Retaining Wall Base Excavation in Calgary

The Problem It Solves

A retaining wall holds back a wedge of soil that is always trying to move downhill, and in Calgary that soil is often clay that holds water and heaves when it freezes. Walls that fail usually show it the same way: the top course leans out, the face bulges partway along, blocks rotate or step apart, timbers bow, and water seeps from the joints after rain. Usually that is not the fault of the blocks. It comes from what the wall was built on and what is behind it: a base set on topsoil or wet clay, a levelling pad too thin to spread the load, or no drainage, so water pressure and frost push on the back of the wall every spring.

The search for a retaining wall in Calgary is mostly a search for someone to build the finished wall, and that is not what this page offers. What can get skimped on small and mid-sized walls is the earthwork underneath. Hand-digging a long base trench in heavy clay is slow, so it ends up shallow. Cutting back into a slope far enough for the geogrid on a taller wall is real excavation, with spoil to store or truck away. And the digging often runs beside house foundations, fences, trees and a neighbour's property line, where a careless cut does its own damage.

When the base is wrong, the fix is not a repair. The wall comes down, the blocks or timbers are set aside or thrown out, the base is excavated again and the wall is rebuilt. A tall wall that fails can drop soil onto a neighbour's yard, a walkway or the side of a garage. A wall at or over the City's 1.2 m building permit height that went up without a permit and engineer-stamped plans is out of step with the City's requirements, and that is a harder conversation after it is built than before. And a wall that dams surface water or sends it sideways can put water on a neighbouring lot, which Calgary's Guide to Lot Drainage says foundation drainage discharge must not do.

How It Works

Aspen Excavating does the ground work and hands over a base; we do not build the wall. We excavate the base trench to the width and depth the block system's installation guide or the engineer's drawing calls for, including the buried portion of the wall below finished grade. Sod, topsoil, roots and soft or wet soil come out until the trench bottom is firm, undisturbed ground. Where a soft spot goes deeper, it is sub-excavated and replaced with compacted granular. We then place the granular levelling pad in compacted lifts and bring it to line and grade, stepping it where the wall climbs a slope.

Taller walls, and any wall with geogrid, need the slope behind cut back far enough for the grid lengths on the drawing, plus working room. We excavate that reinforced zone to the design line, stockpile what will be reused and haul away what will not. Where a cut is deep enough that a worker will stand in it beyond 1.5 m, the bank is sloped to the OHS Code angle for the soil or supported. If the wall builder wants machine support as courses go up, we can return to place and compact backfill and drainage stone in lifts on their schedule.

Missing or blocked drainage is a common reason walls fail, so we treat it as part of the base, not an extra. Perforated drain pipe is set at the back of the base with a fall to outlets at the ends or low points, and clear drainage stone goes in behind the wall face as the design calls for, so water drains down and out rather than building pressure. The water has to go somewhere sensible: to daylight away from the wall, and never onto a neighbour's lot. At the top and toe of the wall, grades are shaped so surface water runs away from the house, tying into the same rules that apply to foundation drainage.

What Decides the Specification

The wall type sets the base. Segmental block walls follow the manufacturer's installation guide or an engineer's drawing for pad thickness, embedment depth and geogrid length. Timber walls commonly need trenches for tiebacks or deadmen running back into the slope. Boulder walls carry far heavier individual pieces, so the bottom row is typically seated in an excavated, compacted base rather than on the surface. Height changes everything: the taller the wall, and the more it carries above it, such as a driveway, parking area, slope or building, the deeper the embedment, the longer the grid and the more likely an engineer must design it. We do not invent those numbers on site. They come from the design.

The soil sets how hard drainage and compaction have to work. Clay holds water and heaves when it freezes, and Calgary frost typically reaches around 2 m (7 ft) and reached 2.4 m (8 ft) in the severe winter of 2019. A wall base is typically not placed below that depth, so drainage stone, pipe and a real outlet are what keep ice and water pressure off the wall. Where the design does not set lifts, the City's roads specification is a sensible benchmark: granular layers no thicker than 300 mm under rollers, hand-compacted layers no thicker than 200 mm loose, and no thicker than 100 mm in the top 600 mm. Frozen or saturated soil is never used under or behind a wall.

Around Calgary

Outside the city, sloped lots are where walls become hard to avoid. Cochrane's Rivercrest area plan describes land immediately south of the Bow River with post-glacial kettle topography, moderately steep slopes of 15 to 20 degrees and about 50 m of relief. On lots cut into ground like that, a retaining wall is often the only way to create a level yard, a driveway pad or a walkout grade, and the cut-back behind a gridded wall can be a large excavation in its own right. The Town of Cochrane issues its own permits, so wall heights and engineering requirements are confirmed with the Town rather than assumed from Calgary's rules.

Inside Calgary, the same work shows up on a smaller footprint. In inner-city communities such as Altadore and South Calgary, a new infill house can be built to design grades that do not match the older yard next door, and the difference ends up held by a wall along the side or rear property line. Those walls sit in narrow side yards, beside fences, beside neighbours' garages and within reach of both houses' foundations, so the base trench is dug with small equipment and careful spoil handling. The City's combined-height rule matters here: a low wall with a raised garden box within 1 m of it can add up to 1.2 m and trigger a permit.