Criteria for safety and design of structures subject to underground blasts

IS 6922 - Scope: Key Specifications & Formulas

Scope (Clause 2.0): Defines terms and applicability related to underground blasting and its effects on structures.

Safe Distance from Blast (Clause 4.2.2.1)

• Seismic wave velocity (C) for different media (m/s):

• Charge per delay (Q) and range (R) are related to ensure safety against threshold damage (see Fig.1 in code).

Notes

• If delay time is less than specified, consult additional references.
• Safe vibration velocity values are conservative for masonry and apply to concrete M150 quality.
• Monitoring subsidence of structures is recommended (Clause 6.4).

Formula (General form for safe distance)

[ R = k \times Q^{1/3} ]

Where:

• ( R ) = safe distance (m)
• ( Q ) = charge per delay (kg)
• ( k ) = constant depending on soil/rock type and seismic velocity

graph LR
A[Charge per Delay (Q)] --> B[Seismic Wave Velocity (C)]
B --> C[Safe Distance (R)]
C --> D[Threshold Damage Safety]

For detailed design, refer to IS 6922 Fig.1 and tables.

IS 6922: Key Definitions and Specifications

1. Definitions (Clause 2.0)

• The code defines terms related to underground blasting, seismic effects, and safety distances for structures.

2. Longitudinal Seismic Wave Velocity, C (Clause 4.2.2.1)

• Used in calculating safe distances and seismic effects from blasts.

3. Design Acceleration for Large Charges (Clause 5.1)

For charges Q > 100 kg/delay, design horizontal acceleration a is:

[ a = \frac{K_2 \times Q^{1/3}}{R} ]

Where:

• (a) = design acceleration (cm/s²)
• (K_2) = constant (4 for soil/weathered rock, 6 for hard rock)
• (Q) = charge per delay (kg)
• (R) = distance from blast (m)

Notes:

• Safe vibration velocity values (Clause 4.1.1.1) are conservative for masonry and lower than human tolerance.
• Use provided wave velocities if actual site data is unavailable.

graph LR
A[Blast Charge Q] --> B[Seismic Wave Velocity C]
B --> C[Safe Distance R]
Q & R & K2 --> D[Design Acceleration a]

This summarizes key definitions and formulas for safe blasting design per IS 6922.

IS 6922: General Principles - Key Formulas & Specifications

1. Threshold Damage (Clause 2.11)

• Defined as formation of new plaster cracks or widening of old cracks due to blasting vibrations.

2. Safe Vibration Velocity (Clauses 4.1.1.1)

• Safe particle velocity v values given are conservative for masonry and concrete (M150).
• These values are lower than human tolerance, ensuring structural safety.

3. Safe Distance from Blast (Clause 4.2.2.1)

• Longitudinal seismic wave velocity (C) is critical for evaluating safe distances.
• Typical values of C (m/s):

• Safe distance R relates to charge per delay Q and velocity C (see Fig.1 in code).

4. General Formula (conceptual)

[ R \propto \sqrt[3]{Q} ]

• Larger charge requires larger safe distance to prevent threshold damage.

flowchart LR
    A[Blasting Charge Q] --> B[Seismic Wave Velocity C]
    B --> C[Calculate Safe Distance R]
    C --> D[Check Threshold Damage]
    D -->|No Damage| E[Safe]
    D -->|Damage| F[Increase R or Reduce Q]

Summary: Use seismic velocity C and charge Q to determine safe distance R to avoid threshold damage (new/widened plaster cracks). Use conservative velocity values for design.

IS 6922: Safety Criteria Against Threshold Damage

Key Specifications:

• Ground Particle Velocity Limits (Clause 4.1.1.1):

• Longitudinal Seismic Wave Velocity, C (m/s) (Clause 4.2.2.1):

Important Formula:

For safety against threshold damage, safe distance R from blast charge Q (kg) is related by:

[ R \propto \sqrt[3]{Q} ]

Exact safe distances depend on medium velocity C and charge per delay Q, as per Clause 4.2.2.1 and Fig.1 (charge vs. range curve).

