This article explains what makes Bangkok soil special, how it differs from other regions, and how this affects foundation selection from an engineering perspective, with a diagram comparing soil profiles in three regions and a 3D model of the effects of land subsidence. All figures are typical values for illustration, not design values for any specific project.

What Is Bangkok Clay, and Why Do Geotechnical Engineers Worldwide Know It?

The soil profile of Bangkok and its surrounding provinces is known in international geotechnical engineering as "Bangkok Clay", recognized as one of the examples of coastal soft clay (Marine Clay) that are the deepest and most complex in Southeast Asia.

Soil investigation for a condominium project in Bangkok on the Bangkok Clay soft clay layer
Soil investigation for a condominium project in Bangkok, built on a soft clay layer (Photo: SPN Soil Engineering)
📚 Technical background

Bangkok Clay is a marine clay deposited in a shallow-sea to estuarine environment on the Chao Phraya floodplain during the Holocene marine transgression. It is a soft to medium stiff clay with a high Plasticity Index.

What Are the Soil Layers Under Bangkok, from Top to Bottom?

Inner Bangkok soil typically starts with topsoil or fill about 2 m thick, followed by soft clay to about 15 m depth, then medium stiff clay, and the first sand layer at about 21–28 m. Actual layer depths vary by location, so every project must confirm them with a soil investigation.

← Scroll to see the full table →

Soil layerDepthCharacteristicsSPT N-Value
Weathered Crust0-2 mFill / desiccated clay5-15
Soft Bangkok Clay2-15 mGray soft clay0-4
Medium Stiff Clay15-21 mMedium stiff clay5-15
1st Sand Layer21-28 mFirst dense sand layer30-50+
Stiff Clay28-35 mStiff clay15-30
2nd Sand Layer35-50+ mSecond dense sand layer50+

Special Properties of Soft Bangkok Clay

  • High water content of 60-100%, meaning the soil holds more water than dry solids by weight
  • Low undrained shear strength (Su) of 10-25 kPa, so it resists little shear
  • High compressibility, so it compresses easily under load
  • Sensitive clay, which becomes several times weaker once disturbed

How Do Soil Profiles in Other Regions Differ from Bangkok?

1. The North (Chiang Mai, Chiang Rai, Lampang)

Mostly old alluvium (Old Alluvial) and weathered rock:

  • Surface soils are mostly medium to stiff clay
  • Rock is found close to the surface (5-15 m) in many areas
  • Piles are much shorter; 8-15 m lengths are sufficient
  • Some areas can use spread footings directly

2. The Northeast (Khorat, Khon Kaen, Udon Thani)

Sandy to loamy soils mixed with gravel in the upper layers:

  • Upper soils have good bearing capacity; SPT N-values often exceed 10 from the surface
  • The main issue is expansive soil through the wet and dry seasons
  • Piles 6-12 m long in most areas

3. The South (Songkhla, Phuket, Krabi)

Highly variable, depending on terrain:

  • Coastal areas: soft clay similar to Bangkok Clay but thinner (5-10 m)
  • Hills: residual soil with granite, very good bearing capacity
  • Low-lying plains: watch for high groundwater in the rainy season

4. The Bangkok Metropolitan Region (Samut Prakan, Nonthaburi, Pathum Thani, Ayutthaya)

Similar to Bangkok, but with local features:

  • Samut Prakan: thicker soft clay than Bangkok, about 18-22 m, so typical piles are often about 25-28 m long
  • Western Nonthaburi: soft clay 12-18 m, piles 21-26 m
  • Pathum Thani: soft clay thins toward the north, 8-15 m
  • Ayutthaya: soft clay interbedded with old sand layers, requiring detailed soil boring

The figure below places the Bangkok soil profile side by side with the North and the Northeast at the same depth scale, making it immediately clear why pile lengths differ several-fold.

