Why the law defines soil layers by numbers

Because the values the law allows for design — such as the allowable bearing pressure or pile skin friction — are tied to the "soil layer type". If the definitions were vague, everyone would interpret them differently. The Ministerial Regulation on Building Foundations and Supporting Ground B.E. 2566 (2023) (read the overview in The 2023 Foundation Ministerial Regulation, simply explained) therefore ties soil layer definitions to values measured by 2 standard tests performed in almost every soil investigation:

  • SPT (Standard Penetration Test) — a standard sampler is driven with a standard hammer weight and drop height, and the blows needed to drive it the final 30 cm are counted as the N-Value. The higher the value, the denser or harder the soil — the test method and interpretation are covered in detail in How to interpret the SPT N-Value
  • Undrained shear strength (Su) — from laboratory tests (e.g. Unconfined Compression) or in the field with the Field Vane Shear Test; used mainly for clay

Soil layer classification under the Ministerial Regulation (simplified)

The "clay and hard soil" group is classified by shear strength Su together with the SPT N-Value. This table summarizes the guidelines to show which numbers each soil name is tied to (sand is classified by N-Value alone — see the next table):

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Guideline classification of clay and hard soil by Su and SPT N-Value
Soil layer typeBrief descriptionNumerical criteria (guideline)
Very soft clayVery wet, high water content; found in the top layer of lowland plainsSu < 12.5 kPa (water content > 100%)
Soft clayEasily molded, deforms when squeezed; common in the Central Plains lowlandsSu 12.5–25 kPa / N-Value ≤ 4 (water content > 70–100%)
Medium stiff clayFirmer; a thumb still leaves an impressionSu 25–50 kPa / N-Value 4–8
Stiff clayThumb barely penetratesSu > 50 kPa / N-Value 8–30
Hard soil (hardpan)Very dense/hard layer; often the pile end-bearing layerN-Value > 30

For clay, the Ministerial Regulation also uses "water content" as a criterion together with Su and N-Value, especially for the softest layer — very soft clay is soil with a water content above about 100% and Su below 12.5 kPa, a layer that settles easily and needs particular care in foundation work.

Sand (cohesionless soil): density by SPT N-Value

For cohesionless soils such as sand and gravel, the SPT N-Value is the main classifier of "density" (relative density), without using shear strength Su:

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Density of sand by SPT N-Value
SPT N-ValueDensity (relative density)Relative density Dr (%)
0–4Very loose0–15
5–10Loose15–35
11–30Medium dense35–65
31–50Dense65–85
> 50Very dense85–100
⚠️ Important

This table is a guideline for understanding only. For figures and wording used for legal reference, rely on the definitions in the text of the Ministerial Regulation, and the classification of soil layers for an actual project is the responsibility of the engineer responsible for the soil investigation results.