Notes:

• Resonance unlikely due to impulsive, irregular ground motion (Clause 3.7).
• Use velocity limits to check if ground vibration is within safe limits.
• If delay time is less than recommended, consult additional references.

graph LR
A[Blast Charge Q (kg)] --> B[Ground Vibration]
B --> C{Medium Type}
C -->|Soils| D[Max v = 50 mm/s, C=200-1800 m/s]
C -->|Weathered/Soft Rock| E[Max v = 50 mm/s, C=1800-3200 m/s]
C -->|Hard Rock| F[Max v = 70 mm/s, C=3200-7500 m/s]
D & E & F --> G[Safe Distance R from Blast]

This summary helps ensure design safety against threshold damage from underground blasting vibrations per IS 6922.

IS 6922 - Ground Acceleration for Design

Key Formula (Clause 5.1)

For charges > 100 kg/delay, design horizontal ground acceleration ( a ) is:

[ a = \frac{K_2 \sqrt{Q}}{R} ]

Where:

• ( a ) = design acceleration (cm/s²)
• ( K_2 ) = constant:
  • 4 for soil, weathered, soft rock
  • 6 for hard rock
• ( Q ) = charge per delay (kg)
• ( R ) = distance from blast point (m)

Important Notes

• Apply the design acceleration uniformly to the structure (Clause 5.1.1).
• Peak ground particle velocity is a key damage index (Clause 3.2).
• When using millisecond delay detonators, reduction factor ≥ 0.5 (Clause 4.2.2.2).

Summary Table for ( K_2 ):

flowchart LR
    Q[Charge per delay (Q)] -->|sqrt(Q)| A[Design Acceleration (a)]
    R[Distance (R)] -->|1/R| A
    K2[Constant (K2)] -->|Multiply| A
    A -->|Uniform application| Structure[Structure Design]

This formula helps design structures to withstand seismic effects from large blasting charges.

IS 6922 Monitoring - Key Points & Specifications

Clause 6.4:

• Subsidence of the structure must be monitored to ensure stability.

Ground Vibration Monitoring (Clause 4.1.1.2):
Peak ground particle velocity limits for vibration control:

Monitoring Instruments (Clause 6.2.1):

• Use peak velocity sensors for ground vibration monitoring.
• Sensors can trigger warning systems:
  • Visual (light indicators)
  • Audible alarms

Practical Monitoring Setup

flowchart LR
    A[Ground Vibrations] --> B[Peak Velocity Sensors]
    B --> C{Threshold Exceeded?}
    C -- Yes --> D[Activate Warning System]
    D --> E[Visual Alarm]
    D --> F[Audible Alarm]
    C -- No --> G[Continue Monitoring]

Summary:

• Monitor subsidence continuously.
• Control vibrations within specified peak velocities.
• Use sensors with alarm systems for real-time alerts.

IS 6922: Pilot Tests and Charge Control Key Points

1. Ground Particle Velocity Formula (Clause 4.1)

[ v = K_1 \times \frac{\sqrt{Q}}{R} ]

• v = ground particle velocity (mm/s)
• K₁ = constant (typical values):
  • Soils, weathered/soft rock = 880
  • Hard rock = 1400
• Q = charge per delay (kg)
• R = distance from blast (m)

2. Longitudinal Seismic Wave Velocity (Clause 4.2.2.1)

• Used for determining safe distances and delay times.

3. Pilot Tests (Clause 5.1)

• Determine constant (K_a) for particle velocity calculation.
• Measure longitudinal wave velocity (C).
• Establish maximum charge (Q) to keep particle velocity within safe limits near structures.

4. Charge Control (Clause 5.1c)

• Control charge during excavation to keep ground particle velocity ≤ safe values (Clause 4.1.1.2).
• Essential in built-up areas to avoid structural damage.

Summary Diagram: Charge vs Distance for Safe Blasting

graph LR
A[Charge per Delay, Q (kg)] --> B[Ground Particle Velocity, v (mm/s)]
B --> C[Safe Limits for Structures]
C --> D[Control Charge and Distance]

Use pilot tests to calibrate (K_a) and (C), then apply the formula to control charge (Q) and distance (R) ensuring safe particle velocity near structures.

IS 6922 Key Points: Instrumentation & Measurement Methods

1. Instruments for Ground Vibration Measurement (Clause 6.2):

• Velocity pick up: Best for small charges & short ranges.
• Accelerometer: Best for large charges & long ranges.
• Displacement meter: Also used but less common.
• Frequency response:
  • Velocity pick up: Flat above 10 c/s (cycles per second)
  • Accelerometer: Flat in 0 to 100 c/s range

2. Vibration Limits (Clause 4.1.1.2):
Peak ground particle velocity (PPV) limits to avoid damage:

3. Monitoring (Clause 6.4):

• Subsidence of structure should be monitored alongside vibrations.

