What Is a Boring Log?
A boring log (borehole log) is a report that records all soil layers encountered in each borehole, from the ground surface down to the specified depth. It covers soil description, density/consistency and field test results.
The boring log is the key document structural engineers use to design foundations, select pile type and length, and assess how the soil beneath a building will behave.
The model below shows where the data in a boring log comes from. Every line in the report comes from a real borehole: the depth of each layer boundary, each SPT test point and each SS/ST sample is recorded at the depth it was taken, then arranged into columns on the boring log sheet.
Got Your Boring Log? The 3 Most Important Things to Check First
For project owners who are not engineers, there are 3 things to look for first:
- 1. The depth where SPT-N becomes consistently high (N > 30–50) — this is the "bearing layer" where the pile tips will sit. This figure directly sets the pile length.
- 2. The thickness of the soft layer near the top — look for descriptions such as "Very Soft / Soft CLAY". The thicker it is, the more you need to watch for settlement of the ground around the building and of fill.
- 3. Groundwater level (GWL) — if shallow (e.g. less than 1–2 m), excavation for foundations, basements or water tanks needs dewatering and waterproofing planned in advance.
These three points answer almost all of a project owner's main questions: how long must the piles be, will the ground around the building settle, and will excavation hit water?
A Real Boring Log Example (BH-1)
Below is a real boring log from an SPN soil investigation in Ratchathewi District, Bangkok (borehole BH-1, depth 30.45 m, groundwater level 0.50 m), showing every column as found in an actual report.
How to Read Each Column of a Boring Log
An actual boring log has many columns arranged from left to right, each giving different information:
- Depth (m) — depth below ground surface; read the layers from top to bottom
- Symbol / Graphic Log — graphic symbols showing the soil type of each layer
- Sample Type / Number — type and number of each sample, e.g. ST (Shelby tube, undisturbed clay sample) and SS (split spoon from the SPT) — read about the difference at SS vs ST soil samples: what's the difference?
- Soil Description — description of the layer: type, color and consistency, e.g. "Very Soft to Medium Stiff CLAY, Grey (CH)" down to "Hard CLAY, Brown (CL)"
- Su (t/m²) — undrained shear strength of clay, from Unconfined Compression (UC) and Field Vane Shear (FV) tests
- SPT-N (blows/ft) — number of blows in the Standard Penetration Test; the higher the value, the denser the soil
- Water Content / LL / PL (%) — natural water content and Atterberg limits (Liquid Limit, Plastic Limit)
- Total Unit Weight (t/m³) — total unit weight of the soil
Note: In this real example, Su (undrained shear strength) was measured in the upper soft-to-medium clay layers (by UC □ and FV ■ tests), at about 1–3 t/m². The stiff soil and sand layers deeper down are reported as rising SPT-N values (from ~17 to over 70 blows/ft), which form a good bearing layer — sand (cohesionless soil) has no Su value.
What Is the SPT N-Value and How Do You Read It?
The SPT (Standard Penetration Test) N-value per ASTM D1586 is the number of blows of a 63.5 kg hammer falling freely from 76 cm needed to drive a split-spoon sampler into the soil. The sampler is driven a total of 45 cm (in 3 increments of 15 cm). The N-value is the sum of blows for the last two 15 cm increments (30 cm in total); the first 15 cm is the seating drive and is not counted. The higher the value, the denser or stiffer the soil — for a deeper look at turning N-values into bearing capacity estimates, see How to interpret SPT N-values
← Scroll to see the full table →
| N-Value | Density / consistency of soil | Engineering meaning |
|---|---|---|
| 0–4 | Soft clay / very loose sand | Low capacity; not suitable as a bearing layer |
| 5–10 | Medium stiff clay / loose sand | Low bearing capacity; not yet suitable |
| 11–30 | Stiff clay / medium dense sand | Moderate bearing capacity |
| 31–50 | Hard soil / dense sand | Suitable as a pile-tip bearing layer |
| >50 | Hard soil / very dense sand | Best bearing layer |
Look for the depth where the N-value rises consistently above 30–50. That is the "bearing layer" where the engineer will design the pile tips to sit. The depth of this layer varies widely from area to area across Thailand — for a detailed look at dense-sand depths by region, see Bangkok soft clay vs. the provinces
Soil Consistency and Density Classification by SPT N-Value
Geotechnical engineers use the SPT N-value to interpret the "consistency" of clay and the "compactness" (relative density) of sand according to international practice (based on the Terzaghi & Peck tables). These two tables are the key to reading a boring log and understanding how stiff or soft each layer is.
