Experimental investigation of blasting compaction of a cohesionless pit mine fill

The mechanical behaviour of a very loose cohesionless open pit mine fill during and after single and group charge blasting compaction is experimentally investigated. Considering the distance from the charges the parts of the fill affected by single or group charge blastings can be classified into near, intermediate and far fields. In the near field the rapid cavity expansion originated by the blasting induces a shock front whose propagation velocity decreases with the distance from the source resulting in obliteration of the grain skeleton. Soil compaction takes place mainly in the intermediate field. The compression of the soil skeleton left behind by the shock wave leads to a rapid increase of the effective pressure followed by a delayed increase of pore water pressure due to the presence of gas bubbles. The loss of effective pressure (liquefaction) starts a short time after detonation and reaches distances of about 20 m, leading to a collapse of the grain skeleton and hence to a compaction zone within the fill. Simultaneous measurements of earth and pore water pressures enabled the detection of liquefaction and its duration.