How many ways can a retaining wall fail, and what FS does the law require?
Retaining walls do not fail in just one way. There are 3 main failure mechanisms, and Clause 31 of the 2023 Foundation Ministerial Regulation sets a separate minimum factor of safety for each of them for retaining walls with a base. The wall must pass all three, not just one. The forces acting on the wall are to be calculated as prescribed by the Minister (Clause 31, paragraph one):
1. Sliding — FS ≥ 1.50
Earth pressure behind the wall pushes it forward. Resistance comes from friction under the base and passive resistance of the soil in front of the wall. If the soil under the base is soft or the groundwater is high, resistance drops sharply. The ratio of resisting force to driving force must be not less than 1.50.
2. Overturning — FS ≥ 2.00
Earth pressure creates a moment that rotates the wall about the front edge of the base. The self-weight of the wall and the soil bearing on the base resist it. The taller the wall, the faster the overturning moment grows. The ratio of resisting moment to overturning moment must be not less than 2.00.
3. Bearing capacity of the foundation soil — FS ≥ 3.00
The wall weight and earth pressure combine to press eccentrically on the soil under the base, so the front edge of the base takes the highest stress. If the soil under the base cannot carry it, the wall will settle and tilt even without sliding or overturning. This criterion is consistent with the FS = 3 principle for ordinary spread footings (also related to the Plate Bearing Test condition; read When Does a Plate Bearing Test Become Mandatory?)
← Scroll to see the full table →
| Check | Minimum factor of safety under the 2023 Ministerial Regulation (Clause 31) |
|---|---|
| Sliding | 1.50 |
| Overturning | 2.00 |
| Bearing capacity of foundation soil | 3.00 |
These criteria are part of the Ministerial Regulation Prescribing Building Foundations and Ground Supporting Buildings B.E. 2566 (2023), which covers everything from soil investigation, spread footings and piles to retaining walls — for an overview of the whole regulation, see The 2023 Foundation Ministerial Regulation Simply Explained
What soil data do you need before designing a retaining wall?
A key point many people overlook: none of the three FS values can be calculated without knowing the actual soil properties — of the soil behind the wall and under the base — and the groundwater level:
← Scroll to see the full table →
| Check | Soil parameters needed | Obtained from |
|---|---|---|
| Lateral earth pressure | Unit weight, internal friction angle (φ), cohesion (c) | Soil boring + laboratory testing |
| Sliding | Base friction, properties of soil under the base | Soil boring + SPT + shear strength testing |
| Bearing capacity | Bearing capacity of soil under the base | Soil boring, SPT, or Plate Bearing Test |
| Effect of water | Groundwater level, seasonal variation | Boreholes / observation wells |
Water pressure behind a wall that cannot drain fast enough adds enormous thrust, far beyond what the design allowed for. Many retaining wall failures happen after heavy rain. Knowing the groundwater level from a soil investigation and designing the drainage behind the wall are therefore as important as the structure itself. Read more in How Groundwater Affects Foundations
For soft clay layers, shear strength can be measured directly in the field with a Field Vane Shear Test, together with SPT results and laboratory testing, to make the design parameters as reliable as possible.
Which projects need a retaining wall FS check?
The three FS criteria in Clause 31 are written for retaining walls with a base, such as gravity walls and cantilever walls. Systems without a base, such as sheet piles, use other checks such as embedment depth and anchor forces (compare each type in Types of Retaining Walls and How to Choose One). Common examples:
- Retaining walls along roads or between land at different levels — in housing estates or hillside homes
- Basement walls of buildings — carrying both earth pressure and permanent water pressure
- Riverbank and canal-bank walls — facing water-level changes on both sides throughout the year
- High fills close to a neighboring property line — many neighbor disputes over slumping fill start with a retaining wall that was not engineered properly
Not sure whether your job needs an engineer to design it? Read the criteria in Building a Retaining Wall: When Is an Engineer Required? And if you are comparing construction systems, see the cost factors in Precast vs Cast-in-Place Retaining Walls: What Drives the Price
How do you start a retaining wall project legally? Soil boring → design → FS check
- Soil investigation along the wall alignment — collect soil layer data, SPT values, samples for laboratory testing, and the groundwater level
- Engineer designs from actual soil data — sets the wall section and drainage system and checks that all three FS values meet the criteria
- Prepare design calculations that trace the source of the soil values — for permit applications or confirmation with the parties involved
SPN covers both stages: soil investigation and testing and geotechnical engineering design, which includes retaining structures — so design is based on the site's actual soil data, all in one place.
The 3D view below links the soil data to the three FS checks: soil boring along the wall alignment, SPT testing and sampling to determine γ, φ, c, establishing the groundwater level, and then calculating FS against the minimum criteria.
Summary: FS must pass all 3 checks and be calculated from actual soil
The 2023 Foundation Ministerial Regulation clearly sets the safety criteria for retaining walls: sliding (1.50), overturning (2.00) and bearing capacity of the foundation soil (3.00), with every value calculated from actual soil properties. A safe and legally compliant retaining wall therefore starts with a soil investigation, followed by design by an engineer who has actually seen the soil data for that site — not an off-the-shelf design copied from elsewhere.
This article summarizes the Ministerial Regulation for general understanding only. It is not legal or engineering advice for any specific project. For practical use, refer to the official text of the Ministerial Regulation as published in the Royal Gazette and consult the licensed engineer for your project.