🛣️ CBR Bearing Capacity Index
Determination of California Bearing Ratio (CBR) of soils and mineral mixtures. Fundamental parameter for road pavement design.
Overview
CBR (California Bearing Ratio) is empirical soil bearing capacity index, expressed as ratio of force required to press standard plunger into tested soil to force required for same penetration into reference material (crushed aggregate from California). Result given in percent.
CBR is main geotechnical parameter used in road pavement design — pavement layer thickness depends on it (subgrade, base, surface). Typical values: cohesive soils 2–10%, sands 10–30%, gravels 30–80%, stabilized aggregate 80–200%.
Catalog of typical road pavement structures (GDDKiA) requires subgrade CBR ≥25% (G1), ≥10% (G2), ≥5% (G3), <5% (G4 — requires reinforcement).
Method principle
Soil is compacted in steel Proctor cylinder (ø150 mm) by Proctor method (standard or modified) at optimum moisture. Specimen is immersed in water for 96 h (4 days) with surcharge load simulating pavement weight. Swelling is measured.
After soaking, specimen is placed in press and plunger ø50 mm is pressed at rate 1.27 mm/min. Force at penetration 2.5 mm and 5.0 mm is recorded.
CBR = (F / Fref) × 100%
Fref for 2.5 mm = 13.2 kN; for 5.0 mm = 20.0 kN. Higher of two CBR values is adopted.
Applications
- Road pavement design (GDDKiA pavement catalog)
- Subgrade classification (G1–G4)
- Base assessment from aggregates and mineral mixtures
- On-site compaction control (CBR in-situ)
- Soil stabilization — reinforcement effect assessment (cement, lime)
- Airport pavement and maneuvering area design
Key parameters
| Parameter | Value |
|---|---|
| Plunger diameter | ø 50 mm (19.6 cm²) |
| Penetration rate | 1.27 mm/min |
| Measurement penetration | 2.5 mm and 5.0 mm |
| Reference force 2.5 mm | 13.2 kN |
| Reference force 5.0 mm | 20.0 kN |
| Soaking time | 96 h (4 days) in water |
Standard
- Standard number
- PN-EN 13286-47:2021
- Title (PL)
- Mieszanki niezwiązane i związane hydraulicznie — Część 47: Metoda badania kalifornijskiego wskaźnika nośności (CBR), wskaźnika nośności bezpośredniej i pęcznienia liniowego
- Title (EN)
- Unbound and hydraulically bound mixtures — Part 47: Test method for the determination of California bearing ratio, immediate bearing index and linear swelling
Step-by-step procedure
1. Specimen preparation
Compact soil in CBR cylinder (ø150 mm) by Proctor method at optimum moisture. Prepare min. 3 specimens (25, 55, 25 blows per layer).
2. Soaking
Place cylinders in water tank (level ~25 mm above specimen). Apply surcharge load. Mount swelling dial gauge. Soak 96±2 h.
3. Swelling measurement
Read swelling after 96 h. Swelling = (Δh / h₀) × 100%. Allowable swelling: ≤2% for base.
4. Plunger penetration
Place cylinder in press. Apply surcharge. Press plunger ø50 mm at rate 1.27 mm/min. Record force every 0.5 mm penetration up to 12.5 mm.
5. CBR calculations
CBR₂.₅ = (F₂.₅ / 13.2) × 100%. CBR₅.₀ = (F₅.₀ / 20.0) × 100%. Adopt higher value. If CBR₅.₀ > CBR₂.₅ — repeat test.
6. Reporting
Report CBR with accuracy to 1%. Prepare force-penetration graph. Classify subgrade: G1 (≥25%), G2 (10–25%), G3 (5–10%), G4 (<5%).
Required equipment and apparatus
| Equipment | Example | Indicative price |
|---|---|---|
| CBR set (cylinder + plunger + surcharge) | Controls 27-T0110, ELE International | 5,000–12,000 PLN |
| CBR press | Controls Digimax, 50 kN, rate 1.27 mm/min | 20,000–50,000 PLN |
| Proctor set | For specimen compaction | 3,000–8,000 PLN |
| Soaking tank | With swelling dial gauge | 2,000–5,000 PLN |
| Displacement transducer (LVDT) | For penetration measurement with accuracy 0.01 mm | 3,000–8,000 PLN |
Reagents, media and consumables
| Reagent | CAS | Details |
|---|---|---|
| Tap water | — | For specimen soaking (96 h) |
Health and safety (OHS)
- Hydraulic press — crushing hazard, do not insert hands under plunger
- Heavy cylinders — lifting ergonomics
- Water in tank — wet floor, slip risk