Methods of test for soils, Part 31: Field determination of California Bearing Ratio

IS 2720 Part 31: Scope & Key Specifications

Scope (Clause 3.0)

• Defines procedures for In-place Bearing Ratio (IPBR) test on soils.
• Uses definitions from IS 2809:1972 (Glossary of soil engineering terms).

Related IS Codes (Annex A)

• IS 2720 Part 2: Water content determination
• IS 2720 Part 11: Shear strength by triaxial compression
• IS 2720 Part 16: CBR determination
• IS 2720 Part 28 & 29: In-place density by sand replacement & core cutter methods

Key Tables & Formats

Important Formula

Corrected Load (N) = Measured Load - Surcharge Load

Summary of Test Procedure

• Measure penetration in mm.
• Record proving ring dial gauge readings.
• Calculate corrected load.
• Determine bearing ratio from load and penetration data.

flowchart TD
    A[Start Test] --> B[Measure Moisture Content]
    B --> C[Determine Density (Sand/Core Cutter)]
    C --> D[Apply Surcharge Weight]
    D --> E[Penetration Test with Extensions]
    E --> F[Record Dial Gauge Readings]
    F --> G[Calculate Corrected Load]
    G --> H[Compute In-place Bearing Ratio]
    H --> I[Report Results]

Use IS 2720 Part 31 along with referenced parts for complete soil bearing capacity evaluation.

IS 2720 Part 31 references several key Indian Standards essential for soil testing. These standards provide methods and terminology crucial for in-situ and laboratory soil investigations.

Key Referenced IS Codes (Annex A, Clause 2.1)

Important Notes:

• These referenced standards are necessary adjuncts to IS 2720 Part 31.
• They cover water content, shear strength, dry density, and CBR tests.
• Use the latest editions and amendments for compliance.
• The Bureau of Indian Standards (BIS) governs the use and revision of these standards.

Summary Table for Number of Field Tests (Clause 2.5 excerpt)

Practical Use

• For soil bearing capacity: use CBR test (IS 2720 Part 16).
• For density: use sand replacement (Part 28) or core cutter method (Part 29).
• For shear strength: use triaxial test (Part 11).

flowchart TD
    A[IS 2720 Part 31] --> B[Water Content -

IS 2720 Part 31 - Definitions & Key Specifications

• Definitions: As per Clause 3.0, definitions from IS 2809:1972 (Glossary of terms and symbols relating to soil engineering) apply alongside this standard.

• Referenced IS Codes (Clause 2.1 & Annex A):

  • IS 2720 Part 2: Water content determination
  • IS 2720 Part 11: Shear strength parameters by triaxial test
  • IS 2720 Part 28 & 29: Dry density by sand replacement and core cutter methods
  • IS 2720 Part 16: Laboratory CBR determination

• Bearing Ratio Calculation (Clause 6.2):

[ \text{Bearing Ratio} = \frac{P_t}{P_s} \times 100% ]

Where:

• (P_t) = Corrected test load at penetration depth (MPa or N)

• (P_s) = Standard load at same penetration from Table 1

• Standard Loads (Table 1):

• Field Test Proforma (Annex B) includes:
  • Location, material, depth, moisture content
  • Surcharge weights, penetration readings, corrected loads

flowchart TD
    A[Penetration Depth] --> B[Measure Load (Pt)]
    B --> C[Correct Load for Shape/Conditions]
    C --> D[Refer Table 1 for Standard Load (Ps)]
    D --> E[Calculate Bearing Ratio = (Pt/Ps)*100%]
    E --> F[Assess Soil Bearing Capacity]

This concise summary provides the key definitions, references, formulas, and tables essential for understanding and applying IS 2720 Part 31.

IS 2720 Part 31: Apparatus - Key Formulas & Tables

Apparatus (Clause 4.11)

• General apparatus include: spirit level, pick, spade, scoop, brush.
• Moisture determination apparatus: See IS 2720 (Part 2): 1973.
• Density determination apparatus: See IS 2720 (Part 28): 1974 (Sand replacement) and IS 2720 (Part 29): 1975 (Core cutter).

