Code of practice for subsurface investigation for foundations

IS 1892: Scope Summary & Key Specifications

Scope (Clause 1.1)

• Subsurface investigations for multi-storeyed building foundations.
• Determine:
  • Sequence & extent of soil/rock strata.
  • Engineering properties affecting design & construction.
  • Groundwater location & corrosive effects on foundation materials.
• Applicable also to multi-purpose river valley projects.

Key Formulas & Definitions (Clause 4.1.1)

• Max 20% for stiff formations <br> - Prefer ≤10% for soft sensitive clays |

Soil Exploration Methods (Appendix A)

Laboratory Tests (Table 3, Clause 8.1)

IS 1892: Preliminary Investigation & Existing Information

Key Objectives (Clauses 1.1 & 3.1.1)

• Identify soil/rock strata depth, thickness, extent, and composition.
• Locate rock and groundwater levels.
• Obtain approximate soil strength and compressibility.
• Use geophysical methods, cone penetrometers, sounding rods for guidance.
• Understand site geological features (overburden, streams, outcrops).

Depth of Exploration (Clause 2.3.2.1, Table 1)

Important Notes

• Determine seasonal water table fluctuations to design drainage/intercepting drains.
• Preliminary exploration helps decide investigation methods when reconnaissance isn't possible.
• Detailed site investigation is essential before final design.

flowchart TD
    A[Preliminary Exploration] --> B[Soil Strata Identification]
    A --> C[Rock & Groundwater Location]
    A --> D[Soil Strength Approximation]
    B --> E[Depth, Thickness, Extent]
    C --> F[Water Table Fluctuation]
    D --> G[Geophysical & Penetrometer Tests]

This summary aligns with IS 1892 requirements for preliminary site investigation.

IS 1892: Methods of Site Exploration — Key Points

1. Phases of Exploration & Tests (Table 3, Clause 8.1)

2. Current Methods of Subsoil Exploration (Appendix A)

3. Seismic Refraction Key Formula

• Critical distance (x_c) relates to depth (h) and velocity ratio (V2/V1):

[ x_c = 2h \sqrt{\frac{V_2^2}{V_1^2} - 1} ]

where:

• (V_1) = velocity in upper layer
• (V_2) = velocity in lower layer

4. Typical Seismic Wave Velocities (m/s)

IS 1892: Sampling Tools and Techniques - Key Points

1. Types of Samples (Clause 5.1)

2. Sampling Tool Design (Clause 4.1)

• Cutting Edge: Minimizes soil disturbance.
• Inside Wall Friction: Should be low to avoid sample disturbance.
• Non-return Valve: Prevents sample loss during extraction.

3. Cutting Edge Parameters (Clause 4.1.1)

• Inside Clearance (CI):

[ CI = D_e - D_s ]

• Outside Clearance (Co):

[ Co = D_w - D_T ]

• Area Ratio (Ar):

[ Ar = \frac{D_w^2 - D_e^2}{D_e^2} \times 100% ]

• Recommended Values:
  • Ar ≤ 20% for stiff formations
  • Ar ≤ 10% for soft sensitive clays (prefer piston sampler if clearance insufficient)

4. Sampling Procedure

• Use tools that cause minimal displacement and remoulding.
• Choose sampler type based on soil/rock nature and sample type required.
• Special methods required for undisturbed sand samples below water table.

flowchart TD
    A[Nature of Ground] --> B{Sample Type}
    B -->|Disturbed| C[Hand/Auger/Shell Samples]
    B -->|Undisturbed| D[Chunk/Tube Samples or Cores]
    C --> E[Testing & Analysis]
    D --> E

Summary: Use appropriate samplers with low area ratio and proper cutting edge design to minimize disturbance, ensuring reliable soil/rock sample quality per IS 1892 guidelines.

IS 1892: Methods of Sampling - Key Points

1. Types of Samples (Clause 5.1)

• Disturbed Samples: Natural structure modified/destroyed; moisture content can be preserved.
• Undisturbed Samples: Natural structure and properties preserved; easier from rocks, special methods for soils.

2. Sampling Methods by Ground Nature (Clause 5.1)

3. Procedure for Taking Samples (Clause 6.2.1)

• For soft clay strength, supplement sampling with in-situ vane shear test (IS:4434-1978).

4. Quantity of Soil Sample Required (Clause 6.1)

Summary Diagram of Sampling Types and Methods

graph TD
  A[Nature of Ground] --> B[Soil]
  A --> C[Rock]
  B --> D[Disturbed]
  B --> E[Undisturbed]
  C --> F[Disturbed]
  C --> G[Undisturbed]

  D --> H[Hand samples, Auger, Shell]
  E --> I[Chunk, Tube samples]
  F --> J[Wash samples]
  G --> K[Core samples]

References:

• IS 1892 Clauses 4.2, 5.1,

IS 1892: Procedure for Taking Soil and Rock Samples

1. Types of Samples (Clause 5.1)

• Disturbed Samples: Natural structure altered; moisture content can be preserved.
• Undisturbed Samples: Natural structure and properties preserved; easier for rocks, special methods needed for soils.

