What a retaining wall holds: earth, water and load
A retaining wall holds three things at once: the soil pressure behind it, the pressure of water pooling in the backfill, and surface loads — a driveway, a car, sometimes a neighbour's foundation. Soil pressure grows with the square of height: a 1.5-metre wall pushes 4 times more than a 75-centimetre one, and that progression commands the whole design.
Water is the walls' silent enemy: a waterlogged backfill exerts up to twice the pressure of a dry one, and most walls that lean or slump were built without a drain or drain stone. The wall is therefore never designed alone: it is designed with its drainage, its geogrid and its bearing soil. That is the base both wall families share, and the one that decides their lifespan.
The segmental block wall: the geogrid family
The segmental block wall — big interlocking concrete blocks stacked in setback — stands by its own weight and, beyond 1.2 metres, by geogrids: sheets of synthetic mesh laid in the backfill every 2 courses and anchored in the soil behind, holding the earth mass like a net. The block is only the facing; the geogrid carries the slope. It is the technique of the 1-to-3-metre residential walls.
Its concrete advantages: mortar-free installation — blocks stack and lock, absorbing differential settlement without cracking — curve flexibility, and easy replacement of a damaged block. Its limits: a maximum height around 3 metres without an engineer, a facing that ages if unsealed, and a higher cost per square metre at purchase — offset by shallower foundations and faster installation.
The standard concrete block wall: the mason's wall
The standard concrete block wall — 20 or 25-centimetre hollow blocks laid on a reinforced concrete footing and filled with concrete and rebar every 2 to 3 blocks — is the classic wall of foundations and basements. It reaches any height by one means only: vertical reinforcement in the filled cavities and a thick gravity wall. Above 1.5 metres, its footing widens to carry the weight.
It costs less per square metre at purchase - cheaper per square metre than segmental - and its raw surface finishes with coating or paint. But it demands a poured concrete footing, flawless drainage and careful jointing: a crack in a filled block is a structural crack, and rebar cannot be added later. It is the right wall for mid heights and shared-boundary constraints.
Drainage and stone: the wall's real heart
Whichever family you choose, the wall lives or dies by its drainage. Behind the facing: a 4-inch drain at the foot, wrapped in geotextile, in a bed of clean ¾-inch stone 15 to 30 centimetres wide, and weep holes at the wall's base to let rising water out. The stone backfill rises at least 60 centimetres behind the wall, the geotextile separating soil from stone so nothing clogs.
The wall-killing mistake is skimping on this section: a well-built wall without drain stone holds water against its back, the clay swells in frost, and pressure doubles. Conversely, an average wall with good drainage holds for decades. The geogrid installs every 2 courses for walls beyond a metre, anchored 90 to 150 centimetres into the backfill — it is what turns a stack of blocks into a structural wall.
Height, soil and permit: the decision rule
The decision rule plays on three factors. Height: under 1.2 metres, a well-drained standard block wall suffices and costs less; from 1.2 to 3 metres, segmental with geogrids dominates for flexibility and holding; beyond 3 metres, both families move to engineer design with calculated rebar. Soil: swelling clay and a high water table demand reinforced drainage and wider footings, whatever the facing.
And the permit: in most Greater Montreal municipalities, a retaining wall over 1.2 metres requires a construction permit, sometimes an engineer plan depending on the neighbourhood — a wall on the property line touches the neighbour's land and regulates more strictly. The AssiseSolide quote integrates these three factors, names the wall family, the geogrid, the drain and the stone, and documents the installation photo by photo. Free diagnosis: 438-544-6364.
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