Methods of test for soils, Part 16: Laboratory determination of CBR
1987 Edition

Overview of IS 2720 Part 16 (1987) – California Bearing Ratio Testing

• Objective: To determine CBR values and expansion ratios to evaluate soil suitability for pavement foundations.
• Sample Types: Both undisturbed and remoulded soils, with options for soaked and unsoaked conditions.
• Compaction Techniques: Use of static and dynamic compaction, including light and heavy compaction variants.
• Result Reporting:
  • CBR determined at penetration depths of 2.5 mm and 5.0 mm.
  • Expansion ratio derived from dial gauge measurements.

Fundamental Equations

• 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) = Original specimen height (mm)

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

Data Recording Templates (Appendices A & B)

• Sample Information: Soil identification, specimen condition, compaction method, moisture content pre- and post-soaking.
• Mass and Density: Weights of mould plus soil, mould alone, soil mass, volume, bulk and dry densities.
• Penetration Measurements: Load recorded at 2.5 mm and 5.0 mm penetration, surcharge mass, soaking duration.

Units Utilized (SI System)

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

flowchart LR
    A[Soil Sample] --> B[Compaction (Static or Dynamic)]
    B --> C[Soaking (Soaked or Unsoaked)]
    C --> D[Penetration Testing]
    D --> E[Load and Penetration Data]
    E --> F[CBR and Expansion Ratio Computation]
    F --> G[Reporting of Results]

This standard ensures consistent methodology and results presentation essential for pavement design.

Key Definitions and Specifications in IS 2720 Part 16 (1987)

• Definitions: Refer to Clause 2.0; terms comply with IS 2809-1972 and additional definitions within this part.

• CBR Test Outputs (Clause 8.1):

  • CBR Value: Ratio of the measured load to the standard load at designated 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):

  • Variants: Undisturbed or remoulded, soaked or unsoaked.
  • Compaction Types: Static or dynamic; light or heavy.
  • Measurements: Weight of mould with soil, soil mass, specimen volume, bulk density, moisture content, dry density.

• Penetration Data Documentation (Appendix B):

  • Load values at specified penetrations.
  • Details of surcharge weight and soaking duration.
  • CBR calculated at 2.5 mm and 5.0 mm penetration depths.

Parameter Summary Table

Additional Notes:

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

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

Overview of Apparatus and Materials for IS 2720 Part 16 (1987)

Equipment (Clause 3.11)

• Essential tools include:
  • Mixing bowl for soil preparation
  • Straightedge for specimen trimming
  • Precision scales
  • Soaking tank or pan for immersion
  • Oven for drying samples
  • Filter paper
  • Dishes for sample handling
  • Calibrated measuring jar

Sample Preparation (Clause 6.1 & Appendix A)

• Document specimen details such as:
  • Soil identification and condition (undisturbed/remoulded, soaked/unsoaked)
  • Compaction technique (static/dynamic, light/heavy)
  • Weight of soil fraction larger than 20 mm replaced
  • Moisture content and density before and after soaking
• Calculate bulk and dry densities:

[ \text{Bulk density} = \frac{\text{Soil weight}}{\text{Specimen volume}} \quad (g/cm^3) ]

[ \text{Dry density} = \frac{\text{Bulk density}}{1 + \frac{\text{Moisture content}}{100}} \quad (g/cm^3) ]

Penetration Data and CBR Computation (Clause 6.2 & Appendix B)

• CBR values 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 specimen height (mm)

Reporting Results (Clause 8.1)

• Include:
  • CBR percentage
  • Expansion ratio percentage

Measurement Summary Table

Guidelines for Specimen Preparation in IS 2720 Part 16

1. Obtaining Undisturbed Samples (Clause 4.2)

• Use a steel mould with a 150 mm internal diameter.
• Carefully push the mould into the soil; if resistance is encountered, excavate around the mould.
• Extract the mould by digging beneath it.
• Trim the specimen’s top and bottom to the required length with flat surfaces.
• For loose soil samples, fill any annular voids with paraffin wax to provide support.

