Methods of test for soils, Part 10: Determination of unconfined compressive strength

IS 2720 Part 10: Scope & Key Specifications

This part of IS 2720 covers the Determination of Unconfined Compressive Strength of soil specimens.

Key Parameters & Initial Data (Clause A-4):

Important Formulas:

• Axial Strain (e): [ e = \frac{\Delta L}{L_0} = \frac{L - L_0}{L_0} ]

• Cross-sectional Area under deformation (A): [ A = A_0 (1 - e) ]

• Compressive Stress (σ): [ \sigma = \frac{P}{A} \quad \text{where } P = \text{axial force (N or kgf)}, A = \text{area (cm}^2) ]

• Unconfined Compressive Strength (qu): [ q_u = \text{Maximum compressive stress at failure (kPa)} ]

• Undrained Shear Strength (su) (if applicable): [ s_u = \frac{q_u}{2} ]

Observations & Remarks (Clause A-5 & A-6):

• Record deformation dial readings, axial strain, and compressive stress.
• Sketch and describe failure mode.
• Measure water content after test from failure zone.

Summary Table for Compression Test Observations:

| Deformation Dial Reading (mm) | Axial Strain (e) | Area (cm²) | Proving Ring Reading | Axial Force (N) | Compressive Stress (kPa) | Remarks | |-------------------------------|------------------|------------|---------------------|-----------------|

IS 2720 Part 10 references several key standards and provides essential test data formats and specimen details for unconfined compression tests on soils.

Referenced Standards (Clause 2.1)

• IS 2132:1986 — Code of practice for thin-walled tube sampling of soils.
• IS 2720 (Part 2):1973 — Determination of water content.

Key Test Data & Formulas (Clause 5.2, Tables A-2, A-5)

Observations & Remarks

• Maximum particle size in specimen must be noted.
• Water content after test is determined from failure zone samples.
• Unconfined compressive strength ( q_u ) and undrained shear strength (if applicable) are reported.

Summary Diagram of Test Parameters

flowchart TD
    A[Initial Specimen] --> B[Measure Do, Lo]
    B --> C[Calculate Ao = π(Do/2)^2]
    C --> D[Apply axial load P]
    D --> E[Measure deformation ΔL]
    E --> F[Calculate strain e = ΔL/Lo]
    F --> G[Calculate current area A = Ao(1 - e)]
    G --> H[Calculate stress σ = P/A]
    H --> I[Determine qu, undrained shear strength]

Note: Always ensure use of latest revisions and amendments from BIS for compliance and accuracy.

IS 2720 Part 10: Definitions & Key Formulas (SI Units)

Key Definitions (Clause 3.0 & 3.1)

• Soil Specimens:
  • Undisturbed, remoulded, or compacted
• Initial Dimensions:
  • Diameter, ( D_0 ) (mm)
  • Length, ( L_0 ) (mm)
  • Area, ( A_0 ) (cm²)
  • Volume, ( V ) (cm³)
• Physical Properties:
  • Specific gravity ( G )
  • Initial mass ( m ) (g)
  • Initial density ( \rho = \frac{m}{V} ) (g/cm³)
  • Water content ( w ) (%)
  • Degree of saturation ( S_r ) (%)
  • In-situ density and water content or maximum dry density & optimum water content

Compression Test Observations (Table A-5)

Strength Parameters

• Unconfined Compressive Strength ( q_u ): Maximum compressive stress (kPa)
• Undrained Shear Strength: If applicable, derived from ( q_u )

Summary Formulae

[ \text{Axial strain}, e = \frac{\Delta L}{L_0} ] [ \text{Area at strain}, A = A_0 (1 - e) ] [ \text{Compressive stress}, \sigma = \frac{F}{A} ]

Notes:

• Remarks should include max particle size per Clause 5.2.
• Water content after test is from failure zone

IS 2720 Part 10: Apparatus & Key Parameters for Unconfined Compression Test

Apparatus Details (Clause 3.1 & Table A-2)

• Calibration factor for proving ring (A-3.1)
• Specimen dimensions:
  • Initial diameter, ( D_0 ) (mm)
  • Initial length, ( L_0 ) (mm)
  • Initial cross-sectional area, ( A_0 ) (cm²)
  • Initial volume, ( V ) (cm³)
• Soil properties:
  • Specific gravity, ( G )
  • Initial mass (g)
  • Initial density, water content (%), degree of saturation (%)
• Specimens can be undisturbed, remoulded, or compacted.

Observations & Calculations (Table A-5)

Key Strength Parameters

• Unconfined compressive strength, ( q_u ) (kPa)
• Undrained shear strength (if applicable)

Miscellaneous Equipment (Clause 4.8)

• Specimen trimming tools
• Remoulding apparatus
• Water content cans
• Data sheets

Summary Formulae

Axial strain:  e = ΔL / L₀

Area during test:  A = A₀ (1 - e)

Compressive stress:  σ = F / A

flowchart TD
    A[Initial Specimen] --> B[Measure D₀, L₀, A₀]
    B --> C[Apply Load]
    C --> D[Record Dial Gauge ΔL]
    D -->

IS 2720 Part 10: Preparation of Test Specimen - Key Points

Specimen Size (Clause 5.2)

• Minimum Diameter (D₀): 38 mm
• Maximum Particle Size: ≤ 1/8 of specimen diameter (i.e., ≤ 4.75 mm for 38 mm diameter)
• Height to Diameter Ratio (L₀/D₀): Between 2 and 2.5
• Measurement Accuracy: Vernier calipers or similar, to nearest 0.1 mm

Initial Specimen Data to Record (Clause 3.1)

Compression Test Observations (Clause A-5)

Important Remarks

• If larger particles than permitted are found post-test, note under remarks.
• Water content of specimen after test should be determined from failure zone samples.
• Unconfined Compressive Strength ( q_u ) and Undrained Shear Strength (if applicable) must be reported.

