What is soil boring and what does it involve?

Soil boring means drilling a borehole 10–30 m deep or more (depending on the building size and ground conditions) to collect soil samples and test the soil density at every depth interval. The data tells you which soil types lie beneath your land, how deep the hard bearing layer is, and where the groundwater level sits.

Most fieldwork uses wash boring combined with the Standard Penetration Test (SPT) to ASTM D1586 at intervals of approximately 1.5 m, with soil samples collected for laboratory testing. Site time is approximately 1–2 days per borehole for a residential house — for the detailed steps and the duration of each stage, see How long does soil boring take? A soil investigation timeline

Soil boring rig at work at an open-excavation project site in Chon Buri
Soil boring fieldwork at an open-excavation project site in Chon Buri (Photo: SPN Soil Engineering)

Soil boring steps

  1. Planning and setting out borehole locations — the engineer specifies the number and depth of boreholes based on the building size and site conditions
  2. Mobilizing and setting up the drilling rig — the rig can access both open sites and confined spaces, such as inside a house or factory
  3. Sampling and SPT testing — wash boring with Standard Penetration Tests (SPT) at intervals of approximately 1.5 m down to the specified depth
  4. Measuring the groundwater level — the water level in the borehole is recorded for designing below-ground works
  5. Laboratory testing of soil samples — e.g. Atterberg's Limits, moisture content, Unconfined Compression
  6. Preparing an engineer-certified report — including the boring log, SPT values, lab results and foundation recommendations; takes approximately 7–14 working days after fieldwork is completed

Fieldwork for a residential house takes approximately 1–2 days per borehole — see the detailed timeline in How long does soil boring take

Standards used in soil boring

Soil boring and testing follow international standards and Thai requirements: the Standard Penetration Test (SPT) to ASTM D1586, undisturbed thin-walled tube sampling to ASTM D1587-15, rock core drilling to ASTM D2113 when rock is encountered, laboratory soil property testing to the relevant ASTM standards, and the guidelines of the Department of Public Works and Town & Country Planning (DPT). Certain building types are also governed by the Ministerial Regulation on Building Foundations and Supporting Ground B.E. 2566 (2023). The report must be signed and certified by a licensed civil engineer (COE) to be usable for building permit applications.

The ground beneath your feet is more complex than you think

Even when two plots are right next to each other, the soil layers beneath them can differ greatly. Choosing a foundation type and pile length without soil data is like ordering a tailored shirt without taking measurements — and according to the DPT's technical guidance, soil data from neighboring sites can serve only as a guide; it cannot replace a soil investigation on the actual site, because it may be wrong and unsafe.

The model below gives an overview of a real soil boring day. A tripod rig stands over the borehole while the crew performs SPT tests and takes soil samples at intervals down to the stiff clay or dense sand layer. The data from this borehole is what tells you how long the piles of the house to be built must be.

3D model of soil boring on a plot before building a house, with the soil layers beneath the plot and piles that must reach the bearing layer A 3D view of a plot of land cut away through the soil layers on the front and right faces. On the surface are a tripod wash-boring rig, a 63.5 kg SPT hammer, a winch, a water pump and tank, a mud-return tub and a three-person crew. At the back right is the location of the two-storey house to be built, shown as a transparent house over stakes and string lines. The front section shows the borehole extending to a depth of approximately 30 m, with SPT tests and soil sampling approximately every 1.5 m and the groundwater level measured in the borehole. The soil layers from top to bottom are fill and topsoil, Bangkok-type soft clay approximately 12 to 15 m thick, stiff clay, and a dense sand layer at a depth of approximately 21 to 30 m. The right-hand section beneath the house shows the piles, which must extend down to the dense sand layer, a depth that can only be known from the soil investigation results. Depths in the figure are compressed. 3D Model Not to scale 0 m 10 m 20 m 30 m Tripod rig (wash boring) Flushes hole base in stages SPT 63.5 kg, 76 cm drop ASTM D1586 Pump + water tank Circulates in hole Planned 2-storey house Foundation not yet designed Soft clay Bangkok: ~12–15 m thick Stiff clay Dense sand (bearing) approx. 21–30 m in Bangkok GW level In borehole SPT + sampling every ~1.5 m Must reach bearing layer Length set by boring data only SS (SPT driven) ST (soft clay) Depth compressed · depths are a Bangkok-area example
3D model — soil boring on a plot before building a house. The borehole passes through soft clay down to the dense sand layer, with SPT tests and sampling approximately every 1.5 m, while the piles beneath the house must reach the bearing layer, which can only be known from the boring results (depths are a Bangkok-area example and compressed)
⚠️ A startling fact

Most buildings with serious settlement and cracking problems are caused by foundation designs that do not match the actual soil conditions — and almost all of them could have been prevented by a soil investigation before construction.

