Methods of test for soils, Part 16: Laboratory determination of CBR

Scope of IS 2720 Part 16 (1987) - California Bearing Ratio (CBR) Test

• Purpose: Determines the CBR value and expansion ratio of soil to assess bearing capacity for pavement design.
• Specimens: Undisturbed or remoulded, soaked or unsoaked.
• Test Conditions: Static/dynamic compaction, light/heavy compaction.
• Results Presentation:
  • CBR value at 2.5 mm and 5.0 mm penetration.
  • Expansion ratio calculated from dial gauge readings.

Key Formulas

• Expansion Ratio (%)
  [ \text{Expansion Ratio} = \frac{d_e - d_s}{h} \times 100 ] Where:
  (d_e) = Final dial gauge reading (mm)
  (d_s) = Initial dial gauge reading (mm)
  (h) = Initial height of specimen (mm)

• CBR Value (%)
  [ \text{CBR} = \frac{\text{Measured load at penetration}}{\text{Standard load}} \times 100 ]

Data Recording Tables (Appendices A & B)

• Specimen Details: Soil ID, condition, compaction type, water content before/after soaking.
• Weight & Density: Mould + soil, mould only, soil weight, volume, bulk & dry density.
• Penetration Data: Load readings at 2.5 mm and 5.0 mm penetration, surcharge weight, soaking period.

Units (SI)

• Force: Newton (N)
• Pressure/Stress: Pascal (Pa) = N/m²
• Length: millimeter (mm)

flowchart LR
    A[Soil Sample] --> B[Compaction (Static/Dynamic)]
    B --> C[Soaking (Soaked/Unsoaked)]
    C --> D[Penetration Test]
    D --> E[Load & Penetration Data]
    E --> F[Calculate CBR & Expansion Ratio]
    F --> G[Results Presentation]

This standard ensures uniform procedures and reporting for CBR tests critical for pavement design.

IS 2720 Part 16 (1987) — Definitions & Key Specifications

• Definitions: As per Clause 2.0, definitions follow IS 2809-1972 and additional terms in this part.

• CBR Test Results (Clause 8.1):

  • CBR Value: Ratio of measured load to standard load at specific penetrations (2.5 mm and 5.0 mm).
  • Expansion Ratio:
    [ \text{Expansion Ratio} = \frac{d_e - d_s}{h} \times 100 ] where:
    (d_s) = initial dial gauge reading,
    (d_e) = final dial gauge reading,
    (h) = initial specimen height (mm).

• Specimen Conditions (Appendix A):

  • Types: Undisturbed/Remoulded, Soaked/Unsoaked.
  • Compaction: Static/Dynamic, Light/Heavy.
  • Key measurements: Weight of mould + soil, soil weight, volume, bulk density, water content, dry density.

• Penetration Data Recording (Appendix B):

  • Load readings at penetration depths.
  • Surcharge weight and soaking period noted.
  • CBR calculated at 2.5 mm and 5.0 mm penetration.

Summary Table: Key Parameters

Notes:

• Use IS 9669 for weights and IS 2720 Part 16 for mould and compaction equipment specifications.
• SI units are standard: Force (N), Pressure (Pa), etc.

flowchart LR
    A[Start: Soil Sample] --> B{Condition}
    B -->|Undisturbed|

IS 2720 Part 16 (1987) — Apparatus & Materials Key Points

Apparatus (Clause 3.11)

• Miscellaneous apparatus include:
  • Mixing bowl
  • Straightedge
  • Scales
  • Soaking tank or pan
  • Drying oven
  • Filter paper
  • Dishes
  • Calibrated measuring jar

Specimen Preparation (Clause 6.1 & Appendix A)

• Record specimen data including:
  • Soil ID, condition (Undisturbed/Remoulded, Soaked/Unsoaked)
  • Type of compaction (Static/Dynamic, Light/Heavy)
  • Soil fraction > 20 mm replaced (kg)
  • Water content and density before and after soaking
• Bulk density and dry density calculated as:

[ \text{Bulk density} = \frac{\text{Weight of soil}}{\text{Volume of specimen}} \quad (g/cc) ]

