Methods of test for soils, Part 35: Measurement of negative pore water pressure

IS 2720 Part 35 (1974) - Scope & Key Specifications

This part covers methods for preparation and testing of soil specimens, including accessories and apparatus used.

Key Points from Scope & Clauses:

• Clause 2.1: Terminology follows IS 2809-1972 (Glossary of soil engineering terms).

• Clause 3.4.6: Seamless Rubber Membrane

  • Used for specimen extrusion.
  • Tube open at both ends.
  • Internal diameter = specimen diameter.
  • Length = specimen height + 50 mm.
  • Thickness: 0.2 to 0.3 mm (depends on soil strength and size).

• Clause 3.4.20: Accessories

  • Includes tools for specimen preparation, extrusion, trimming.
  • Measurement tools for size, weight, water content.

Typical Dimensions for Rubber Membrane

Summary Diagram: Specimen with Rubber Membrane

flowchart LR
    A[Soil Specimen] -->|Encased by| B[Seamless Rubber Membrane]
    B -->|Length = Specimen height + 50 mm| C[Membrane Length]
    B -->|Diameter = Specimen diameter| D[Membrane Diameter]
    B -->|Thickness 0.2-0.3 mm| E[Membrane Thickness]

This standard ensures consistent specimen preparation for reliable soil testing results. For detailed procedures, refer to full IS 2720 Part 35 text.

IS 2720 Part 35: Apparatus Specifications

Apparatus Components (Clause 3.4)

• The apparatus typically includes:
  • A loading frame
  • A dial gauge or displacement measuring device
  • A container for soil sample
  • Plates or porous stones for drainage
  • Water supply system for saturation and consolidation
  • Pressure gauge for measuring pore water pressure

Preparation (Clauses 6.1 & 7.1)

• Apparatus should be prepared as per Fig. 1 (refer IS 2720-35 for detailed schematic).
• Ensure all components are clean, calibrated, and assembled correctly.
• Follow the dimensions and setup as indicated in Clauses 3.2 and 3.3.

Environmental Conditions (Clause 3.6)

• Maintain constant ambient temperature during measurements to ensure accuracy.

Key Notes:

flowchart LR
    A[Soil Sample] --> B[Porous Stone]
    B --> C[Loading Plate]
    C --> D[Loading Frame]
    D --> E[Dial Gauge]
    F[Water Supply] --> G[Sample Container]
    G --> B
    H[Pressure Gauge] --> G

For exact dimensions and detailed apparatus setup, refer to Fig. 1 of IS 2720 Part 35.

IS 2720 Part 35: Saturation & De-airing of Fine Ceramic Porous Stone and Pore Water Pressure Lines

Key Points from the Code:

• Ceramic Porous Stone:

  • Diameter = 32 mm
  • Thickness = 10 mm
  • Air entry value > maximum negative pore water pressure to be measured (Clause 3.4.7)

• Saturation & De-airing (Clause 4.2):

  • Essential for measuring negative pore water pressure.
  • Use de-aired water to saturate the ceramic stone and pore pressure lines.
  • All valves start closed to avoid air entry.
  • Thorough removal of air from lines is critical to avoid measurement errors.

Procedure Summary:

1. Connect the apparatus with valves closed.
2. Fill pore pressure lines and ceramic stone with de-aired water (water boiled and cooled in a vacuum to remove dissolved gases).
3. Open valves gradually to allow water to saturate the stone and displace air.
4. Repeat de-airing until no air bubbles appear in the lines.

Important Specification:

Conceptual Mermaid Diagram:

flowchart LR
    A[De-aired Water Reservoir] --> B[Valve Closed]
    B --> C[Fine Ceramic Porous Stone (32 mm x 10 mm)]
    C --> D[Pore Water Pressure Line]
    D --> E[Pressure Transducer]
    style B fill:#f9f,stroke:#333,stroke-width:2px
    style C fill:#bbf,stroke:#333,stroke-width:2px

Summary: Saturate the ceramic stone and pore lines with de-aired water under closed valve conditions to ensure accurate negative pore water pressure measurement.

IS 2720 Part 35 (1974) - Soil Sample for Test: Key Points

Accessories (Clause 3.4.20)

• Includes tools for extrusion, trimming, size & weight measurement, water content determination.
• Essential for preparing undisturbed soil specimens.