How the SPT gives the N-value, and SPT N-Value ranges for clay consistency and sand density The left image shows the SPT principle per ASTM D1586: a 63.5 kg hammer free-falls from a height of 76 cm onto the anvil at the top of the drill rods, transmitting the blow to drive a split-spoon sampler with a 50.8 mm outside diameter at the bottom of the borehole over three increments of 15 cm each. The first increment is the seating drive and is not counted; the N-value is the number of blows in the second plus third increments, i.e. 30 cm of penetration. The right image shows N-value range bars. Clay is divided into soft N not more than 4, medium stiff 4 to 8, stiff 8 to 30, and hard soil (hardpan) N greater than 30, while very soft clay is classified by shear strength Su less than 12.5 kPa, not by N. Sand is divided into very loose 0 to 4, loose 5 to 10, medium dense 11 to 30, dense 31 to 50, and very dense above 50. N greater than 30 marks the dense sand or hard soil layer commonly used for pile end bearing. The bottom strip summarizes that the higher the N, the harder or denser the soil; for soft clay, Su from Unconfined Compression or Vane Shear should be used as well; and calculations use the corrected N60, not the raw N. The figures are guidelines for understanding; legal soil classification follows the text of the Ministerial Regulation. SPT N-Value: how it is obtained and what it tells us Not to scale Getting N (ASTM D1586) 63.5 kg hammer 76 cm free fall 1st 15 cm: seating (excl.) 2nd 15 cm 3rd 15 cm N N = blows, incr. 2 + 3 (30 cm) Split spoon OD 50.8 mm ↓ N ranges vs soil type Common pile toe layer Clay — consistency Soft N ≤ 4 Medium 4–8 Stiff 8–30 Hard soil N > 30 Sand — density Very loose 0–4 Loose 5–10 Medium dense 11–30 Dense 31–50 Very dense > 50 0 4 8 10 30 50 SPT N (blows / 30 cm) → Very soft clay: Su < 12.5 kPa (N not used) N in clay varies widely → confirm with Su from UC / Vane Shear Guideline only — legal definitions follow the Ministerial Regulation 1 Higher N Harder / denser soil 2 Soft clay Check Su, not N alone 3 Design uses N60 Not raw N
Left: the N-value comes from driving a split-spoon sampler with a 63.5 kg hammer free-falling 76 cm, counting the blows in the 2nd and 3rd 15 cm increments per ASTM D1586 · Right: N-value ranges and soil classification — clay: soft N ≤ 4 · medium stiff 4–8 · stiff 8–30 · hard soil N > 30 | sand: very loose 0–4 · loose 5–10 · medium dense 11–30 · dense 31–50 · very dense > 50 (very soft clay is classified by Su < 12.5 kPa)
  • Choosing the foundation type — thick soft clay like Bangkok's usually requires piles to transfer loads to the dense sand or hard soil below, while areas with shallow stiff soil may use spread footings (read Bangkok soft clay and foundation work)
  • Setting pile length — the pile tip should sit in a layer with a sufficiently high N-Value, and the depth of that layer can only be known from the Boring Log (see how to read one in the Boring Log reading guide)
  • Calculating load capacity as required by law — every value the engineer uses in the structural calculations must be consistent with the soil layer type proven by test results (for the basics of soil bearing capacity, read What Is Bearing Capacity)
  • Assessing settlement — soft clay keeps settling over a long period (consolidation). Knowing the thickness of the soft layer helps assess and prevent house settlement problems.

The flip side to watch out for: without soil investigation results, the law limits you to presumptive bearing values, which are very low. Read the details in Without a soil test, what bearing values does the law allow?

Bangkok vs other provinces: why soil borings find different soils

The Chao Phraya lowland plain has a layer of Bangkok Soft Clay about 12–15 m thick before reaching stiff soil or dense sand, while many provinces in the Northeast, the East or foothill areas may encounter stiff soil or weathered rock at shallow depth. Classification under the legal definitions therefore gives very different results from area to area, which is why the pile length for the same house design in two provinces can differ by several meters — see the appropriate number and depth of boreholes for each job in How many boreholes, and how deep, is enough

Comparison of soil layers and SPT N-values between the Bangkok lowland plain and upland areas in other provinces, and the effect on pile length Two cross-sections compared. Left: the Chao Phraya lowland plain, Bangkok and its vicinity, with a thin fill layer followed by soft clay about 12 to 15 m thick with SPT N not more than 4, then stiff clay with increasing N, and a hard soil or dense sand layer with N greater than 30, so house piles must extend down to the hard layer. Right: upland or foothill areas in many provinces of the Northeast and East, with stiff clay from shallow depth and weathered rock or hard soil not far below, so piles for the same house design can be several meters shorter; a dashed line shows the Bangkok-type pile length for comparison. Horizontal bars beside each section show SPT N with depth, and a dashed line at N equal to 30 marks the hard soil criterion. Values in the image are hypothetical, for illustration. Bangkok vs provinces — different soils when bored Not to scale Bangkok & vicinity Chao Phraya lowland 0 5 10 15 20 25 Depth (m) SPT N N = 30 ≥50 Soft clay Stiff clay Hard soil / Dense sand Provinces (upland/foothill) e.g. parts of the Northeast, East 0 5 10 15 20 25 Depth (m) SPT N N = 30 ≥50 Stiff clay Hard soil / Weath. rock Differs by meters BKK-type pile SPT N values shown are hypothetical · actual layers require a soil investigation of that plot
Example comparison — the Chao Phraya lowland plain has soft clay about 12–15 m thick with low SPT N before reaching stiff clay and the hard soil/dense sand layer (N > 30), while upland or foothill areas in many provinces encounter stiff soil or weathered rock at shallow depth, so piles for the same house design can differ in length by several meters (N values shown are hypothetical)

Summary

Under the 2023 Foundation Ministerial Regulation, "soft soil" or "hard soil" is no longer a loose expression but a legal status tied to the measured SPT N-Value and shear strength Su. For anyone planning to build, a soil investigation is the only way to know which category your soil falls into under the legal definitions, so the engineer can design the foundations with correct values that have a traceable reference in the structural calculations.

📌 Disclaimer

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.