2. Methods of Sampling

3. Procedure Highlights (Clause 6.3.2)

• Undisturbed Samples: Use specialized tools (e.g., Shelby tubes) to preserve soil structure.
• Sampling below water table in sands requires advanced techniques.

4. Record Keeping (Appendix D)

Maintain detailed records including:

• Boring details (diameter, inclination, type)
• Soil strata description and classification
• Depth and thickness of strata and samples
• Type and depth of samples taken
• Groundwater level and remarks

5. Handling and Labelling (Appendix E)

• Samples must be carefully handled to avoid disturbance.
• Proper labelling with location, depth, date, and sample type is mandatory.

flowchart TD
    A[Start Sampling] --> B{Nature of Ground}
    B -->|Soil| C{Sample Type}
    B -->|Rock| D{Sample Type}
    C -->|Disturbed| E[Hand/Auger/Shell Samples]
    C -->|Undisturbed| F[Chunk/Tube Samples]
    D -->|Disturbed| G[Wash Samples]
    D -->|Undisturbed| H[Cores]
    E & F & G & H --> I[Record Data]
    I --> J[Handle & Label Samples]
    J --> K[Send for Testing]

Summary: Follow IS 1892 clauses for selecting sampling methods based on soil/rock type and sample type, maintain detailed records, and ensure careful handling and labelling to preserve sample integrity.

IS 1892 - Protection, Handling, and Labelling of Samples

Key Points from Clause 7.1 & Appendix E

1. Handling of Samples (Appendix E)

• Undisturbed samples in liners or seamless tubes must be:

  • Carefully sealed at both ends immediately after extraction.
  • Protected against vibration, shock, and temperature changes.
  • Stored vertically or horizontally with proper cushioning.
  • Transported promptly to avoid disturbance.

• Disturbed samples should be placed in airtight containers or bags and labelled clearly.

2. Labelling of Samples

• Each sample must be labelled with:
  • Trial pit or boring identification
  • Depth and thickness of sample
  • Date and time of sampling
  • Type of sample (disturbed/undisturbed)
  • Location details (map reference, RL, etc.)

3. Protection

• Use rigid containers for protection.
• Avoid exposure to moisture loss or contamination.
• Samples should be kept in a fit state for testing.

Sample Record Format (Appendix D)

Summary Diagram of Sample Handling

flowchart TD
    A[Sample Extraction] --> B[Sealing in Liner/Tube]
    B --> C[Label Sample with Details]
    C --> D[Protection in Container]
    D --> E[Transport to Lab]
    E --> F[Testing & Examination]

References:

• IS 1892: Clause 7.1, Appendix D & E
• Ensure full labelling and protection for reliable test results.

IS 1892: Recording & Reporting of Subsurface Data — Key Points

1. Recording Borings (Clause 6.5.1 & Appendix D)

• Use a standard record sheet (Appendix D) including:
  • Name, organization, type & diameter of boring
  • Date, ground surface level, inclination
  • Detailed description of strata with:
    • Soil classification (e.g., SP, CI)
    • Thickness & depth of each stratum
    • Sample type (undisturbed/disturbed), depth & thickness
    • Groundwater level and remarks
• Attach a site plan showing boring locations.
• For deviated deep borings, provide plan & section views.

2. Testing of Samples (Clause 8.1 & Table 3)

• Tests depend on exploration phase:

3. Subsoil Exploration Methods (Appendix A)

• Reconnaissance: Electrical resistivity, seismic refraction, SPT, static/dynamic cone penetrometer.
• Exploratory: Shell & auger drilling for various soil types.

Summary Diagram: Subsurface Data Recording Flow

flowchart TD
    A[Start Subsurface Exploration] --> B[Conduct Borings]
    B --> C[Record Data on Standard Sheet]
    C --> D[Attach Site Plan & Deviations]
    D --> E[Collect Soil/Rock Samples]
    E --> F[Laboratory Testing]
    F --> G[Report Engineering

Geophysical Investigation Methods per IS 1892

1. Seismic Refraction Method (Clause 1.1, Appendix B)

• Principle: Shock waves generated at ground surface travel through soil layers. Primary waves travel directly; refracted waves bend at layer interfaces.
• Critical Distance: Distance where primary and refracted waves arrive simultaneously, depends on layer depth and velocity ratio.
• Velocity Ranges (m/s):

• Output: Time-distance graph with break indicating critical distance.

2. Electrical Resistivity Method (Clause 3.2, 2.1.2)

• Measures variation in apparent resistivity to identify soil/rock types.
• Mean Resistivity (ohm.m):

3. Testing Samples (Clause 8.1, Table 3)

• Reconnaissance: Visual classification (IS:1498)
• Detailed Tests: Liquid/plastic limits, grain size, specific gravity, moisture content, unit weight, consolidation, shear strength, permeability, chemical tests (chlorides, sulphates

IS 1892 - Dynamic Penetration Tests Key Points

Overview (Clauses 2.1, 2.1.2)

• Dynamic penetration tests assess soil resistance by driving a tool into the soil using a hammer.
• The Standard Penetration Test (SPT) is the most common dynamic test (refer IS 2131-1963).
• Useful for non-cohesive soils where bearing capacity is hard to estimate otherwise.
• Results help compare soil properties across boreholes on the same site.

