Wave Propagation in Porous Media as a Function of Fluid Saturation

-An experimental investigation is conducted using dynamic photoelasticity and high speed photography to study the wave propagation due to blast loading in porous media as a function of fluid saturation. The porous media have been modeled as a continuous solid containing particular arrays of holes or voids. The study has focused mainly on the effect of the porous structure on transient pulse propagation as well as the effect of the moisture in the pores on wave propagation. A series of experiments have been conducted using a sheet of Homalite 100 with different geometry of the periodic array of holes. A small amount of explosive was used to generate the stress wave. Dynamic photoelastic photographs were taken with the high speed camera as the wave propagated across the holes. These data are analyzed to obtain the wave velocity as well as the stress-wave attenuation in the porous media. Introduction The problem of interaction of elastic waves with discontinuities or boundaries of complex shapes arises in situations where waves propagate through a medium having cavities, inclusions or cracks. Due to material inelasticity and inhomogeneity, the wave propagation in a discontinuous medium is much more involved than homogeneous elastic wave propagation and shows directi, onal as well as frequency dependence. This phenomenon becomes significant for step-loading pulses where the wavelengths are of the order of the size of the discontinuities. Such problems denoted as scattering and diffraction problems have long standing interest in acoustics and electromagnetic wave theory. Composite materials such as concrete, ceramics, etc., axe characterized by the number of pores, voids or fluidfilled cavities. The influence of these pores on the deformation and the failure of these materials has not been interpreted uniquely. Since areas of stress concentration may arise in the vicinity of these pores, it is believed that the pores may play a role in influencing the crack and wave propagation in these materials. Moreover, porous materials are used extensively for shock isolation as they are capable of absorbing large quantities of energy during impact loading. Hence the behavior of these materials under impulsive loading has been of substantial interest to engineers. Wave propagation in a discontinuous medium has also been of interest to the soil and rock-mechanics community. A. ShuMa (SEM Member) is Professor, University o f Rhode Island, Department o f Mechanical Engineering and Applied Mechanics, Kingston, RI 02881. V. Prakash is GttMuate Research Assistant, Brown University, Division of Engineering, Providence, RI 02912. Paper was presented at the 1988 SEM Spring Conference on Experimental Mechanics held in Portland, OR on June 5-10. Original manuscript submitted: December 13, 1988. t~nal manuscript received: August 31, 1989. The propagation of elastic waves in the earth's crust is most intimately related to the properties of soil and rock. The elastic properties of these substances are greatly affected by the amount of water contained in them, packing density, porosity, the size of the particles that form the substances, and the binding material which they contain. Current interest in geomechanics is focused on the transient phenomena occurring in earthquakes, wave loading and consolidation. Moreover the increasing needs for urban and resource development demand faster, safer and more efficient procedures for underground excavations of rock. Most methods of rapid excavation in hard rock use some form of dynamic loading, such as explosive or water jet. This type of loading produces stress waves which induce crack initiation and propagation. The initial attempts to study rock media and soil structure as arrays of elastic particles (e.g., spheres and disks) were made by Iida, ''2 Takahas]~i and Sato, ~ Gassman' and Brandt3 They investigated the propagation velocity as a function of confining pressure, particle size and aggregate geometry. The effect of water content in the pores on elastic velocity has been studied by several investigators. OliphanP and Owen 7 found that slight additions of water caused a sharp drop in velocity with a slow decrease as the saturation approached 100 percent. Hughes and Jones' measured the dilation wave velocity of samples of very low porosity, less than one percent. Using the same apparatus and methods, Hughes and Cross ~ measured the velocity in Solenhofen Limestone (porosity four percent) and Caplen Dome Sandstone (porosity five percent) for dry and saturated samples. A considerable amount of research is also under way in determining the internal structure of porous and granular media by various sounding techniques. For example, acoustic emission methods have been presented by Hardy, 1~ while Allison and Lama" have discussed a low amplitude vibration technique to predict rock structure. Current research in wave propagation in such media has involved, for example, statistical theories by Varadan et al. ~2 or mixture theories by Junger23 Analytical approaches axe often limited as they cannot fully account for the material inhomogeneity, isotropy and defects. Most of the previous work focuses on wave-propagation phenomena in general without going into the details of relating specific microstructure to the associated wave motion. This paper reports on an experimental study of wave propagation due to explosive loading in a porous medium. The porous medium was modeled as an array of holes machined in a continuous sheet of a brittle polyester material Homalite-100. The study looked at the wavepropagation phenomenon from a microscopic point of view by going into the details of the geometric nature of the porous structure. The geometry of the pores was