Methods of test for soils, Part 6: Determination of shrinkage factors

IS 2720 Part 6: Scope - Key Points

• Scope: This part of IS 2720 covers the determination of shrinkage factors of soils, essential for volume change calculations in geotechnical design.

• Definitions: Refer to IS 2809-1972 for terms not defined here.

• Referenced Standards:

  • IS 2720 Part III Section 2 (1980) for specific gravity determination of fine, medium, and coarse-grained soils.
  • Equipment specifications for shrinkage factor determination are also included.

• Formula for Shrinkage Factor (Clause 5.4):

  [ S = \frac{V_w - V_d}{V_w} ]

  Where:

  • ( S ) = Shrinkage factor
  • ( V_w ) = Volume of wet soil
  • ( V_d ) = Volume of dry soil

• International Coordination: The standard aligns with international practices while considering Indian field conditions.

Summary Table: Key References

flowchart LR
    A[Soil Sample] --> B[Determine Volume of Wet Soil (Vw)]
    B --> C[Dry Soil Sample]
    C --> D[Determine Volume of Dry Soil (Vd)]
    D --> E[Calculate Shrinkage Factor S = (Vw - Vd)/Vw]

This concise overview helps understand the scope and key formulas in IS 2720 Part 6.

IS 2720 Part 6: Definitions - Key Points

• Clause 2.1: Definitions used in this standard refer primarily to IS:2809-1972 for terms not explicitly defined here.

• General Scope: This part deals with determination of shrinkage factors of soils.

• Related Standards:

  • IS:2720 Part III/Section 2 (1980) is the updated reference for specific gravity determination of soils (fine, medium, coarse).
  • Equipment specifications for shrinkage factor tests are also referenced.

Important Definitions (from IS 2809 and IS 2720 Part 6 context):

Key Formula for Shrinkage Factor (from Clause 5.4):

[ S = \frac{V_w - V_d}{V_s} ]

Where:

• ( S ) = Shrinkage factor
• ( V_w ) = Volume of wet soil
• ( V_d ) = Volume of dry soil
• ( V_s ) = Volume of solids

Notes:

• The standard emphasizes international coordination and local practice relevance.
• Use IS 2720 Part III/Section 2 (1980) for specific gravity tests.
• Equipment specifications are detailed for shrinkage factor determination.

flowchart LR
    A[Soil Sample] --> B[Determine Volume of Wet Soil (Vw)]
    B --> C[Dry Soil Sample]
    C --> D[Determine Volume of Dry Soil (Vd)]
    A --> E[Determine Volume of Solids (Vs)]
    B & D & E --> F[Calculate Shrinkage Factor S = (Vw - Vd)/Vs]

This concise summary should help clarify definitions and related formulae in IS 2720 Part 6.

IS 2720 Part 6 — Apparatus Specifications

Key Apparatus Dimensions & Details (Clause 3.6)

• Glass Plates: Two plates, each 75 mm x 75 mm, thickness 3 mm
  • One plain glass plate
  • One glass plate with 3 metal prongs (inert to mercury)
• Other Apparatus:
  • Evaporating dish, spatula, shrinkage dish, straight edge, glass cup, prong plate, plain plate, measuring cylinder per IS 10077-1982

Apparatus Purpose

• Used for determining volumetric changes in soil by measuring mercury displacement.

Key Notes:

• Metal prongs help hold soil pat firmly.
• Mercury displacement method is used to measure volume change before and after shrinkage.

Summary Table of Apparatus Dimensions

Visual Concept (simplified):

flowchart LR
    SoilPat[Soil Pat on Glass Plate]
    GlassPlate1[Plain Glass Plate]
    GlassPlate2[Glass Plate with Metal Prongs]
    Mercury[Mercury in Evaporating Dish]
    
    SoilPat --> GlassPlate2
    GlassPlate2 --> Mercury
    Mercury --> VolumeChange[Measure Displaced Mercury]

This setup allows precise volumetric shrinkage measurement by mercury displacement.

For detailed apparatus specs, refer to IS 10077-1982 and IS 2720 Part 6 (Clause 3.6).

