Retaining Walls in Manitoba: Why They Fail and How to Build One That Holds

Oct 22

7 min read

A retaining wall in Manitoba fails for one of three reasons: water built up behind it, the base under it moved, or it was never built to hold back as much as it is holding back. Height and appearance are what people choose. Drainage, base preparation and the material behind the wall are what decide whether it is still straight in ten years. A wall that leans, bulges or steps apart is telling you about water, not about blocks.

Retaining walls look like a landscaping product and behave like a structure. That gap is where most of the trouble comes from.

Somebody buys a pallet of blocks, levels a row on the existing ground, stacks them up, and backfills with the clay that came out of the hole. It looks good the day it is finished. Then the first spring arrives, the clay behind the wall takes on water and swells, the ground under the first course heaves, and the wall starts to lean out at the top.

None of that is a product failure. Every one of those walls failed at something that was invisible once the job was done.

If you are planning a wall, or looking at one that has already gone out of line, here is what actually governs it.

At a Glance

Water pressure behind a wall is the primary cause of failure here
Drainage stone and a drain at the base are not optional extras
Never backfill a retaining wall with the clay you excavated
The base course sits below grade on compacted granular fill, and it must be level
Frost acts on the base and on saturated backfill both
Taller walls, sloped ground above, and anything carrying load nearby change the design entirely
Terracing two shorter walls is often better than building one tall one

Why Do Retaining Walls Fail in Manitoba?

Because of what is behind them and underneath them, in that order.

A retaining wall is holding back soil, and soil that is full of water is much heavier and pushes much harder than soil that drains. Clay is the problem material here. Much of Winnipeg and southern Manitoba sits on heavy clay that takes on water, swells, and holds it. Backfill a wall with that clay and every wet period loads the wall further, every freeze expands it, and the wall has nothing to push back with.

The result is the classic failure you can see all over the city. The top of the wall leans outward, the courses step apart, and the wall bulges in the middle where the pressure is highest.

The second failure is the base. A wall stacked on topsoil or on unprepared ground has nothing stable to stand on. Frost lifts it unevenly, and the whole wall goes out of level, which puts the courses out of alignment and lets the pressure behind do the rest. Our post on why concrete cracks in Manitoba and what actually prevents it describes the same base preparation principle in a different application, and it transfers exactly.

The third is a design problem. A wall built for a low garden bed will not hold a four-foot cut, and a wall with a slope above it or a driveway near the top is carrying load that a simple stacked wall was never designed for.

What Does Proper Drainage Behind a Wall Look Like?

The whole point is to never let water build up in the first place.

Free-draining granular backfill directly behind the wall, not the excavated clay, in a zone wide enough to actually work
A perforated drain pipe at the base of that zone, laid so it slopes to an outlet, with an outlet that actually daylights somewhere useful
Geotextile separating the drainage stone from the surrounding soil so the soil does not migrate into the stone and plug it
The surface above the wall graded so water runs away from the top rather than into the backfill
Downspouts and any yard drainage directed well clear of the wall, not over the top of it

The drain outlet is the part that quietly gets skipped. A drain pipe that has nowhere to discharge is just a pipe full of water. It needs an exit, and it needs one that will not be buried by future landscaping.

If your yard already has water problems, solve them before you build the wall rather than after. Our post on why Winnipeg backyards flood every spring covers the wider drainage picture.

How Is the Base Actually Built?

Below grade, on compacted granular fill, and dead level.

The first course sits below the finished ground level in front of the wall, which does two things. It puts the bottom of the wall into the ground so it cannot simply slide forward, and it means the soil in front is holding the base rather than the base holding itself.

Under that course goes a layer of compacted granular fill. Not sand, not the native soil, and not a layer dumped and driven over once. Compacted in lifts, thick enough for the wall and the soil conditions, and levelled carefully.

Level matters more on the base course than anywhere else. Every error in the first course multiplies as the wall goes up, and the courses above are relying on full contact with the ones below. A base that is out by a small amount at the bottom is out by a large amount at the top.

When Does a Wall Need to Be Engineered?

When it gets tall, when there is a slope above it, or when there is load near it.