[ \text{Dry density} = \frac{\text{Bulk density}}{1 + \frac{\text{Water content}}{100}} \quad (g/cc) ]

Penetration Data & CBR Calculation (Clause 6.2 & Appendix B)

• CBR Value at 2.5 mm and 5.0 mm penetration:

[ \text{CBR} = \frac{\text{Test load}}{\text{Standard load}} \times 100% ]

• Expansion Ratio:

[ \text{Expansion ratio} = \frac{d_e - d_s}{h} \times 100 ]

Where:

• ( d_s ) = Initial dial gauge reading (mm)
• ( d_e ) = Final dial gauge reading (mm)
• ( h ) = Initial height of specimen (mm)

Presentation of Results (Clause 8.1)

• Results include:
  • CBR value (%)
  • Expansion ratio (%)

Summary Table: Key Measurements

IS 2720 Part 16: Preparation of Test Specimen - Key Points

1. Specimen Preparation (Clause 4.2)

• Undisturbed Specimens:
  • Use a steel cutting edge mould of 150 mm internal diameter.
  • Push mould gently into soil; dig around if needed.
  • Remove mould by under digging.
  • Trim top & bottom surfaces for required length.
  • If specimen is loose, fill annular cavity with paraffin wax for support.

2. Density & Water Content Determination

• Density can be found by:
  • Weighing soil with mould when full.
  • Measuring soil dimensions and weighing.
  • Field density methods per IS 2720 Part 28 or Part 29.
• Water content per IS 2720 Part 2.

3. Data Recording (Clause 6.1 & 6.2)

• Record specimen data including:
  • Soil ID, compaction type, soil fraction >20 mm replaced.
  • Water content and density before/after soaking.
• Record penetration data for expansion ratio and load penetration (Appendices A & B).

4. Apparatus (Clause 3.11)

• Mixing bowl, straightedge, scales, soaking tank, drying oven, filter paper, calibrated jar.

Summary Table for Specimen Dimensions

flowchart TD
    A[Start: Soil Sampling] --> B[Fit 150 mm mould]
    B --> C[Push mould gently into soil]
    C --> D{Is mould full?}
    D -->|Yes| E[Remove mould by under digging]
    D -->|No| F[Dig around & extract lump]
    F --> G[Trim specimen to size]
    E --> G
    G --> H[Trim top & bottom surfaces flat]
    H --> I{Is soil loose?}
    I -->|Yes| J[Fill annular gap with paraffin wax]
    I -->|No| K[Proceed to density & water content

IS 2720 Part 16: Test for Swelling – Key Formulas & Specifications

1. Expansion Ratio Calculation (Clause 7.1)

The expansion ratio quantifies soil expansiveness after soaking.

[ \text{Expansion ratio} = \frac{d_r - d_s}{h} \times 100 ]

• ( d_r ) = final dial gauge reading (mm)
• ( d_s ) = initial dial gauge reading (mm)
• ( h ) = initial height of specimen (mm)

2. Swelling Test Procedure (Clause 5.1 & 5.2)

• Specimen Preparation: Undisturbed or compacted soil specimen in mould.
• Soaking: Specimen soaked as per procedure.
• Measurement: Dial gauge readings before and after soaking.
• Penetration Test (Optional):
  • Load applied at 1.25 mm/min penetration rate.
  • Loads recorded at penetrations: 0.5, 1.0, 1.5, 2.0, 2.5, 4.0, 5.0, 7.5, 10.0, 12.5 mm.
  • Surcharge weights simulate overburden pressure.
  • Water content measured post-test (IS 2720 Part 2).

3. Test Setup (Fig. 1 Summary)

• Components: Proving ring (load), dial gauge (penetration), penetration plunger (50 mm diameter), surcharge weights.
• Load application: Initial 4 kg seating load, zeroed before penetration.
• Soil sample: Top 30 mm layer sampled for water content.