2D model comparing typical soil profiles and foundations in Bangkok, the North and the Northeast Two-dimensional section comparing soil profiles in three areas at the same depth scale, zero to forty meters. Inner Bangkok: topsoil or fill from zero to two meters, soft clay from two to fifteen meters with N of zero to four, medium stiff clay from fifteen to twenty-one meters, the first sand layer from twenty-one to twenty-eight meters, stiff clay from twenty-eight to thirty-five meters, and the second sand layer below thirty-five meters; houses typically use piles twenty-one to twenty-five meters long. The North, e.g. Chiang Mai: medium to stiff clay at the surface, with weathered rock at five to fifteen meters in many areas; spread footings or short piles of eight to fifteen meters are used. The Northeast, e.g. Khorat: upper layers of sand or loam with gravel, N often above ten from the surface; spread footings can be used in many areas, or piles of six to twelve meters, with care needed for expansive soil. Figures are typical values for illustration, not design values. 2D model: Bangkok vs North vs Northeast soil profiles Typical values, not for design 0 5 10 15 20 25 30 35 40 ← Depth (m) Topsoil/fill · N 5–15 Soft clay · N 0–4 Soft Bangkok Clay Medium stiff clay · N 5–15 1st sand · N 30–50+ Stiff clay · N 15–30 2nd sand · N 50+ (to 50+ m) Pile 21–25 m Inner Bangkok Soft clay approx. 13 m Medium–stiff clay Weathered rock Often at 5–15 m Pile 8–15 m Spread footing North (Chiang Mai) Shallow weathered rock Sand/loam with gravel N often >10 from surface Denser with depth Pile 6–12 m Northeast (Khorat) Expansive soil in places Spread footing 1 Bangkok: long piles Through soft clay to stiff layer 2 Chiang Mai: short pile/footing Depends on rock depth 3 Khorat: footings often OK Beware expansive soil
2D model — soil profiles compared at a depth scale of 0–40 m. Inner Bangkok: soft clay at about 2–15 m (N 0–4); houses typically use piles of about 21–25 m. North: many areas find weathered rock at about 5–15 m and use spread footings or short piles. Northeast: dense sand/loam from the surface; spread footings can be used in many areas. Figures are typical values, not design values (Image: SPN Soil Engineering)

Why Must Foundations in Bangkok and the Provinces Be Designed Differently?

1. Pile Lengths Differ by 3-4 Times

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AreaTypical pile lengthReason
Inner Bangkok21-25 mThrough about 13 m of soft clay into the stiff layer / first sand
Samut Prakan25-30 mThicker soft clay, so piles must go deeper
Thonburi side28-32 mThe stiff layer / first sand is deep or thin in places, so piles often go deeper
Chiang Mai8-15 mRock close to the surface
Khorat6-12 mUpper soil has good bearing capacity

2. Different Foundation Types Are Used

  • Bangkok / surrounding provinces: almost all permanent buildings use driven piles or bored piles, because spread footings on soft clay settle a lot and keep settling (compare pile types in Driven, bored or spun piles: which to choose)
  • North / Northeast: spread footings can be used in many areas, saving a lot of construction cost
  • Hills / weathered rock areas: a mat foundation or strip footing is sufficient

3. Foundation Construction Costs Differ

For the same project (e.g. a 2-storey, 200 m² house), foundation costs vary greatly between areas, depending on pile length and the foundation type required:

  • Bangkok: 💰💰💰 High — long piles through the soft clay are required
  • Samut Prakan: 💰💰💰💰 Highest — piles longer than in Bangkok
  • Chiang Mai: 💰💰 Moderate — spread footings or short piles
  • Khorat: 💰 Low — spread footings on sandy soil

*The 💰 levels are a relative comparison only, to illustrate differences between areas, and are not actual prices. Foundation construction cost depends on the structural design, building load, material prices and contractor at the time.

Is Land Subsidence Still a Risk for Bangkok?

Another factor in Bangkok is land subsidence, which was once very severe, reaching about 10-12 cm/year in the early 1980s due to excessive groundwater pumping. After groundwater control measures were enforced, the rate in inner Bangkok fell sharply. A study analyzing InSAR satellite data from Dec 2014–Aug 2023 found that inner Bangkok subsided no more than about 1 cm/year (many points were stable or subsiding only millimeters per year) (Source: Ahmed, Hiraga & Kazama, 2024, Science of the Total Environment). Another study using InSAR data from 2014–2024 found that most of its study area was still subsiding slowly, averaging about 0.3 cm/year (Source: Oh et al., 2026, Geosciences Journal)

