Retaining wall under construction showing base preparation and drainage stone

Retaining walls · 7 min read

Why retaining walls lean, bulge and fail

Almost every failed wall we are called to has the same three causes. None of them is the block.

Introduction

We get called out to failing walls fairly regularly, and there is a pattern. The blocks are almost never the problem. What fails is what went in behind and beneath them — and by the time a wall is visibly leaning, the cause has usually been at work for years.

(01)

Cause one: water had nowhere to go

This is the big one. Soil holding water is dramatically heavier than dry soil, and that weight pushes horizontally against the back of the wall. It is called hydrostatic pressure, and it is what takes walls down.

A wall built properly has an escape route designed in: clean crushed stone behind the block rather than backfilled soil, a perforated pipe at the base, and an outlet where that water actually leaves. Fabric between the stone and the soil so the stone does not silt up over time.

A wall built without it is a dam. It will hold for a while. Then a wet winter arrives and it does not.

Clean stone backfill, not soil, behind the wall
Perforated drain pipe at the base of the stone
A real outlet — daylight or a drainage system
Filter fabric so fines do not clog the stone

(02)

Cause two: the base was not good enough

A wall is only as level as the trench it started in. That first course has to sit on compacted aggregate, buried below grade, and it has to be dead level. Every course above amplifies whatever the first course got wrong.

Walls that were started on soil, or on a base that was not compacted, settle unevenly. You see it as a dip along the top of the wall, or as courses that no longer line up.

(03)

Cause three: no reinforcement where it was needed

A gravity wall holds by mass alone — its own weight against the soil. That works to a point, and past that point it does not.

Where the wall is taller, or where there is a slope above it, or a driveway, or anything else adding load, the wall needs to be tied back into the soil it is holding. That is what Geo-Grid does: layers of reinforcement laid back into the fill as the wall goes up, turning wall and soil into a single mass.

Skipping it does not cause an immediate failure. It causes a slow forward lean that becomes obvious years later. We cover the two types in more detail on our <a href="/retaining-walls">retaining wall page</a>.

(04)

How to read your own wall

You do not need to be a contractor to spot a wall in trouble. Walk it and look for these.

A top course that is no longer straight when you sight along it
Blocks tipping forward, especially near the middle of a run
Gaps opening between blocks that used to be tight
Soil, mulch or water seeping through the face
Ground behind the wall dropping or cracking away from it
Water standing behind the wall after rain and staying there

(05)

Repair or rebuild?

The honest answer depends on the cause, and this is where homeowners are most often sold the wrong thing.

If the wall is leaning because it has no drainage and no reinforcement, resetting the face does not fix anything. The cause is still behind it, and you will be looking at the same lean again. That is a rebuild.

If a wall is fundamentally sound and has settled in one localised area — a washout at one end, a failed outlet — a targeted repair can be genuinely worthwhile.

Our position when we look at one: we will tell you which of the two you have, even when the answer is the cheaper one.

(06) Questions

Asked often.

How long should a retaining wall last? +

A properly built segmental wall with drainage and appropriate reinforcement should last decades. Walls that fail early almost always failed on water or base, not on materials.

Does every retaining wall need a permit? +

Not every wall, but taller walls and walls carrying additional load commonly require engineered drawings and permitting. We assess this at the site visit and tell you which side of the line your project sits on.

My wall is leaning slightly. Is it urgent? +

Movement means something is acting on it, and that something rarely stops on its own. It is worth having looked at before a wet season rather than after one.

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