Before excavation at SURF’s 4850L, it is of utmost importance to understand how the energy from blast events propagates through the rock mass and the air. Starting in December 2015, LBNF initiated a test blast program to allow other experiments to assess the impact LBNF/DUNE excavation will have on them, as well as to calibrate the LBNF excavation blasting plan itself, and to validate the construction schedule and cost estimates.
The test blast technique consists of drilling a pattern of holes into the rock, and filling most of them with explosives that get triggered by programmable electronic detonators. A set of holes in the center of the pattern, called the burn cut, is left empty. The pattern is designed such that energy from the blasts in the outer holes propagate radially inward towards the burn cut.
While the December test blast successfully indicated how the energy propagates through the rock mass, it did not provide a complete understanding of air blast overpressures and impacts on existing facilities and experiments. The burn cut pattern did not adequately allow inward radial dissipation of blast energy, and spikes in the air blast overpressures occurred. Also, a few of the detonators failed, resulting in incomplete rock breakage. Air pressure alarms were triggered near the Davis Campus and two or three ceiling tiles fell in a space at the Ross Campus.
A second phase of the program was planned with a redesigned blast pattern, non-electronic detonators, and reinforced air control doors throughout the 4850L. Test blasts on March 8 and 9, 2016 yielded readings of peak particle velocity very similar to those recorded in December, providing confirmation of LBNF’s understanding of the blast energy propagation, and air blast overpressure readings were roughly 20% of the December readings, in line with expectations. No alarms or impacts were reported.
These two March blast events were successful and representative of construction production blast events. The blasts fully satisfy program objectives and therefore a planned third blast has been cancelled. The test blast field work is complete!
— Tracy Lundin