Typical cross-sections of hill roads

The cross-section of a hill road depends on the slope of the natural ground, the type of soil or rock, and the cost. The typical types are:

1. Cross-section in cutting (full cutting)

The road is wholly cut into the hill. The cut slope is made stable by flattening (about 1:4 (H:V) in rock and 1:1, 1.5:1 or flatter in soil) and benching, with a catch drain above. It is suited to steep, rocky slopes. It is the most stable section, since the road rests entirely on natural ground with no fill, but spoil must be disposed of and the cost of excavation is high.

        catch drain
          \_/___
            \    \  cut slope
             \    \
    side drain\____\___ road ___

2. Cross-section in filling (full embankment)

The road is wholly on fill, retained by a toe wall or side slope. It is used on gentle ground and in valleys, and may need toe walls and slope protection. It is avoided on steep ground, since fill on steep slopes may slide.

        ___ road ___
       /            \  fill slope 1.5:1
 _____/              \_____
 natural ground, benched

3. Balanced cut and fill (side-hill / half-cut half-fill)

Part of the road width is excavated and the spoil is used to build the other part, with walls where needed (a breast wall on the hill side, a retaining wall on the valley side). This is the most economical section on moderate slopes (up to about 25 to 30°), since it minimises haulage and spoil.

          breast wall
        ___/|
 cut   /    |__road__|
 _____/     fill  |retaining wall
 hill slope         \_______

4. Cross-section with retaining wall (full width on a shelf)

Used on steep ground with an unstable slope or where land is limited. The walls retain the cut on the hill side and the fill on the valley side.

5. Cross-section on rock (shelf or half-tunnel / gallery)

On very steep rock, the road may be a shelf cut with overhead protection, or a half tunnel. This is expensive and used only at exceptional places.

Common elements

• A side drain on the hill side (minimum width about 0.6 to 1.0 m), a catch drain above the cut, and a camber or an inward cross slope of the road towards the hill for drainage, with a parapet or guard stones on the valley side.
• Cut slopes, fill slopes and wall heights chosen according to the soil and geology.