5 main reasons engineers recommend soil boring

1. Know how deep the piles must go — not guess

The core of pile design is knowing how deep the "bearing stratum" is, which differs from site to site and cannot be seen from the ground surface. Soil boring with SPT testing locates the stiff clay/dense sand layer that can actually carry the load. If the piles are too short, they will "float" in the soft soil and the building will settle later. Learn how to interpret soil density values in How to interpret the SPT N-Value, and see soil layer depths by region across Thailand in Bangkok soft clay vs the provinces

2. Choose the right foundation type

Different soil conditions call for different foundations. In some areas driven piles work well, in others bored piles are needed to reduce the impact on adjacent buildings, and in some areas the soil is stiff enough for a spread footing, which is much more economical — all of this can only be decided from soil investigation data.

3. Know the groundwater level — the invisible variable

Soil boring measures the groundwater table, which directly affects soil strength, basement design, waterproofing and the water pressure acting on the structure — especially for projects with deep excavation such as swimming pools, water tanks or basements. Without knowing the groundwater level in advance, you may face water inflow during construction that brings work to a halt.

4. Save construction costs in the long run

Accurate soil data allows the engineer to design a foundation that is "just right" — not so small that it is risky, nor so large that it is wasteful. Many projects with complete soil data can reduce pile length or the number of piles while remaining safe to standard, so the cost of soil boring is often less than the savings. Conversely, without soil data the engineer must design "with a large margin" using a presumptive bearing value, which the law sets low for safety; the result is a foundation that is larger and more expensive than necessary — read more in Can you skip soil boring? Statutory soil bearing values without a soil investigation

5. Get documents you can actually use

A soil boring report certified by a licensed civil engineer registered with the Council of Engineers Thailand (COE), with signature and registration number, can be used for design and for building permit applications to the relevant authorities, and many financial institutions also request it when assessing loans for high-value projects.

Does the law require soil boring?

The Ministerial Regulation on Building Foundations and Supporting Ground B.E. 2566 (2023) requires certain building types (e.g. high-rise buildings and extra-large buildings) to include a soil investigation report with the structural calculations. For ordinary residential houses the law does not require it directly, but without boring results the engineer must use the presumptive soil bearing values set by law, which are very low for safety, making the foundation more expensive — so economically it is "strongly recommended". For a full explanation of the legal provisions, the bearing value conditions without boring and a table of borehole numbers for houses, see Does a 2-storey house need soil boring? An engineer's answer

How many boreholes, and how deep?

The number of boreholes depends on the building size and the variability of the soil layers. For a typical residential house, 1–3 boreholes are common, while large buildings, factories or warehouses add boreholes according to the building area to cover variations in the soil beneath the building footprint. The standard depth in Bangkok and its vicinity usually reaches the dense sand layer at approximately 21–30 m.

The appropriate figures for each project must be specified by an engineer based on the actual building and site — see the detailed criteria for setting the number and depth of boreholes in How many boreholes and how deep?

How much does soil boring cost?

SPN's soil boring service for residential houses starts at approximately THB 20,000–40,000 (2–3 boreholes, including basic laboratory testing and an engineer-certified report). The actual price depends on the number of boreholes, the depth and the lab tests required.

Note: this price does not include travel and equipment mobilization, which are charged extra according to the distance to site. — see the detailed price structure by building type in House soil boring prices 2026, see the full scope of services in Soil investigation and testing services, or contact us for a free quotation within 24 hours.

Boring vs no boring — what's the difference?

The figure below compares 3 cases on the same soil profile; the difference lies in whether the designer "knows" or "guesses" the depth of the bearing layer. The following table summarizes all the other differences.

Soil boring vs no boring: how pile lengths differ, comparing 3 cases Three soil cross-section panels on the same soil profile: fill, soft clay, stiff clay and a dense sand layer from about 21 m deep. In the first panel there is no soil investigation and the piles are too short; the pile tips float in the soft clay and the house settles unevenly, tilting with cracked walls. In the second panel there is no soil investigation and a large margin is added to pile length and foundation size; the piles extend beyond the required layer and the foundation cost is higher than necessary. In the third panel soil boring is done first, giving SPT values at every depth interval and the depth of the dense sand layer, so the pile length is set so that the tips sit right in the bearing layer. The bottom strip summarizes the data gained from soil boring: the bearing layer depth, the groundwater level and actual soil bearing values instead of the statutory presumptive values. Boring vs no boring — where does pile length come from? Not to scale A No boring · short guess B No boring · big margin C Bored first Tips float in soft clay Stiff clay Dense sand (bearing) No data Add margin ฿ ฿ ฿ Excessive SPT N @ ~1.5 m Tips right in bearing Uneven settlement · cracks Safe, but costly foundation Safe and matched to soil 1 Bearing layer depth Sets pile length 2 GW level Plan excavation, waterproofing 3 Actual soil bearing value not low statutory presumed value
Comparison of 3 cases on the same soil profile — (a) piles guessed too short, tips floating in soft clay, the house settles unevenly; (b) a large margin added to length and size, safe but with a high foundation cost; (c) soil boring first, so the actual bearing layer depth is known and the pile length can be set just right

