Hydrodynamic extension of radial fractures by explosive gas loading

Abstract

The motivation of this dissertation is to improve understanding about explosively drivennradial fracture dynamics in rock and to provide data for any subsequent mathematicalnmodelling efforts. Of particular interest is radial fracture extension due to explosive gasnloading in the near field region of the explosive charge. The near field is defined to be thenrockmass region within about ten explosive charge diameters of the explosive charge.Consequently, part of the thesis work involves a critical review of experimentalnmeasurements dealing with explosively generated radial fracturing and associatednphenomena. Many factors from this review are identified as being potentially significant tonany experimental study or application of blast driven radial fracture propagation. Importantnrockmass properties include internal discontinuities, permeability, porosity and in-situnstresses. The dynamics of gas flow within the radial fracture itself significantly influencesnthe radial fracture's propagation. Furthermore, spallation at the radial fracture surface cannresult during fracture propagation and fractures form transverse to radial fractures becausenof explosive gas loading over the radial fracture surfaces. Deficiencies observed in previousnexperimental work include a lack of dynamic measurements associated with actual radialnfracture propagation and an absence of information about gas dynamics within thenpropagating fracture. An experimental design is therefore developed to rectify this situation.Single explosive charges, of constant volume 200cm3, are detonated in instrumented andnmassive geologically simple rockmasses containing no macroscopic discontinuities,nisotropies or inhomogeneities. Two contrasting rock types, marble and granite, form thenexperimental rockmasses. Dynamic measurements of phenomena associated with thendetonating explosive and subsequent radial fractures are made in the near field region of thendetonated explosive charge. These include explosive gas pressure and energy, accelerationnof the rockmass and radial fracture propagated speeds. TNT, Ammonit and Granonit arenused as the experimental explosives.Radial fracture arrivals are determined at positions within the rockmass close to thenexplosive charge. These are calculated from seismic waves generated at the pressurenmonitoring borehole surface subsequently intercepted by the mounted pressure transducers.nFrom plots of the arrival times of radial fracture against the smallest separation between thenpressure borehole and the blasthole wall, a linear relationship is observed. This is confirmednfrom a statistical analysis. Constant fracture propagation speeds are determined in marblenand granite. Their values are 79.1ms-1 and 58.3ms-1 respectively.The plot intercept for arrival times of radial fractures against the closest pressure boreholendistance to the blasthole wall gives distances of about 1f and 2 explosive charge diametersnin marble and granite respectively. These conform to distances for radial fractures emergingnfrom a zone of crushing or for initial radial fractures forming to these distances immediatelynafter detonation and prior to explosive gas penetration.Radial fractures observed in the near field of the explosive charge are bifurcating by reasonnthat they are propagating at constant speeds and emanating from constant positions near thenblasthole wall. Furthermore, fracture propagation speeds are constant over varyingndistances away from the blasthole, the explosive type used and varying peak pressures andntimes of recorded peak pressures in the pressure monitoring cavities. Energy losses from thenexplosive gas products during their transport through the radial fracture are one or twonmagnitudes lower than that required for formation of the main fracture surface. Thisnindicates mechanisms consuming explosive gas energy that are more significant than thenmain surface generation. Fracture bifurcation is a likely mechanism by which energy fromnthe transported explosive gas products is consumed. Of considerable importance is that thendetermined terminal speeds of radial fracture propagation are only a small fraction of thenRaleigh wave speed, their values being respectively 2.2 and 1.5% in marble and granite.nSuch values are lower than though consistent with those measured during laboratorynexperiments and tabulated in Petroysan (1994).n n n n n n n n n