2. Determining Density and Moisture Content

• Density can be assessed by:
  • Weighing the soil along with the mould when full.
  • Measuring dimensions and weighing soil.
  • Field density tests as per IS 2720 Part 28 or Part 29.
• Moisture content determined in accordance with IS 2720 Part 2.

3. Data Documentation (Clauses 6.1 & 6.2)

• Record details such as soil ID, compaction method, replacement of particles >20 mm.
• Note moisture and density before and after soaking.
• Document penetration measurements for calculating expansion ratio and load penetration (Appendices A & B).

4. Equipment Utilized (Clause 3.11)

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

Specimen Dimension Summary

flowchart TD
    A[Begin Soil Sampling] --> B[Place 150 mm diameter mould]
    B --> C[Press mould into soil gently]
    C --> D{Is mould fully inserted?}
    D -->|Yes| E[Extract mould by under digging]
    D -->|No| F[Excavate soil around mould & retrieve]
    E --> G[Trim specimen surfaces]
    F --> G
    G --> H[Level top and bottom surfaces]
    H --> I{Is soil loose?}
    I -->|Yes| J[Fill annular cavity with paraffin wax]
    I -->|No| K[Proceed to density and moisture testing]

IS 2720 Part 16: Procedures for Measuring Soil Swelling and Expansion

1. Calculation of Expansion Ratio (Clause 7.1)

The expansion ratio quantifies the volumetric increase of soil after soaking.

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

• ( d_r ) = Final dial gauge measurement (mm)
• ( d_s ) = Initial dial gauge measurement (mm)
• ( h ) = Original specimen height (mm)

2. Swelling Test Method (Clauses 5.1 & 5.2)

• Sample Preparation: Use undisturbed or compacted soil specimens in moulds.
• Soaking: Immerse specimens as specified.
• Measurements: Record dial gauge readings before and after soaking.
• Penetration Testing (Optional):
  • Apply load at a penetration rate of 1.25 mm/min.
  • Record loads at multiple penetration depths: 0.5, 1.0, 1.5, 2.0, 2.5, 4.0, 5.0, 7.5, 10.0, 12.5 mm.
  • Apply surcharge weights to simulate overburden pressure.
  • Determine moisture content after testing (per IS 2720 Part 2).

3. Test Setup Details (Refer to Figure 1)

• Components include a proving ring for load measurement, dial gauge for penetration, a 50 mm diameter penetration plunger, and surcharge weights.
• Initial seating load of 4 kg applied; zero calibrated before penetration.
• Sample top ~30 mm layer used for moisture analysis.

4. Additional Considerations

• Swelling test may be omitted if not required.
• Ensure no oversized soil particles beneath the plunger to prevent inaccurate readings.
• Record specimen data including compaction type, moisture content, and densities before and after soaking (Appendix A).

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

Guidelines for Recording Observations in IS 2720 Part 16

1. Specimen Information (Appendix A, Clause 6.1)

Record the following parameters:

• Project name, test number, sample identification, date, and tester’s name
• Soil identification
• Sample condition: undisturbed or remoulded; soaked or unsoaked
• Compaction method: static or dynamic; light or heavy
• Weight of soil particles replaced (> 20 mm fraction)
• Moisture content (%) before soaking and post-test (top, center, bottom sections)
• Weight measurements necessary for moisture determination:
  • Weight of container plus wet soil
  • Weight of container plus dry soil
  • Weight of water, container, and dry soil
• Specimen condition before and after soaking:
  • Weight of mould plus soil (kg)
  • Weight of mould alone (kg)
  • Soil weight (kg)
  • Volume of specimen (cc)
  • Bulk density (g/cm³)
  • Average moisture content (%)
  • Dry density (g/cm³)

2. Penetration Data (Appendix B, Clause 6.2)

• Record load values specifically at 2.5 mm and 5.0 mm penetration depths.
• Calculate CBR values for these depths.
• Compute 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 specimen height (mm)

3. Reporting of Outcomes (Clause 8.1)

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

Moisture Content Calculation Table

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

1. California Bearing Ratio (CBR) Definition

• Expressed as a percentage ratio of the load per unit area required to penetrate a soil sample using a 50 mm diameter plunger at a rate of 1.25 mm/min relative to a standard reference material.