← Scroll to see the full table →

IssueSoil investigationNo boring (guessed from neighboring sites)
Pile lengthSet from the actual bearing stratumGuessed — risk of too short (settlement) or too long (costly)
Foundation typeAppropriate choice; in some cases a spread footing is more economicalLarge safety margin required, higher cost
Soil bearing value used in designActual value from test resultsStatutory presumptive value (very low)
GW levelKnown in advance; excavation/waterproofing can be plannedUnknown — risk of problems on site
Documents for permits/loansEngineer-certified report ready to useNone
Risk of house settlementLow — designed from actual dataHigh — foundation repairs often cost many times more than boring

Which sites are high-risk and most in need of boring?

High-risk sites are those where the soft soil layers are thick or variable enough that guessing the pile length can easily go wrong. Engineers recommend soil boring in every case, without exception, for these sites:

  • Bangkok and its vicinity — the Bangkok Soft Clay layer is approximately 12–15 m thick with very low bearing capacity; piles must reach the dense sand layer deeper than 21 m
  • Riverside areas, lowlands and former rice fields — thick deposits of soft sediment whose thickness changes rapidly over a few tens of meters
  • Newly filled land — the fill has not yet consolidated, and its weight continues to compress the underlying soft soil, causing ongoing settlement for many years
  • Provinces with variable soil layers — some areas have thin hard layers sitting above soft soil (a "false hard layer"), which you cannot detect without boring

We summarize soil profiles by region, pile depths by area and locations of anomalous soil layers in Bangkok in the article Bangkok soft clay vs the provinces — why foundations must be designed differently

When in a project should soil boring be done?

The best time is "before structural design" — that is, after you have the land and know the rough building layout (if you have not yet bought the land, a screening borehole before transfer can also help you decide; see Buying vacant land: should you do soil boring first?), but before the engineer starts the foundation calculations, because soil data is the starting input for the design. If boring is done after the design is finished, the whole set of drawings may need to be redone when the soil turns out to differ from what was assumed. For a second-hand house or an extension to an existing building where settlement is found, soil boring can also be done afterward to find the cause and design the remedy — see guidance for extensions in House extension settling: do you need soil boring?

What do you get from soil boring?

The final output of soil boring is a "soil investigation report" that compiles the field data and laboratory test results together with engineering recommendations, certified by a licensed civil engineer.

What you receive when the work is complete:

  • Boring Log — a layer-by-layer diagram of the soil showing thickness, description, color and density (How to read a boring log)
  • SPT N-Value — soil density at each level, used to assess bearing capacity
  • Groundwater level — for designing below-ground works and waterproofing
  • Lab test results — engineering properties of the soil, e.g. Atterberg's Limits, Unconfined Compression
  • Engineering recommendations — the appropriate foundation type and pile depth
📋 Good to know

Reports certified by SPN's engineers can be used for building permit applications to the relevant authorities — no need to pay for the work twice.

How to choose a soil boring company without getting cheated?

The quality of a soil boring report depends on the company that does it. Before hiring, check whether a licensed civil engineer (COE) signs and certifies the report, whether the tests are genuinely performed to ASTM standards, whether the company has its own soil testing laboratory or subcontracts it, and whether it has verifiable references. A report without an engineer's certification cannot be used for building permit applications, and the design engineer cannot use it either — read the full 10-point checklist in How to choose a soil boring company

Soil boring cost vs repair cost if you skip it

✅ Cost of prevention vs cost of repair

Soil boring for a residential house usually costs less than 1% of the total construction cost, while fixing a faulty foundation often costs many times more than the boring, and in some cases the house must be demolished and rebuilt — see price ranges by building type and number of boreholes in House soil boring prices 2026

🚫 Warning signs you should not ignore

If a building starts to show cracks in columns and beams, doors and windows that no longer close properly, or tilting floors, the foundation may already have a problem — check all the symptoms in 5 signs of foundation problems

📌 Note

This article provides basic knowledge to help you prepare for construction. Decisions on foundation type and pile length must be based solely on calculations by a licensed engineer.