Why test in the lab after soil boring?
Because field tests and lab tests answer different questions. A field test such as the SPT (Standard Penetration Test) tells you the density or consistency of the soil with depth, but not what type of soil it is or how much a building will settle and over how many years. The table below compares them side by side.
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| Aspect | Field tests (e.g. SPT) | Laboratory tests |
|---|---|---|
| What is measured | Overall density/consistency of the soil with depth | Soil type, shear strength, compressibility |
| Question answered | "How dense is the soil? How deep is the hard layer?" | "What soil type is it? How much load can it carry? How much will it settle?" |
| Where / when | On site, during the soil investigation | In the lab, after the soil samples are delivered |
| Where the results go | N-value column in the boring log | Lab results columns in the boring log + appendix at the end of the report |
These two sets of data are not an "either/or" choice; they are two halves that must be combined in the boring log and the complete soil investigation report that design engineers actually use.
How many groups of lab soil tests are there?
Laboratory soil tests fall into 2 main groups: Index Properties and Engineering Properties, which differ in purpose, the type of sample used and the time needed to get results.
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| Aspect | Group 1: Index Properties | Group 2: Engineering Properties |
|---|---|---|
| Purpose | "Get to know" the soil — classify each soil layer | Turn soil into "design numbers" — bearing capacity, settlement |
| Sample used | Samples from the SPT sampler, available at almost every depth | Undisturbed samples from key soil layers |
| Main tests | Moisture Content, Unit Weight, Atterberg Limits, Sieve Analysis | Unconfined Compression, Consolidation, Direct Shear, Triaxial |
| Approximate turnaround | Fast — a matter of days | Longer — some tests such as consolidation take weeks |
| What the results are used for | USCS soil classification, the basis for interpreting everything else | Direct calculation of foundations and piles, and settlement prediction |
To show what equipment each group of tests uses, the model below shows the 5 main test stations in a soil laboratory and what each station measures.
Group 1: What are the index property tests?
Index properties are the group of tests used to "get to know" each soil layer. They are performed on samples from almost every depth. Each test costs little but cannot be skipped, because they are the basis for interpreting everything else.
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| Test | Standard | What it gives | What engineers use it for |
|---|---|---|---|
| Moisture content (Moisture / Water Content) | ASTM D2216 | Proportion of water in the soil mass (%) | A first indication of how soft or stiff the soil is — soft clay usually has high moisture content — and an input to the calculations of almost every other test |
| Unit weight (Unit Weight / Density) | ASTM D7263 | Weight of soil per unit volume | Used to calculate overburden stress in the soil layers; a starting input for almost every analysis |
| Atterberg limits (Atterberg Limits) | ASTM D4318 | Liquid Limit (LL), Plastic Limit (PL), Plasticity Index (PI) | Shows how clay behaves as its moisture content changes — a high PI means a highly plastic clay with a risk of swelling and shrinkage — and is used for USCS soil classification |
| Grain size analysis (Sieve Analysis) | ASTM D6913 (Hydrometer: D7928) | Proportions of gravel, sand and fines | Classifies the soil as sand or clay — dense sand layers are suitable for pile toe bearing, while fine-grained soils need further settlement analysis |
| Specific gravity (Specific Gravity, Gs) | ASTM D854 | Specific gravity of the soil solids | Used to calculate the void ratio for compressibility analysis |
The results from this group are used to classify the soil under the Unified Soil Classification System (USCS), the common language engineers worldwide use in soil boring reports.
Group 2: What are the engineering property tests?
Engineering properties are the group of tests that turn soil samples directly into "design numbers". They are performed on undisturbed samples from the soil layers that affect foundation design.
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| Test | Standard | What it gives | What engineers use it for |
|---|---|---|---|
| Unconfined compression (Unconfined Compression, UC) | ASTM D2166 | Undrained shear strength (Su) of clay | Calculating the bearing capacity of foundations on clay, pile skin friction, and the stability of excavations and fills |
| Consolidation (Consolidation) | ASTM D2435 | Time-dependent compressibility parameters of clay | Predicting building settlement — how much and how long — which is critical in Bangkok's soft clay |
| Direct shear (Direct Shear) | ASTM D3080 | Shear strength parameters (c, φ) | Analyzing slope stability, retaining walls and work where the soil carries shear under drained conditions |
| Triaxial compression (Triaxial UU/CU/CD) | ASTM D2850 / D4767 / D7181 | Shear strength under various drainage conditions | Complex work such as deep excavations, dams and advanced stability analysis — specified by the design engineer to suit the project |
Good to know: the consolidation test takes the longest, because the soil must drain naturally under each load increment and this cannot be rushed. This is one reason a complete soil boring report takes time to prepare (see How Long Does Soil Boring Take, from Mobilization to Report) — and in Bangkok's soft clay this test is practically the heart of the design (read more in Bangkok Soft Clay and Foundations)
What extra tests do roads, compacted fills and slabs on ground need?
