The excavation process follows a systematic workflow:
Depth Calculation: Determine the depth based on the structural drawing (foundation depth). Always ensure you reach the "hard strata" (safe bearing capacity layer) identified in the soil report.
Cutting & Removal:
Side Sloping/Benching: If the soil is loose or sandy, the sides must be sloped or stepped to prevent collapse.
Working Space: Excavate approximately 300mm–600mm extra around the perimeter of the footing to allow for formwork (shuttering) installation and proper concrete pouring.
Dewatering: If the water table is high or it has rained, water must be pumped out of the pit to prevent soil liquefaction and to ensure the concrete can be poured on a dry base.
3. The "Bottom-of-Pit" Treatment
The soil at the bottom of the excavation pit is the actual surface carrying the building's weight. It requires careful preparation:
Compaction: After achieving the required depth, the bottom of the pit must be rammed or mechanically compacted to ensure the soil is dense and uniform.
Level Checking: Use a leveling instrument (Auto Level or Dumpy Level) to ensure the bottom is perfectly flat and level across all footing pits.
PCC (Plain Cement Concrete): A layer of PCC (usually 1:4:8 or 1:5:10 mix) of 75mm–100mm thickness is placed at the bottom. This provides a clean, level, and hard surface for the reinforcement cage and formwork.
4. Safety Measures (Critical)
Excavation is one of the most hazardous activities on a construction site.
Shoring & Strutting: For deep excavations (usually >1.5m) in unstable soil, provide timber or steel shoring to support the pit walls and prevent cave-ins.
Fencing: Provide temporary barricades or caution tape around the perimeter of open pits to prevent workers or animals from falling in.
Access: Ensure safe entry/exit points (e.g., proper ladders) are available for workers inside deep pits.
Storage: Never store excavated soil or heavy materials right at the edge of the pit (this adds surcharge weight and causes wall failure). Keep them at least 1 meter away.
5. Technical Checklist for the Engineer
Before finalizing the excavation, verify the following:
Item
Requirement
Grid Lines
Match the drawing exactly to avoid structural misalignment.
Soil Type
Does it match the foundation design assumptions?
Bottom Level
Is the pit bottom level as per the site datum?
Loose Soil
Has all loose/slushy soil been removed and replaced/compacted?
Safety
Are sides supported if the pit is deep?
6. Common Mistakes to Avoid
Over-excavation: If you dig deeper than required, do not refill with loose soil. It will settle unevenly. Refill with PCC or lean concrete up to the required level.
Working in Rain: Unprotected excavation pits can fill with water, destroying the bottom soil compaction. Always try to schedule excavation immediately before the PCC and reinforcement work begins.
Ignoring Water Table: Failure to address groundwater will lead to floating foundations or massive settlement during the rainy season.
Pro-Tip for Site Engineers: Always keep the foundation drawings accessible at the pit side and cross-verify the excavation width against the footing size plus the space required for your specific shuttering material (e.g., plywood or steel plates).
