No soil boring results: what values does the law allow?

The 2023 Foundation Ministerial Regulation sets out a straightforward principle: the allowable bearing pressure of foundation soil, or the resistance of a pile, must have a documented basis. With a soil investigation report, the engineer can calculate from actual soil properties. Without one, the law only allows values no higher than those prescribed in Clauses 7–9 of the regulation (for buildings outside the mandatory group under Clause 5). These values are deliberately set low for safety, because no one can confirm what the soil beneath the building is really like.

The values prescribed by the regulation can be summarized from the text as follows (soil types are defined by SPT N per 300 mm, undrained shear strength su, or water content w, per Clause 2):

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Soil type as defined in the regulationApplicable up toNo.
Sound rock with no fissures, cracks or cavities250 kPa (25 t/m²)7 (1)
Dense gravel or hardpan (stiff clay N > 30)200 kPa (20 t/m²)7 (2)
Stiff clay (N > 8–30 or su > 50 kPa), dense sand, or medium-dense gravel100 kPa (10 t/m²)7 (3)
Medium stiff clay (N > 4–8 or su 25–50 kPa), medium-dense sand, or loose gravel50 kPa (5 t/m²)7 (4)
Soft clay (N ≤ 4 or su 12.5–25 kPa) not located at river mouths, bay mouths, sea coasts, mangroves, or areas where clay may become very soft20 kPa (2 t/m²)7 (5)
Pile skin friction in clay (outside the above areas), depth ≤ 10 m / 10–15 m / over 15 m500 / 500 + 160 × y / 1,300 kg/m²8
Very soft clay (w > 100% or su < 12.5 kPa)su ≤ 4 kPa or friction ≤ 160 kg/m²9

In Clause 8, y is the length of the pile portion deeper than 10 m but not deeper than 15 m below the original ground level. Clause 9 also requires long-term settlement to be considered. For details of application, rely on the text published in the Royal Gazette; the design engineer is responsible for applying the values correctly (for background, see What is soil bearing capacity?)

No soil investigation vs. soil investigation: the legal cap values force larger foundations, with a comparison of allowable bearing pressure by soil type The left panel compares two footings carrying the same column load. Footing area equals load divided by allowable bearing pressure. The left footing has no soil investigation results, so the engineer must use the conservative cap values of the 2023 Foundation Ministerial Regulation, which are low; the footing must therefore be larger, or the piles longer or more numerous. The right footing has soil investigation results and an actual boring log, so it uses values from actual soil properties, which are usually higher, and the footing size suits the real soil. The right panel is a proportional bar comparison of allowable bearing pressure: soft clay is lowest, capped at about 2 tonnes per square meter when there are no soil investigation results; stiff clay, dense sand, and gravel or rock are progressively higher. Design values must come from soil investigation results and the design engineer. The strip at the bottom shows three situations where skipping soil boring costs more: soft-soil lowlands, newly filled land, and heavy buildings or buildings with basements. No boring = low values = costlier footing Not to scale Same column load P — only the design soil value differs Footing area A = P ÷ q allow → low q, big footing P Low q (cap value) P q from real soil ? ? ? ? Log No boring Legal cap value (low) → Bigger footing / more or longer piles Boring Real soil values (usually higher) → Footing sized to real soil Mandatory buildings need ≥ 3 boreholes anyway Allowable bearing pressure by soil type Stiffer = higher capacity → Clay Soft Clay Stiff Sand Dense Gravel / rock Soft clay ≤ 2 t/m² Cl. 7 · non-coastal/estuary Bars are relative — real values from boring 1 Soft lowlands Firm layer may be shallower 2 New fill land Untested = lowest value 3 Heavy bldg / basement Gap becomes big money
Proportional comparison: the softer the soil, the lower the allowable bearing pressure. Soft clay outside river-mouth, coastal or mangrove areas is limited by law to no more than 2 t/m² when there are no soil investigation results (Clause 7), while stiff clay, dense sand, and gravel/rock layers carry progressively more — design values must come from soil investigation results and the design engineer

How does soil boring differ from using the cap values?

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IssueNo boring (legal cap values)Boring (values from actual investigation)
Design soil bearing valueLow (conservative)Based on actual soil properties, usually higher
Spread footing sizeLarger to compensate for low valuesSuited to actual soil
Pile length/numberMore allowance neededCan be designed accurately
Risk of unexpected soil layersHigh (no data)Low (actual boring log available)
Total foundation costUsually higherUsually lower, even including soil boring cost

📌 Note: Actual figures for each project depend on soil conditions, building size and the responsible engineer's design. This table illustrates the principle of comparison only. This article is a summary for understanding, not legal or project-specific engineering advice.

