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How walls fail

How retaining walls fail

A retaining wall can slide, tip, sink, or get pushed out by water. Those are checks on the wall itself. The one that gets missed is global stability: the wall holds together and the ground it sits in moves.

The ways a wall fails

Short animations, exaggerated so they are easy to see. Every project is screened for all of these.

SlidingThe soil pushes the wall sideways and friction under the base holds it back. If the push wins, the wall slides out along its base.
OverturningThe soil behind the wall pushes hardest near the bottom. If that push beats the weight of the wall, the wall tips forward over its toe.
Bearing and settlementThe wall presses down hardest under its toe. If the ground there is soft or overloaded, the wall sinks and leans.
Water pressureWith the drainage clogged or missing, water builds up behind the wall and adds its own push on top of the soil.
Block walls with geogridThe grid can pull out of the soil or tear, and the face bulges or the top comes off.
Global failureThe wall stays in one piece, but the ground gives way along a deep curve under it and everything above that curve moves out together.

Global stability

The wall passes. The ground doesn’t.

The checks above are about the wall. Global stability is about the hillside the wall sits in. The analysis looks at deep, curved surfaces that run behind the wall, under it and out through the slope below, and asks one question of each: does the soil along that curve hold, or does everything above it slide?

When it doesn’t hold, the wall can be in perfect shape and still end up at the bottom of the slope, with the yard, the fence and the pool deck going with it.

400+global stability analyses since 2015, on tall, tiered and water-side walls across Texas.

What global stability checksA gravity retaining wall at the top of a steep slope, with a pool or drive behind it. A deep curved slip surface runs behind the wall, under its base and out through the slope below. Everything above the curve moves as one block. The analysis cuts the block into slices and compares the weight pushing each slice with the soil strength holding it back.Pool, drive or houseWall, soil and yardmove as one blockSteep slopebelow the wallSlip surfaceSoil below stays put
Gray arrows: the weight pushing each slice down the slope. Orange arrows: the soil strength along the curve holding it back. The analysis tries many curves and reports the weakest one.

Global stability in under two minutes

What it is and when to ask for it.

When a job needs one

Not every wall needs a full analysis. These are the conditions where we recommend one.

Steep slope belowGround that drops away in front of the wall, especially down to a pond, creek or channel.
Tiered wallsWalls stepped up a hill closer together than about twice the lower wall’s height act like one tall wall.
Tall wallsMore height means deeper slip surfaces and more load on the ground under the wall.
WaterDetention ponds, creeks, swales and high groundwater. Saturated soil can cut the safety margin by as much as half.
Loads near the topPools, houses, driveways, parking and fire lanes close to the top of the wall.
Soft or weak soilUndocumented fill, or a geotechnical report that shows soft layers deeper down.
Tiered walls on one slip surfaceTwo gravity walls stepped up a slope. The upper wall sits closer to the lower wall than twice the lower wall height, so they load each other. One deep slip surface runs under both walls and the ground between them, and they move out together.HDD less than 2H:the tiers load eachother and act asone tall wallUpper wallLower wallOne slip surfaceunder both tiers
Tiered walls · closer than 2H, the tiers share one slip surface
What tips the balance toward a global failureA wall at the top of a slope with four conditions marked: 1, a steep slope below the wall; 2, a load near the top; 3, water in the soil; 4, a soft layer deeper down.1234
What tips the balance
  1. 1Steep slope below the wall
  2. 2Load near the top
  3. 3Water in the soil
  4. 4Soft layer deeper down

Why it gets missed

Standard wall calculations stop at the wall and its footing. Wall plans often list global stability as “by others,” and on many jobs nobody else picks it up.

And there is nothing to see until it moves. The slip surface is underground, so the first signs show up in the yard, often after a wet season. By then the repair can cost many times what the analysis would have.