Summary Table: Instrument Selection & Frequency Response

Additional Notes:

• Use velocity pick up for sensitive monitoring near structures.
• Accelerometers are preferred for detailed high-frequency vibration analysis.

flowchart LR
    A[Ground Vibration] --> B{Choose Instrument}
    B -->|Small charge, short range| C[Velocity pick up]
    B -->|Large charge, long range| D[Accelerometer]
    B -->|General| E[Displacement meter]
    C --> F[Frequency > 10 c/s flat]
    D --> G[Frequency 0-100 c/s flat]

For detailed procedures & calibration, refer to full IS 6922 text.

Safe Distance from Blast (IS 6922 - Clause 4.2)

Key Points:

• Charge per delay (Q): Up to 100 kg considered for safe distance calculations.
• Safe distance (R): Depends on charge Q and seismic wave velocity (C) of the medium.
• Seismic wave velocity (C): Used to estimate wave propagation and damage threshold.

Seismic Wave Velocity (C) Values (Clause 4.2.2.1):

Safe Distance Estimation:

• Use Fig. 1 (not shown here) to find safe distance R for a given charge Q.
• If delay time < prescribed, refer to additional references for safety.

General Formula (conceptual):

[ R \propto Q^{1/3} \times \frac{1}{C} ]

• Safe distance increases with charge and decreases with higher seismic velocity.

Summary:

• Identify medium type → select velocity C.
• For given charge Q (≤100 kg), use Fig. 1 or formula to find safe distance R.
• Ensure delay times conform to Clause 4.2.2 for safety.

graph LR
A[Charge Q (kg)] --> B[Seismic Velocity C (m/s)]
B --> C[Safe Distance R (m)]
A --> C
C --> D[Structure Safety Against Blast Damage]

Note: Refer to IS 6922 Fig.1 for precise Q-R relationship.

IS 6922: Design Considerations for Structures Subject to Underground Blasts

Key Points from IS 6922:

• Design acceleration from underground blasts should be uniformly applied to the structure (Clause 5.1.1).
• The standard focuses on maximum permissible vibrations to avoid damage like cracking.
• Applies to structures on homogeneous soil/rock; complex conditions require vibration monitoring.
• Pre-blast surveys of existing cracks are recommended to monitor potential damage.
• Shielding devices should be installed to protect from flying debris.

Design Acceleration and Safety Criteria

• Use effective acceleration (a_eff) derived from blast-induced vibrations for structural design.
• Apply acceleration uniformly to the structure as a base excitation.
• Maximum permissible vibration levels depend on structure type and condition (refer to IS 6922 tables for specific limits).

Typical Design Formula for Blast-Induced Acceleration:

[ a = \frac{A}{r^n} ] Where:

• (a) = acceleration at structure (m/s²)
• (A) = blast intensity constant (depends on charge weight)
• (r) = distance from blast to structure (m)
• (n) = attenuation exponent (usually between 1.5 to 2)

Recommendations:

• Conduct pre- and post-blast structural surveys.
• Use vibration monitoring instruments during blasting.
• Design structure to resist uniform acceleration corresponding to blast vibrations.
• Install protective shielding to prevent debris impact.

flowchart TD
    Blast --> Vibration[Blast-Induced Vibrations]
    Vibration --> Acceleration[Effective Acceleration (a_eff)]
    Acceleration --> Structure[Apply Uniform Acceleration to Structure]
    Structure --> DamageCheck{Damage?}
    DamageCheck -- Yes --> Monitoring[Monitor Cracks & Vibrations]
    DamageCheck -- No --> Safe[Structure Safe]

For detailed permissible vibration limits and design accelerations, refer to IS 6922 tables (not provided here).

IS 6922: Vibration Control Techniques – Key Points

1. Ground Particle Velocity Limits for Safety

• Clause 4.1.1.1: Safe limits to avoid threshold damage.
• Clause 4.1.1.2: Monitoring limits using instruments.

