Factors in construction stage of trench excavation

Factors to be considered in the construction stage of trench excavation:

1. Setting out and alignment: Mark the trench lines with pegs, string and lime from the approved drawing; check dimensions and levels (profile boards, sight rails).
2. Depth and width: Excavate to the required foundation level; width is the footing width plus working space of 150 to 300 mm for formwork.
3. Type of soil: Soft soil needs slopes or shoring; hard soil or rock needs picking, drilling or blasting.
4. Side stability and shoring: Timber planking and struts for trenches deeper than 1.5 m in loose soil to prevent collapse and for the safety of workers.
5. Ground water: Dewatering by sumps, ditches, or well-point system; the bottom is kept dry.
6. Safety: Barricades, warning signs and lights, ladders for exit, spoil heaped at a safe distance from the edge (at least 1 m), no heavy loads near the trench, and inspection after rain.
7. Disposal of spoil: Store for backfilling and remove surplus; do not block drainage.
8. Existing services and adjacent structures: Locate underground pipes and cables; support nearby foundations.
9. Trench bottom: Level, clean, and compact; remove loose soil; do not over-excavate (fill with PCC if so).
10. Weather: Avoid the rainy season; protect the trench from rain run-off by diversion bunds.
11. Method and equipment: Manual or machine; cost and time.
12. Inspection and approval of the trench bed by the engineer before concreting.