1) Clay (Cohesive Soil) — Consistency and Su
← Scroll to see the full table →
| Consistency | SPT N-Value | Su (t/m²) | Field observation |
|---|---|---|---|
| Very Soft | < 2 | < 1.25 | Easily penetrated by fingers; mud-like |
| Soft | 2–4 | 1.25–2.50 | Can be molded; thumb penetrates easily |
| Medium | 4–8 | 2.50–5.00 | Thumb penetrates with moderate effort |
| Stiff | 8–15 | 5.00–10.00 | Thumb indents, but with difficulty |
| Very Stiff | 15–30 | 10.00–20.00 | Indented only by a thumbnail |
| Hard | > 30 | > 20.00 | Barely marked by a thumbnail |
2) Sand (Cohesionless Soil) — Compactness and Relative Density
← Scroll to see the full table →
| Compactness | SPT N-Value | Relative Density (%) | Field observation |
|---|---|---|---|
| Very Loose | 0–4 | < 15 | Loose sand; flows easily |
| Loose | 4–10 | 15–35 | Easily dug with a shovel |
| Medium Dense | 10–30 | 35–65 | Medium dense |
| Dense | 30–50 | 65–85 | Requires much effort to dig |
| Very Dense | > 50 | 85–100 | Barely diggable by hand; a good bearing layer |
Note: Values in the tables are qualitative reference ranges from international practice (1 t/m² ≈ 10 kPa), intended for a basic understanding of boring log results. Actual design Su and density values must come from field and laboratory testing and be analyzed by a geotechnical engineer case by case.
Other Parts of a Boring Log You Should Know
Groundwater Level
The groundwater level is recorded at the time of drilling and again 24 hours later. This data is important for designing basements and waterproofing and for assessing groundwater pressure on structures.
Soil Classification
Soil type is identified using the USCS (Unified Soil Classification System) or AASHTO, e.g. CH (high-plasticity clay), CL (low-plasticity clay), MH (high-plasticity silt), ML (low-plasticity silt), SP (poorly graded sand), SM (silty sand), SC (clayey sand)
Physical Description of the Soil
Color, moisture, odor and inclusions are recorded, e.g. "dark grey, organic odor" or "reddish brown, with roots". This information helps confirm the soil character and assess the suitability of each layer.
Lab Test Results
Some samples are sent to the laboratory to determine additional engineering properties, such as Atterberg limits (LL, PL, PI), natural water content, unconfined compression strength and grain size analysis — read what each test measures and what it is used for in the laboratory soil testing guide
Warning Signs in a Boring Log to Raise with Your Engineer Right Away
Some boring logs contain data that points to risks the project owner should notice and ask the design engineer about:
- A thin hard layer with soft soil beneath it — e.g. the N-value jumps up and then drops low again at the next depth. If pile tips are founded on this false hard layer, the building will settle later.
- Large differences in N between boreholes — indicates variable ground under the building footprint; additional boreholes or zoned foundation design may be needed.
- Organic soil or waste fill — descriptions such as "Organic Clay" or "Fill Material" indicate layers that cannot carry load and may keep compressing.
- Unusually shallow groundwater — affects excavation, bored pile construction and uplift (buoyancy) on underground structures.
These observations do not mean you cannot build, but they do mean the foundation design must deal with them properly, by a licensed engineer.
Once you receive the boring log, ask the design engineer: "At what depth is the main bearing layer?" and "What pile type and length do you recommend?" The answers will help you understand the foundation design much better.
Why Does the Bearing Layer Depth Differ from Area to Area?
The depths in your boring log will not always match other projects, because geology varies greatly across Thailand's regions — Bangkok and its surrounding provinces have a thick soft clay layer known as "Bangkok Clay", so piles must be several times longer than in other regions, while some areas in the North or Northeast may find stiff soil or rock much shallower. There is no "standard" depth that applies everywhere; even within one province, conditions can change within a few hundred meters — for a full comparison of Bangkok soils with other regions, including a pile-length table by area, see Bangkok soft clay vs. the provinces — why foundations must be designed differently
Real Provincial Boring Log Examples: Phuket vs. Khon Kaen
Boring logs from the provinces look clearly different from Bangkok. The two real examples below from SPN soil investigations — borehole BH-6 in Phuket and borehole BH-1 in Khon Kaen — show how soil type, the depth to stiff strata and even the data columns in the report change with local geology.
