🎓 Lesson 5
D2
Cone Penetration Test (CPT) Interpretation: qc, fs, and u₂ Profiling
The Cone Penetration Test (CPT) is like pushing a sharp, instrumented cone into the ground to measure how hard the soil resists — telling us what’s underground without digging.
🎯 Learning Objectives
- ✓ Interpret qc, fs, and u₂ profiles to classify soil layers using standard CPT-based charts
- ✓ Calculate friction ratio (Rf) and normalized pore pressure ratio (Bq) from raw CPT data
- ✓ Apply Robertson’s soil behavior type (SBTn) chart to distinguish between clay, silt, sand, and gravel strata
- ✓ Analyze trends in qc and u₂ to identify groundwater level and potential liquefaction zones
📖 Why This Matters
In mining and open-pit blasting design, knowing subsurface conditions isn’t optional—it’s foundational. A misclassified weak clay layer beneath a proposed blast bench could cause slope failure or poor fragmentation. CPT delivers rapid, high-resolution soil profiling with minimal disturbance—critical for designing stable haul roads, waste dumps, and blast pad foundations. Unlike boreholes, CPT provides continuous data every 2 cm, revealing subtle transitions (e.g., thin silt seams in sand) that control drainage and stability.
📘 Core Principles
CPT relies on measuring three forces during steady-state penetration: (1) qc — the axial resistance at the 10 cm² cone tip, primarily reflecting soil strength and stiffness; (2) fs — the lateral friction along the 150 cm² cylindrical sleeve, sensitive to soil density and roughness; and (3) u₂ — pore water pressure measured behind the cone shoulder, indicating drainage conditions and excess pore pressure generation. The friction ratio Rf = (fs / qc) × 100% helps discriminate fine-grained (low Rf < 1%) from coarse-grained (Rf > 2%) soils. Normalized pore pressure Bq = (u₂ − u₀) / (qc − σᵥ₀), where u₀ is hydrostatic pressure and σᵥ₀ is effective overburden stress, reveals soil sensitivity and consolidation state. Robertson’s SBTn classification integrates qc, Rf, and Bq to assign Soil Behavior Type numbers (1–7), directly linked to engineering properties.
📐 Key Calculations: Rf and Bq
Friction ratio (Rf) quantifies relative contribution of skin friction to tip resistance — essential for soil type discrimination. Normalized pore pressure ratio (Bq) corrects for overburden and hydrostatic effects, enabling assessment of soil compressibility and drainage response. Both are foundational for SBTn chart interpretation.
💡 Worked Example
Problem: At depth 8.2 m in a coastal alluvial deposit: qc = 4.8 MPa, fs = 0.12 MPa, u₂ = 95 kPa, σᵥ₀ = 142 kPa, u₀ = 80 kPa.
1.
Step 1: Compute Rf = (fs / qc) × 100 = (0.12 / 4.8) × 100 = 2.5%
2.
Step 2: Compute Δu₂ = u₂ − u₀ = 95 − 80 = 15 kPa
3.
Step 3: Compute Bq = Δu₂ / (qc − σᵥ₀) = 15 / (4800 − 142) = 15 / 4658 ≈ 0.0032
Answer:
Rf = 2.5% and Bq ≈ 0.003 — placing this point in SBT Zone 4 (sand with traces of silt) per Robertson (2010).
🏗️ Real-World Application
At the Diavik Diamond Mine (Northwest Territories, Canada), CPT was deployed across a proposed waste rock dump alignment to assess underlying glaciolacustrine clays. qc values dropped from >5 MPa in till to <0.8 MPa at 6–9 m depth, while u₂ surged and Rf exceeded 8% — signaling highly sensitive, low-strength clay. This prompted redesign of the dump’s foundation layer (including surcharge preloading and wick drains), avoiding post-construction settlement exceeding 1.2 m. CPT profiling identified a previously unmapped 0.5-m-thick silt lens at 7.3 m that would have caused localized lateral spreading during loading — a detail missed by 30-m-spaced boreholes.