Why Does the Law Specify the Content of Soil Investigation Reports?
A Soil Investigation Report is the document summarizing the results of a soil investigation and of both field and laboratory tests, which the design engineer uses as the “source” of every figure in the foundation calculations. Before the 2023 Foundation Ministerial Regulation, report quality on the market varied widely — some reports had only a few lines of N-Value tables, no borehole location plan and no laboratory test results, so neither the design engineer nor the local building official could verify where the values came from. The Ministerial Regulation therefore clearly sets “minimum standards” for report content and for who is entitled to certify it.
This matters especially for the 4 building groups that the law requires to have a soil investigation report with at least 3 investigation points supporting the calculations: high-rise buildings, extra-large buildings, large controlled-use buildings 4 storeys or higher, and buildings 4 storeys or higher in land allocation (housing estate) projects. For these buildings, an incomplete report is not just time lost fixing documents; it risks the permit application failing from the outset. (For more on the building permit side, read Do You Need Soil Boring Results for a Building Permit?)
What Is on the Legal Checklist for Soil Investigation Report Content?
Under the Ministerial Regulation, the minimum content of a soil investigation report consists of 8 main parts. The table below summarizes “what must be included” in each part and how a non-engineer project owner can check it personally before signing off on the work or paying the final installment:
← Scroll to see the full table →
| Report component | What must be included | How to check it yourself |
|---|---|---|
| 1. Borehole location plan | Location of every borehole, referenced to the project site plan, with a reference level | Compare the plan with the actual land — boreholes must be within the construction area, and the number of holes must match what was contracted |
| 2. Boring and testing methods | States the boring method, e.g. Wash Boring or Rotary Drilling, and field tests, e.g. Standard Penetration Test (SPT) per ASTM D1586 | Find the Methodology page — it must clearly cite the test standards, not just say “soil boring” |
| 3. Soil layer data (Boring Log) | Soil layer sequence, thicknesses, a description of each layer, and N-Values by depth, for every borehole | Scan the N-Values: they must continue at intervals down to the bottom of the hole (see how to read them in the Boring Log reading guide) |
| 4. Laboratory test results | E.g. moisture content, Atterberg Limits (ASTM D4318), unit weight, shear strength, as needed for design | Lab test result sheets must be attached at the back of the report, not just summary figures with no visible source |
| 5. Groundwater level | Groundwater level at the time of investigation for each borehole, with a note on seasonal variation if data are available | Look for the water level symbol in the Boring Log (see how groundwater affects foundations in Groundwater Level and Foundations) |
| 6. Summary table of soil properties for design | Parameters the engineer can actually use to calculate bearing capacity or pile resistance | Look for a Summary of Soil Properties table — without it, the design engineer has to extract the values from the whole report |
| 7. Analysis and recommendations | E.g. suitable foundation type, pile toe levels to consider (for information, not a substitute for design) | There is an analysis chapter at the end that makes clear which foundation approach should be considered for this soil profile |
| 8. Certifying engineer's signature | Engineer's signature with professional engineering license number (COE) | Open the certification page — it must have both the signature and the license number, which can be verified with the Council of Engineers |
Always check these 3 things first: is there a borehole location plan, are the N-Values complete down the depth, and does the last page have the engineer's signature with license number? A report missing any one of these 3 is very likely to be unusable for a building permit application.
Who Is a “Reliable Institution” Entitled to Certify Reports?
A reliable institution is the Ministerial Regulation's definition of who is entitled to prepare or certify a soil investigation report. It covers government agencies or educational institutions with relevant duties, and juristic persons holding a professional engineering license, with an engineer of the grade required by law as the responsible person. A report from a private company can therefore be submitted if the company holds a juristic-person license and its certifying engineer meets the conditions — the details of the definition and engineer grades should be checked by a civil engineer against the full text of the Ministerial Regulation.
What Should a Project Owner Check Before Hiring a Soil Boring Contractor?
Screening soil investigation providers from the start is the cheapest way to prevent an unusable report. Check these 3 points before agreeing to hire:
- Does the company hold a juristic-person professional engineering license? — this is a condition of a “reliable institution” as defined by law
- What grade of Council of Engineers (COE) license does the certifying engineer hold? — ask for the license number in advance; its status can be verified
- Has the company produced reports that have passed permit applications with the relevant authorities? — ask to see a sample report (with client details redacted) and compare it item by item with the checklist above
On budget, soil investigation for a 2–3 storey house starts at approximately THB 20,000–40,000 for the whole job (2–3 boreholes, including basic lab testing and an engineer-certified report), depending on depth and area. See details in Soil Boring Prices 2026 and guidance on the number of boreholes in How Many Boreholes and How Deep Is Enough? (These prices exclude travel and equipment mobilization, which are charged according to the distance to site.)
How Many Days Should You Allow to Get the Report?
A complete soil investigation report generally takes about 7–14 working days from the start of boring. After boring is complete (about 1–2 days per hole), the soil samples must first go through laboratory testing before the engineer can analyze them, summarize the parameters and sign off the report. If you have a building permit submission deadline, plan the soil boring at least 3–4 weeks ahead, allowing time for document revisions — see the step-by-step timeline in How Long Does Soil Investigation Take?
What Can Happen If the Report Is Incomplete?
The consequences of a report with missing components do not end at “the documents need fixing”; they affect time, budget and foundation design:
- Rejected at permit submission — the local building official requests more documents, review rounds are lost, and the construction schedule of the whole project is affected
- The design engineer cannot use the values — if the report lacks sufficient parameters, the engineer must fall back on the legal ceiling values, which are more conservative, so the foundations become unnecessarily expensive (read No Soil Boring: What Values Does the Law Allow?)
- Re-boring — the worst case is paying twice: for new boring and for the opportunity cost of delayed work
For the values engineers derive from the report, such as interpreting SPT into soil strength, read more in How to Interpret SPT N-Values
Summary: Complete the Checklist Before Accepting the Report
In the era of the 2023 Ministerial Regulation, a soil investigation report is not just a “paper attachment” but a technical document whose minimum content and certifying parties are clearly defined by law. Before hiring a soil boring contractor, check the provider's 3 qualifications, and before accepting the report, go through all 8 checklist items — especially the borehole location plan, Boring Logs, lab results and the engineer's signature with license number — so the building permit application to the relevant authority goes smoothly from the first round.
This article is a summary for basic understanding only, not legal or project-specific engineering advice. The report components and legal definitions should be checked by a civil engineer against the full text of the Ministerial Regulation before actual use.