Everything within a radius of 300 metres has been cleared and secured. It is unnaturally quiet, time seems to be frozen. Seconds later Gerhard Meyer triggers the blasting by radio. Shortly thereafter a dull bang occurs, dust fountains shoot meter-high into the air, one hears the loud rumbling of slipping stone masses. Then everything is quiet. The cloud of dust has moved. Around 47,000 tons of loose rock remain. One of the biggest explosions in the Wiesenhofen quarry is history.
Three days of preparation for almost two seconds of detonation lie behind Gerhard Meyer and Franz Wild. The experienced blasters inspect their work. They are satisfied and release the sighted material for further use. A few minutes later, the first excavator arrives and begins feeding the new tracked primary crusher. There the rock is first pre-crushed, i.e. further crushed, and then processed into mineral concrete, gravel, chippings and crushed sand in the processing plant - valuable raw materials for road construction or the production of precast concrete parts. The material will last for two to three weeks, after which it will have to be blown up again. More in summer, less in winter.
Digital wall measurement in 3D
Gerhard Meyer creates the drilling plan on the computer with special software according to the scan specifications. For the blasting he determines the number and spacing of the drill holes, their diameter and the angle at which they are drilled into the rock. In this case, this means: 16 boreholes, each 115 millimeters in diameter and six meters apart. Using a printout of the drill grid, Franz Wild sets to work and drills holes up to 30 meters deep into the rock face. Drilling takes place at an angle of 75 degrees. The drilling work takes two days at summer temperatures, about 45 minutes of drilling time per borehole. A log shows whether cavities were encountered during drilling or whether other problems occurred.
16 boreholes in two days
Nach Vorgaben des Scans erstellt Gerhard Meyer am Computer mit einer speziellen Software den Bohrplan. Für die Sprengung legt er die Anzahl und Abstände der Bohrlöcher, deren Durchmesser und den Winkel fest, mit dem sie in den Fels gebohrt werden. In diesem Fall heißt das: 16 Bohrlöcher mit je 115 Millimetern Durchmesser und sechs Metern Abstand. Mit einem Ausdruck des Bohrrasters macht sich Franz Wild an die Arbeit und treibt mit dem Bohrgerät bis zu 30 Meter tiefe Löcher in die Felswand. Gebohrt wird in einem Winkel von 75 Grad. Zwei Tage dauern die Bohrarbeiten bei sommerlichen Temperaturen, pro Bohrloch rund 45 Minuten Bohrzeit. Ein Protokoll gibt Auskunft, ob beim Bohren Hohlräume angetroffen wurden oder andere Probleme aufgetreten sind.
Delayed blasting in the millisecond range
Meanwhile, Gerhard Meyer sits at the computer again and prepares the blasting plan based on the survey data. This gives an overview of how much explosives have to be filled into the individual boreholes and with what time delay they are detonated. The aim is for the rock to come loose one after the other during the blasting and for the rock face to collapse due to gravity. If all boreholes were ignited at the same time, the vibrations, dust and noise would be too strong and the danger of uncontrolled flying would be too great. However, the time-delayed ignition of the individual explosive charges is hardly visible or audible to the human eye and ear. The whole thing takes place within milliseconds.
Three layers of explosives
Once the boreholes have been drilled, Franz Wild checks whether they are open for filling with explosives. He also measures depth and inclination again with a hand gradient meter, a plummet with a light source. It also rained over the weekend. The water level in the boreholes therefore plays an important role. Early in the morning on the blasting day, the first thing the blasters do in the wet area of all the boreholes is to use around a ton of patronised explosives, 60-centimetre cartridges with a diameter of 80 millimetres. Almost three tons of emulsion explosives are pumped into the cartridge with a hose. The emulsion displaces the water in the boreholes and fills them completely. The components are delivered by a mixed loading vehicle and mixed on site for safety reasons. In the upper area, the boreholes, where it is dry, are filled with around 2.8 tonnes of powdery ANFO explosives and sealed with the appropriate final fill material.
Routine, without falling into routine
In Wiesenhofen, the explosives are detonated by radio with a redundant ignition system. This means that two ignitions are carried out per borehole. For the foot charge at depths of up to 30 metres, which is detonated first, an explosive with high energy is used. It is intended to ensure that the rock is pushed forward. Less energy is required for the top charge, which is ignited 25 milliseconds later. Prior to this, both blasting masters fitted the lower and upper impact cartridges in the drilled holes with detonators and detonator hoses vaporized with explosives. They then connect the individual boreholes with 16 ignition retarders, which are connected to the start igniter. The men are perfectly tuned to each other and know exactly what to do. They are experienced, but without falling into routine.
Trust is good, control is better
A final check takes place during which Gerhard Meyer performs a visual inspection of the ignition system. Everything runs perfectly, the ignition circuit is intact. Time to leave the blasting site. He uses walkie-talkie and hand signals to ensure that the safety area is cleared up to 300 metres away. Minutes later, Franz Wild blows the final act with his warning mare. Gerhard Meyer presses the radio remote ignition button. The rest is history ... and a welcome food for the new crushing plant, whose greedy throat wants to be fed.
