Criteria for safety and design of structures subject to underground blasts
1973 Edition

Overview of Scope (Clause 2.0):

Defines terminology and the scope of application regarding underground blasting vibrations and their influence on nearby structures.

Safe Distance Parameters (Clause 4.2.2.1):

• Seismic wave velocity (C) values for various geological materials (m/s):

• Relationship between charge per delay (Q) and safe radius (R) to prevent damage (see Fig.1 in the code).

Additional Notes:

• For delay times shorter than specified, consult supplementary references.
• Prescribed vibration velocity thresholds are conservative and apply primarily to masonry and M150 concrete.
• Monitoring of structural subsidence is recommended (Clause 6.4).

Core Formula for Safe Distance:

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

Where:

• (R): safe distance (meters)
• (Q): charge weight per delay (kilograms)
• (k): constant determined by soil/rock type and seismic velocity

graph LR
Q[Charge per Delay (Q)] --> C[Seismic Wave Velocity (C)]
C --> R[Safe Distance (R)]
R --> S[Threshold Damage Prevention]

Refer to IS 6922 Fig.1 and related tables for detailed design guidance.

Key Terminology in IS 6922 (Clause 2.0)

• Defines essential terms related to underground blasting and structural safety.

Seismic Wave Velocity (C) (Clause 4.2.2.1):

• Used in safe distance calculations and blast effect assessments.

Design Horizontal Acceleration for Large Charges (Clause 5.1):

For charges exceeding 100 kg per delay, design acceleration (a) is given by:

[ 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:

• Vibration velocity limits are conservative for masonry and lower than human tolerance.
• Use typical seismic velocities if site-specific data is unavailable.

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

This section summarizes fundamental definitions and formulas critical for blast safety design.

IS 6922: Core Principles and Key Formulas

Threshold Damage Definition (Clause 2.11):

• Damage is identified by the appearance of new plaster cracks or enlargement of existing ones due to blast-induced vibrations.

Safe Vibration Velocity Limits (Clause 4.1.1.1):

• Particle velocity limits are set conservatively for masonry and M150 concrete.
• These limits are below thresholds for human comfort, ensuring structural safety.

Safe Blast Distance (Clause 4.2.2.1):

• Longitudinal seismic wave velocity (C) is a critical factor in calculating safe distances.

• Safe distance (R) is related to charge per delay (Q) and seismic velocity (C), as shown in Fig.1.

General Relationship:

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

• Larger charge necessitates greater safe distance to avoid threshold damage.

flowchart LR
Q[Blast Charge Q] --> C[Seismic Wave Velocity C]
C --> R[Calculate Safe Distance R]
R --> D{Damage Assessment}
D -->|No Damage| S[Safe]
D -->|Damage| M[Adjust Q or R]

Summary: Utilize seismic velocity and charge data to determine safe distances preventing plaster cracking.

IS 6922: Criteria to Prevent Threshold Damage

Ground Particle Velocity Limits (Clause 4.1.1.1):

Longitudinal Seismic Wave Velocity (Clause 4.2.2.1):

Key Equation for Safety Distance:

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

• Safe distance depends on charge per delay and medium velocity (refer Clause 4.2.2.1 and Fig.1).

Additional Guidance:

• Resonance effects are minimal due to the impulsive nature of blasting (Clause 3.7).
• Use velocity thresholds to verify ground vibration safety.
• Consult additional sources if delay times are shorter than recommended.

graph LR
Q[Blast Charge Q] --> V[Ground Vibration]
V --> M{Medium Type}
M --> S1[Soil: Max v=50 mm/s, C=200-1800]
M --> S2[Weathered/Soft Rock: Max v=50 mm/s, C=1800-3200]
M --> S3[Hard Rock: Max v=70 mm/s, C=3200-7500]
S1 & S2 & S3 --> R[Safe Distance R]

This section aids in designing blasting operations to safeguard against structural damage.

IS 6922: Calculating Design Ground Acceleration

Formula for Large Charges (Clause 5.1):

For charges exceeding 100 kg per delay, horizontal design acceleration (a) is calculated as:

[ a = \frac{K_2 \sqrt{Q}}{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 point (m)

Important Notes:

• Apply this acceleration uniformly across the structure (Clause 5.1.1).
• Peak particle velocity is a key damage indicator (Clause 3.2).
• Millisecond delay detonators provide a reduction factor of at least 0.5 (Clause 4.2.2.2).

Summary of Constants:

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

This calculation is vital for engineering structures resilient to seismic effects from blasting.

