Methods of test for soils, Part 5: Determination of liquid and plastic limit

IS 2720 Part 5 (1985) - Scope Summary

This part of IS 2720 covers determination of the liquid and plastic limits of soils, key for soil classification and engineering properties.

Key Specifications & References:

• Grooving Tool: Must conform to IS 9259-1979 (Clause 3.1.2).
• Reporting: Test results and calculations are reported as per Appendix A proforma (Clause 12.1).
• Liquid Limit Apparatus: Must meet IS 2720 Part 5 specifications.

Important Indices Defined:

Basic Formulas:

• Plasticity Index (PI)
  [ PI = LL - PL ]

• Liquidity Index (LI)
  [ LI = \frac{w - PL}{PI} ]

• Consistency Index (Ic)
  [ Ic = \frac{LL - w}{PI} ]

Where:

• ( w ) = natural water content
• ( LL ) = liquid limit
• ( PL ) = plastic limit

flowchart TD
    A[Soil Sample] --> B[Determine LL]
    A --> C[Determine PL]
    B --> D[Calculate PI = LL - PL]
    D --> E[Calculate LI and Ic using natural water content]
    E --> F[Classify soil & report results]

This scope ensures consistent measurement of soil plasticity parameters for geotechnical design.

IS 2720 Part 5 (1985) - Key Formulas for Soil Consistency Indices

The code provides formulas related to liquid and plastic limits, crucial for soil classification and engineering behavior:

• LL = Liquid Limit (%)
• PL = Plastic Limit (%)
• W = Natural water content (%)

These indices help assess soil behavior under varying moisture conditions.

Reporting:
Use the proforma in Appendix A for standardized test result presentation (Clause 12.1).

flowchart LR
    LL[Liquid Limit (LL)]
    PL[Plastic Limit (PL)]
    W[Water Content (W)]
    PI[Plasticity Index (PI)]
    If[Flow Index (If)]
    TI[Toughness Index (TI)]
    LI[Liquidity Index (LI)]
    Ic[Consistency Index (Ic)]

    LL --> PI
    PL --> PI
    PI --> TI
    If --> TI
    W --> LI
    PL --> LI
    LL --> Ic
    W --> Ic

IS 2720 Part 5 (1985) — Key Definitions & Indices

This part defines key soil consistency indices related to liquid and plastic limits:

Key Indices & Formulas

Notes:

• LL = Liquid Limit (%)
• PL = Plastic Limit (%)
• W = Natural water content (%)
• All indices help classify soil behavior and consistency states.

Reference:

• Test results and calculations should be reported as per Appendix A proforma.
• Grooving tool specifications per IS 9259:1979.

flowchart LR
    LL --> PI[Plasticity Index = LL - PL]
    PL --> PI
    PI --> TI[Toughness Index = PI / If]
    If[Flow Index] --> TI
    W --> LI[Liquidity Index = (W - PL)/PI]
    PL --> LI
    PI --> LI
    W --> Ic[Consistency Index = (LL - W)/PI]
    LL --> Ic
    PI --> Ic

This concise summary aids understanding of soil plasticity and flow characteristics as per IS 2720 Part 5.

IS 2720 (Part 5) - Liquid Limit by Cone Penetration Method

Key Specifications & Procedure:

• Liquid Limit (LL) is the moisture content at which the cone penetrates 20 mm into the soil.
• Plot water content (%) (y-axis) vs. cone penetration (mm) (x-axis).
• Draw the best-fit straight line through test points.
• LL corresponds to moisture content at 20 mm penetration.
• Report LL to the nearest 0.1% (first decimal place).
• Also report soil sample history (natural, air-dried, oven-dried, unknown), test method, and soaking period after mixing.

Important Notes:

• The revised cone is used, improving over Casagrande’s method by avoiding groove cutting and soil slipping.
• LL depends on clay content and affects soil classification and cohesion properties.

Formula for Liquid Limit by Cone Penetration:

[ LL = W_c \quad \text{at} \quad P = 20 \text{ mm} ]

Where:

• (W_c) = Moisture content (%) at cone penetration (P)
• (P) = Cone penetration (mm)

Reporting:

• Round off as per IS 2-1960.
• Include soaking time and soil condition.

graph LR
A[Soil Sample Preparation] --> B[Mix with water]
B --> C[Measure cone penetration (P) at different moisture contents (W)]
C --> D[Plot W vs. P]
D --> E[Draw best-fit line]
E --> F[Find W at P=20 mm]
F --> G[Report Liquid Limit (LL = W at 20 mm)]

This method provides a reliable alternative to Casagrande's apparatus, especially for low plasticity soils.

IS 2720 Part 5: Liquid Limit by Mechanical Method (Summary)

Key Specifications:

• Mechanical Liquid Limit Device: Must conform to IS 9259-1979.
• Reporting: Liquid Limit (LL) reported to nearest 0.1%.
• Sample History: Natural state, air-dried, or unknown must be noted.

