Pavement Deflection and Seismic Refraction for Determining Bedrock Type, Depth, and Physical Properties beneath Roads

We examined the relationship between three data types--geologic maps, measurements of pavement deflection under load, and seismic refraction data-- from diverse geologic settings in Texas to determine (1) whether geologic maps and seismic refraction data might be used to interpret deflections and assess pavement condition and (2) whether deflections and refraction data acquired on pavement might have geologic applications. Engineers assess pavement condition by applying a known load to a road and measuring vertical pavement deflection using the Falling-Weight Deflectometer (FWD). Our comparisons of deflections with mapped geologic units in four physiographic regions of Texas revealed differences in FWD response that are related to differences in either bedrock depths or physical properties of geologic units that range from Precambrian to Holocene in age and that include many different sedimentary, igneous, and metamorphic rocks. At the FWD sensor most distant (1.8 m) from the load, deflections are greatest where roads are underlain by elastic sedimentary rocks (sandstones, mudstones, and shales) and unconsolidated alluvium. Lowest deflections are measured over stiffer limestones and igneous and metamorphic rocks. Ratios calculated by dividing the deflections measured 0.3 m (W2 sensor) and 1.8 m (W7 sensor) from the load are better discriminators. These ratios are highest where pavement is underlain by rigid rock types such as granites, gneisses, and schists (ratios of 17:1 to 40:1), are intermediate where the underlying bedrock is limestone (10:1 to 27:1), and are lowest over sandstones, mudstones, and unconsolidated sediments (6:1 to 14:1). These results suggest that geologic maps are useful in FWD analysis and that FWD data alone might be used to predict rock type, allowing the FWD to be used in applications such as geologic mapping and sinkhole detection. We employed the FWD and a modified soil-probe hammer as seismic refraction sources to determine whether the correlation between rock type and pavement deflections is caused by differing rock properties or bedrock depth. These tests revealed that the FWD is a viable seismic source and that refraction data can be acquired on pavement. The refraction experiments and the rock type-deflection relationship show that (1) pavement deflections can help the geologist make geologic maps, (2) geologic maps can help the engineer interpret pavement deflections, and (3)seismic refraction measurements can assist both geologist and engineer in determining bedrock depths, rock properties, and rock types.