DAY 13 – Rock Coring, Core Recovery, TCR, SCR & RQD (Geotechnical Investigation)
🪨 Why Day 13 Is Extremely Important
When soil ends and rock begins, geotechnical investigation becomes more critical. Poor understanding of rock quality can lead to:
Wrong foundation type
Unexpected excavation difficulties
Unsafe slope or tunnel design
Day 13 focuses on rock coring, how cores are recovered, and how engineers evaluate rock quality using TCR, SCR, and RQD.
1️⃣ What Is Rock Coring?
Rock coring is the process of drilling into rock formations to extract a cylindrical sample called a rock core.
These cores provide direct information about:
Rock type
Fractures and joints
Weathering condition
Rock strength and quality
2️⃣ Core Recovery – The First Check
🔹 What Is Core Recovery?
Core recovery is the percentage of rock core recovered from a drilled run.
Formula:
Core Recovery (%) = (Length of core recovered / Length of core run) × 100
📌 High recovery generally indicates competent rock, while low recovery may suggest fractured or weathered rock.
3️⃣ Total Core Recovery (TCR)
🔹 What Is TCR?
TCR represents the total length of all recovered core pieces, including broken and fractured pieces.
Formula:
TCR (%) = (Total recovered core length / Core run length) × 100
🔹 TCR Interpretation
90–100% → Excellent
75–90% → Good
50–75% → Fair
< 50% → Poor
📌 TCR gives a general idea of rock mass condition, but does not reflect fracture spacing.
4️⃣ Solid Core Recovery (SCR)
🔹 What Is SCR?
SCR measures the length of solid, intact core pieces, usually longer than 10 cm.
Formula:
SCR (%) = (Length of solid core pieces / Core run length) × 100
🔹 Why SCR Matters
Indicates degree of fracturing
More reliable than TCR for rock quality
Useful for foundation and excavation decisions
5️⃣ Rock Quality Designation (RQD)
🔹 What Is RQD?
RQD is the most widely used parameter to describe rock mass quality.
It considers only sound core pieces longer than 100 mm (10 cm).
Formula:
RQD (%) = (Sum of core pieces ≥ 100 mm / Core run length) × 100
🔹 RQD Classification
| RQD (%) | Rock Quality |
|---|---|
| 0–25 | Very poor |
| 25–50 | Poor |
| 50–75 | Fair |
| 75–90 | Good |
| 90–100 | Excellent |
📌 RQD is a key input for rock mass classification systems.
6️⃣ Relationship Between TCR, SCR & RQD
TCR → How much core was recovered
SCR → How much intact core was recovered
RQD → How good the rock mass really is
High TCR does NOT always mean high RQD.
7️⃣ Importance in Engineering Design
Rock core data is used to:
Select foundation type (shallow, raft, piles)
Design rock sockets
Assess excavation difficulty
Design tunnels and slopes
Classify rock mass (RMR, Q-system)
8️⃣ Common Mistakes on Site
❌ Mixing soil and rock descriptions
❌ Measuring broken core incorrectly
❌ Ignoring drilling-induced fractures
❌ Poor core handling and storage
📌 Accurate logging is essential for reliable results.
9️⃣ Site Logging Best Practices
✔ Measure core run accurately
✔ Mark depth on core boxes
✔ Identify natural vs drilling breaks
✔ Photograph cores
✔ Log weathering and discontinuities
🔚 Final Takeaway
Rock coring is more than drilling — it’s understanding the ground you are building on. TCR, SCR, and RQD together give engineers a clear picture of rock mass quality, helping ensure safe and economical designs.
Day 13 equips you with core skills every geotechnical professional must master.
📘 *Geotech 30 Days Learning Journey –