Characterization of a site for the prediction and simulation of dynamic events requires the determination of mechanical properties of the rock at stress/strain levels and rates and at a scale appropriate for the events and critical structures involved. The structures of interest can be on the surface, underground, or designed to impact and penetrate into the site (projectiles). Dynamic events of interest are equally diverse: seismic waves from distance earthquakes or explosions; close-in phenomena associated with earthquakes and explosions; vibrations associated with heavy equipment or machinery; and stress transients associated with impact and penetration. The complexity of site characterization depends on two factors not specifically related to the nature of the site itself: the stress levels generated by the dynamic event and the sophistication of the prediction and simulation techniques. For very high stress levels, such as occur near a nuclear detonation, and for low stress levels, such as occur during seismic wave propagation, site characterization is simplified, and often certain details of the site are not particularly important, such as shear strength variations in the case of very high stress levels. In the intermediate stress range, however, all descriptive relations are non- linear, a complete finite strain constitutive model is required, and all details of the site, such as lithology and fracture distribution, become important. The site characterization must be complete enough to support the most sophisticated prediction and simulation technique that will be applied to the analysis of the dynamic event. The procedures of site characterization are illustrated in this paper by a specific dynamic event, projectile impact and penetration, and a specific site, an outcrop of Dakota Sandstone near San Ysidro, New Mexico. The structure of interest was the projectile itself: nominally, 1.5 m in length, 0.165 m in diameter, 180 kg in mass, and 9.25 calibre ogive nose. Objectives were twofold: (1) to predict the penetration depth of the projectile and simulate the motion (acceleration, velocity, and depth versus time) in a rational manner based on impact parameters and properties of the site; and (2) to predict and simulate the stress and vibration environment within the projectile. The site characterization supported three types of prediction and simulation efforts: (1) empirical and analytical techniques to predict the penetration depth and motion of full-scale projectiles in situ; (2) finite difference computer code simulations of the projectile motion and stress/vibration environment within the projectile for the in situ penetration event; and (3) finite difference/finite element computer code simulations of a reverse ballistic test involving a scaled projectile and large-diameter (~0.35 m) sandstone core.
The site characterization consisted of three phases: field investigations, consisting of geological reconnaissance, geo-physical surveys, drilling, logging, and sampling; laboratory investigations; and recommended site model and constitutive properties for the model. Necessarily the presentation of each phase will be very brief; complete details can be found in Butler, et al. (1977).
Site Description. The sandstone penetration test site is located about 19 km southwest of San Ysidro, New Mexico, and 80 km north-west of Albuquerque.