Saturation & Air Entry Value (Clause 4.3.6)

• Ceramic stones used for measuring negative pore water pressure must be saturated and de-aired before use.
• Air entry value of ceramic stone is measured once to define its effective pressure range.

Application of Pressure (Clause 7.9)

• Valves A1 and A2 are opened; pore air pressure applied via A1, cell pressure via C1.
• Both pressures should be equal and increased in 0.5 kg/cm² increments.
• In soils with continuous air phase, equal increments in cell and pore air pressure cause corresponding increases in pore water pressure.

Summary Table: Pressure Application

Practical Notes:

• Ensure saturation and de-airing of ceramic stones before testing.
• Maintain equal magnitude of cell and pore air pressures to simulate in-situ conditions.
• Use accessories for accurate specimen preparation and measurement.

flowchart LR
    A[Start: Soil Sample Preparation]
    B[Use Accessories: Extrusion, Trimming, Measurement]
    C[Saturate & De-air Ceramic Stone]
    D[Open Valves A1 & A2]
    E[Apply Pore Air Pressure (A1)]
    F[Apply Cell Pressure (C1)]
    G[Increase Pressures in 0.5 kg/cm² Steps]
    H[Measure Pore Water Pressure]
    A --> B --> C --> D --> E & F --> G --> H

This ensures reliable soil testing per IS 2720 Part 35.

IS 2720 Part 35: Procedure for Direct Measurement of Negative Pore Water Pressure

Key Points from Clause 5.1 & Procedure Overview

• Sample Preparation:

  • Diameter of soil sample = Diameter of triaxial cell pedestal.
  • Sample height: Any convenient height is acceptable.
  • Applicable for field samples or lab-prepared samples (compacted, remoulded, etc.).

• Measurement Principle:

  • Negative pore water pressure (matric suction) is measured directly using specialized equipment (e.g., tensiometers) embedded in the soil sample.
  • The device measures the tension in pore water, indicating suction.

Typical Measurement Setup

flowchart LR
    A[Soil Sample] --> B[Trimmed to pedestal diameter]
    B --> C[Placed in Triaxial Cell]
    C --> D[Tensiometer inserted]
    D --> E[Direct measurement of negative pore water pressure]

Important Considerations

• Ensure good contact between tensiometer and soil for accurate readings.
• Maintain saturation of tensiometer porous tip.
• Readings typically range from 0 to -100 kPa (limited by tensiometer range).
• For higher suctions, other methods (e.g., filter paper) are used.

Formula (Conceptual)

[ u = -\sigma_w ]

Where:

• ( u ) = Negative pore water pressure (suction) [kPa]
• ( \sigma_w ) = Water tension measured by tensiometer [kPa]

Summary Table: Sample Dimensions

For detailed calibration and apparatus specifications, refer to the full IS 2720 Part 35 document.

IS 2720 (Part 35) - Axis-Translation Technique for Negative Pore Water Pressure

Key Specifications & Setup

• Sample Preparation: Diameter equal to triaxial cell pedestal; height flexible.
• Apparatus: As per Fig. 1 (IS 2720 Part 35), same for direct & indirect measurements.
• Valves: Open B1, D2; close D1, D2, D3, Da, D5 after sample assembly.
• De-airing: Essential if air outgassing occurs (Clause 4.2).

Procedure Summary (Clause 6.11)

1. Open valves B1 and D2.
2. Disassemble soil sample.
3. Reduce pressure in X to zero.
4. Close valves D1, D2, D3, Da, D5.
5. Allow water flow from burette to ceramic stone top.
6. Clean ceramic stone top.
7. Ready for next measurement or de-air if required.

Axis-Translation Principle

• Increase air pressure above atmospheric in the chamber.
• Keep pore water pressure at atmospheric or lower.
• Measure negative pore water pressure indirectly by controlling air pressure and measuring water pressure.

Formula for Negative Pore Water Pressure (u)

[ u = P_a - P_w ] Where:

• (P_a) = Applied air pressure (above atmospheric)
• (P_w) = Measured pore water pressure

Diagram: Simplified Axis-Translation Setup

flowchart LR
    A[Air Pressure Chamber] -->|Pressurizes air| B[Soil Sample]
    B -->|Pore water pressure| C[Ceramic Stone]
    C -->|Water flow| D[Burette]
    D -->|Measurement| E[Pressure Gauge]

Note: Accurate sealing and de-airing are critical to avoid errors in negative pore water pressure measurement.