Terracing a Steep Block in Molonglo or Gungahlin: Retaining Walls, Drainage and Access
Canberra's newest suburbs are on hillsides. How to plan and dig terraces, retaining wall footings and drainage on a sloping new lot with a compact excavator and skid steer.
Denman Prospect, Whitlam, Coombs, Throsby, Googong: Canberra's new suburbs are on the hills the old ones avoided. Most new lots come with a slope, and the first job after the builder leaves is turning that slope into somewhere you can walk.
Plan the levels first
Stand at the back door and decide where the flat areas need to be: the entertaining area, the lawn, the shed. Everything else becomes batters or walls. Cut and fill on the block so that spoil from one terrace builds the next; carting clay off a Canberra block is expensive.
Approvals
In the ACT, retaining walls over a set height and any wall near a boundary need approval, and Access Canberra's exempt development rules set the limits. Walls that hold up a neighbour's land or a driveway are engineered. Check before you dig; a wall built without approval is a wall you may be asked to remove.
Cutting
A 1.5 tonne excavator with a tilt bucket cuts the terraces: the bucket's tilt sets the batter angle and the cross-fall without moving the machine. Work from the top down so the machine always sits on undisturbed ground, and keep the blade downhill.
Footings
Wall footings in Canberra clay go below the reactive zone and are usually engineered. Dig them with a toothed bucket to the width and depth on the drawing, and pour them the same week; an open footing in a Canberra storm or frost is a problem.
Drainage behind the wall
Every retaining wall needs a drain: slotted pipe in gravel, wrapped in geofabric, falling to a legal outlet. Water behind a wall in clay is what pushes walls over in the second winter. The excavator digs the drain trench as part of the footing.
Backfill and compaction
Backfill in layers and compact each one; a skid steer with a 4-in-1 bucket spreads and a plate compactor finishes. Loose backfill settles for years.
Frost and season
Autumn and spring are the terracing months. Winter clay is unworkable and the batters slump; summer clay is hard and dusty but stable.
Machines
The R15 with a tilt bucket and toothed bucket does most residential terracing; the R22 PRO for bigger lots and rock; an RS06 skid steer for moving and spreading. See the Molonglo area guide and the tilt buckets that fit each model.