Controlling blast vibrations 17 Dec 2020

Strict vibration limits often restrict the use of drill+blast excavation in vibration sensitive environments. However, good planning of controlled blasting can often accommodate such restrictions. In a guideline produced by the ITAtech Committee Excavation Group and published by the ITA, International Tunnelling and Underground Space Association, describes practical suggestions of how to utilize vibration measurement in the design of drill charge hole patterns and detonation control.

Due to the complexity of geology, forecasting vibration propagation for a given drill+blast project can be difficult. Multiple factors are described in the guideline that can affect the propagation of vibration waves including:

• receptor distance from the blast
• rock mass properties, the ability to conduct and absorb the vibration
• changes in rock mass which can cause bending and reflection of vibration waves
• rock mass fracturing and orientation towards the vibration waves
• change of water content or temperature, such as in moraine or frosted soil

Fig 1. Example of momentary design and scaled vibration level [mm/sec] where both overshoots and delayed detonation can be detected

Environment and nearby structures define the allowed vibration limits, which are usually set by authorities taking into account factors including:

• the type and age of structures
• use of structures with strict limits for hospitals and scientific facilities
• the prevailing geology
• construction materials of structures and basement type
• the reliability of the as-built data of structures

The total amount of explosives for each drill+blast round of excavation is calculated as a mass per each charge, as this is typically the minimum amount of momentary kilos that can be blasted. Modern detonator systems offer extensive possibilities for the delay timing of the blast, with the more material that blasts per ignition level, the more vibration that will be generated. The tendency therefore is to limit the amount of explosives per ignition level. The momentary reporting of blast vibrations is based on defining hole charges and detonation delays in the blast round plan.

Measurement systems play a vital role in vibration control with triaxial seismograph sensors attached at critical locations where peak particle velocity (PPV) limit-values have been designated. Sensors are connected wirelessly to a web-server and record data when they detect vibration levels above a deigned trigger level. Vibrations are recorded in speeds of mm/sec, amplitude in mm, acceleration in m/s2 and frequency as 1/s. This data is recorded in the x-, y- and z- axes directions with web-server data analysis producing vibration reports together with possible overshoot comparisons.

Based on the vibration analysis the charges, detonation control and location of the holes in the drill plan can be adjusted to improve production and limit the need for shortening the round length or introducing partial face blast rounds.

Download a copy of the guideline

If vibration data is examined against the momentary design (against time-kilos- graph), the correlation between vibration data and the designed hole charge blasting can be found. This will not only express the PPV-value of the round blasted, but the data is also available for detailed study. When the absolute value of vibration data amplitude is scaled to generally match the blasted kilos, the remaining deviations between these two values can be observed (Fig 1). An overshoot might be due to too large a momentary design or incorrect selection of the detonators or misfire in some of the previous charge holes. The overshoot holes can be pinpointed based on the timing value to indicate where the drilling and blasting plan can be adjusted.

A more predictable vibration of the blast can increase drill+blast excavation progress. Some key points that the ITA guideline suggests to keep in mind include the following.

• Reducing the maximum instantaneous charge (MIC) tends to increase the number of holes required in the face.
• Smaller charge hole diameter increases typically increases the partial charge blasting reliability of emulsion explosives.
• Smaller charges increase the profile quality, assuming a good drill plan, of top quality drilling and controlled detonation.
• Smaller charges reduce the blasting damage zone.
• Better quality can reduce the amount of shotcrete required for primary lining and support purposes.
• Better profile quality reduces scaling time and can increase safety.
• Conversely, increasing the number of holes increases the cost of drilling with the additional use of explosives and detonators.

The ITAtech document is produced as an initiative for establishing a common interface between vibration measurement systems and drill+blast design programs to help all parties control blast vibration in a cost-effective way. Download a pdf copy of the Practical approach for controlling blasting vibration guideline produced by the ITAtech Committee Excavation Group of the ITA.