🛤️ CBR Bearing Ratio of Soil
Laboratory determination of California Bearing Ratio (CBR) of soil by standard plunger penetration method. Key test for pavement design.
Overview
The California Bearing Ratio (CBR) is one of the most important geotechnical parameters used in road pavement design. The test involves penetrating a standard 50 mm diameter plunger into a compacted soil sample at a constant rate and comparing the obtained stresses with reference values for standard crushed stone from California.
CBR result is expressed as a percentage — the higher the value, the better the soil bearing capacity. Typical values are: clays and organic soils 2–5%, sands 10–30%, gravels 30–80%, crushed stone 80–100%+. Based on CBR value, the thickness of pavement structure layers is designed — lower subgrade CBR requires thicker layers.
Standard PN-EN 13286-47 describes both CBR (after 4-day soaking) and Immediate Bearing Index IBI (without soaking) as well as linear swelling test. Testing is performed on samples compacted by Proctor method in CBR cylinders of 150 mm diameter and 120 mm height.
CBR index is mandatory when designing roads according to the catalog of typical flexible and semi-rigid pavement structures (GDDKiA). It is also used in assessing soil suitability for construction of embankments, cuts and pavement subgrade.
Method principle
Soil sample is compacted in a CBR cylinder (Ø150 mm) by Proctor method at optimum moisture content. After 4-day soaking in water (or without — for IBI), a 49.6 mm diameter plunger is penetrated into the sample at constant rate of 1.27 mm/min. Force is recorded at penetrations of 2.5 and 5.0 mm. CBR is calculated as percentage ratio of measured stress to standard stress: CBR₂.₅ = (p₂.₅ / 6.9 MPa) × 100% and CBR₅.₀ = (p₅.₀ / 10.3 MPa) × 100%. The higher of the two values is taken as result.
Applications
- Design of pavement layer thicknesses
- Assessment of subgrade bearing capacity for roads and yards
- Classification of soils for road embankment construction
- Quality control of improved subgrade layers
- Design of airport pavements
- Assessment of soil suitability for construction purposes
- Testing of subgrade for industrial floors
Key parameters
| Parameter | Value |
|---|---|
| Plunger diameter | 49.6 mm (area 1935 mm²) |
| Penetration rate | 1.27 mm/min |
| Measurement penetrations | 2.5 mm and 5.0 mm |
| Standard stresses | 6.9 MPa (2.5 mm) and 10.3 MPa (5.0 mm) |
| Soaking time | 4 days (96 h) in water |
| CBR cylinder | Ø150 mm, h = 120 mm |
Standard
- Standard number
- PN-EN 13286-47:2022
- Title (PL)
- Mieszanki niezwiązane i związane hydraulicznie — Część 47: Metoda badania kalifornijskiego wskaźnika nośności, natychmiastowego wskaźnika nośności 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. Soil preparation
Dry soil at 50°C, break up lumps, sieve through 20 mm. Determine optimum moisture by Proctor method.
2. Sample moistening
Add water to achieve optimum moisture (per Proctor result). Mix thoroughly and leave in closed container for 24 h.
3. Compaction in CBR cylinder
Compact soil in CBR cylinder in 5 layers of 56 blows with 4.5 kg rammer (Modified Proctor). Level surface.
4. Density measurement
Weigh cylinder with sample. Calculate bulk density. Compare with maximum density from Proctor (required ≥95%).
5. Soaking (4 days)
Place perforated disc with surcharge on sample. Immerse cylinder in water tank. Mount swelling gauge. Record swelling every 24 h.
6. Removal and drainage
After 4 days remove cylinder from water. Wait 15 min for gravity drainage. Weigh.
7. Plunger penetration test
Place cylinder in CBR press. Apply seating load of 44 N. Penetrate plunger at 1.27 mm/min rate. Record force every 0.5 mm penetration.
8. Results reading
Read force at 2.5 mm and 5.0 mm penetrations. Check curve shape — if inflection point, apply correction.
9. CBR calculation
CBR₂.₅ = (σ₂.₅ / 6.9) × 100%, CBR₅.₀ = (σ₅.₀ / 10.3) × 100%. Result = higher of two values, rounded to 1%.
10. Moisture determination after test
Take 3 soil samples from cylinder (top, middle, bottom). Dry at 105°C to constant mass. Calculate moisture.
11. Documentation
Prepare test report: CBR, IBI, swelling, moisture, density, force-penetration curve.
Required equipment and apparatus
| Equipment | Example | Indicative price |
|---|---|---|
| CBR press with force-displacement recording | Controls 82-T0108, Wykeham Farrance 50 kN, Matest S215 | 25 000–80 000 PLN |
| CBR cylinders (set of 4) | Steel Ø150 mm with perforated plate, extension and surcharge | 1 500–4 000 PLN/set |
| Proctor compaction apparatus | Mold Ø150 mm, rammer 4.5 kg (Modified Proctor) | 3 000–8 000 PLN |
| Soaking tank | Steel or plastic, capacity min. 50 L, with water level gauge | 500–2 000 PLN |
| Dial gauge displacement sensors | Mitutoyo 2046S, range 10 mm, division 0.01 mm | 300–800 PLN/pc |
| Laboratory balance 30 kg | Radwag WLC 30/A2, accuracy 1 g | 2 000–5 000 PLN |
| Laboratory oven | POL-EKO SLW 240, 250 L, 300°C max | 6 000–15 000 PLN |
Reagents, media and consumables
| Reagent | CAS | Details |
|---|---|---|
| Tap water | — | For preparing samples at optimum moisture and for soaking (4 days) |
| Filter paper | — | Placed on sample before soaking — prevents washout of fine particles |
| Technical vaseline | — | For lubricating cylinder inner surface before compaction |
Health and safety (OHS)
- CBR press — crushing hazard, do not approach plunger zone during operation
- Proctor rammer 4.5 kg — hand injury risk, use protective gloves
- Heavy cylinders with soil (approx. 10 kg) — careful handling
- Soil dust — use dust mask when transferring dry soil