🎓 Lesson 9
D5
Planning a Landslide Site Investigation (Drilling, Sampling, In-Situ Tests)
Planning a landslide site investigation means carefully deciding where, how deep, and what kind of tests to do underground to understand why the slope might be moving.
🎯 Learning Objectives
- ✓ Design a borehole layout that satisfies spacing, depth, and orientation requirements per ISRM and ASTM standards
- ✓ Analyze soil/rock core recovery and RQD data to classify rock mass quality and estimate shear strength parameters
- ✓ Apply correction factors to SPT-N values for overburden pressure and rod length to estimate effective friction angle and cohesion
- ✓ Explain how piezometer placement depth and filter zone design influence pore-water pressure monitoring reliability in saturated landslide zones
📖 Why This Matters
Every major landslide failure—like the 2014 Oso landslide (USA) or 2017 Xinmo landslide (China)—was preceded by unrecognized subsurface conditions: hidden weak layers, rising pore pressures, or weathered shear zones. Skipping or misdesigning field investigations leads to underestimating risk, failed stabilization projects, and preventable loss of life and infrastructure. This lesson equips you to ask the right questions *before* the first drill rig arrives.
📘 Core Principles
A robust landslide investigation follows three interdependent pillars: (1) *Geological targeting*—using surface mapping, aerial LiDAR, and historical failure data to identify likely slip surfaces and lithologic boundaries; (2) *Representative sampling*—ensuring core recovery ≥85% in rock and undisturbed sampling (Shelby tubes, piston samplers) in soft soils to preserve structure and moisture content; and (3) *In-situ validation*—using tests like vane shear (clays), dilatometer (fine-grained soils), and pressuremeter (weathered rock) to calibrate lab-derived strength parameters. Depth planning must extend ≥5 m below inferred slip surface; borehole inclination (typically 15–30° upslope) improves intersection probability with planar failure surfaces.
📐 Corrected Standard Penetration Test (SPT) Value
The raw SPT blow count (N) is corrected for overburden stress and equipment inefficiency to estimate effective soil strength. This corrected value (N60 or N1,60) correlates empirically with friction angle (φ') and undrained shear strength (su).
💡 Worked Example
Problem: Given: Measured SPT N = 24 blows/30 cm at depth 8.5 m in sandy silt; effective overburden stress σ'ᵥ = 115 kPa; hammer energy ratio = 60%; rod length = 12 m.
1.
Step 1: Apply energy correction: N₆₀ = N × (Er/60) = 24 × (60/60) = 24
2.
Step 2: Apply overburden correction: CN = 0.77 × log₁₀(2000/σ'ᵥ) = 0.77 × log₁₀(2000/115) ≈ 0.77 × 1.24 ≈ 0.95
3.
Step 3: Compute N₁,₆₀ = N₆₀ × CN = 24 × 0.95 = 22.8 ≈ 23
4.
Step 4: Estimate φ' using Kulhawy & Mayne (1990): φ' ≈ 27.1 + 0.3 × N₁,₆₀ − 0.00054 × N₁,₆₀² = 27.1 + 6.9 − 0.28 ≈ 33.7°
Answer:
The corrected N₁,₆₀ is 23, yielding an estimated effective friction angle of 33.7°, within the typical range of 30–36° for dense sandy silt.
🏗️ Real-World Application
During the 2018 Mud Creek landslide (California, USA), investigators deployed 14 inclined boreholes (20° upslope) across the headscarp and toe. Core logging revealed a previously unmapped 0.3–0.8 m thick clay-rich shear zone at 12–15 m depth—confirmed by low vane shear strengths (su = 12–18 kPa) and elevated piezometric levels (>80% of overburden). This discovery redirected stabilization from surface drainage alone to deep toe berms and targeted dewatering, preventing reactivation during subsequent El Niño rains.
✏️ Field Layout Design Exercise
You are tasked with investigating a 200-m-long, 45° steep translational landslide in foliated schist. Surface cracks suggest a shallow failure (5–8 m deep), but historical photos indicate deeper movement. Using ISRM (2007) guidelines and slope geometry, design a minimum borehole array: (a) number and spacing of boreholes; (b) minimum depth for each; (c) recommended inclination and justification; (d) required in-situ tests at 3 m, 6 m, and 10 m depths—and explain why each test is selected.
🔧 Interactive Calculator
🔧 Open Slope Stability & Landslide Risk Calculator📋 Case Connection
📋 Post-Earthquake Landslide Stabilization — Kaikōura, New Zealand
Multiple deep-seated rockslides blocking critical transport corridor; unstable toe conditions and high pore pressures
📋 Tailings Storage Facility (TSF) Slope Reinforcement — Pilbara, Australia
Existing FoS < 1.1 under Mw 6.5 scenario; limited space for buttressing; strict environmental containment requirements
📋 Historic Landslide Reactivation Mitigation — Portuguese Riviera
Complex kinematics (translational + rotational), marine clay layer at depth, saltwater intrusion affecting pore pressure...