🔹 DAY 20 – Borehole Log Correction (Phase 1 vs Phase 2)
Why Revisions Matter in Geotechnical Investigation
Many fresh engineers believe that once a borehole log is prepared on site, the job is finished.
❌ This is a dangerous misunderstanding.
In real geotechnical practice, borehole logs go through two critical stages:
✔ Phase 1 – Field (Initial) Borehole Log
✔ Phase 2 – Corrected (Final) Borehole Log
Understanding the difference between these two phases is essential for accurate design and reporting.
🧠 Why Borehole Log Correction Is Important
Borehole logs are the foundation of all geotechnical decisions, including:
• Foundation type and depth
• Pile design and termination level
• Soil parameters selection
• Settlement and bearing capacity calculations
• Groundwater assessment
⚠️ If the borehole log is wrong, the entire design can be unsafe or uneconomical.
1️⃣ Phase 1 Borehole Log (Field Log)
🔍 What Is Phase 1 Logging?
Phase 1 logging is the initial record prepared on site during drilling.
It reflects:
• Visual soil/rock description
• Drilling observations
• Field test results
👉 It is preliminary and subject to change.
🛠 What Is Recorded in Phase 1?
During drilling, the site engineer or geologist records:
• Depth of strata changes
• Soil type (visual/manual classification)
• Color and consistency
• SPT N-values
• Groundwater level (initial)
• Drilling method used
• Core recovery and RQD (for rock)
• Water loss, cavities, or obstructions
⚠️ Limitations of Phase 1 Logs
Phase 1 logs have several limitations:
❌ Soil classification based only on visual judgment
❌ No laboratory confirmation
❌ Groundwater may not be stabilized
❌ Rock strength not yet tested
❌ Possible human error due to time pressure
👉 Phase 1 logs must never be used directly for final design.
2️⃣ Phase 2 Borehole Log (Corrected / Final Log)
🔍 What Is Phase 2 Logging?
Phase 2 logging is the final corrected borehole log, prepared after laboratory testing and detailed review.
It represents the true subsurface condition.
🧪 Data Used for Phase 2 Corrections
Corrections are made using:
• Grain size analysis
• Atterberg limits
• USCS soil classification
• Chemical test results
• UCS & point load tests
• RQD verification
• Stable groundwater readings
• Engineer’s geological judgment
📝 What Changes in Phase 2 Logs?
Common corrections include:
✔ Soil name (e.g., “Silty Sand” → “Poorly Graded Sand with Silt (SP-SM)”)
✔ Layer boundaries refined
✔ Consistency or density updated
✔ Rock description refined
✔ Strength values added
✔ Groundwater level corrected
🆚 Phase 1 vs Phase 2 Borehole Logs (Comparison)
| Feature | Phase 1 (Field Log) | Phase 2 (Final Log) |
|---|---|---|
| Prepared at | Site | Office |
| Basis | Visual + field tests | Lab + field + interpretation |
| Soil Classification | Approximate | USCS-based |
| Groundwater Level | Initial | Stabilized |
| Use in Design | ❌ No | ✔ Yes |
| Reliability | Moderate | High |
💧 Groundwater Level Correction – A Critical Change
One of the most important corrections between Phase 1 and Phase 2 is groundwater level.
Why?
• Water level during drilling may be disturbed
• Drilling fluid affects readings
• Aquifers take time to stabilize
👉 Final groundwater level is taken:
• After 24–48 hours
• Or from observation wells
🪨 Rock Logging Corrections
Rock logs are also corrected in Phase 2:
• RQD recalculated
• Core loss verified
• UCS / point load values added
• Weathering grade refined
Example:
“Weathered Limestone” → “Moderately Weathered Limestone, UCS = 55 MPa, RQD = 70%”
🧠 Engineer’s Interpretation – The Key Element
Phase 2 logging is not copy-paste work.
It requires:
✔ Experience
✔ Engineering judgment
✔ Correlation between logs
✔ Understanding of site geology
⚠️ Two engineers may interpret the same data differently — documentation is critical.
❌ Common Mistakes Fresh Engineers Make
❌ Submitting Phase 1 logs as final
❌ Not updating soil classification
❌ Ignoring lab results
❌ Keeping incorrect groundwater levels
❌ Not revising layer boundaries
📝 How Phase 2 Logs Appear in Final Reports
Typical report note:
“Borehole logs have been reviewed and corrected based on laboratory test results and final groundwater observations.”
🎯 Key Takeaways – Day 20
✔ Phase 1 logs are preliminary only
✔ Phase 2 logs represent true ground conditions
✔ Lab tests drive corrections
✔ Final design must use Phase 2 logs only
✔ Borehole correction is a professional responsibility
📘 Up Next – Day 21:
Trial Pit Logging & CBR / Proctor Tests – Explained Simply