However, some points in the surrounding provinces, such as Samut Prakan, Samut Sakhon and Ayutthaya, are still subsiding faster, at more than 2 cm/year in some areas and up to about 5 cm/year according to the same study. Within the city, at some locations such as the Ramkhamhaeng area, the Director-General of the Department of Mineral Resources stated that subsidence is still about 2–3 cm/year (Matichon, Sep 2025). Subsidence therefore varies greatly from area to area. The main causes of subsidence are:

  • Past groundwater pumping, which lowered pore water pressure in the soil layers
  • Natural consolidation of the soft clay
  • Increasing loads from buildings and roads
⚠️ Design implications

Because land subsidence lowers the ground around a house while the building on piles does not settle, a gap forms under the house floor (a "floating floor"), which is common in Bangkok. Slab on Pile or Slab on Grade must be designed to tolerate this separation. Soil settling around the piles can also cause negative skin friction, which must also be considered in design.

The 3D model below shows what happens to a house on piles when the ground around it subsides, along with past versus present subsidence rates.

3D model of a house on piles in Bangkok when the surrounding ground subsides Conceptual 3D view of a house on piles that pass through soft clay into the stiff layer and the first sand layer. The dashed line is the original ground level. The ground around the house has subsided below the dashed line, but the house on piles has barely settled, so a gap has formed under the house floor, exposing the piles in the gap; the walkway beside the house is cracked and the pipes entering the house are misaligned. The right panel is a bar chart of land subsidence rates: about ten to twelve centimeters per year in the early 1980s; InSAR satellite data from late 2014 to 2023 show inner Bangkok at no more than about one centimeter per year, while some points in the surrounding provinces exceed two centimeters per year, up to about five centimeters per year; plus typical mitigation measures. 3D model: House on piles as the ground subsides Not to scale Soft clay Medium stiff clay 1st sand 1 Ground subsides Dashed = original GL 2 Gap under floor House on piles stays Pipe off Cracks Land subsidence rate Unit: cm/yr (approx.) Early 1980s Before pumping control 10–12 Inner Bangkok, today InSAR Dec 2014–Aug 2023 ≤ ~1 Some suburbs Samut Prakan, Sakhon, Ayutthaya 2–5+ Current: InSAR satellite data Source: Ahmed et al. (2024) Science of the Total Environment Typical mitigation • Key slabs on piles (Slab on Pile) • Flexible pipe joints • Separate yard/extensions • Details by engineer Conceptual view, not to scale
3D model — the ground around the house has subsided below its original level (dashed line), but the house on piles has barely settled, so a gap forms under the floor, the yard cracks and pipes come out of alignment. Right panel: subsidence in the early 1980s was about 10–12 cm/year; InSAR data for Dec 2014–Aug 2023 show inner Bangkok at no more than about 1 cm/year and some points in the surrounding provinces above 2 cm/year (Ahmed et al., 2024). Conceptual image (Image: SPN Soil Engineering)

How Should Projects in Each Region Get Started?

Bangkok and Surrounding Provinces

  • Carry out a soil investigation for every project, large or small
  • Have the engineer set the pile tip layer from the boring results — usually the stiff clay or the first sand layer, depending on building load
  • Consider pile testing such as a Static Load Test or Dynamic Load Test (PDA) for medium to large projects
  • Design the structure to accommodate ground movement around the building

North / Northeast

  • Drill at least 1-2 boreholes to confirm the soil conditions
  • Watch for expansive soil in the Northeast
  • Watch for erosion of sandy soils in the rainy season

Southern Thailand

  • Coastal areas: design as in Bangkok, but shallower
  • Hill areas: check slope stability and soil erosion
  • Flood-prone areas: design foundations to withstand water pressure

Summary: Why Every Project Needs Its Own Soil Boring Report

Soil profiles in Thailand are highly variable; even within the same province they can differ. Using guesswork or data borrowed from elsewhere risks errors in foundation design.

SPN Soil Engineering has over 20 years of experience in soil investigation across Thailand, understands the particular soil characteristics of each region, and can give advice suited to your project. Contact us for a free consultation.

📌 Note

This article is provided as engineering knowledge. Soil layer thicknesses and pile lengths are typical values for each area. Actual foundation design is the responsibility of a licensed design engineer, based on the project's actual soil data.