2. Parameters for Vibration

• Ground vibration characterized by acceleration, velocity, displacement, and their frequencies.
• Peak ground particle velocity (v) is the best general index for damage, independent of frequency. (Clause 3.2)

3. Structural Response Factors

• Response depends on:
  • Natural frequency of structure (N)
  • Frequency of ground motion (f)
  • Damping in structure material
  • Duration and intensity of vibration
• Resonance unlikely due to impulsive and irregular nature of explosions. (Clause 3.7)

Formula for Peak Ground Particle Velocity (v)

If acceleration ( a(t) ) is known, velocity can be approximated by integration:

[ v(t) = \int a(t) , dt ]

Or from displacement ( d(t) ) and frequency ( f ):

[ v_{peak} = 2 \pi f \times d_{peak} ]

Summary Diagram of Vibration Control Concept

flowchart LR
    A[Ground Vibration] --> B[Measure Acceleration]
    B --> C[Calculate Velocity (v)]
    C --> D{v < Limit?}
    D -- Yes --> E[Safe for Structure]
    D -- No --> F[Vibration Control Measures]
    F --> G[Reduce Intensity or Modify Construction]

Use these velocity limits and parameters for monitoring and controlling vibration to prevent structural damage as per IS 6922.

IS 6922: Damage Assessment & Crack Monitoring Key Points

1. Threshold Damage (Clause 2.11)

• Defined as formation of new plaster cracks or widening of old cracks.
• Damage assessment focuses on visible crack changes.

2. Safety Criteria Based on Ground Vibration (Clause 3.2)

• Peak Ground Particle Velocity (PPV) is the key parameter for damage prediction.
• Independent of frequency, PPV correlates well with damage severity.

3. Safe Distance & Charge per Delay (Clause 4.2.2.1)

• Safe distance ( R ) relates to charge per delay ( Q ) and seismic wave velocity ( C ).
• Use the following velocity values if unknown:

• Refer to Fig. 1 (Charge per delay vs. range) for design safety.

4. Monitoring (Clause 6.4)

• Monitor subsidence and crack width changes.
• Use instruments like crack gauges and vibration sensors to track structural response.

Summary Formula for Safe Distance ( R ):

[ R = k \times \left( \frac{Q^{1/3}}{C} \right) ]

• Where ( k ) is an empirical constant based on site data.
• ( Q ) = charge per delay (kg)
• ( C ) = seismic wave velocity (m/s)

graph LR
A[Blast Charge Q] --> B[Seismic Wave Velocity C]
B --> C[Ground Particle Velocity]
C --> D[Damage Threshold (PPV)]
D --> E[Crack Monitoring & Structural Assessment]

Note: For detailed tables and exact values, refer to IS 6922 Clause 4.2.2.1 and Clause 6.4.

IS 6922: References and Notes - Key Formulas and Tables

1. Longitudinal Seismic Wave Velocity (Clause 4.2.2.1)

Used to estimate safe distances from underground blasting vibrations:

• C = Longitudinal seismic wave velocity of the medium (m/s).

2. Safe Distance and Charge per Delay (Clause 4.2.2.1)

• Charge per delay Q (kg) and range R (m) relate to safety against threshold damage.
• Refer to Fig. 1 in IS 6922 for charge vs. range curves.

3. Notes on Safe Vibration Values (Clause 4.1.1.1)

• Safe vibration levels given are conservative for masonry; concrete M150 quality tolerates slightly higher vibration.
• Values are lower than human tolerance to ensure structural safety.

4. Monitoring (Clause 6.4)

• Structural subsidence should be monitored during blasting operations.

Summary Formula for Safe Distance (Conceptual)

[ R = k \times Q^{1/3} ]

• R = Safe distance (m)
• Q = Charge per delay (kg)
• k = Constant depending on medium and vibration limits (from Fig. 1)

Contact Information for Reference and Testing

• Headquarters: Manak Bhavan, New Delhi
• Central Laboratory: Sahibabad Industrial Area
• Multiple regional and branch offices across India for testing and consultation.

graph LR
A[Charge per Delay Q (kg)] --> B[Safe Distance R (m)]
B --> C[Threshold Damage Safety]
C --> D[Depends on Medium Velocity C]
D --> E[Soil, Weathered Rock, Hard Rock]

For detailed charge vs. range curves and vibration limits, refer to **Fig.