Ground Penetrating Radar as a Sentinel Device

Abstract

Geophysical methods provide a non-destructive non-intrusive method of subsurface investigation. Oftentimes, however, the target is difficult to “see” due to lack of contrast between the target and background. This lack of contrast can be rectified through the use of a sentinel device. A series of sentinel devices were used in downtown Pittsburgh, Pennsylvania to determine potential subsidence beneath a building plaza. This sentinel strategy was performed to predict future deformation of the sidewalk and to prevent foot-traffic hazards. The building was installed in 1983 and the building footprint was over-excavated to provide for the construction of the basement. Soldier piles were installed to keep the loose sand, clay and gravel deposits from collapsing into the construction area. Prior to the installation of the perimeter sidewalk, 0.15 meter compacted lifts of fineto medium-grained sand were placed in the annular space between the soldier piles and the native soil. A 0.3 meter (1 ft.) thick grout and 0.3 meter (1 ft.) thick concrete pad were placed on the fineto medium-grained sand and overlain with a 4-inch sand sub-base prior to the installation of the decorative granite pavers (Figure 1). Figure 1: Profile of the plaza and underlying materials, showing the sentinel device for GPR detection. Unfortunately, 15 years later the sand, cement/grout layer and pavers have subsided in many areas (Figure 2). The cost of the removal of the pavers and/or replacement of the pavers coupled with the cost of rehabilitating the subsurface was costing over $100,000 each event. Building management was keen