Bearing Ratio Test (Clause 6.2 & Annex B)

Bearing Ratio Formula:

[ \text{Bearing Ratio} = \frac{P_t}{P_s} \times 100% ]

Where:

• (P_t) = Corrected test load at penetration depth (MPa or N)
• (P_s) = Standard load at same penetration depth (MPa or N)

Table 1: Standard Load (P_s) for Penetration Depth

Typical Field CBR Apparatus Components (Fig. 1)

• Penetration dial gauge
• Adjustable post
• Surcharge weights (slatted, annular)
• Folding datum bar
• Proving ring with dial gauge

Summary

• Use corrected load values from load-penetration curves for (P_t).
• Refer to standard loads from Table 1 for (P_s).
• Apparatus and procedures align with IS 2720 Parts 2, 28, and 29 for moisture and density.

flowchart LR
    A[Field Test] --> B[Measure Penetration]
    B --> C[Record Load (Proving

IS 2720 Part 31: Test Procedure Key Formulas & Tables

1. Proforma for In-Place Bearing Ratio Test (Annex B, Clause 7.1)

• Record details: Location, material, depth, moisture content, density method, surcharge weight, and soaking period.
• Penetration test readings at specified depths (mm): 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 4.0, 5.0, 7.5, 10.0, 12.5.
• Record proving ring dial gauge readings and calculate Corrected Load (see Clause 6.1).

2. Bearing Ratio Calculation (Clause 6.2)

[ \text{Bearing Ratio} = \frac{P_t}{P_s} \times 100% ]

• ( P_t ) = Corrected test load at penetration depth (MPa or N)
• ( P_s ) = Standard load at same penetration (MPa or N) from Table 1

3. Standard Load Table (Table 1, Clause 6.2)

4. Test Procedure Summary

• Perform penetration test using field CBR apparatus.
• Record dial gauge readings and calculate corrected loads.
• Calculate bearing ratio at 2.5 mm and 5 mm penetration.
• Repeat test if results are inconsistent or rejected.

flowchart TD
    A[Start Test] --> B[Record Location, Material, Depth]
    B --> C[Measure Moisture Content & Density]
    C --> D[Apply Surcharge Weight &

IS 2720 Part 31: Calculation and Interpretation of In-Place CBR Test Results

Key Formula:

The Bearing Ratio (CBR) at a given penetration is calculated as:

[ \text{Bearing Ratio} = \frac{P_t}{P_s} \times 100% ]

Where:

• (P_t) = Corrected unit or total test load at the chosen penetration (MPa or N)
• (P_s) = Standard unit or total load for the same penetration from Table 1 (MPa or N)

Table 1: Standard Loads (P_s) (Clause 6.2)

Procedure Summary:

• Record proving ring dial gauge readings at standard penetration depths (0.5 to 12.5 mm).
• Calculate corrected load values (Clause 6.1).
• Plot or refer to the load-penetration curve.
• Use corrected load at desired penetration to compute CBR using the formula.
• Compare with standard load from Table 1 to get % bearing ratio.

Interpretation:

• Higher CBR values indicate better subgrade strength.
• Typical CBR values:
  • <5%: Weak soils
  • 5-10%: Medium strength

  • 10%: Good strength

Proforma includes:

• Location, material, depth, moisture content, density method.
• Test conditions (soaked/unsoaked), surcharge weight.
• Penetration readings and corresponding load data.

graph LR
A[Penetration Test] --> B[Record Dial Gauge Readings]
B --> C[Calculate Corrected Load]
C --> D[Plot Load-Penetration Curve]
D --> E[Select Pen

IS 2720 Part 31: Number of Field Tests & Key Specifications

Number of Field Tests (Clause 8)

• The number of field tests depends on the bearing ratio values at specific penetrations.
• Tests should be conducted at various locations to ensure representativeness.