IS 2720 Part 6 — Soil Sample for Test: Key Points

Sampling & Testing Procedure

• Clause 8.2:
  • Perform minimum 3 tests per soil sample.
  • Calculate the average of results.
  • Discard any test result differing by ±2% from the average and retest.

Specific Gravity

• Clause 6.3:

  • Determine Specific Gravity (G) as per IS 2720 Part 3 (1964).
  • Typically, use the formula:

  [ G = \frac{\text{Weight of soil solids}}{\text{Weight of equal volume of water}} ]

Equipment & Conditions

• Clause 3.11:
  • Use a desiccator with a desiccating agent (other than sulphuric acid) for moisture control.

Summary Table: Test Repetition Criteria

Notes:

• Repeat tests ensure accuracy and reliability of soil properties.
• Specific gravity is essential for further calculations like void ratio, porosity, and degree of saturation.

flowchart TD
    A[Soil Sample] --> B[Perform 3 Tests]
    B --> C{Check Variation ±2%?}
    C -- Yes --> D[Discard Outlier]
    D --> B
    C -- No --> E[Calculate Average]
    E --> F[Report Results]

This ensures consistent and reliable soil characterization per IS 2720 Part 6.

IS 2720 Part 6: Shrinkage Limit (Remoulded Soil) - Key Points

Sample Preparation

• Use 100 g of soil passing 425 micron IS sieve (Clause 4.1).
• Prepare soil pats of 45 mm diameter and 15 mm height (Clause 4.2.2).
• Round edges to avoid air entrapment during mercury displacement.

Shrinkage Limit (Remoulded Soil), ( w_s )

• Calculated by:

[ w_s = \frac{W_s - W_d}{W_d} \times 100 ]

Where:

• ( W_s ) = Weight of water in the soil at shrinkage limit
• ( W_d ) = Weight of dry soil solids

Allied Properties Calculated

• Shrinkage Ratio (Sr)
• Shrinkage Index (Is)
• Volumetric Shrinkage (Vs)

Typical Procedure Summary

flowchart TD
    A[Sample Preparation] --> B[Weigh Soil]
    B --> C[Form Soil Pats]
    C --> D[Dry Soil Pats]
    D --> E[Measure Volume & Weight]
    E --> F[Calculate Shrinkage Limit]
    F --> G[Calculate Allied Properties]

For detailed formulas of allied properties, refer to IS 2720 Part 6 Clauses 7.3-7.5.

IS 2720 Part 6: Procedure for Determination of Shrinkage Limit (Undisturbed Soil)

Sample Preparation (Clause 4.2.2)

• Trim undisturbed soil pats:
  • Diameter ≈ 45 mm
  • Height ≈ 15 mm
• Round edges to avoid air entrapment during mercury displacement.

Mercury Displacement Method (Clause 5.5.1)

• Fill glass cup with mercury, remove excess using a glass plate with three prongs.
• Place oven-dried soil pat on mercury surface, gently press under mercury using the plate.
• Weigh displaced mercury to 0.1 g accuracy.
• Calculate volume of oven-dried soil pat:
  [ V_o' = \frac{\text{Weight of displaced mercury}}{\text{Unit weight of mercury}} ]

Shrinkage Limit Calculation (Clause 7.6)

[ W_{su} = \frac{(W_{os} - W_{od}) \times V_{os}}{V_o'} \times 100 ]

Where:

• (W_{su}) = Shrinkage limit of undisturbed soil (%)
• (W_{os}) = Water content of soil sample before drying (%)
• (W_{od}) = Water content of oven-dried soil (usually 0%)
• (V_{os}) = Volume of soil sample before drying (mercury displacement)
• (V_o') = Volume of oven-dried soil pat (mercury displacement)

Summary Table

flowchart TD
    A[Trim Soil Pat (45mm x 15mm)] --> B[Place on Mercury]
    B --> C[Press Pat Under Mercury]
    C --> D[Measure Displaced Mercury Weight]
    D --> E[Calculate Volume of Oven-Dried Soil Pat]
    E --> F[Calculate Shrinkage Limit using formula]

This procedure ensures accurate measurement of shrinkage limit

IS 2720 Part 6: Calculation of Shrinkage Factors

Key Definitions & Formulas

• Shrinkage Limit, ws (%): Water content below which soil ceases to shrink.

