On a hillside, cut into the uphill side, use that soil to build up the downhill side, slope the cut back at 1.5 horizontal to 1 vertical, and compact every fill layer of 15-20 cm. The earthwork grows steeply with the ground slope: on a 3 m wide bench a 25% side slope needs 0.45 m² of cut per metre of road, a 50% side slope needs 2.25 m², and a 60% slope needs 6.75 m². Past a slope of about 67%, the cut face never meets the ground, so the road needs a retaining wall.
Part of Road Construction Basics
Building a level roadbed on a sloping hillside by digging into the uphill side and building up the downhill side with the same soil.
A Steeper Hillside Costs Far More Earthwork Than It Looks
The parent article gives the method: set stakes along the centreline, measure half the road width in both directions, cut into the slope on the uphill side and use that material to build up the downhill side, compacting in layers. The uphill bank must be sloped back and not cut vertical: 1.5 horizontal to 1 vertical for most soils, steeper for rock. If the slope is steep, build a retaining wall or place large stones at the foot of the fill. Its critical rule: never build on uncompacted fill, and compact every 15-20 cm layer before adding the next.
The parent does not say how quickly the amount of digging grows with the steepness of the hill. It grows very fast, and that is the main reason to choose a route that follows the contour. See Route Selection.
What You Need
- Stakes and string to mark the centreline and the edges.
- A way to measure slope — a water level or an A-frame. See A-Frame Level and Grade Calculation.
- Picks, shovels, barrows or carts.
- Stone for a retaining wall or fill toe where the slope is steep.
- A compaction tool. See Compaction Methods.
Method: Cut And Fill A Sidehill Road
Step 1: Stake the centreline and the edges Drive a stake at the centreline every few metres. Measure half the road width each way.
Step 2: Cut the uphill side Dig from the top of the bank down, shaping the cut face back at 1.5:1. Never undercut the bank.
Step 3: Place the soil on the downhill side Build it up in layers of at most 15-20 cm. Compact each before adding the next.
Step 4: Support the toe of the fill On a steep slope, set large stones or a wall at the base of the fill.
Step 5: Check the finished width and slope The surface should be at its design width, with the crossfall toward the ditch.
Worked Example: How Much Dirt, At Two Slopes
Inputs, mine: the road bench is 3 m wide, so half-width b = 1.5 m; the road surface is level across the bench and meets the original ground at the centreline; the cut face slopes at 1.5:1 (horizontal:vertical), which is 1 / 1.5 = 0.667 of rise per metre run, an angle of 33.7 degrees; the fill face uses the same slope, an assumption since the parent does not give one. Ground slope s is rise over run.
s = 0.25 (25%, 14 degrees).
- Cut area over the bench: the ground at the bench edge is 0.25 × 1.5 = 0.375 m above the bench, so the area is 0.25 × 1.5² / 2 = 0.281 m² per metre of road.
- The cut face meets the ground at a distance x beyond the edge where 0.667 x = 0.375 + 0.25 x, which is x = 0.375 / 0.417 = 0.90 m.
- Extra triangle: 0.5 × 0.90 × 0.375 = 0.169 m².
- Total cut: 0.281 + 0.169 = 0.45 m² per metre, which is 45 m³ for 100 m of road.
- The fill side is the mirror image: 0.45 m² of fill.
s = 0.50 (50%, 27 degrees).
- Bench: 0.5 × 1.5² / 2 = 0.562 m². Height at the edge 0.75 m.
- x = 0.75 / (0.667 − 0.50) = 4.5 m. Triangle: 0.5 × 4.5 × 0.75 = 1.69 m².
- Total cut: 0.562 + 1.69 = 2.25 m² per metre, 225 m³ per 100 m. That is five times the 25% case, for a doubling of the ground slope.
s = 0.60 (60%). x = 0.90 / (0.667 − 0.60) = 13.5 m; triangle 6.08 m²; total 6.75 m² per metre, 675 m³ per 100 m. As s approaches 0.667 (a slope of about 67%) the cut face and the ground run parallel and the earthwork has no limit.
Compaction. Grade Calculation advises allowing about 20% more fill volume than cut because fill compacts. Applying that to the 25% case: material needed from the cut is 0.45 × 1.2 = 0.54 m², against 0.45 m² available, so the section is 0.09 / 0.54 = 17% short. Moving the centreline slightly into the hill gives more cut and less fill; an equal-area section does not balance on its own.
Common Mistakes
- Cutting the bank vertical — it slumps onto the road. Slope it back.
- Dumping fill in one lift — loose soil settles and ruts. Compact every 15-20 cm layer.
- Ignoring the compaction allowance — you end up short of fill.
- Building across a steep slope — the earthwork grows rapidly; route along the contour.
- No toe support — a fill on a steep slope slides. Use stones or a wall.
Safety
Bank collapse and falling soil
A cut bank can fall without warning and bury a worker. Never dig under an overhang, never work below a fresh cut, keep spoil back from the edge, and stop work after heavy rain or when cracks open at the top of the cut. Keep people off the downhill side of a fill being tipped.
What’s Next
- Learn next: Compaction Methods — firming each layer of fill
- Combine with: Side Ditches — draining the cut face