During 2007-2008 the project Optimal Fragmentation in Blasting II was conducted at the Långåsen quarry of NCC Roads AB near Arlanda. The main goals were:
• An evaluation of the effect of using EDDs in quarry blasting.
• A validation of the design curve concept introduced at the Vändle quarry.
Five production blasts were monitored. After a first trial round, the remaining 4 rounds each had about 100, Ø89 mm holes in a 14-19 m high bench charged with Titan 6075 or 6080 SME emulsion. In rounds 1 and 2 a tighter pattern than normal was used in one half of the rounds, raising the specific charge there from 0.7 to 1.0 kg/m3. Nonel initiation was used with two holes per 25 ms delay in-row and 67 ms between rows. In rounds 3 and 4 electronic delay detonators were used instead, with 10 and 5 ms in-row delays respectively.
The monitoring encompassed drill hole collaring and alignment, in-hole deviations, the rock structure in the bench faces from 3D imaging, 1st row burdens etc. Eleven, 400 tonne samples were taken from the muck piles and sorted in the quarry and sieved in the laboratory (lab). The lab data made up a fines tail that was grafted onto the in-pit sorting data to form sieving curves for the test piles. These curves were then compensated for the absence of boulders to construct the sieving curves for the blasted muck pile.
These have been used to derive KCO model formulas for the Nonel rounds, also called design formulas. They are based on blast geometry, charging data and the properties of the rock mass. The formulas may be used to estimate the effect of changing blast hole pattern, bore hole diameter and stemming length for example.
The use of EDDs gave several unexpected results. In comparison with Nonel initiated rounds with a somewhat lower specific charge the fragmentation was much coarser; x50 increased from 160 to 200 mm. The effect of raising the specific charge in the Nonel initiated rounds from 0.72 to 0.99 kg/m3 was a much finer fragmentation; x50 decreased from 160 to 120 mm.