas pipes, water supply pipes, sewer pipes, cables, etc.) near the pile driving area.
II. Technical Preparation
(1) Preparation of construction plan. Before construction, a construction plan should be prepared, specifying pile driving machinery, pile driving methods, construction sequence, and protection measures for adjacent buildings or underground pipelines.
(2) Construction schedule. Determine the pile foundation construction plan based on the overall project schedule, including progress plan, labor demand plan, and material and equipment demand plan.
(3) Formulate quality assurance, safety technology, and civilized construction measures.
(4) Conduct process test piles. To determine reasonable construction technology, process test piles should be conducted before construction to determine process parameters. Mechanical equipment preparation: before construction, select appropriate machinery and equipment based on the designed pile type and soil conditions, and conduct process test piles.
III. Site Preparation
(1) Remove on-site obstacles. Before pile driving, remove high-altitude and underground obstacles that hinder construction, such as utility poles, overhead wires crossing the construction area, old building foundations, or other underground structures. This is crucial for ensuring smooth pile driving.
(2) Site leveling. Pile foundations for high-rise buildings usually involve densely arranged pile groups. Before the pile driver enters the site, the entire work area must be leveled to ensure the verticality of the pile driver and facilitate its stable movement.
02. Full-Process Management
I. Preparation of Special Safety Construction Plans
Check whether the procedures for preparation, review, and expert demonstration of special safety construction plans for hazardous major projects comply with regulations, whether the plan content is complete and effective, and whether relevant units and personnel have signed and sealed as required.
II. Implementation of Special Safety Construction Plans
Check whether the construction unit conducts technical disclosure before implementing hazardous major projects as required; whether construction is organized strictly according to the plan during the process, and whether dedicated personnel are assigned to supervise and monitor the implementation of the special plan; whether relevant personnel are organized for acceptance as required.
III. Certified Personnel
Check whether project managers and full-time safety production management personnel of construction enterprises hold valid safety production assessment certificates; whether special operation personnel such as construction crane operators, installers and dismantlers, riggers, and scaffolders hold certificates.
IV. Responsibilities of Construction and Supervision Units
Check whether the construction unit provides a list of hazardous major projects during safety supervision registration as required; whether the supervision unit includes hazardous major projects in the supervision plan and implementation rules; whether the special plan is reviewed as required; whether on-site supervision of the special plan implementation is conducted; whether they participate in the acceptance of hazardous major projects.
03. Emergency Handling
I. Emergency Preventive Measures for Foundation Pit Accidents
1. Excessive displacement or collapse of foundation pit system
[Measures] Conduct earthwork excavation according to the special construction plan for foundation pit support and earthwork excavation, guide excavation and support according to the "time-space effect" theory, and require "layered, block, symmetrical, balanced, and time-limited" excavation. Control the excavation thickness of each layer to not exceed 2.5m, and use multiple excavators to excavate from the periphery to the center. Maximize compliance with the design concept of foundation pit support to ensure the stress conditions of the support system. In addition, strengthen surface drainage around the foundation pit and take effective measures to prevent surface water from intruding into the pit.
2. Base heave and soil boiling
[Measures] When earthwork excavation reaches the pit bottom, set a settlement observation point every 20m at the bottom, and arrange for a construction worker to observe each point twice a day. The warning value for base heave settlement observation is 10mm per observation. If the warning value is exceeded three consecutive times, report to the project department promptly and take corresponding measures.
3. Defects in retaining structure
[Measures] Ensure the construction quality of the waterproof curtain, mainly by controlling the quality of cement mixing piles. The overlap of mixing piles must meet design requirements, and cold joints should be avoided during construction. The cement content and water-cement ratio of mixing piles must meet design requirements: water-cement ratio 1:1.5, cement content 22%. The injected cement slurry must be uniformly mixed, filtered, and then high-pressure air jet is used to stir the cement in the hole, so that the cement-soil mixing pile is fully mixed before initial setting, ensuring full mixing of cement and soil and the cement dosage of each pile.
4. Cracking and uneven settlement of surrounding buildings
[Measures] Construct according to the expert-demonstrated construction plan for foundation pit support and earthwork excavation, ensure the construction quality of the support system, strengthen foundation pit monitoring, and avoid exceeding warning values. The warning value for building tilt is 1%, the warning value for building settlement rate is 5mm/d, and the cumulative total settlement is 30mm.
II. Emergency Technical Measures for Foundation Pits
1. Excessive displacement or collapse of foundation pit system
(1) Excessive displacement of foundation pit system. Based on monitoring data provided by the monitoring unit, if excessive displacement occurs, approaching or reaching the warning value, stop earthwork excavation on the side with excessive displacement, adjust the excavation position, and excavate on the opposite side. If displacement continues to expand, stop excavation, backfill that area, and use sandbags or local soil as backfill materials.
(2) Foundation pit collapse. In case of collapse, immediately provide temporary support for the collapsed slope using 14# channel steel @200 sheet piles, and spray 80mm thick C20 concrete on the outside of the sheet piles to prevent water from leaking into the pit.
2. Base heave and surrounding soil boiling
(1) If base heave occurs, immediately stop construction, use prepared sandbags and pit soil for backfill loading, reinforce the base soil according to actual conditions, and then excavate to the design elevation. Add settlement monitoring points outside the pit.
(2) If surrounding soil boiling occurs, insert sheet piles along the periphery inside the pit to prevent external soil from squeezing inward, increase the embedded depth of the retaining structure, and dewater the base soil to reduce hydrodynamic pressure and prevent soil boiling.
3. Defects in retaining structure
(1) If the leakage point is limited above the excavation surface and the leakage is not large, use quick-setting cement to chase the groove and inject polyurethane for sealing.
(2) If the leakage point is limited above the excavation surface and the leakage is large, drive a drainage pipe into the leakage point, seal around the drainage pipe with steel pipe and quick-setting cement, and close the drainage pipe valve after the sealing material reaches strength.
(3) If the leakage point extends from above the excavation surface to below, inject dual-fluid grout near the leakage point outside the pit. Grouting pressure should be controlled, with a maximum pressure not exceeding 0.3MPa.
(4) If the leakage point extends from above to below the excavation surface and the flow is large, backfill locally inside the pit until the flow decreases, then inject polyurethane near the leakage point outside the pit.
(5) If the leakage point is unknown and water flows upward from below the excavation surface, immediately stop excavation, backfill locally until leakage stops, and then adopt the above-mentioned dual-fluid grouting outside the pit.
(6) If the leakage water is turbid and lasts for a long time, be aware of possible severe soil loss and voids near the leakage point. Heavy machinery must be kept away, and vibratory pipe grouting should be used immediately to fill the voids.
4. Cracking and uneven settlement of surrounding buildings
(1) Immediately stop construction, invite experts to analyze building cracking and settlement, identify causes, formulate and implement a protection plan for the buildings, and continue construction after eliminating hidden dangers.
(2) Immediately stop foundation pit excavation, strengthen foundation pit support. Ground reinforcement measures include grouting within 5.0m around the pit to reinforce the soil, using pure cement slurry as grouting material, with grouting pressure of 0.5~1.0MPa and soil reinforcement depth of 8.0m.