Standard Penetration Test (SPT) Essentials

• Procedure: Drive a split spoon sampler 150 mm into soil using a 63.5 kg hammer dropping 760 mm.
• N-value: Number of blows required for last 300 mm penetration (after initial 150 mm).
• Used to estimate soil bearing capacity and relative density.

Related IS Codes

• IS 2131-1963: Detailed SPT procedure.
• IS 4968 (Parts I-III): Sounding tests measuring toe and skin resistance for pile driving.

Typical SPT N-value Interpretation (Approximate)

Formula for Approximate Bearing Capacity from SPT N-value (for sands):

[ q_u = N \times C ]

Where:

• ( q_u ) = Ultimate bearing capacity (kPa)
• ( N ) = SPT blow count
• ( C ) = Empirical factor (varies 10-20 kPa depending on soil)

flowchart TD
    A[Start: Borehole Drilling] --> B[Insert Split Spoon Sampler]
    B --> C[Drive with Hammer (63.5 kg, 760 mm drop)]
    C --> D[Record Blows for 150 mm penetration (Ignore)]
    D --> E[Record Blows for Next 300 mm (N-value)]
    E --> F[Interpret N-value for Soil Bearing Capacity]

Vane Shear Test (IS 1892 & IS 4434-1978) Key Points

• Purpose: Measures in-situ shear strength (Cu) of soft clays up to ~30m depth.
• Procedure: A 4-bladed vane (cylindrical shape) is pushed into clay and rotated; maximum torque (T) to cause rotation is recorded.
• Shear strength formula:

[ C_u = \frac{T}{(2 \pi r^2 h + 2 \pi r h^2)} ]

Where:

• (T) = Torque (N·mm or N·m)

• (r) = Vane radius (m)

• (h) = Vane height (m)

• Interpretation: Torque equals moment from shear strength acting on the cylindrical surface (top + side).

Typical Vane Dimensions (IS 4434)

Test Procedure Summary (IS 4434)

• Insert vane vertically into soil at test depth.
• Rotate at a constant rate (~6°/sec).
• Record maximum torque before rotation.
• Calculate shear strength using formula above.

Notes

• Useful for soft, sensitive clays.
• Non-empirical, direct shear strength measurement.
• Supplement sampling for comprehensive soil strength profiling.

flowchart LR
    A[Vane inserted into clay] --> B[Rotate vane at constant speed]
    B --> C[Measure max torque (T)]
    C --> D[Calculate Cu using formula]
    D --> E[Shear strength of soft clay]

For detailed procedure and calibration, refer to IS 4434-1978.

IS 1892: Record of Boring – Key Specifications & Format

1. Essential Information (Clause 6.5.1 & Appendix D)

• Boring ID & Organization
• Soil Sampler Used
• Diameter of Boring
• Inclination (usually vertical)
• Date Completed
• Ground Surface Level
• Type of Boring (e.g., Wash Boring)

2. Description Table Format (Appendix D)

3. Additional Requirements

• Site plan showing boring locations.
• For deviated deep borings, provide plan & section views.
• Record water table level and crowd (penetration force).

4. Sample Handling (Appendix E)

• Proper labelling and handling to avoid contamination.
• Maintain undisturbed samples for sensitive tests.

Visual Summary of Record Sheet Structure

flowchart TD
    A[Start Boring Record] --> B[Basic Info: ID, Date, Diameter]
    B --> C[Strata Description Table]
    C --> D[Sample Details: Type, Depth]
    D --> E[Water Level & Crowd]
    E --> F[Attach Site Plan & Deviations]
    F --> G[Sample Handling & Labelling]

This format ensures comprehensive, standardized documentation for geotechnical investigations per IS 1892.

IS 1892 - Handling and Labelling of Soil Samples

Key Specifications (Clause 7.1 & Appendix E)

• Protection & Handling:

  • Samples must be protected to preserve natural state.
  • Avoid disturbance during extraction, transport, and storage.
  • Use suitable containers (e.g., sealed jars for cohesive soils).
  • Transport samples promptly to avoid moisture loss or contamination.

• Labelling:

  • Each sample must be fully labelled with:
    • Trial pit or boring number.
    • Depth and thickness of sample.
    • Date and time of sampling.
    • Location details (map reference, RL of ground surface).
    • Type of sample (disturbed/undisturbed).
  • Labels should be waterproof and securely attached.

Record Format (Appendix D - Record of Boring)

Summary Diagram

flowchart TD
    A[Sample Extraction] --> B[Proper Handling]
    B --> C[Sealed Containers]
    C --> D[Labelling with Details]
    D --> E[Transport & Storage]
    E --> F[Examination & Testing]

Note: Refer Appendix E of IS 1892 for detailed handling methods and sample preservation techniques. Proper labelling ensures traceability and reliability of test results.