The two groups above mainly answer the questions of foundation and pile work. But if the work is a road, a parking area, a slab on ground or a compacted fill layer, the questions change: "At what moisture content should it be compacted to be densest?" and "How well will this soil layer support the pavement structure when it gets wet?" These require tests on soil freshly compacted in the laboratory, not undisturbed samples.
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| Test | Standard | What it gives | What engineers use it for |
|---|---|---|---|
| Compaction (Compaction / Proctor) | ASTM D698 (Standard) ASTM D1557 (Modified) | Optimum moisture content (OMC) and maximum dry density | Sets the target moisture and density for field compaction, and is the reference value used to calculate % compaction from field density test results |
| CBR (CBR — California Bearing Ratio) | ASTM D1883 (AASHTO T193) | CBR value as a percentage, plus swell during soaking | Assesses the strength of subgrade and subbase soils as a design input for pavement structure thickness |
CBR is measured by pushing a standard piston into soil compacted in a mold and comparing the resistance with that of a standard crushed rock at the same penetration, so the result is a percentage — the higher the percentage, the better the soil carries load. The acceptance criteria for each pavement layer follow the project specifications or the owner agency's requirements and are set by the design engineer.
Compaction and CBR tests use bulk samples of several tens of kilograms, not undisturbed samples from a sampling tube. If the project also includes roads or slabs on ground, specify this in the scope of work from the start so the field team collects enough sample for testing.
How do undisturbed samples differ from SPT samples?
Borehole soil samples sent to the lab come in 2 main types: undisturbed samples (UD) and disturbed samples from the SPT sampler (SS). If there is road or fill work, bulk samples are a 3rd type. Each type is used for a different group of tests — if the wrong type of sample is collected, the engineering test results cannot be relied on.
The diagram below traces each type of soil sample from collection in the borehole to the group of laboratory tests that uses it — note that engineering tests can only use undisturbed (UD) samples.
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| Aspect | Undisturbed sample | Disturbed sample (SPT) |
|---|---|---|
| Sampling method | Special sampling tube that preserves the natural soil structure | Split spoon driven in the SPT |
| Soil structure | Close to in-situ condition | Disturbed by driving |
| Used for | Engineering Properties — UC, Consolidation, Triaxial | Index properties — moisture, Atterberg, sieve |
| Taken from which layers | Clay layers that affect the design | Almost every depth interval in the borehole |
Which test answers which design question?
Looking at it from the questions owners and engineers want answered, each question has a test that answers it directly:
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| Design question | Test that answers it |
|---|---|
| What type of soil is this? | Atterberg Limits + Sieve Analysis + Moisture Content |
| How much load can the soil carry? | Unconfined compression (clay) + SPT N-value (field) |
| How much will the building settle, and over how long? | Consolidation Test + Unit Weight |
| Which layer is suitable for pile toe bearing? | Sieve analysis + SPT N-value together |
| Is the excavation/slope safe? | Unconfined Compression / Direct Shear / Triaxial |
| Does the compacted fill meet the criteria yet? | Compaction test (Proctor) + field density test |
| How thick does the road/slab on ground need to be? | CBR + Compaction Test |
The question "how much load can the soil carry?" involves more detail than you might think — read on in What Is Soil Bearing Capacity?
How do you choose the right tests, no more and no less?
The number and types of tests depend on the soil actually encountered and the nature of the project. The principles are:
- Soil samples from every depth should receive basic (index) tests (moisture content, soil classification)
- Engineering tests are performed on undisturbed samples from the layers that affect the design, such as clay layers that the foundations or piles rely on
- A good quotation should clearly list the tests and the reference standards
Unusually cheap soil boring prices usually come from cutting out important tests, resulting in a report the design engineer cannot actually use. Soil investigation for a residential house with basic tests starts at about THB 20,000–40,000 (excluding travel and equipment mobilization, which are charged extra based on the distance to the site) — see the detailed price breakdown in Soil Boring Prices 2026 and how to choose a contractor in How to Choose a Soil Investigation Company
Summary
Laboratory soil testing turns the "soil cores" from a borehole into numbers engineers can actually use to design foundations — index properties tell you what type of soil it is, and engineering properties tell you how much load it can carry and how much it will settle, while road and compacted-fill work uses compaction and CBR tests to set compaction criteria and pavement thickness. A soil boring report without supporting lab results therefore cannot fully answer the design questions.
The SPN Soil Engineering laboratory tests to ASTM standards, covering both index properties and engineering properties, with results certified by civil engineers licensed by the Council of Engineers Thailand (COE). See the full scope of our soil investigation and laboratory testing services at Soil Investigation and Testing Services
This article is provided as engineering knowledge. The selection of tests and the interpretation of results should follow the project specifications and the judgment of a licensed engineer.