Effects of porosity and saturation on shock-wave response in tuffs

Abstract

The attenuation of a stress-wave as it propagates through tuffaceous rocks of various porosities and degrees of saturation is investigated by a calculational parameter study. First, the equations of state for the various porous and saturated tuffs considered are constructed from (1) approximations of the loading curves for tuffs at various grain densities, (2) the weight per cent of water under consideration, (3) a loading curve for water, and (4) estimates of loss of gas-filled porosity during loading. Then, these constructed equations of state are used in one-dimensional calculations to examine how peak radial stress and peak particle velocity are affected by porosity and saturation. The investigation was limited to the regions where peak radial stress is below about 40 kbar. The results show that the peak stress at a given distance from the working point decreases over the range from rock at grain density to rock with small amounts of either water or open void space. The first 5 per cent of volume given up to either void or water is most effective in decreasing the stress at a given distance; additional proportions of either void or water produce relatively less effect. Open void is substantially more effective than water-filled space in producing stress-wave attenuation: a few per cent of dry void volume has the same effect on attenuation as several times as much volume of water with no void. When either the gas-filled or total porosity of a material drops below about 5 per cent, peak stress at a given distance is markedly increased. Above about 5 per cent, the peak stress is relatively insensitive to either the gas-filled or the total porosity of a material. The attenuation of peak particle velocities is somewhat similar to stress attenuation through the rock. However, the particle velocity is less sensitive than stress to changes in void fraction or saturation of the rock.