Reporting & Proforma (Annex B)

• Bearing ratio to be reported correct to the first decimal place.
• Record details including:
  • Location, material, depth, moisture content, test conditions (unsoaked/soaked), surcharge weight.
  • Penetration (mm), Proving ring dial gauge readings, Load (N), Corrected Load (N).

Key Test Parameters

• Penetration depths: 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 4.0, 5.0, 7.5, 10.0, 12.5 mm.
• Load correction as per Clause 6.1.
• Moisture content and density determination methods (sand replacement, core cutter) referenced in IS 2720 Parts 2, 28, 29.

Summary Table for Field Tests

References for Related Tests

• IS 2720 Part 2: Water content determination
• IS 2720 Part 28: Density by sand replacement
• IS 2720 Part 29: Density by core cutter
• IS 2720 Part 16: Laboratory CBR determination

flowchart TD
  A[Start Field Test] --> B{Select Test Location}
  B --> C[Measure Moisture Content]
  B --> D[Determine Density]
  B --> E[Conduct Penetration Test]
  E --> F

IS 2720 Part 31: Reporting of Test Results - Key Points

1. Reporting Precision (Clause 7.1)

• Bearing ratio must be reported correct to the first decimal place.

2. Proforma for Test Reporting (Annex B)

Include the following details:

• Location and Tester name
• Material at test point
• Depth of test point
• Condition (unsoaked/soaked)
• Soaking period and Surcharge weight during soaking
• Moisture content
• Density determination method
• Penetration test details with surcharge weight

3. Number of Field Tests (Clause 2.5)

• At 2.5 mm penetration: minimum 1-2 tests
• At 5 mm penetration: minimum 7-23 tests depending on conditions

4. Additional References (Annex A)

• IS 2720 Part 2: Water content determination
• IS 2720 Part 28: In-place density by sand replacement
• IS 2720 Part 29: In-place density by core cutter
• IS 2720 Part 16: Laboratory CBR determination

Summary Diagram of Reporting Flow

flowchart TD
    A[Test Conducted] --> B[Record Location & Material]
    B --> C[Record Depth & Condition]
    C --> D[Measure Moisture & Density]
    D --> E[Perform Penetration Test]
    E --> F[Record Dial Gauge & Load]
    F --> G[Calculate Corrected Load]
    G --> H[Report Bearing Ratio (1 decimal place)]

Note: Always use the proforma in Annex B for consistent and complete reporting as per IS 2720 Part 31.

IS 2720 Part 31: List of Referred Indian Standards

This part references essential IS codes related to soil testing methods, critical for field and laboratory investigations:

Usage Notes:

• These standards are mandatory adjuncts to IS 2720 Part 31.
• Refer to these for detailed test procedures, formulas, and definitions.
• Always confirm you have the latest edition or amendments from BIS.

Quick Reference Formula Example (Water Content):

[ w = \frac{W_w}{W_d} \times 100% ]

• (w): water content (%)
• (W_w): weight of water
• (W_d): weight of dry soil

For further details or updates, contact BIS regional offices or visit the BIS website.

IS 2720 Part 31: In-Place Bearing Ratio (CBR) Test Proforma & Key Formulas

Proforma Details (Annex B, Clause 7.1)

• Test Info: Location, tester, material, test depth, condition (unsoaked/soaked), soaking period, surcharge weight.
• Moisture Content & Density Method
• Penetration Test Data Table:

Key Formula (Clause 6.2)

[ \text{Bearing Ratio} = \frac{P_t}{P_s} \times 100% ]

• (P_t) = Corrected test load at penetration (MPa or N)
• (P_s) = Standard load at same penetration (from Table 1)

Table 1: Standard Load (P_s) (Clause 6.2)

Testing Specifications (Clause 8.1 & 6.2.1)

• Perform 3 tests per location; if results vary beyond tolerance, perform 3 more.
• Tolerances depend on bearing ratio value (e.g., ±3% if <10%).
• Use bearing ratio at 2.5 mm penetration for design unless 5 mm penetration value is higher consistently.
• If 5 mm penetration bearing ratio > 2.5 mm, repeat test; if consistent, use 5 mm value.

flowchart TD