Building into a hillside lets the slope act as the rear and side walls, which is very effective, but water running down the slope is aimed straight at the building and the cut itself can fail. Put a drain across the slope above the building to intercept water, avoid slopes that are already moving, and cut the slope back or retain it so the excavation cannot slide.
Hillside integration sets a building into a slope so that earth covers the rear, the sides and often the roof, while the front faces out.
The Hill Gives Walls And Delivers Water
A slope provides the earth cover without a deep pit, and the front can face the sun. The same slope carries rain and groundwater downhill. A building cut into the hillside sits in the path of that water and, below the cut, may sit on ground that has seeps and springs. The parent article states the rule for any underground building, that drainage comes first; on a hillside the first drain belongs upslope, not just around the footing.
A cut into a slope also changes the slope’s balance. The soil above the cut loses its support and can slide into the excavation, especially when wet.
What You Need
- A site survey of slope direction, seeps, springs and signs of past movement.
- An interceptor drain across the slope above the building, with an outlet that carries water around it.
- A retained or sloped cut, and a retaining wall where needed; see Retaining Wall Design.
- A perimeter drain and waterproofing; see French Drains and Perimeter Drainage.
For stepped ground and the problems of moving water on slopes, see Terracing; for choosing ground, see Permanent Settlement Site Selection.
Method: Survey, Intercept, Cut, Retain
Step 1: Walk the slope after heavy rain. Note where water emerges, where it runs and whether the ground shows cracks, leaning trees or slumps. Do not build on ground that is moving.
Step 2: Cut the interceptor drain first, and make sure its outlet discharges clear of the building and of the neighbours below.
Step 3: Excavate in stages with sloped or shored sides; see Cut-and-Cover Method.
Step 4: Retain the cut with a wall at the rear, or lay the slope back.
Step 5: Waterproof and drain the buried walls, then backfill.
Common Mistakes
- Cutting without checking for seeps. The cut can open a spring into the room.
- An interceptor drain with no outlet.
- Building on a slope that is already creeping.
Safety
Slope cuts fail suddenly
A fresh cut in a hillside can slide, and any excavation deeper than about 1.2 m can collapse onto workers. Slope or shore the cut, never work below an overhanging bank, and stop work in heavy rain.
What’s Next
- Learn next: Waterproofing and Drainage
- Combine with: South-Facing Glazing — the open front of the building