Example 1: Phuket Boring Log (BH-6, Thalang District) — Interbedded Sand and Clay
Borehole BH-6 in Thalang District, Phuket, drilled to 20.00 m, shows an "interbedded sand and clay" profile common in southern coastal areas, unlike the thick continuous soft clay of Bangkok:
- 0–3.5 m — loose to medium dense silty sand with gravel (SM, SP), N = 6–14
- 3.5–8.0 m — dense to very dense sand (SP), N = 27–55
- 8.0–10.0 m — soft clay (Soft CLAY, CH), N = 4 lying beneath the dense sand
- 10.0–15.5 m — medium dense sand (SP-SM, SM) and stiff to very stiff sandy clay (CL), N = 13–24
- 15.5–20.0 m — very dense clayey sand (SC), N = 52 up to 50/6"
The most interesting feature of this borehole is the soft clay layer (N = 4) lying beneath dense sand (N = 39–55) — exactly the first "warning sign" described above. If drilling had stopped at 6 m because dense sand had been reached, nobody would know there was a soft layer hidden below. This is why borehole depth must be set by an engineer, not stopped as soon as the first hard layer is found.
Example 2: Khon Kaen Boring Log (BH-1) — Shallow Hard Soil and Weathered Rock
Borehole BH-1 in Khon Kaen reached rock at only 10.00 m — the complete opposite of Bangkok:
- 0–0.5 m — medium dense clayey sand with gravel (SC), N = 13
- 0.5–5.0 m — hard gravelly clay (Hard Gravelly CLAY, CL); SPT recorded as 50/3" to 80/11", i.e. so hard the sampler could not be driven the standard distance
- 5.0–10.0 m — highly to slightly weathered sandstone and claystone; sampling switched to rock coring (RC-1 to RC-5), giving RQD = 38–71%
Note that this boring log has different columns from the Bangkok and Phuket examples — an RQD (%) graph is added for the rock section, and the sample type changes from SS (split spoon) to RC (rock core), which is the standard format when an investigation encounters rock.
How to Read 50/6" and RQD in a Boring Log
A value such as 50/6" means that after 50 blows the sampler penetrated only 6 inches (about 15 cm), short of the standard 30 cm. The supervisor stops driving to avoid damaging the equipment and records the number of blows with the actual penetration instead of a normal N-value. Such a value indicates soil so dense or hard that it is considered practical refusal.
RQD (Rock Quality Designation) is the percentage of intact core recovered, calculated as the total length of core pieces 10 cm or longer divided by the length of the cored run. The higher the value, the more intact the rock — typical criteria are RQD < 25% = Very Poor, 25–50% = Poor, 50–75% = Fair, 75–90% = Good, 90–100% = Excellent. In the Khon Kaen example, RQD of 38–71% falls in the poor-to-fair range, which is common in weathered rock.
Comparison of Boring Logs from 3 Areas (Real Boreholes)
← Scroll to see the full table →
| Area (example borehole) | Dominant soil profile | Stiff/bearing layer found | Borehole depth |
|---|---|---|---|
| Bangkok (BH-1, Ratchathewi) | Thick soft clay (Bangkok Clay) continuous in the upper part | Stiff soil/dense sand at depth; N consistently high toward the bottom of the hole | 30.45 m |
| Phuket (BH-6, Thalang) | Interbedded sand and clay, with a soft layer at 8–10 m | Very dense clayey sand at 15.5–20 m (N ≥ 52) | 20.00 m |
| Khon Kaen (BH-1) | Hard gravelly clay from ~0.5 m | Weathered rock from ~5 m (RQD 38–71%) | 10.00 m |
Note: All data come from specific real boreholes and are shown only for learning to read boring logs. They are not representative of whole provinces — soil layers can differ even a few hundred meters apart. Actual foundation design requires a soil investigation at the project location, analyzed by an engineer case by case.
Boring Log Examples and Forms — Download a Sample File
For anyone who wants to see a full report, SPN offers a sample boring log file from a real soil investigation (boring test) to use alongside this article, or to compare with reports you have received from other soil boring contractors. The file is in the same format delivered to actual clients, with every column: soil symbols, SPT N-values, groundwater level and basic test results.
⬇ Download sample boring log (PDF)
Where Can You Find Public Boring Log Data?
If you have not yet carried out a soil investigation and want a rough idea of the soil profile in your area first, there are public boring log sources you can use for preliminary reference:
- Department of Public Works and Town & Country Planning (DPT) — maintains a database of soil investigation results from government projects nationwide, useful for seeing area-level soil trends (search for "DPT soil layer data")
- Universities and research — many soil mechanics theses and research papers publish boring logs, especially for Bangkok and the surrounding provinces
- Owners of large public projects — e.g. road, bridge or government building projects often attach soil investigation results to their tender documents (TOR), which are public documents
Caution: This public data is only suitable for "preliminary planning" — soil layers can differ even a few hundred meters apart. Actual foundation design still requires a soil investigation at your own project location to ASTM D1586, with results certified by an engineer.