A terrace retaining wall doubles as a barrier and a vantage point, but its main job is holding soil, and water is what breaks it: a 1.5 m wall faces about 6.4 kN of soil thrust per metre of length when the soil is dry and about 14.5 kN when it is saturated with no drainage, 2.3 times as much. Build it as a battered dry-stone wall with gravel and weep holes behind it, leave marked gaps and steps for your own people, and keep children off the top edge.
Part of Defensive Architecture
A terrace wall is a retaining wall that holds a levelled strip of ground for crops; in a dual-purpose design its height and position also make it a barrier along an approach.
The Retaining Job Comes First And Water Decides It
The defensive-architecture guide lists terracing walls that retain agricultural terraces and also function as barriers and firing positions among its dual-purpose features. A terrace wall of 1-1.5 m is a real obstacle to a person climbing up from below and a drop for anyone coming from above, so it fits a slope facing an approach. It has to be built to hold the earth behind it first, and a wall that fails is both a lost terrace and an injury.
The earth load guide gives the method and constants for soil against a wall (Earth Load Calculations): the thrust per metre of wall is P = ½ K γ H², acting H ÷ 3 above the base, with K = 0.333 for a friction angle of 30 degrees, unit weight 17 kN/m³, and for a saturated wall with no drainage the submerged weight 9.19 kN/m³ plus the full water pressure of 9.81 kN/m³.
Worked Example: A 1.5 m Terrace Wall
Inputs: wall height H = 1.5 m; dry-stone wall 1.0 m thick (1.5 m³ of stone per metre of length, the household wall guide’s figure, Walls, Stone and Earth); stone unit weight 25 kN/m³ (assumed, 2,500 kg/m³); friction between wall and ground 0.5 (assumed).
- Dry thrust: P = ½ × 0.333 × 17 × 1.5² = 6.37 kN, about 6.4 kN, per metre, at 0.5 m above the base. Overturning moment = 6.37 × 0.5 = 3.2 kN·m per metre.
- Wall weight: 1.0 × 1.5 × 25 = 37.5 kN per metre. Resisting moment about the toe = 37.5 × 0.5 = 18.75 kN·m. Factor of safety against overturning = 18.75 ÷ 3.2 = 5.9. Against sliding: 0.5 × 37.5 ÷ 6.37 = 2.9.
- Saturated, no drainage: pressure at the base = 0.333 × 9.19 × 1.5 + 9.81 × 1.5 = 4.6 + 14.7 = 19.3 kPa. Thrust = ½ × 19.3 × 1.5 = 14.5 kN per metre, 14.5 ÷ 6.37 = 2.3 times the dry value. Moment = 14.5 × 0.5 = 7.2 kN·m: factor against overturning 18.75 ÷ 7.2 = 2.6; against sliding 0.5 × 37.5 ÷ 14.5 = 1.3.
The wall that was safe by a factor of 6 and 3 dry is down to 2.6 and 1.3 when waterlogged, and real dry-stone walls have air gaps and are not a solid block, so the real margins are smaller. A surcharge, such as a loaded cart or a pile of spoil on the terrace above, adds to it again. Drainage is therefore not an extra; it is the thing that makes the wall hold.
What You Need
- Stone, with long tie stones, laid with each stone bridging the joint below it (Stone Foundation gives the two-over-one rule)
- Gravel for the backfill, and a drain at the foot
- Weep holes through the wall
- Steps or ramps through the wall line
- A plan of where the wall is the barrier and where it is the path
Method: Build It To Hold Water-Free Soil
Step 1: Mark the line along the contour and the face of the slope that looks toward the approach.
Step 2: Dig a firm footing and lay the first course of the largest stones.
Step 3: Build up with the wall leaning back a little (a batter), tie stones every course or two, and gravel backfill as you go.
Step 4: Put weep holes at the base and a gravel drain behind the foot of the wall.
Step 5: Build in steps or gaps for your own people, with no more than the number the approach needs.
Step 6: Keep carts, spoil and stored goods off the terrace edge, and inspect after every heavy rain.
Common Mistakes
- A solid wall with no drainage. The soil turns it into a dam face.
- A tall vertical wall. It overturns sooner than a leaning one.
- Heavy loads at the top edge. They add a surcharge.
- No gaps. The wall traps your own people in a fire or a flood.
Safety
Retaining walls fail suddenly, and edges drop
A retaining wall that has bulged or leaned is near failure; keep out of the zone as wide as the wall is high, below it as well as above. A wall of 1.5 m is a fall that breaks bones, so rail or mark the edge near children. Wet soil behind the wall, a weeping crack and a bulge are all warnings; stop using the terrace and relieve the water.
What’s Next
- Learn next: Observation Points as Water Towers — the second shared feature
- Combine with: Drainage Ditches — getting the water away from the foot