• Calculation formula:

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

Where:

• (P_s) = Corrected soil load at specified penetration (kg/cm²)

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

• Use the penetration depth (2.5 mm or 5 mm) that yields the higher CBR value.

2. Expansion Ratio

• Provides a measure of soil's potential for swelling.

• 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) = Original specimen height (mm)

3. Presentation of Computed Values (Clause 8.1)

• Report both the CBR percentage and expansion ratio.

4. Standard Load Table for CBR

Visual Representation:

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

Presentation of CBR Test Results According to IS 2720 Part 16

Essential Outputs (Clause 8.1):

• CBR Value (California Bearing Ratio)
• Expansion Ratio

Key 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 (from 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) = Original specimen height (mm)

Reporting Format (refer Appendices A & B):

• Soil identification and condition (undisturbed/remoulded; soaked/unsoaked)
• Compaction type
• Moisture content before and after soaking at top, middle, bottom
• Weight and density values pre- and post-soaking
• Load readings and CBR calculations at 2.5 mm and 5 mm penetration
• Expansion ratio data
• Final figures rounded as per IS 2-1960

Notes:

• Generally, the CBR value at 2.5 mm penetration is utilized unless the 5 mm value is considerably higher and verified by repeat testing.
• All data should be systematically recorded using the prescribed data sheets.

flowchart TD
    A[Commence Test] --> B[Measure Load at Specified Penetrations]
    B --> C[Apply Load Corrections]
    C --> D[Calculate CBR at 2.5 mm and 5 mm]
    D --> E{Is CBR at 2.5 mm greater than at 5 mm?}
    E -- Yes --> F[Adopt CBR at 2.5 mm]
    E -- No --> G[Repeat Test]
    G --> H{Does Repeat Confirm Higher CBR at 5 mm?}
    H -- Yes --> I[Adopt CBR at 5 mm]
    H -- No --> F

Essential Formulas and Parameters for CBR Testing per IS 2720 Part 16

1. Formula for Calculating CBR:

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

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

2. Standard Load Values (from Table/Figure 2):

3. Penetration Depths for CBR Computation:

• Calculations performed at 2.5 mm and 5.0 mm penetration depths.
• Generally, the 2.5 mm value is preferred.
• If the 5 mm value is higher, repeat tests are recommended.
• Upon confirmation, the 5 mm value can be used.

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 specimen height (mm)

5. Data Sheet Contents:

• Soil ID, condition (undisturbed/remoulded, soaked/unsoaked)
• Compaction method (static/dynamic, light/heavy)
• Moisture content before and after soaking
• Weight and density measurements (bulk, dry)
• Load versus penetration data
• Surcharge weight and soaking duration

Summary Diagram of CBR Test Procedure:

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

IS 2720 Part 16: Procedures for Recording and Correcting Load-Penetration Data

Important Details and Formulae

• Recording Data: Document penetration depth alongside corresponding load measurements on the data sheet (Appendix B).

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

  • Plot the load against penetration depth.
  • Initial curve may show a downward concavity due to surface unevenness.
  • Draw a tangent at the section with the steepest slope.
  • Shift the penetration axis origin to the tangent's intercept point (defining zero penetration).
  • The corrected curve combines this tangent with the original data from the tangency point onward.

• Standard Load Values (Clause 2.5, Table):

• CBR Calculation:

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

Where:

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

• (P_s) = Standard load from table

• Design CBR Selection: Use CBR value at 2.5 mm penetration unless the 5 mm penetration CBR is higher; in that case, repeat the test. If the repeated test confirms higher 5 mm CBR, use that value.

Diagram Showing Load-Penetration Correction Process

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

Follow this procedure and use the provided table for accurate and consistent CBR evaluation.