GROUND PENETRATING RADAR: AN EFFECTIVE TOOL FOR LOCATING DRY CAVES ALONG A PORTION OF STATE ROUTE 69 NEAR MAYER, YAVAPAI COUNTY, ARIZONA

Widening and realignment of State Route 69 traverses an area of Tertiary-age travertine bedrock near Mayer, Arizona. During the design phase of the project, subsurface exploration encountered small voids within the right-of-way. A moderate-size cave structure in the area was mapped by local speleologists. Arizona Department of Transportation (ADOT) was concerned that cave structures of unknown size might be found within a few feet of the new roadway subgrade. As a result, highway construction specifications contained special provisions requiring geophysical surveys to identify cave structures that could adversely affect the roadway and expose the traveling public to possible subgrade failure hazards. ADOT's concern was realized during construction when a D-9 Caterpillar tractor broke through a cave roof and dropped about six feet into the void. A geophysical survey conducted of the cave-affected alignment identified 130 cave-type anomalies, and recommendations were provided to ADOT, and the contractor, to remediate the cave-affected highway section. Survey monuments were established for monitoring roadway performance and potential subgrade settlement.