🎓 Lesson 1
D1
Why Site Investigation Is the Foundation of Every Geotechnical Project
Site investigation is like taking an X-ray of the ground before building or blasting—to see what’s hidden underground so engineers don’t make dangerous or costly mistakes.
🎯 Learning Objectives
- ✓ Explain the purpose and sequence of key site investigation phases (reconnaissance, desk study, field exploration, lab testing, reporting)
- ✓ Analyze borehole logs and Standard Penetration Test (SPT) N-values to classify soil/rock units and estimate bearing capacity
- ✓ Apply appropriate sampling methods (e.g., Shelby tube vs. rock core) based on material type and required data quality
- ✓ Evaluate groundwater level measurement techniques and their impact on slope stability and excavation dewatering design
- ✓ Design a minimal but adequate investigation program for a small open-pit blast area using ASTM D420 and ISRM guidelines
📖 Why This Matters
In mining and blasting, skipping or underperforming site investigation is like detonating explosives blindfolded—rock mass variability, hidden faults, or unexpected water inflows can cause catastrophic slope failures, misfires, or excessive ground vibration. A single undetected clay seam in a blast bench may trigger sliding; unmeasured joint spacing can ruin fragmentation. Real-world cost: $2M+ in remediation after a 2019 Australian quarry slope failure traced directly to inadequate pre-blast geotechnical characterization. This lesson shows how rigorous subsurface understanding prevents failure—and saves lives, time, and money.
📘 Core Principles
Site investigation rests on four pillars: (1) Geological context—understanding regional tectonics, weathering profiles, and structural geology to anticipate variability; (2) Field exploration—using boreholes, trial pits, geophysics (e.g., seismic refraction), and in-situ tests (SPT, vane shear, RQD) to sample and characterize; (3) Laboratory testing—determining index properties (Atterberg limits, grain size), strength (UCS, triaxial), and deformability (E-modulus); and (4) Interpretation & modeling—integrating data into 2D/3D ground models, identifying critical discontinuities (for blasting), and quantifying uncertainty. Depth and density of investigation follow the 'rule of three': minimum 3 boreholes per 10,000 m² for moderate-risk slopes (per SME Guidelines), with spacing guided by expected geological heterogeneity—not just convenience.
📐 Minimum Borehole Depth Rule-of-Thumb
For slope stability and blast design, boreholes must extend below potential failure surfaces. A widely applied empirical rule ensures adequate depth for analysis while balancing cost and practicality.
Minimum Investigation Depth (H_min)
H_min = H / tan(θ) + ΔHEmpirical depth guideline ensuring boreholes intersect potential failure surfaces beneath a slope or blast bench.
Variables:
| Symbol | Name | Unit | Description |
|---|---|---|---|
| H | Bench or slope height | m | Vertical height from crest to toe |
| θ | Dip angle of dominant discontinuity or failure plane | degrees | Measured from horizontal |
| ΔH | Safety margin | m | Typically 2–3 m to account for data uncertainty and model simplification |
Typical Ranges:
Open-pit blast benches (10–20 m high): 20 – 30 m
Highwall stability assessment: 30 – 50 m
💡 Worked Example
Problem: A proposed 15-m-high blast bench is planned in moderately weathered granite. Preliminary mapping suggests a potential planar failure surface dipping at 35°. What is the minimum recommended borehole depth below bench toe?
1.
Step 1: Identify bench height H = 15 m and dip angle θ = 35°.
2.
Step 2: Apply H_min = H / tan(θ) = 15 / tan(35°) ≈ 15 / 0.700 ≈ 21.4 m.
3.
Step 3: Add 2–3 m safety margin for uncertainty → H_min ≈ 24 m. Verify against typical range: 20–30 m for benches 10–20 m high in competent rock.
Answer:
The result is 24 m, which falls within the safe range of 20–30 m for this context.
🏗️ Real-World Application
At the 2022 Tropicana Gold Mine (Western Australia), pre-blast site investigation revealed previously unmapped subhorizontal quartz veins acting as weak planes within otherwise competent gneiss. SPT and core logging showed RQD <40% across these zones, and direct shear tests confirmed residual friction angles of only 18°—far below the assumed 32°. This discovery triggered redesign of blast patterns (reduced burden, staggered holes) and installation of dowel anchors along the vein trace. Without the investigation, a 120,000-ton slope failure would likely have occurred during first production blast—avoiding ~AUD $45M in losses and ensuring zero fatalities.
🔧 Interactive Calculator
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