4. Additional Notes

• Swelling test may be omitted if unnecessary.
• Ensure no oversize particles beneath plunger to avoid erroneous results.
• Record specimen data (Appendix A) including compaction, water content, density before/after soaking.

flowchart TD
    A[Prepare Soil Specimen] --> B[Soak Specimen]
    B --> C[Measure Initial Dial Gauge (ds)]
    C --> D[Measure Final Dial Gauge (dr)]
    D --> E[Calculate Expansion Ratio]
    E --> F{Perform Penetration Test?}
    F -- Yes -->

IS 2720 Part 16 - Record of Observations: Key Points

1. Specimen Data (Appendix A - Clause 6.1)

Record the following on the data sheet:

• Project, Test No., Sample No., Date, Test By
• Soil Identification
• Condition of Specimen: Undisturbed/Remoulded, Soaked/Unsoaked
• Type of Compaction: Static/Dynamic, Light/Heavy
• Soil fraction > 20 mm replaced (kg)
• Water Content (%) before soaking and after test (top, center, bottom)
• Weight measurements for water content calculation:
  • Weight of can + wet soil
  • Weight of can + dry soil
  • Weight of water, can, dry soil
• Condition of Specimen Before and After Soaking:
  • Weight of mould + soil (kg)
  • Weight of mould (kg)
  • Weight of soil (kg)
  • Volume of specimen (cc)
  • Bulk density (g/cc)
  • Average water content (%)
  • Dry density (g/cc)

2. Penetration Data (Appendix B - Clause 6.2)

• Record load at 2.5 mm and 5.0 mm penetration.
• Calculate CBR values at these penetrations.
• Record Expansion Ratio using:

[ \text{Expansion Ratio} = \frac{d_e - d_s}{h} \times 100 ]

Where:

• (d_s) = Initial dial gauge reading (mm)
• (d_e) = Final dial gauge reading (mm)
• (h) = Initial height of specimen (mm)

3. Presentation of Results (Clause 8.1)

• Present results as:
  • CBR value (%)
  • Expansion ratio (%)

Summary Table for Water Content Calculation

IS 2720 Part 16 (1987) Key Formulas and Tables for CBR and Expansion Ratio

1. California Bearing Ratio (CBR)

• Definition: Ratio (%) of force/unit area to penetrate soil with a 50 mm diameter plunger at 1.25 mm/min, compared to standard material.

• Formula:

[ \text{CBR} = \frac{P_s}{P_t} \times 100 ]

Where:

• (P_s) = Corrected load on soil at penetration (2.5 mm or 5 mm) (kg/cm²)

• (P_t) = Standard load for corresponding penetration (2.5 mm = 1370 kg, 5 mm = 2055 kg)

• Note: Use the penetration (2.5 or 5 mm) giving the higher CBR value.

2. Expansion Ratio

• Purpose: Qualitative measure of soil expansiveness.

• Formula:

[ \text{Expansion Ratio} = \frac{d_t - d_s}{h} \times 100 ]

Where:

• (d_t) = Final dial gauge reading (mm)
• (d_s) = Initial dial gauge reading (mm)
• (h) = Initial height of specimen (mm)

3. Presentation of Results (Clause 8.1)

• Report both CBR value (%) and Expansion Ratio (%).

4. Summary Table for CBR Loads (Standard values)

Visual Summary:

flowchart TD
    A[Penetration Load on Soil, \(P_s\)] --> B[Calculate CBR: \(\frac{P_s}{P_t} \times 100\)]
    B --> C{Choose Penetration (2.5 or 5 mm)}
    C -->|Higher CBR| D[Final CBR Value]
    E[Dial Gauge Readings \(d_s, d_t\)] --> F[Calculate Expansion Ratio: \(\frac{d_t - d_s}{

IS 2720 Part 16 - Presentation of Results for CBR Test

Key Outputs (Clause 8.1):

• CBR Value (California Bearing Ratio)
• Expansion Ratio

Formulas:

1. CBR Calculation:

[ \text{CBR} = \frac{P_T}{P_S} \times 100 ]

Where:

• ( P_T ) = Corrected test load at penetration (kgf)
• ( P_S ) = Standard load at the same penetration (kgf)

Standard Loads (Fig. 2/Table):