Monitoring Requirements as per IS 6922

Clause 6.4:

• Continuous surveillance of structural subsidence is mandated to maintain stability.

Ground Vibration Monitoring (Clause 4.1.1.2):

• Peak particle velocity limits for vibration control:

Instrumentation (Clause 6.2.1):

• Use of peak velocity sensors is recommended.
• Sensor systems can trigger alarms:
  • Visual indicators
  • Audible warnings

Typical Monitoring Flow:

flowchart LR
V[Ground Vibrations] --> S[Peak Velocity Sensors]
S --> C{Threshold Exceeded?}
C -- Yes --> W[Activate Warning System]
W --> VL[Visual Alarm]
W --> AL[Audible Alarm]
C -- No --> M[Continue Monitoring]

Summary: Monitor structural subsidence and maintain vibration levels within permissible limits using appropriate instrumentation equipped with alarm capabilities.

IS 6922 Procedures for Pilot Testing and Charge Control

Ground Particle Velocity Calculation (Clause 4.1):

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

Where:

• (v): ground particle velocity (mm/s)
• (K_1): constant (880 for soil/weathered rock, 1400 for hard rock)
• (Q): charge per delay (kg)
• (R): distance from blast (m)

Seismic Wave Velocity Values (Clause 4.2.2.1):

• Used to calculate safe distances and delay timings.

Pilot Test Steps (Clause 5.1):

• Determine site-specific constant (K_a).
• Measure longitudinal seismic wave velocity (C).
• Establish maximum permissible charge (Q) ensuring particle velocity remains within safety limits.

Charge Control (Clause 5.1c):

• Regulate charge weights during excavation to maintain ground vibrations within permitted values.
• Particularly critical in urban or sensitive zones.

Overview Diagram:

graph LR
Q[Charge per Delay (Q)] --> V[Ground Particle Velocity (v)]
V --> S[Check against Safety Limits]
S --> C[Control Charge and Distance]

Pilot testing validates constants and velocities used in formulas, ensuring that actual blasting operations keep vibrations within safe thresholds.

Instrumentation Guidelines per IS 6922

1. Instruments for Measuring Ground Vibrations (Clause 6.2):

• Velocity Pick-up: Effective for small charges and short distances; flat frequency response above 10 Hz.
• Accelerometers: Suitable for large charges and extended ranges; flat response over 0–100 Hz.
• Displacement Meters: Occasionally used, less common.

2. Vibration Limits (Clause 4.1.1.2):

3. Monitoring Requirements (Clause 6.4):

• Structural subsidence should be tracked alongside vibration measurements.

Instrument Selection and Frequency Response Summary

flowchart LR
V[Ground Vibrations] --> I{Select Instrument}
I --> VP[Velocity Pick-up]
I --> AC[Accelerometer]
I --> DM[Displacement Meter]
VP --> F1[Flat frequency > 10 Hz]
AC --> F2[Flat frequency 0–100 Hz]

For detailed calibration and procedures, consult the full IS 6922 documentation.

Safe Distance Determination (IS 6922 Clause 4.2)

Core Elements:

• Charge per delay (Q) up to 100 kg is primarily considered for safe distance calculations.
• Safe distance (R) depends on charge weight and seismic wave velocity (C) of the surrounding medium.

Typical Seismic Wave Velocity Values (Clause 4.2.2.1):

Estimation Method:

• Use Figure 1 (charge vs. distance curve) to find the safe distance for a given charge.
• If delay timing is less than prescribed, additional precautions and references should be consulted.

Simplified Conceptual Formula:

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

• Safe distance grows with the cube root of charge and inversely with seismic velocity.

Summary:

• Identify geological medium and assign seismic velocity C.
• For given charge Q (≤100 kg), determine safe distance R using code charts or formula.
• Ensure compliance with delay time recommendations.

graph LR
Q[Charge Q] --> C[Seismic Velocity C]
C & Q --> R[Safe Distance R]
R --> S[Protection of Structures]

Refer to IS 6922 Fig.1 for precise charge-to-distance correlation.

Design Guidelines for Structures Affected by Underground Blasts (IS 6922)

Key Points:

• Apply design acceleration uniformly across the structural system (Clause 5.1.1).
• The code focuses on limiting vibration amplitudes to avoid structural damage such as cracking.
• Applies primarily to structures founded on uniform soil or rock; complex site conditions necessitate vibration monitoring.
• Pre-blast surveys of existing cracks are advised to detect any blast-induced changes.
• Installation of protective shielding devices is recommended to guard against flying debris.