Test Procedure (Mechanical Method):

• Soil paste is placed in a cup of the mechanical device.
• The device lifts and drops the cup at a standard rate (about 2 drops per second).
• The number of drops required to close a groove of 12.7 mm (1/2 inch) is recorded.
• Repeat for different moisture contents.

Key Formulas:

• Flow Curve Equation:

[ N = a - b \log(w) ]

Where:

• (N) = Number of blows (drops) to close the groove

• (w) = Moisture content (%)

• (a, b) = Constants from test data

• Liquid Limit (LL): Moisture content at (N = 25) blows, found by interpolation from the flow curve.

Flow Index (If required):

[ I_f = \frac{w_{16} - w_{64}}{\log(64) - \log(16)} = \frac{w_{16} - w_{64}}{0.602} ]

Where (w_{16}) and (w_{64}) are moisture contents at 16 and 64 blows respectively.

Reference Table (Typical Data):

flowchart LR
    A[Prepare Soil Paste] --> B[Place in Mechanical Device]
    B --> C[Drop Cup at 2 drops/sec]
    C --> D[Count Drops to Close Groove]
    D --> E[Repeat at Different Moisture Contents]
    E --> F[Plot Log(N) vs Moisture Content]
    F --> G[Interpolate LL at N=25]

Note: For one-point method using Casagrande apparatus

IS 2720 Part 5 (1985) - Apparatus Key Details

• Porcelain Evaporating Dish: Diameter ~ 12 cm (Clause 7.1.1)
• Flat Glass Plate: Thickness 10 mm, size ~ 45 cm x 45 cm or larger (Clause 7.1.1)
• Apparatus requirements refer back to Clauses 4.3 and 3.1 for detailed specifications (Clauses 6.2, 5.2).

Summary Table of Apparatus Dimensions

Notes:

• The standard does not provide explicit formulas for apparatus but specifies minimum dimensions to ensure consistency in testing.
• For detailed apparatus construction and calibration, refer to Clauses 3.1 and 4.3 of IS 2720.
• Test results should be reported as per Appendix A proforma (Clause 12.1).

flowchart LR
    A[Apparatus Requirements] --> B[Porcelain Evaporating Dish]
    B --> C[Diameter ~12 cm]
    A --> D[Flat Glass Plate]
    D --> E[Thickness 10 mm, Size ≥ 45 cm²]
    A --> F[Refer to Clauses 3.1 & 4.3 for details]

IS 2720 Part 5: Plastic Limit Test - Key Points

Procedure Summary (Clause 7)

• Use soil passing 425-micron IS sieve.
• Take at least three portions of soil in plastic state.
• Roll soil into threads of 3 mm diameter on a glass plate.
• When thread crumbles at 3 mm diameter, that moisture content is the Plastic Limit (PL).
• Determine moisture content as per Clause 7.3 (oven drying method).
• Average the three results, round to nearest whole number.

Formula for Plastic Limit (Moisture Content)

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

Where:

• ( W_w ) = weight of water in soil
• ( W_s ) = weight of dry soil

Reporting (Clause 6.6)

• Report average plastic limit value as percentage moisture content.
• Use this value for plasticity index calculations.

Plasticity Index (from Clause 1.2)

[ PI = LL - PL ]

• LL = Liquid Limit (%)
• PL = Plastic Limit (%)

Summary Table

flowchart TD
    A[Take soil passing 425 micron sieve] --> B[Mix soil with water to plastic state]
    B --> C[Roll soil into 3 mm diameter threads]
    C --> D{Does thread crumble at 3 mm?}
    D -- No --> B
    D -- Yes --> E[Determine moisture content]
    E --> F[Repeat for 3 samples]
    F --> G[Calculate average Plastic Limit]

This concise procedure ensures consistent determination of the plastic limit per IS 2720 Part 5.

Plasticity Index (Ip) Calculation - IS 2720 Part 5

• Formula:

  [ \boxed{ I_p = w_L - w_p } ]

  Where:

  • (I_p) = Plasticity Index
  • (w_L) = Liquid Limit (%)
  • (w_p) = Plastic Limit (%)

• Notes:

  • If (w_p \geq w_L), then (I_p = 0).
  • For sandy soils where plastic limit cannot be determined, report (I_p = N_p) (non-plastic).

