Influence of Bedrock Type and Depth on Road Deflections

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1999

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Geologic maps provide much information about the distribution of rock types at and near the land surface. Deflections of Texas highways measured with the Falling-Weight Deflectometer (FWD) appear to be correlated to bedrock type, particularly at the most distant FWD sensors. To examine this apparent bedrock influence, we compared FWD data with mapped geologic units from six roadway segments in four physiographic regions of Texas. This analysis revealed differences in FWD responses among regions that are likely to be related to systematic differences in either bedrock depths or physical properties of geologic units that range from Precambrian to Holocene in age and include many different sedimentary, igneous, and metamorphic rocks. At the W7 detector (6-ft [1.8-m] offset), average normalized deflections are highest for areas where roads are underlain by siliciclastic sedimentary rocks (sandstones, mudstones, and shales) and unconsolidated alluvial sediments. Lowest normalized W7 deflections are measured in areas underlain by Precambrian igneous and metamorphic rocks that include granites, schists, and gneisses and in areas underlain by chemically precipitated sedimentary rocks such as limestone.

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