Retaining walls look simple. A stack of rock or block holding back a slope — what could go wrong? Quite a lot, as it turns out. Failed retaining walls are a common sight on Vancouver Island properties, and the causes are almost always the same. Understanding why walls fail is the best way to ensure the one you build doesn’t.

The Number One Cause: No Drainage

The most common reason retaining walls fail — by a wide margin — is inadequate drainage behind the wall. When rain saturates the soil behind a retaining wall and there’s nowhere for that water to go, hydrostatic pressure builds up against the back of the wall. Water is heavy, and a saturated clay soil exerts enormous lateral pressure. Over time — sometimes over just a few wet seasons — that pressure overcomes the wall’s resistance and the wall begins to lean, crack, or overturn.

Proper drainage behind a retaining wall means a layer of free-draining gravel aggregate against the back of the wall, weeping tile at the base of that gravel to carry water away, and outlet points that allow water to escape freely without undermining the wall’s foundation. This drainage layer is not optional — it’s what makes the difference between a wall that lasts decades and one that fails in five years.

Inadequate Base Preparation

A retaining wall is only as stable as its foundation. Walls built directly on soft or organic material will settle unevenly, causing sections to drop, lean, or separate. The base course of any retaining wall should be set on firm, compacted native material or a compacted gravel pad — below the frost line at higher elevations — and level in both directions so the wall has a consistent bearing surface from the start.

Insufficient Wall Mass or Depth

The taller the wall and the heavier the soil being retained, the more mass and embedment depth the wall requires to resist the lateral forces acting on it. Walls that are undersized for the load they’re carrying — either too thin, too short, or with insufficient batter (backward lean into the slope) — are structurally compromised from the start. For walls over a certain height, engineering is required in BC precisely because the forces involved require proper calculation.

Surcharge Loading

Many homeowners don’t think about what’s at the top of the slope behind their retaining wall. A driveway, a structure, parked vehicles, or heavy equipment operating near the top of a wall all add surcharge load — additional downward pressure on the soil behind the wall that increases the lateral forces the wall must resist. A wall designed without accounting for surcharge loading may be undersized for actual conditions.

Vancouver Island Conditions Make This More Challenging

The combination of heavy seasonal rainfall, clay-heavy soils that hold water and expand when wet, and freeze-thaw cycles at higher elevations makes drainage and foundation preparation even more critical here than in drier climates. A retaining wall built without proper drainage that might last 15 years in a dry climate may fail in three or four Vancouver Island winters.

What Good Retaining Wall Construction Looks Like

Excavate to firm bearing material. Set base course level and well-embedded. Backfill with free-draining gravel aggregate rather than native soil. Install weeping tile at the base of the gravel. Ensure outlets are clear and functional. Build with appropriate batter. Size the wall for the actual load — including any surcharge. These aren’t exotic requirements — they’re standard practice for walls that are meant to last.