Cut-and-cover means excavating a pit, building and waterproofing the structure inside it, then backfilling and covering it with earth. Dig the pit with sides sloped back to about 1.5 m horizontal for every 1 m of depth or shored with planks and cross-braces, and keep the spoil at least 0.6 m from the edge, because an unsupported pit wall deeper than about 1.2 m can kill.
Cut-and-cover builds a structure in an open excavation and buries it, the most direct route to a truly underground building.
The Pit Is The Most Dangerous Part Of The Whole Project
Cut-and-cover works because the structure is built in the open, with room to work and to waterproof every face. The price is the pit. Soil weighs about 1,500-2,000 kg per cubic metre, so even a small slab of pit wall falling in can bury and crush a worker. The usual approach is to slope the sides back, because a sloped pit needs no timbering, at the cost of a wider hole and more spoil.
What You Need
- Digging tools and a way to remove spoil to a stockpile away from the edge.
- Shoring materials where the pit cannot be sloped: planks, upright boards and cross-braces.
- A ladder from the pit floor to the surface, always in place.
- A drain and sump for water entering the pit.
- Structural and waterproofing materials, and a staged plan for backfilling.
Method: Dig, Build, Seal, Cover
Step 1: Set out the pit wide enough that the structure, its waterproofing and the drain fit inside with working room.
Step 2: Dig with sloped or shored sides. For soil you cannot identify, assume the weakest class and slope the sides back to about 1.5 m horizontal for every 1 m of depth, the slope used by US excavation rules for the weakest soil. Shore with planks and braces where space prevents sloping.
Step 3: Put spoil at least 0.6 m back from the edge (the US excavation-rule minimum) and farther for deeper pits, because spoil adds a surcharge load on the pit wall.
Step 4: Drain the pit floor and keep water out of the excavation.
Step 5: Build the structure, then apply the waterproofing and the drain; see Waterproofing and Drainage.
Step 6: Backfill in even lifts, compacting, with the roof structure designed for the soil load; see Roof Structures for Earth Loading.
Common Mistakes
- Working in an unsupported pit “just for a minute”.
- Spoil piled at the lip.
- Leaving the pit open through rain. Wet soil is heavier and weaker.
- Backfilling a wall that is not yet braced. See Bermed Walls.
Safety
Pit walls collapse without warning
Any excavation deeper than about 1.2 m needs sloped or shored walls. Never work in a pit alone, keep a ladder in it, keep spoil, tools and vehicles at least 0.6 m back from the edge, and leave the pit at once if you see cracks along the top, soil trickling from the wall or water seeping in. Check the walls again after rain and after any heavy traffic nearby. For lined shafts, see Well Digging. Never enter a pit or shaft to test the air with a flame.
What’s Next
- Learn next: Earth Load Calculations — sizing the shell
- Combine with: French Drains and Perimeter Drainage — drainage around the finished building