🎓 Lesson 8
D2
Interpreting Lab Reports: Atterberg Limits, Grain Size, and CBR
Atterberg Limits, Grain Size, and CBR are lab tests that tell us how soil behaves when wet or dry, what it’s made of, and how strong it is under load — critical for deciding if it can safely support a building foundation.
🎯 Learning Objectives
- ✓ Explain the physical significance of liquid limit, plastic limit, and plasticity index in relation to foundation settlement and swelling potential
- ✓ Analyze grain size distribution curves to classify soil per USCS and infer drainage, compressibility, and compaction characteristics
- ✓ Calculate CBR values from laboratory test data and interpret results against design thresholds for shallow foundations
- ✓ Apply Atterberg and grain size data to select appropriate foundation type (e.g., spread footing vs. mat) for cohesive vs. cohesionless soils
📖 Why This Matters
In shallow foundation design, getting the soil wrong means risking differential settlement, cracking, or even collapse — not from poor structural design, but from misreading the lab report. A mining/blasting engineer may oversee site preparation where overburden removal exposes variable soil layers; misinterpreting Atterberg limits could lead to inadequate excavation or improper backfill specification. These three tests are the 'vital signs' of soil — and every foundation starts with them.
📘 Core Principles
Soil behavior is governed by particle size and interparticle forces. Atterberg Limits reflect colloidal clay activity: high plasticity (PI > 20) indicates expansive clays prone to volume change with moisture — dangerous beneath slabs. Grain size analysis separates particles by diameter (ASTM D422/D2487): coarse-grained soils (>50% gravel/sand) rely on friction for strength, while fine-grained soils (<50% passing No. 200 sieve) depend on cohesion and water content. CBR (ASTM D1883) simulates wheel load penetration; a CBR < 3% signals highly compressible soil requiring deep excavation or stabilization, whereas CBR > 15% often permits direct-on-grade foundations. Together, these tests inform soil classification (USCS), allowable bearing pressure, and need for remediation.
📐 CBR Calculation
CBR is calculated as the ratio of the measured soil resistance at specific penetrations (2.5 mm and 5.0 mm) to the resistance of standard crushed rock aggregate, expressed as a percentage. The lower value governs design unless otherwise specified by agency criteria.
💡 Worked Example
Problem: A CBR test yields 4.2 MPa at 2.5 mm penetration and 5.8 MPa at 5.0 mm penetration. Standard crushed rock resistance is 7.0 MPa at 2.5 mm and 10.5 MPa at 5.0 mm.
1.
Step 1: Compute CBR at 2.5 mm: (4.2 MPa / 7.0 MPa) × 100 = 60%
2.
Step 2: Compute CBR at 5.0 mm: (5.8 MPa / 10.5 MPa) × 100 = 55.2%
3.
Step 3: Per ASTM D1883, the reported CBR is the lesser value unless the 5-mm curve is steeper — here, 55.2% governs.
Answer:
The result is 55%, which falls within the safe range of 10–80% for granular subgrades supporting shallow foundations on stable terrain.
🏗️ Real-World Application
At the Telfer Gold Mine (Western Australia), pre-foundation investigation of a new workshop pad revealed LL = 68, PL = 29, PI = 39, and CBR = 2.1% in the upper 1.2 m of grey clay. Grain size showed 72% passing No. 200 sieve. Engineers classified the soil as CH (high-plastic clay) and rejected direct shallow footings. Instead, they specified 1.5 m of imported select fill (CBR ≥ 20%) compacted to 95% Proctor, with geotextile separation — reducing long-term settlement risk by >80% compared to untreated clay. This decision was directly driven by integrating all three lab tests.