Design Acceleration and Safety:

• Use blast-induced effective acceleration ((a_{eff})) as a uniform base excitation in structural analysis.
• Permissible vibration levels vary by structure type and condition (refer to IS 6922 tables).

Typical Attenuation Formula for Acceleration:

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

Where:

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

Recommendations:

• Conduct structural crack surveys before and after blasting.
• Employ vibration monitoring instruments during blasting operations.
• Design structural elements to withstand uniform acceleration due to blast vibrations.
• Implement shielding to mitigate debris hazards.

flowchart TD
B[Blast Event] --> V[Blast-Induced Vibrations]
V --> A[Effective Acceleration a_eff]
A --> S[Uniform Acceleration Applied to Structure]
S --> D{Damage Assessment}
D -- Yes --> M[Monitor and Mitigate]
D -- No --> Safe[Structure Considered Safe]

Refer to IS 6922 tables for specific vibration limits and design accelerations.

IS 6922: Guidelines on Controlling Blast Vibrations

1. Maximum Ground Particle Velocity:

• Refer to Clauses 4.1.1.1 and 4.1.1.2 for limits.

2. Vibration Parameters:

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

3. Structural Response Factors:

• Influenced by natural frequency (N), vibration frequency (f), damping, and duration.
• Resonance is unlikely due to the impulsive and irregular nature of blasting (Clause 3.7).

Velocity Calculation:

• Velocity (v(t)) can be obtained by integrating acceleration (a(t)):

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

• Or from displacement (d_{peak}) and frequency (f):

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

Conceptual Flow for Vibration Control:

flowchart LR
A[Ground Vibration] --> B[Measure Acceleration]
B --> C[Compute Velocity v]
C --> D{Is v below Limit?}
D -- Yes --> E[Safe Zone]
D -- No --> F[Implement Vibration Mitigation]
F --> G[Reduce Blast Intensity or Modify Design]

Use established velocity limits and vibration parameters to monitor and control blast-induced vibrations effectively.

IS 6922: Damage Evaluation and Crack Monitoring Procedures

Threshold Damage (Clause 2.11):

• Defined as the appearance of new plaster cracks or the widening of existing cracks caused by blast vibrations.

Safety Parameters Based on Ground Vibration (Clause 3.2):

• Peak Ground Particle Velocity (PPV) is the primary predictor for damage severity, independent of frequency.

Safe Distance and Charge Relationship (Clause 4.2.2.1):

• Safe distance (R) is a function of charge per delay (Q) and seismic wave velocity (C).

• Refer to Fig. 1 for charge versus distance correlations.

Monitoring (Clause 6.4):

• Track subsidence and any changes in crack widths.
• Employ crack gauges and vibration sensors to assess structural response.

Formula for Safe Distance:

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

• (k): empirical constant based on site data
• (Q): charge per delay (kg)
• (C): seismic wave velocity (m/s)

graph LR
Q[Blast Charge Q] --> C[Seismic Wave Velocity C]
C --> V[Ground Particle Velocity]
V --> D[Damage Threshold Evaluation]
D --> M[Crack Monitoring & Structural Assessment]

Refer to IS 6922 for detailed values and monitoring protocols.

IS 6922 Reference Data and Important Notes

1. Longitudinal Seismic Wave Velocity (Clause 4.2.2.1)

• Key for estimating safe distances:

2. Safe Distance and Charge (Clause 4.2.2.1)

• Charge per delay (Q) and safe radius (R) relate to prevent threshold damage.
• Consult Fig. 1 for charge vs. distance curves.

3. Vibration Safety Notes (Clause 4.1.1.1)

• Vibration limits specified are conservative, primarily for masonry.
• Concrete of M150 grade can tolerate slightly higher vibrations.
• Limits are below human comfort thresholds.

4. Monitoring (Clause 6.4)

• Structural subsidence monitoring is essential during blasting.

Summary of Safe Distance Formula:

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

• (R): safe distance (m)
• (Q): charge per delay (kg)
• (k): constant dependent on geological medium and vibration limits

Contact Details for Testing and Consultation:

• Headquarters: Manak Bhavan, New Delhi
• Central Laboratory: Sahibabad Industrial Area
• Numerous regional and branch offices across India

graph LR
Q[Charge per Delay Q] --> R[Safe Distance R]
R --> S[Threshold Damage Prevention]
S --> M[Depends on Medium Velocity C]
M --> G[Soil, Weathered Rock, Hard Rock]

Refer to IS 6922 for detailed charts and tables.