Day 9 – Drilling Logs Explained (Geotechnical Investigation)
Welcome to Day 9 of the Geotech 30 Days Learning Journey. In this lesson, we focus on one of the most important site documents in geotechnical investigation – the Drilling Log.
A drilling log is not just paperwork. It is a technical record of what is actually happening below the ground, and it forms the backbone of geotechnical design decisions.
What Is a Drilling Log?
A drilling log is a systematic, continuous record of subsurface conditions encountered during drilling of boreholes. It documents soil and rock layers, groundwater conditions, sampling details, and in-situ test results.
In simple words:
If you did not record it in the drilling log, it never happened.
Why Drilling Logs Are Extremely Important
Drilling logs are used by:
Geotechnical design engineers
Structural engineers
Foundation designers
Contractors and project managers
They help to:
Understand soil stratification
Identify problematic layers (soft clay, loose sand, fill)
Decide foundation type and depth
Design ground improvement solutions
Verify site conditions during construction
A wrong or poorly prepared log can lead to unsafe designs and costly failures.
Who Prepares the Drilling Log?
On site, drilling logs are usually prepared by:
Site Geologist
Geotechnical Engineer
Sometimes assisted by Drilling Supervisor
The log must be:
Written during drilling, not later
Clear, neat, and technically correct
Signed and verified
Main Components of a Drilling Log
Let’s break down a standard drilling log step by step.
1. General Borehole Information
This section includes:
Project name
Borehole number (BH-01, BH-02, etc.)
Location and coordinates
Ground level (GL)
Drilling method (rotary, wash boring, auger)
Borehole diameter
Drilling start and end date
This information helps identify where and how the borehole was drilled.
2. Depth and Soil Stratification
This is the heart of the drilling log.
For each depth interval, record:
Depth range (e.g., 0.0–1.5 m)
Soil or rock type
Layer boundaries
Soil layers should be described clearly and consistently.
3. Soil Description (As per Visual–Manual Classification)
Soil description typically includes:
Soil type (sand, clay, silt, gravel)
Color
Density or consistency (V-loose, loose, medium dense, dense, V dense / soft, Firm, stiff, V stiff, hard)
Moisture condition (dry, moist, wet)
Plasticity (low, medium, high)
Any inclusions (shells, gravel, organics)
Example:
Brown, medium dense SAND with traces of silt, moist
4. Sampling Details
This section records:
Type of sample (disturbed, undisturbed)
Sampling method (SPT split spoon, Shelby tube)
Sample depth
Sample number
Sample recovery
Sampling is critical because laboratory test results depend on it.
5. Standard Penetration Test (SPT) Data
If SPT is conducted, the log should include:
Test depth
Blow counts for each 150 mm
Final N-value
Remarks (refusal, partial penetration)
SPT values are widely used to:
Estimate soil strength
Assess bearing capacity
Evaluate liquefaction potential
6. Groundwater Level
Groundwater information includes:
Depth where water is first encountered
Stabilized groundwater level
Time of measurement
Groundwater conditions can change foundation design significantly.
7. Drilling Remarks
This section captures important observations such as:
Caving of borehole
Loss of drilling fluid
Hard layers or obstructions
Odor or contamination
These remarks are often as important as test data.
Common Mistakes in Drilling Logs
Fresh engineers often make these mistakes:
Copy-pasting soil descriptions
Missing groundwater observations
Writing logs after drilling is complete
Using non-standard terminology
Ignoring drilling difficulties
Avoid these to become a reliable geotechnical professional.
Best Practices for Good Drilling Logs
Observe soil carefully at the rig
Use standard soil classification systems
Write logs clearly and immediately
Ask questions if unsure
Cross-check with samples and SPT data
Good logs reflect good engineering judgment.
Key Takeaway – Day 9
A drilling log is not just a form—it is a technical story of the ground.
If you master drilling logs:
You understand the site better
Your designs become safer
Your professional value increases
Tomorrow in Day 10, we will discuss Groundwater Conditions & Water Losses during Drilling.
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