Why do retaining walls in Bangkok so often tilt, crack or fail?
The main cause is Bangkok Soft Clay, a low-strength layer about 12–18 m thick that blankets Bangkok and its vicinity, with low bearing capacity and high compressibility. When fill is placed at a different level for a boundary fence, a property line or raising the site, two effects occur at the same time:
- Settlement — the weight of the fill compresses the soft clay layer
- Lateral earth pressure — the fill pushes against the wall
If the retaining system is not properly designed, the wall will gradually tilt, crack or collapse within a few years of construction, and repairs usually cost many times more than designing it correctly from the start.
The thickness and strength of the soft clay layer differ from plot to plot, so retaining walls on soft soil cannot be guessed. You must know the actual soil profile from a soil investigation first, then have an engineer design a system specifically for that plot. Read more on soft-soil basics at Bangkok soft clay and foundations
How high can fill be before an engineer should design the retaining wall?
There is no fixed number, but from design experience on Bangkok soft clay, once the fill height difference reaches about 1.5–2 m or more, lateral earth pressure becomes high enough that simple walls or a single row of piles usually cannot hold it, and a double-pile system or a stronger wall is needed.
Whatever the height, a permanent retaining wall must be checked for all 3 stability modes — overturning, sliding and soil bearing capacity — as required by law. Read the details in Retaining wall safety factors under the Ministerial Regulation B.E. 2566 (2023) and How high can a retaining wall be before an engineer must design it? For a comparison of wall types, see Types of retaining walls
What does SPN's retaining wall design service cover?
It covers 5 parts, from soil investigation to material quantity take-off, as part of SPN's geotechnical engineering design, all done by one team (SPN carries out investigation, testing and design only; no construction work).
- Soil investigation and testing — collecting actual soil profile data for your site, with laboratory test results and an engineer-certified report. See details at Soil investigation services
- Stability and settlement analysis — modeling soil–structure behavior with finite elements (PLAXIS) following the actual construction sequence
- Wall system selection and design — single cantilever piles, King & Anchor double piles, RC walls, or lightweight fill to reduce load, depending on fill height, soil conditions and budget
- Construction drawings and calculations — certified by a civil engineer licensed by the Council of Engineers Thailand (COE), with signature and license number, suitable for supporting building permit applications to the relevant authorities
- Bill of quantities (BOQ) — a BIM model is built to take off pile, concrete and reinforcement quantities, giving budget figures close to the actual work
How to engage SPN for retaining wall design
The design work has 5 steps, from sending preliminary information to receiving construction drawings with calculations.
- Contact us and send preliminary information — site location, approximate fill height and wall length. Receive a free quotation within 24 hours.
- On-site soil investigation — the team drills, collects soil samples and tests them in the laboratory
- Analysis and design — the engineer selects the wall system and analyzes stability and settlement from actual soil data
- Delivery of construction drawings and calculations — signed by a licensed (COE) engineer, with a BOQ for contractor tendering
- Follow-on services during construction (optional) — pile testing and instrumentation to monitor actual movement
Real project example: retaining walls for 2.00 m and 2.50 m fill on 18 m of soft clay
This example is a boundary fence and retaining wall project in the Bangkok metropolitan area, where the soil investigation found a continuous soft clay layer about 18 m thick. The most demanding sections were the 2.00 m and 2.50 m high fill lines, where the earth pressure was too high for a single row of piles.
How does a King & Anchor Pile system work?
King & Anchor Pile is the double-pile retaining wall system the engineer chose for this project. Each pair divides the work as follows:
- King Pile — stands at the front and resists the earth pressure
- Anchor Pile — installed behind, holding back the head of the king pile
As a result, the system resists more load than a single cantilever pile without having to enlarge the section excessively. For the 2.50 m fill case only, lightweight fill (rice husk ash) also replaced part of the ordinary soil fill to reduce both the surcharge on the soft clay and the lateral pressure at the same time — the same approach as soft soil improvement methods.
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| Fill height | Retaining wall system | Pile | Length | Spacing | Remarks |
|---|---|---|---|---|---|
| 2.50 m | King & Anchor Pile | I-350×350 | 18 m | 1.125 m | Partly filled with lightweight material (rice husk ash) |
| 2.00 m | King & Anchor Pile | I-300×300 | 15 m | 1.50 m | — |
With fill just 50 cm higher, the system had to be heavier in pile size, length and closer spacing. This is why retaining walls must be designed from each project's actual soil data — there is no one-size-fits-all formula.
How is stability analyzed with PLAXIS 3D?
The engineer models the whole system in PLAXIS 3D, a finite element (Finite Element Method, FEM) program, using staged construction, i.e. simulating each step in the actual construction sequence: original soil → pile driving → soil or lightweight fill → casting the fence → applying surcharge, wind load and water level.
Stability is then checked against the F.S. ≥ 1.50 criterion, a commonly used stability criterion for retaining structures/slopes in geotechnical engineering, subject to each project's specifications (the project's filling and compaction works reference DPT 1918-62, the standard for earth filling and compaction issued by the Department of Public Works and Town & Country Planning).
What were the design results?
Both cases met the F.S. ≥ 1.50 stability criterion, with F.S. 1.52 (2.50 m fill) and 1.55 (2.00 m fill), summarized below:
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| Case | Max. lateral displacement | Max. settlement | F.S. | Criteria | Result |
|---|---|---|---|---|---|
| Fill 2.50 m | 17.56 cm | 36.66 cm | 1.52 | ≥ 1.50 | Pass |
| Fill 2.00 m | 15.44 cm | 29.49 cm | 1.55 | ≥ 1.50 | Pass |
The key point to understand is that passing stability does not mean no settlement. Although both F.S. values meet the criterion, maximum settlement is still 29.49–36.66 cm, which is the nature of thick soft clay. The construction drawings therefore have to allow for the service level and design joints that can move with the settlement without cracking.
What the client received from this project: construction drawings with calculations certified by a civil engineer licensed by the Council of Engineers Thailand (COE), with signature and license number, plus a BOQ of material quantities taken off from the BIM model. The owner could therefore compare contractor prices immediately and reduce the risk of the wall repeating the tilting and cracking commonly seen on soft soil.
How much does retaining wall design cost?
There is no fixed standard price for retaining wall design, because it depends on wall length, fill height and the complexity of the soil profile. SPN offers a free cost estimate and quotation within 24 hours — just send the project location, approximate fill height and wall length to the engineering team.
For soil investigation, the essential first step, fees start at about THB 20,000–40,000 (2–3 boreholes, including basic lab tests and an engineer-certified report). This price excludes travel and equipment mobilization, which are charged extra according to the distance to the site.
Summary: why should an engineer design retaining walls on soft clay?
Because retaining walls on the soft clay of Bangkok and its vicinity cannot be built from standard drawings: soft soil differs in thickness and bearing capacity from area to area. The correct process is soil investigation → select the wall system for the fill height → analyze stability with FEM → design and certification by a licensed (COE) engineer, as shown in this case study.
Note that all figures in this article (pile size and length, spacing, displacements and F.S.) are design results specific to this project and its actual site soil profile, and cannot be applied directly to other projects. Every filling and retaining wall project needs its own soil investigation and a new design by the responsible engineer.
SPN Soil Engineering provides services from soil investigation to geotechnical engineering design (retaining walls, foundations, ground improvement, slope stability), with over 20 years of experience and more than 500 projects, serving Bangkok, its vicinity and all 77 provinces nationwide. Consult us about your project for free.