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LabCoda / Knowledge Base / CBR Bearing Capacity Index

🛣️ CBR Bearing Capacity Index

Construction PN-EN 13286-47

In short

Determination of California Bearing Ratio (CBR) of soils and mineral mixtures. The test is performed according to PN-EN 13286-47:2021; Plunger diameter: ø 50 mm (19.6 cm²). The procedure comprises 6 steps (about 4 days 1 h 45 min in total); it is used for: Road pavement design (GDDKiA pavement catalog), Subgrade classification (G1–G4), Base assessment from aggregates and mineral mixtures.

At a glance

  • Standard: PN-EN 13286-47:2021
  • Category: Construction
  • Procedure steps: 6
  • Total time of stages: 4 days 1 h 45 min
  • Plunger diameter: ø 50 mm (19.6 cm²)
  • Penetration rate: 1.27 mm/min
  • Measurement penetration: 2.5 mm and 5.0 mm

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

Key parameters

ParameterValue
Plunger diameterø 50 mm (19.6 cm²)
Penetration rate1.27 mm/min
Measurement penetration2.5 mm and 5.0 mm
Reference force 2.5 mm13.2 kN
Reference force 5.0 mm20.0 kN
Soaking time96 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).

    ⏱ Time: 1 h

  2. Soaking

    Place cylinders in water tank (level ~25 mm above specimen). Apply surcharge load. Mount swelling dial gauge. Soak 96±2 h.

    ⏱ Time: 96 h

  3. Swelling measurement

    Read swelling after 96 h. Swelling = (Δh / h₀) × 100%. Allowable swelling: ≤2% for base.

    ⏱ Time: 5 min

  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.

    ⏱ Time: 10 min

  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.

    ⏱ Time: 15 min

  6. Reporting

    Report CBR with accuracy to 1%. Prepare force-penetration graph. Classify subgrade: G1 (≥25%), G2 (10–25%), G3 (5–10%), G4 (<5%).

    ⏱ Time: 15 min

Required equipment and apparatus

EquipmentExampleIndicative price
CBR set (cylinder + plunger + surcharge)Controls 27-T0110, ELE International5,000–12,000 PLN
CBR pressControls Digimax, 50 kN, rate 1.27 mm/min20,000–50,000 PLN
Proctor setFor specimen compaction3,000–8,000 PLN
Soaking tankWith swelling dial gauge2,000–5,000 PLN
Displacement transducer (LVDT)For penetration measurement with accuracy 0.01 mm3,000–8,000 PLN

Reagents, media and consumables

ReagentCASDetails
Tap water—For specimen soaking (96 h)

Health and safety (OHS)

Frequently asked questions

Which standard describes this test?

The test is performed according to PN-EN 13286-47:2021 — “Unbound and hydraulically bound mixtures — Part 47: Test method for the determination of California bearing ratio, immediate bearing index and linear swelling”.

How does this method work?

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.

What is the measuring range and accuracy?

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.

How long does the test take?

The times given for the individual stages add up to approximately 4 days 1 h 45 min. The procedure comprises 6 steps.

What equipment is required?

CBR set (cylinder + plunger + surcharge), CBR press, Proctor set, Soaking tank, Displacement transducer (LVDT). Indicative cost of the core instrument (CBR set (cylinder + plunger + surcharge)): 5,000–12,000 PLN.

Where is this test used?

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.

What safety precautions apply?

Hydraulic press — crushing hazard, do not insert hands under plunger; Heavy cylinders — lifting ergonomics; Water in tank — wet floor, slip risk.

Which laboratory can perform this test?

The test is performed by laboratories accredited to ISO/IEC 17025. On LabCoda you can find them by the standard number PN-EN 13286-47.

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