Between November 2019 and May 2021, controlled blasting was used to excavate bedrock on the Paskapoo Slope and another small area south of Crestmont Boulevard S.W. As of April 2021, a total of 211 controlled blasts have cleared 1.7 million cubic metres of rock.
The excavation to the designed grades is now complete and daily controlled blasts are no longer needed. Localized, intermittent blasting will still be required to excavate two stormline trenches on both sides of Stoney Trail between the Trans-Canada Highway and Old Banff Coach Road S.W.
None of the daily blasts in April 2021 generated ground vibrations of a sufficient amplitude to produce a seismograph waveform report (associated values are still retrieved from the histogram data and published on this page). Given this, the shallow depth of the localized blasts and the sporadic timing, notice will not be provided for intermittent blasts going forward. However, seismographs will continue to monitor ground vibrations and the reports will continue to be published each week.
Controlled blasting involves drilling into the rock and placing a controlled amount of explosive charges below ground at engineered locations to fracture the rock. Quantity of charges, drill lengths, locations and cover are all engineered to minimize or eliminate impacts like flying debris, vibrations, sound and dust.
Controlled blasting was used on the West Calgary Ring Road where the excavation could not be completed using conventional heavy construction equipment. While this method of fragmenting rock for excavation is less common in Calgary, particularly near residential areas, it is very common in Ontario and British Columbia where the landscape is rocky.
Daily controlled blasting in this area is now complete. Localized, intermittent blasting to excavate two stormline trenches on both sides of Stoney Trail between the Trans-Canada Highway and Old Banff Coach Road S.W. will continue until Fall 2021.
A small area south of Crestmont Boulevard S.W. was excavated using controlled blasting between April and September 2020. Blasting in this area is now complete.
What to expect
Most of the explosive energy during a controlled blast is expended in fragmenting and heaving the heavy rock, but a small portion of this energy passes beyond the immediate blasting area in the form of elastic ground vibrations and air overpressure.
If blasting is occurring near your home, you can expect to feel vibrations and perhaps hear some noise. However, the vibration levels are normally lower and no more potentially damaging to your building than those caused by slamming a door, windstorms and thunderstorms, or children running and jumping around the house.
The energy in a blast which does not contribute to breaking rock is dissipated in the form of vibration either travelling through the ground (ground vibrations) or through the air (air overpressure). Ground vibrations are elastic waves that cause ground particles to displace momentarily, however, at the frequency and intensity of vibrations created by blasting, the ground particles rarely experience any permanent change in their relative position. As in, once the ground stops vibrating, it will be situated in the exact same position as where it began. Ground vibrations are complex and are measured by intensity (velocity) and frequency in three dimensions.
Intensity (velocity) is the speed at which the ground vibration waves cause particles to move. This is called Peak Particle Velocity (PPV) and is measured in millimetres per second.
Frequency is the number of ground vibration waves within one second. This is measured in Hertz (Hz) (cycles per second).
Air overpressure is the vibrations caused by the blast that travel through the air. Typical residential structures will respond to the air overpressure wave by producing higher frequency secondary noise on internal walls. It is this response along flat walls in a structure that causes much of the rattling and other observed effects such as movement of pictures, clocks, etc. This experience is often mistaken for ground vibrations.
In technical terms, the air overpressure generated by controlled blasting is quite similar to the effects of wind. However, the rapid pressure change produced by an air pressure wave can be startling and much more noticeable compared to a more constant and sustained period of wind. A 40 km/h wind can generate equivalent stress on a structure as a blast-generated air overpressure of 131.7 dB(L). To provide context, the seismographs installed in the Cougar Ridge blasting area since November 2019 have regularly measured air overpressures just from wind exceeding 130 dB(L).
While the blast design will dictate the levels of air overpressure generated, the dissipation of overpressure energy compared to ground vibrations is heavily dependent on environmental conditions such as wind, temperature and other current weather conditions.
Alberta Transportation and Economic Corridors held four information sessions about controlled blasting on the Paskapoo Slope in November 2019 (view comment summary), January (view comment summary), February and March 2020.
We heard residents near the blasting area were uneasy about the vibrations resulting from the blasts and noticed increased rodent activity in their yards. We also heard residents would like advanced notice about when blasting will occur and more communication. In response, Alberta Transportation and Economic Corridors:
- Established a consistent blasting schedule so residents knew when to expect vibrations.
- Committed to publishing the seismograph reports for the previous week every Tuesday.
- Held four information sessions for residents to speak with the experts.