Summary Formulae:

[ e = \frac{\Delta L}{L_0} ]

[ A = A_0 (1 - e) ]

[ \sigma = \frac{

IS 2720 Part 10 - Test Procedure Key Points

Specimen Details (Clause A-4)

• Type: Undisturbed, remoulded, or compacted soil
• Specific gravity (G)
• Initial dimensions:
  • Diameter, ( D_0 ) (mm)
  • Length, ( L_0 ) (mm)
  • Area, ( A_0 = \pi (D_0/2)^2 ) (cm²)
  • Volume, ( V = A_0 \times L_0 ) (cm³)
• Mass (g), Density, Water content (%), Degree of saturation (%)

Compression Test Observations (Clause A-5)

Strength Parameters

• Unconfined Compressive Strength, ( q_u ) (kPa): Maximum compressive stress before failure.
• Undrained Shear Strength, ( s_u = q_u / 2 ) (if applicable)

Additional

• Water content after test from failure zone samples
• Sketch and description of failure mode

Apparatus & Miscellaneous (Clause 3.1, 4.8)

• Calibrated proving ring, deformation dial gauge
• Specimen trimming tools, water content cans, remoulding apparatus

flowchart TD
    A[Prepare Specimen] --> B[Measure Initial Dimensions]
    B --> C[Apply Axial Load]
    C --> D[Record Dial Reading & Load]
    D --> E[Calculate Strain & Stress]
    E --> F[Determine \(q_u\) and \(s_u\)]
    F --> G[Analyze Failure & Water Content]
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IS 2720 Part 10: Calculations and Plotting for Unconfined Compression Test

Key Parameters & Formulas

Observations & Plotting

• Plot stress-strain curve: (\sigma) vs. (e).
• Sketch failure pattern on specimen, measure angle of failure surface to horizontal.
• Record water content before and after test (from failure zone).
• Note rate of strain and deformation dial readings.

Summary Table for Test Data

graph TD
  A[Initial Specimen] --> B[Measure \(D_0, L_0\)]
  B --> C[Calculate \(

IS 2720 Part 10: Recording of Observations for Unconfined Compression Test

Key Specifications & Pro Forma (Annex A)

1. Soil Sample Details:

   • Visual description
   • Date of sampling

2. Apparatus Details:

   • Calibration factor of load measuring device

3. Soil Specimen Details:

   • Type: Undisturbed, remoulded, or compacted
   • Specific gravity (G)
   • Initial dimensions: Diameter (Do, mm), Length (Lo, mm)
   • Initial area (Ao, cm²), volume (V, cm³), mass (g)
   • Initial density, water content (%), degree of saturation (%)
   • In-situ density and water content or max dry density and optimum water content, if applicable

4. Compression Test Observations:

5. Post-Test:
   • Sketch and description of failure mode
   • Water content of specimen after test (from failure zone)
   • Unconfined compressive strength ( q_u ) (KPa)
   • Undrained shear strength (if applicable)

Important Formulae

[ \text{Axial strain}, e = \frac{\Delta L}{L_0} ]

[ \text{Area at any time}, A = A_0 (1 - e) ]

[ \text{Compressive stress}, q_u = \frac{P}{A} \quad \text{where } P = \text{axial load

IS 2720 (Part 10) - Pro Forma for Record of Observations:

The pro forma (Annex A) for unconfined compression test records the following:

A-1: Soil Sample Details

• Visual description
• Date of sampling

A-2 & A-3: Apparatus Details

• Equipment used
• Load measuring device & calibration factor

A-4: Soil Specimen Details

• Type: Undisturbed/remoulded/compacted
• Specific gravity (G)
• Initial dimensions: Diameter (Do, mm), Length (Lo, mm)
• Initial area ( A_0 = \pi (D_0/2)^2 ) (cm²)
• Initial volume ( V = A_0 \times L_0 ) (cm³)
• Mass, density, water content (%), degree of saturation (%)
• Relation to in situ or maximum dry density/optimum water content

A-5: Compression Test Observations

A-6 to A-9:

• Sketch & description of failure
• Water content after test
• Unconfined compressive strength ( q_u ) (kPa)
• Undrained shear strength (if applicable)

Key Formulas:

• Initial Area: [ A_0 = \pi \left(\frac{D_0}{2}\right)^2 ]

• Axial Strain: [ e = \frac{\Delta L}{L_0} ]

• Area during test: [ A = A_0 (1 - e) ]

• Compressive Stress: [ \sigma = \frac{Force}{Area} ]

• Unconfined Compressive Strength: [ q