Dig the ditch V-shaped, 2 m wide and 1.5 m deep, and heap the spoil on the settlement side with a flat step between lip and toe: its 1.5 m² of soil per metre swells to about 1.9 m² loose and builds a berm about 1.1 m high, a combined obstacle of about 2.6 m. The defensive-architecture guide’s 3-4 m obstacle needs a square-sided ditch of 3-4 m² per metre, which collapses unless shored, and any berm on the inside must have drainage gaps or it dams the settlement’s own rain.
Part of Defensive Architecture
Ditches, berms and earthworks are excavated trenches with their spoil heaped beside them, forming obstacles that stop vehicles, slow people and channel movement, and that also drain and control erosion.
The Spoil Sets The Height, And The Slope Limits It
The defensive-architecture guide gives the layout: dig a ditch 1.5-2 m deep and 2 m wide; pile the excavated earth on the inside to form a berm; plant the berm with a thorny hedge; and the ditch serves as drainage and water collection in peacetime. It says the combined obstacle is 3-4 m from the ditch bottom to the berm top, and in the deterrence layer it says a 1.5 m ditch with a berm makes a 3 m or greater obstacle. The household ditch guide gets 2.5 m from a 1.5 m ditch and a 1 m berm and warns that the spoil goes inside, with a flat step between lip and toe so it does not slide back (Ditches and Berms).
The two figures can be reconciled with the soil volume, and the result shows what the guide’s larger number costs.
Worked Example: How High Can The Spoil Go
Inputs: ditch 2 m wide; loosened soil swells by about 25% (a recalled typical figure for earth, 20-40%, not from this corpus); the heap’s sides lie at the angle of repose of loose earth, taken as 30-35 degrees (recalled for dry loose soil, check on your own soil); a triangular berm of height h has sides of run h ÷ tan θ, base 2h ÷ tan θ and area ½ × base × h = h² ÷ tan θ.
- V ditch, 2 m wide and 1.5 m deep: cross-section ½ × 2 × 1.5 = 1.5 m² per metre; loose spoil 1.5 × 1.25 = 1.9 m².
- Berm height: h = √(loose area × tan θ). At 35 degrees, tan = 0.70: h = √(1.9 × 0.70) = 1.15 m. At 30 degrees, tan = 0.58: h = √(1.9 × 0.58) = 1.05 m.
- The berm is therefore about 1.1 m high. Combined obstacle: 1.5 + 1.05 to 1.5 + 1.15 = 2.6 m, matching the household guide’s 2.5 m.
- Square-sided 2 m × 1.5 m: 3 m² per metre, loose 3.75 m², berm h = √(3.75 × 0.70) = 1.6 m, obstacle 3.1 m.
- Square-sided 2 m × 2 m: 4 m² per metre, loose 5.0 m², berm h = √(5.0 × 0.70) = 1.9 m, obstacle 3.9 m.
So the guide’s 3-4 m is reachable only with a square-sided ditch of 3-4 m² per metre, twice the soil of a V (the V halves the digging, as the household guide computes), and with sides that stand vertical at 1.5-2 m depth. In loose or wet soil those sides slump; and a 1.5 m deep trench is at about the depth where workplace rules, as I recall, begin to require shoring.
What You Need
- Shovels, picks, baskets or barrows, and pegs and string to mark the line
- A flat step of about a stride between lip and toe, left unheaped
- Planks for your own crossings, marked by touch
- An outfall for the ditch, and drainage gaps in the berm
- Turf or planting to hold the berm
Method: Dig, Heap, Drain
Step 1: Mark the line and leave the step.
Step 2: Dig from the top down, sides sloped back; in loose soil, flatten the sides.
Step 3: Throw the spoil to the settlement side, compact it in layers and plant it.
Step 4: Leave drainage gaps or culverts through the berm at the low points. A berm with the ditch outside and no gaps turns the settlement’s side into a basin.
Step 5: Grade the ditch to an outfall so it does not stand full.
Step 6: Cut gaps for gates and emergency crossings, and mark them.
Common Mistakes
- Spoil on the outside. It becomes a ramp for the intruder.
- Vertical sides in loose soil. They slump and kill the digger.
- A ditch full of water. It drowns children and breeds mosquitoes.
- Sharpened stakes in the bottom. They injure your own people in the dark.
Safety
Trenches collapse and standing water drowns
A trench deeper than about 1.2 m can bury a person in seconds: slope the sides, never undercut the face, never dig alone, and keep out of it after rain. Cover or fence ditches near children and mark them for night walking; 30 cm of water can drown a toddler. Keep spoil heaps at least 0.6 m from the edge. A ditch line is also a barrier on your own escape routes, so mark and rehearse the crossings.
What’s Next
- Learn next: Controlled Access Points — where the ditch is crossed
- Combine with: Drainage Ditches — keeping the ditch from flooding