Page 56

2014SumShotcreteEMag

Safety Shooter Fallouts Can Be Deadly! By Joe Hutter Mas popular as it is today, I witnessed a mine impacted someone. No experienced miner wouldconcrete from even a moderate height would mostlikely have fatal consequences if it directlyany years ago, when the use of shotcretein an underground support system was not foreman walk into an underground opening to ever consider standing beneath freshly excavated admire the job his shotcrete crew had done placing and unsupported ground. For the same reason, no shotcrete only an hour or two earlier. He knew one should ever consider walking into an area of that the area had already been supported with freshly placed, nonsupported shotcrete. screen and bolts (the shotcrete was placed over At the time when the fallout incident occurred, the screen as part of the ground control engineer’s there was no way of knowing the value of the design) so he felt safe knowing that he was not compressive strength of the shotcrete at an early entering an area of unsupported ground. Just as a age. There was an easy test method for deter- member of the shotcrete crew warned him to step mining final set time but to accurately measure back from the area, a small section of shotcrete early-age compressive strength, engineers would fell from above and landed about 15 ft (5 m) from require that a test panel be shot, cored a minimum where he was standing. This incident was a serious of 8 hours later, and the core broken to determine reminder that even though a shotcreted area may the compressive strength value. Although this test appear safe, there are many variables that impact procedure provided accurate results, the time the time required (after overhead placement) before lapse before coring meant the earliest age result shotcrete can be considered structurally sound. was 8 to 12 hours. Today, early-age compressive To illustrate the danger posed by unexpected strength can be obtained using specialized equip- fallouts, it is important to understand the mass ment (end-beam testers) (Fig. 1) that allow com- associated with even a small section of shotcrete. pressive strength to be measured as early as 1 or A 2 ft x 2 ft x 4 in. (0.6 m x 0.6 m x 100 mm) area 2 hours after placement. These results provide the of in-place shotcrete will weigh approximately ground control engineer with confidence that the 175 lb (80 kg). To drop a 175 lb (80 kg) slab of in-place shotcrete has gained sufficient strength Fig. 1: Today, early-age compressive strength can be obtained using specialized equipment (end-beam testers) that allow compressive strength to be measured as early as at 1 or 2 hours after placement 54 Shotcrete • Summer 2014


2014SumShotcreteEMag
To see the actual publication please follow the link above