When does skipping soil boring usually cost more? 3 situations

1. Soft-soil areas, such as the Central Plains lowlands

If you design with the soft-clay cap value, the footings and piles will carry a large allowance, even though actual boring may find dense sand or stiff soil at a shallower depth than expected, allowing shorter piles to be chosen.

2. Newly filled land

There is no way to know the properties of fill without testing. Assuming the lowest value is the only option the law leaves open, which automatically means the most expensive design.

3. Heavy buildings or buildings with basements

The heavier the load on the foundations, the more the gap between the "cap value" and the "actual value" turns into money spent on piles, concrete and steel.

Soft-soil areas need even more care, because the law separates ordinary soft clay (Clause 7) from very soft clay near river mouths and coasts (Clause 9), which allows even lower values. The 3D view below shows how the usable values change from upland areas to the coast.

3D model of the values the 2023 Foundation Ministerial Regulation allows when there is no soil investigation report, by soil type from upland to coast Conceptual 3D view: a soil block cut in section from upland areas on the left to the sea coast and mangroves on the right. The soft clay layer is thin on the left and grows thicker toward the coast, with stiff clay and sand below. The left house sits on a spread footing on stiff soil; under Clause 7 the allowable bearing pressure may not exceed 5 to 10 tonnes per square meter depending on soil type. The middle house sits on soft clay outside the coastal zone, limited to 2 tonnes per square meter. The right house is in the very soft clay zone near the coast; under Clause 9 pile skin friction may not exceed 160 kilograms per square meter. A soil boring rig stands behind; with soil boring, values from the actual soil can be used. The right panel shows bars comparing the Clause 7 values for five soil types, 25, 20, 10, 5 and 2 tonnes per square meter, the pile skin friction values under Clauses 8 and 9, and a note that buildings under Clause 5 must have a soil investigation report of at least three points and cannot use these values. 3D model: legal values with no boring results Not to scale 1 Stiff soil / dense sand Cl. 7: ≤ 5–10 t/m² 2 Soft clay Cl. 7 (5): ≤ 2 t/m² 3 Very soft clay Cl. 9: friction ≤ 160 kg/m² 4 Soil investigation Use real soil values Section: soft clay thickens toward coast Dashed = very soft clay zone starts (concept) Coast · estuary Mangroves Legal values (no boring) Cl. 7 · allowable bearing Sound rock, no cracks 25 t/m² Dense gravel · hardpan 20 t/m² Stiff clay · dense sand 10 t/m² Med. stiff clay · med. sand 5 t/m² Soft clay * 2 t/m² * Outside estuary, coast, mangroves Cl. 8 pile friction (clay) ≤ 10 m: 500 · > 15 m: 1,300 kg/m² Cl. 9 very soft clay su ≤ 4 kPa or friction ≤ 160 kg/m² Clause 5 bldgs can't use these Need soil report ≥ 3 points
3D model — ① A house on stiff soil or dense sand may use Clause 7 values of no more than 5–10 t/m² depending on soil type ② Soft clay outside the coastal zone may use no more than 2 t/m² ③ Very soft clay near the coast, under Clause 9, may use pile skin friction of no more than 160 kg/m² ④ With soil investigation, the engineer can use values from the actual soil. Right panel: values under Clauses 7, 8 and 9, and a note that buildings under Clause 5 cannot use these values. Conceptual illustration (Image: SPN Soil Engineering)

Which buildings cannot use the cap values?

Remember that high-rise buildings, extra-large buildings, large controlled-use buildings of 4 storeys or more, and buildings of 4 storeys or more in housing estate projects have no choice — the regulation already requires a soil investigation report of at least 3 investigation points to accompany the design calculations (see details of the mandatory building groups in the 2023 Foundation Ministerial Regulation summary and guidance on number and depth of boreholes). Clauses 7–9 state clearly that they apply to buildings "other than buildings under Clause 5", so the question of "to bore or not to bore" only concerns small to medium-sized buildings.

Summary: cap values are allowed, but usually cost more than soil boring

The 2023 Foundation Ministerial Regulation does not prohibit building without soil boring, but it "charges interest" on not knowing by forcing the use of the lowest soil bearing values. Soil investigation is therefore not an added cost but an investment that usually pays back right away at the foundation design stage — spend tens of thousands of baht to avoid hundreds of thousands in extra allowance.