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LabCoda / Knowledge Base / CBR Bearing Ratio of Soil

🛤️ CBR Bearing Ratio of Soil

Construction PN-EN 13286-47

In short

Laboratory determination of California Bearing Ratio (CBR) of soil by standard plunger penetration method. The test is performed according to PN-EN 13286-47:2022; Plunger diameter: 49.6 mm (area 1935 mm²). The procedure comprises 11 steps (about 7 days 1 h 5 min–8 days 1 h 5 min in total); it is used for: Design of pavement layer thicknesses, Assessment of subgrade bearing capacity for roads and yards, Classification of soils for road embankment construction.

At a glance

  • Standard: PN-EN 13286-47:2022
  • Category: Construction
  • Procedure steps: 11
  • Total time of stages: 7 days 1 h 5 min–8 days 1 h 5 min
  • Plunger diameter: 49.6 mm (area 1935 mm²)
  • Penetration rate: 1.27 mm/min
  • Measurement penetrations: 2.5 mm and 5.0 mm

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

Key parameters

ParameterValue
Plunger diameter49.6 mm (area 1935 mm²)
Penetration rate1.27 mm/min
Measurement penetrations2.5 mm and 5.0 mm
Standard stresses6.9 MPa (2.5 mm) and 10.3 MPa (5.0 mm)
Soaking time4 days (96 h) in water
CBR cylinderØ150 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.

    ⏱ Time: 1–2 days • 🌡 Temperature: 50°C

  2. Sample moistening

    Add water to achieve optimum moisture (per Proctor result). Mix thoroughly and leave in closed container for 24 h.

    ⏱ Time: 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.

    ⏱ Time: 20 min

  4. Density measurement

    Weigh cylinder with sample. Calculate bulk density. Compare with maximum density from Proctor (required ≥95%).

    ⏱ Time: 5 min

  5. Soaking (4 days)

    Place perforated disc with surcharge on sample. Immerse cylinder in water tank. Mount swelling gauge. Record swelling every 24 h.

    ⏱ Time: 96 h

  6. Removal and drainage

    After 4 days remove cylinder from water. Wait 15 min for gravity drainage. Weigh.

    ⏱ Time: 20 min

  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.

    ⏱ Time: 10 min

  8. Results reading

    Read force at 2.5 mm and 5.0 mm penetrations. Check curve shape — if inflection point, apply correction.

    ⏱ Time: 5 min

  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%.

    ⏱ Time: 5 min

  10. Moisture determination after test

    Take 3 soil samples from cylinder (top, middle, bottom). Dry at 105°C to constant mass. Calculate moisture.

    ⏱ Time: 24 h • 🌡 Temperature: 105°C

  11. Documentation

    Prepare test report: CBR, IBI, swelling, moisture, density, force-penetration curve.

Required equipment and apparatus

EquipmentExampleIndicative price
CBR press with force-displacement recordingControls 82-T0108, Wykeham Farrance 50 kN, Matest S21525 000–80 000 PLN
CBR cylinders (set of 4)Steel Ø150 mm with perforated plate, extension and surcharge1 500–4 000 PLN/set
Proctor compaction apparatusMold Ø150 mm, rammer 4.5 kg (Modified Proctor)3 000–8 000 PLN
Soaking tankSteel or plastic, capacity min. 50 L, with water level gauge500–2 000 PLN
Dial gauge displacement sensorsMitutoyo 2046S, range 10 mm, division 0.01 mm300–800 PLN/pc
Laboratory balance 30 kgRadwag WLC 30/A2, accuracy 1 g2 000–5 000 PLN
Laboratory ovenPOL-EKO SLW 240, 250 L, 300°C max6 000–15 000 PLN

Reagents, media and consumables

ReagentCASDetails
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)

Frequently asked questions

Which standard describes this test?

The test is performed according to PN-EN 13286-47:2022 — “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 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.

What is the measuring range and accuracy?

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).

How long does the test take?

The times given for the individual stages add up to approximately 7 days 1 h 5 min–8 days 1 h 5 min. The procedure comprises 11 steps.

What equipment is required?

CBR press with force-displacement recording, CBR cylinders (set of 4), Proctor compaction apparatus, Soaking tank, Dial gauge displacement sensors, Laboratory balance 30 kg, Laboratory oven. Indicative cost of the core instrument (CBR press with force-displacement recording): 25 000–80 000 PLN.

Where is this test used?

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.

What safety precautions apply?

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.

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.

🔍 Find a laboratory performing this test