2. Expansion Ratio:

[ \text{Expansion Ratio} = \frac{d_e - d_s}{h} \times 100 ]

Where:

• ( d_s ) = Initial dial gauge reading (mm)
• ( d_e ) = Final dial gauge reading (mm)
• ( h ) = Initial height of specimen (mm)

Presentation Format (Appendix A & B):

• Soil identification, condition (undisturbed/remoulded, soaked/unsoaked), compaction type.
• Water content before/after soaking at top, center, bottom.
• Weights and densities before and after soaking.
• Penetration load readings and calculated CBR at 2.5 mm and 5 mm.
• Expansion ratio data.
• Use IS 2-1960 for rounding off final values.

Notes:

• Generally, use CBR at 2.5 mm penetration unless 5 mm CBR is higher and repeat tests confirm.
• Record all data systematically as per Appendix B data sheet.

flowchart TD
    A[Start Test] --> B[Measure Load at Penetrations]
    B --> C[Correct Load Values]
    C --> D[Calculate CBR at 2.5 mm & 5 mm]
    D --> E{CBR 2.5 mm > CBR 5 mm?}
    E -- Yes --> F[Take CBR at 2.5 mm]
    E -- No --> G[

Key Formulas and Specifications for California Bearing Ratio (CBR) Test (IS 2720 Part 16)

1. CBR Calculation Formula:

[ \text{CBR} = \frac{P_s}{P_t} \times 100 ]

• (P_s) = Standard load for penetration (kg/cm² or kgf)
• (P_t) = Corrected load from the test at the same penetration

2. Standard Loads for Penetration (from Table/Fig. 2):

3. Penetration Depth for CBR Values:

• CBR is calculated at 2.5 mm and 5.0 mm penetration.
• Usually, the higher CBR value at 2.5 mm is used.
• If CBR at 5 mm > CBR at 2.5 mm, repeat the test.
• If results are consistent, use the 5 mm CBR.

4. Expansion Ratio Calculation:

[ \text{Expansion Ratio} = \frac{d_e - d_s}{h} \times 100 ]

• (d_s) = Initial dial gauge reading (mm)
• (d_e) = Final dial gauge reading (mm)
• (h) = Initial height of specimen (mm)

5. Test Data Sheet Includes:

• Soil identification, condition (undisturbed/remoulded, soaked/unsoaked)
• Type of compaction (static/dynamic, light/heavy)
• Water content before and after soaking
• Weights and densities (bulk, dry)
• Load vs penetration data
• Surcharge weight, soaking period

Summary Diagram of CBR Test Process:

flowchart TD
    A[Prepare Soil Specimen] --> B[Apply Surcharge Weight]
    B --> C[Penetrate with 50mm Plunger at 1.25 mm/min]
    C --> D[Record Load

IS 2720 Part 16: Penetration Data Recording & Calculation

Key Points & Formulas

• Penetration Data Recording:
  Record penetration depth and corresponding load on the data sheet (Appendix B).

• Correction for Load-Penetration Curve (Clause 7.2):

  • Plot load vs. penetration curve.
  • Initial curve may be concave downward due to surface irregularities.
  • Draw tangent at point of greatest slope.
  • Shift origin to tangent intercept on penetration axis (zero penetration).
  • Corrected curve = tangent + original curve from point of tangency.

• Corrected Load Values (Clause 2.5, Table):

• CBR Calculation:

[ \text{CBR} = \frac{P_r}{P_s} \times 100 ]

Where:

• (P_r) = Corrected test load at penetration (2.5 or 5 mm)

• (P_s) = Unit or total standard load from table above

• Design CBR Value:
  Use CBR at 2.5 mm penetration unless 5 mm penetration CBR is higher (then repeat test). If repeated test confirms, use 5 mm CBR.

Summary Diagram of Load-Penetration Correction

graph LR
A[Original Load-Penetration Curve] --> B[Draw Tangent at Max Slope]
B --> C[Find Tangent Intercept on Penetration Axis]
C --> D[Shift Origin to Intercept (Zero Penetration)]
D --> E[Corrected Load-Penetration Curve]

Use this procedure and table for accurate CBR determination and design.