Liquidity Index (IL) Calculation:

[ \boxed{ I_L = \frac{w_o - w_p}{I_p} } ]

Where:

• (I_L) = Liquidity Index
• (w_o) = Natural moisture content (%)
• (w_p) = Plastic limit (%)
• (I_p) = Plasticity index (%)

Summary Table

flowchart LR
    WL[Liquid Limit (wL)]
    WP[Plastic Limit (wp)]
    WO[Natural Moisture (wo)]
    Ip[Plasticity Index (Ip = wL - wp)]
    IL[Liquidity Index (IL = (wo - wp) / Ip)]

    WL --> Ip
    WP --> Ip
    WP --> IL
    WO --> IL
    Ip --> IL

References: IS 2720 Part 5, Clauses 8.1, 8.2, 10.1

Toughness Index (IT) Calculation - IS 2720 Part 5

• Formula (Clause 9.1):
  [ I_T = 1 - I_p ]
  where,
  (I_p) = Plasticity Index

• Plasticity Index (Ip) (Clause 8.2):
  [ I_p = W_L - W_P ]
  (W_L) = Liquid Limit (%)
  (W_P) = Plastic Limit (%)

• If Plastic Limit cannot be determined for sandy soils, report as Np (Non-plastic).

• If (W_P \geq W_L), then (I_p = 0).

• Flow Index (If), Liquidity Index (IL), Consistency Index (Ic) are related parameters (Clauses 3.5.2, 11.1):
  [ I_c = \frac{W_L - W_o}{I_p} ]
  (W_o) = Natural Moisture Content (%)

Summary Table

This index helps classify soil toughness for engineering applications.

Liquidity Index (IL) Calculation
As per IS 2720 Part 5, Clause 10.1:

[ \boxed{ I_L = \frac{w - w_p}{I_p} } ]

Where:

• ( w ) = Natural moisture content of the soil (%)
• ( w_p ) = Plastic limit of the soil (%)
• ( I_p ) = Plasticity index = ( w_L - w_p ) (Liquid limit - Plastic limit)

Key Related Indices (IS 2720 Part 5)

Interpretation of Liquidity Index

• ( I_L < 0 ): Soil is drier than plastic limit (semi-solid to solid state)
• ( 0 < I_L < 1 ): Soil is in plastic state
• ( I_L > 1 ): Soil is in liquid state (very soft)

flowchart LR
    A[Natural Moisture Content (w)] --> B[Calculate Plasticity Index \(I_p = w_L - w_p\)]
    B --> C[Calculate Liquidity Index \(I_L = \frac{w - w_p}{I_p}\)]
    C --> D{Interpretation}
    D -->|IL < 0| E[Semi-solid to solid soil]
    D -->|0 < IL < 1| F[Plastic soil]
    D -->|IL > 1| G[Liquid soil]

This index helps assess soil consistency and predict engineering behavior.

Consistency Index (Ic) - IS 2720 Part 5

The Consistency Index indicates the relative consistency of fine-grained soils and is calculated as:

[ \boxed{ I_c = \frac{W_L - W_o}{I_p} } ]

Where:

• ( W_L ) = Liquid Limit (%) of the soil
• ( W_o ) = Natural Moisture Content (%) of the soil
• ( I_p ) = Plasticity Index (%) = ( W_L - W_p ) (Plastic Limit)

Related Indices:

Interpretation of Consistency Index:

• ( I_c = 0 ) : Soil at liquid limit (very soft)
• ( I_c = 1 ) : Soil at plastic limit (semi-solid)
• ( I_c > 1 ) : Soil is in semi-solid to solid state (stiffer)

flowchart LR
    A[Liquid Limit (WL)] -->|Subtract| B[Natural Moisture Content (Wo)]
    B -->|Divide by| C[Plasticity Index (Ip)]
    C --> D[Consistency Index (Ic)]

Summary: Use the formula ( I_c = \frac{W_L - W_o}{I_p} ) to assess soil consistency, crucial for understanding soil strength and deformation characteristics in geotechnical design.

IS 2720 Part 5 (1985) - General Report Key Points

1. Consistency Index Formula (Clause 11.1)

[ I_c = \frac{W_i - W_o}{I_p} ]

• (W_i) = Liquid Limit (%)
• (W_o) = Natural Moisture Content (%)
• (I_p) = Plasticity Index (%)

2. General Report Proforma (Clause 12.1 & Appendix A)

The report should include:

• IS No. with year
• Soil sample details & history
• Room temperature during test
• Natural water content (%)
• Soaking period before test

3. Liquid & Plastic Limits Test Data Table (Appendix A)

4. Additional Indices (Appendix A)

Summary Flowchart of

IS 2720 Part 5 - Proforma for Tests and Calculations (Appendix A)

The standard provides a detailed proforma to record soil consistency limits tests (Liquid Limit, Plastic Limit) and related indices. Key elements include:

Sample & Test Details

• IS No. with year
• Soil sample details
• Room temperature
• Natural water content
• History and soaking period of soil sample

Liquid Limit & Plastic Limit Test Data Table

Calculated Indices

Where:

• ( W ) = natural water content (%)
• ( W_L