• Shrinkage Ratio, R (Clause 2.1.4):
  [ R = \frac{\text{Volume change } (%) \text{ w.r.t dry volume}}{\text{Change in water content } (%) \text{ above } w_s} ]

• Volumetric Shrinkage (V₃) (Clause 7.5):
  [ V_3 = (w_1 - w_s) \times R ] where:

  • ( w_1 ) = initial water content (%)
  • ( w_s ) = shrinkage limit (%)
  • ( R ) = shrinkage ratio

• Specific Gravity, G: Determined as per IS 2720 Part 3.

Summary Table for Shrinkage Parameters

Notes:

• The shrinkage ratio ( R ) is dimensionless.
• ( V_3 ) gives volumetric shrinkage relative to dry soil volume.
• Use oven-dried soil weight as basis for water content changes.

flowchart TD
    A[Initial Water Content \(w_1\)] --> B[Subtract Shrinkage Limit \(w_s\)]
    B --> C[Multiply by Shrinkage Ratio \(R\)]
    C --> D[Volumetric Shrinkage \(V_3\)]

This concise framework allows calculation of volumetric shrinkage from measured water contents and shrinkage limits per IS

IS 2720 Part 6: Reporting of Results - Key Points

• Test Repetition & Averaging (Clause 8.2):

  • Perform at least 3 tests per soil sample.
  • Calculate the average of the results.
  • Discard any test result differing by ±2% or more from the average and repeat the test.

• Rounding Off (Clause 0.5):

  • Round off final values as per IS 2-1960 guidelines (generally to the nearest significant digit).

• Record Format (Clause 8.1):

  • Use the pro forma in Appendix A for uniform reporting of observations and results.

Recommended Formula for Average Water Content (Example from Clause 5.4)

[ w = \frac{W_w}{W_s} \times 100 ]

Where:

• (w) = water content (%)
• (W_w) = weight of water
• (W_s) = weight of dry soil

Summary Table for Reporting

flowchart TD
    A[Start Testing] --> B{Perform 3 Tests}
    B --> C[Calculate Average]
    C --> D{Any value ±2% from Average?}
    D -- Yes --> E[Discard & Repeat Test]
    D -- No --> F[Report Average]
    F --> G[Round off as per IS 2-1960]
    G --> H[Record in Appendix A format]
    H --> I[End]

This ensures accuracy, consistency, and clarity in soil water content reporting under IS 2720 Part 6.

IS 2720 Part 6: Pro Forma for Record of Test Results (Appendix A)

• Test Reporting Requirements (Clause 8.1 & 8.2):

  • Perform minimum 3 tests per soil sample.
  • Calculate the average of results.
  • Discard any value deviating by ±2% from the average; repeat test if needed.
  • Round off final values as per IS 2-1960 rounding rules.

• Pro Forma Elements (Appendix A) typically include:

  • Sample Identification: Location, depth, date.
  • Test Details: Type of test, apparatus used, date of test.
  • Observations: Water content, weight measurements, volume, etc.
  • Calculations: Bulk density, dry density, water content.
  • Results: Individual test values, average value.
  • Remarks: Any anomalies or observations during testing.

• Key Formula for Dry Density (Clause 5.4 reference):

[ \text{Dry Density} (\rho_d) = \frac{\text{Bulk Density} (\rho)}{1 + w} ]

Where:

• ( \rho ) = Bulk density (mass/volume)
• ( w ) = Water content (decimal)

Summary Table for Test Result Reporting

flowchart TD
    A[Soil Sample Collection] --> B[Conduct 3 Tests]
    B --> C{Check Variation ±2%}
    C -- Within Limit --> D[Calculate Average]
    C -- Exceeds Limit --> B
    D --> E[Round Off Values (