Low walls that are holding a garden bed or a small grade change are a landscaping job. Beyond a certain height, and the threshold varies by jurisdiction, a retaining wall becomes a structure and needs proper design, which may include reinforcement extending back into the retained soil, a different block system, or a poured wall.

The conditions that push a wall into that category even at modest height:

Ground that slopes up behind the wall, which adds significantly to the load
A driveway, parking area or structure near the top of the wall
Poor or saturated soil conditions
Anything holding back a slope that a building sits on
Water flowing toward or across the wall location

Terracing is the underrated answer. Two shorter walls with a bench between them often do the same job as one tall wall with far less load on each, and they are usually easier to build well and to maintain. The catch is that the benches need to be wide enough to actually separate the loads rather than stacking them, which is a design question rather than a taste question.

Requirements for what needs approval vary by municipality, and your contractor should confirm what applies to your property before design gets far.

Can a Failing Wall Be Repaired?

Sometimes, and the honest answer depends on why it failed.

If a wall has gone out of level because the base moved, it usually has to come apart. You cannot correct a base problem from the top. The blocks may well be reusable, which makes rebuilding less painful than it sounds, but the wall is coming down.

If a wall is leaning because of water pressure, the fix involves opening up the backfill, installing the drainage that should have been there, and resetting whatever has moved. Again, mostly a rebuild of the back of the wall.

If a wall has isolated damage, a failed section, or has lost a few blocks, targeted repair is often reasonable.

What does not work is any attempt to fix a leaning wall by pushing it back, pinning it, or adding weight to the top. The pressure that moved it is still there, and it has not changed its mind.

Real Scenarios

Your wall leans out at the top, and the joints have opened up. That is water pressure. The wall will keep going. The question is whether to rebuild it properly now or manage it until it fails.

You want a wall to level out a sloped yard for a patio or a play area. Good use for one, and this is the case where planning the wall, the drainage, the hard surface and the grading as one project pays off rather than doing them in sequence and hoping they agree.

You are building a wall near the property line. Confirm what is allowed where, and think carefully about where the drainage discharges, because a drain that puts water onto a neighbour's property creates a different kind of problem.

You are doing a full yard after a new build. This is the ideal case, because the grade is not yet set and everything can be designed to work together. That is the approach behind our post on a new build full yard package with sod, fencing and a deck.

You inherited a wall with the house and have no idea how it was built. Look at whether it is straight, whether the courses are in line, and whether anything is growing out of the face. Then look at what is above it and what would be affected if it moved.

FAQ

How tall can a retaining wall be before it needs engineering?

The threshold varies by municipality and by the system being used, and slope and surcharge above the wall can lower it considerably. Your contractor should confirm what applies to your site.

Can I use the soil I dug out as backfill?

Not directly behind the wall. Clay backfill is the most common cause of wall failure here. Free draining granular material goes in that zone, and the excavated material can be used further back or elsewhere.

Do timber retaining walls work in Manitoba?

They are used, and they can work when built with proper drainage and base preparation. The limiting factor is the life of the timber in contact with wet soil rather than the design.

What time of year should a retaining wall be built?

The open ground season. Excavation, base compaction and drainage all require workable ground, which is the same constraint we describe in our post on what construction work can still get done through a Manitoba winter.

Why is my wall bulging in the middle rather than leaning?

Because the pressure behind a wall is highest at the lower middle portion. A bulge is a clear water pressure signal.

Will a wall stop my yard from eroding?

It can be part of the answer, but erosion is a water movement problem. If the water is not being managed, a wall just gives it something to work on.

Ready to Build One That Stays Where You Put It?

Flip Flop Construction builds retaining walls, grading, drainage and full yard work across Winnipeg, Steinbach, Kleefeld, Grand Beach and surrounding Manitoba communities, along with Red Lake, Kenora and Dryden in Northern Ontario. Excavation, base preparation, drainage, concrete and the landscaping around it are handled by the same crew, which is how the wall ends up designed around the water instead of in spite of it.

Have a look at our landscaping work, see past projects in the portfolio, and get in touch before the grade is set.

431-777-5219 flopconstruction@gmail.com flipflopconstructioninc.com One crew, every trade, no excuses.