Warning signs of a global failureExaggerated drawing of a wall starting to move: 1, a crack or step in the yard behind the wall, running parallel to it; 2, a fence or flatwork tilting and gaps opening; 3, the wall leaning or moving out in one piece; 4, the slope face below bulging and staying wet.1234Dashed: where the ground was
Warning signs
  1. 1Crack or step in the yard, running parallel to the wall
  2. 2Fence or flatwork tilting, gaps opening
  3. 3Wall leaning or moving out in one piece
  4. 4Slope face below bulging and staying wet

Running it right

A global stability run is only as good as what goes into it. These are what make the answer real.

  1. Soil from the boringsStrengths and layers from the geotechnical report, extended across the site the way the borings show them, not one assumed soil.
  2. Water where it will beGroundwater, pond levels and wet-season conditions, not a dry day.
  3. Real loads in real placesPools, houses, drives, fire lanes and upper walls where they actually sit, without stacking every worst case on top of each other.
  4. The whole slopeSurfaces behind, under and in front of the wall, through the slope below, with tiers checked together.
  5. Water lines and sprinklersUtility and irrigation plans checked against each wall, so the wet case sits where a leak would actually put water.
  6. A margin on every sectionA target factor of safety met at each section checked, and any fix checked by the same analysis.

Water lines and sprinklers

Know where the water comes from.

Saturated soil is one of the most common triggers for a global failure, and a lot of that water never fell as rain. It comes from a leaking water line or a sprinkler system running at the top of the wall.

So the review doesn’t stop at the grading plan. We check the civil utility plans and the landscape irrigation plan against the wall layout, find the lines that sit in the ground a slip surface would move through, and check those sections with the soil wet around the line.

You get a recommended utility setback for each wall and a plan exhibit showing where lines fall inside it. Where a line can move, moving it is often cheaper than deepening a footing. Where it can’t, the plans call for sleeves, trench plugs, a shutoff on the high side or drip irrigation near the wall.

Utility setback zone along a retaining wallPlan view of a retaining wall along the back of a row of lots, with a pond below it. A shaded band behind the wall marks the utility setback zone, wider where the wall is taller, with a smaller band in front of the wall out to the slope below. 1, the setback zone; 2, a water service line that runs into the zone near the wall; 3, sprinkler heads along the top of the wall; 4, a water main kept outside the zone; 5, a line that has to cross the wall, sleeved with a shutoff on the high side.12345
Utility setback zone, plan view
  1. 1Setback zone, wider where the wall is taller
  2. 2Water line inside the zone
  3. 3Sprinklers along the top of the wall
  4. 4Line kept outside the zone
  5. 5Line crossing the wall: sleeve and shutoff

If it comes up short

Common fixes are a deeper footing, a wider base, a flatter slope, more room between tiers, or better drainage. Each one is checked by the same analysis before it goes on the plans.

A deeper footing works when the weakest surface runs just under the base: setting the base below it forces any slip to go deeper and longer, through more ground that can hold it. How deep comes from the analysis, not a rule of thumb.

What you get

A sealed global stability analysis with the factor of safety for each section, for walls we design and for walls designed by others, plus fix options where a section comes up short. When the utility and irrigation plans are provided, it includes a utility setback exhibit and setback schedule for each wall.

Deepened footing on a steep slopeTwo panels. Left: a wall with a shallow footing at the top of a steep slope; the weakest slip surface runs just under its base. Right: the same wall with the footing set deeper, below that surface; the next slip surface has to go deeper and longer, which gives more ground to hold it.Shallow footingweak surface runsjust under itDeepened footingset below it; the nextsurface is deeper
Deepened footing · one option, checked by the analysis

Not sure if your job needs it?

We can help.

Send the grading plan, geotechnical report, and utility and irrigation plans if you have them, and we will tell you whether a global stability check makes sense for your walls.

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Drawings and animations are simplified and exaggerated for illustration. Every site gets its own analysis. Utility setbacks are based on the plans reviewed and reduce the likelihood of saturated soil in the slip zone; they do not replace leak repair or good irrigation practice.

Plans, photos or just an address. Send what you have for a proposal.

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