In this section, the use of geophysics for condition evaluation of roadways (paved or unpaved) is discussed. This includes a discussion of roadway subsidence in section Mapping Voids, Sinkholes, Abandoned Mines, and Other Cavities, followed by a discussion of mapping of expansive clays in section Roadbed Clay Problems. Slope stability and subsurface Characterization issues are covered in section Subsurface Characterization.
Mapping Voids, Sinkholes, Abandoned Mines, and Other Cavities
Detecting voids and other cavities is a problem often encountered in geophysics. Although this section mentions the use of Ground Penetrating Radar for locating voids under pavements, its focus is mostly on locating deeper voids. Locating voids under pavements is discussed in section Pavements.
The physical properties of the void and host rock and soils are an important consideration when deciding on the particular method to use. For example, when searching for cavities, the physical property of interest may be mass deficiency using the gravity method. However, if the cavity is filled with water or alluvium rather than air, it will be more difficult to locate. However, the water may assist in some seismic methods since compressional waves can travel through water but would not be observed if the cavity were filled with air. Often associated with a cavity are secondary structures that can aid in their delineation. Collapsed structures can occur above a cavity, bringing the low-density c ontrast nearer to the ground surface. Lateral fractures may also be present, making the width of the low-density region greater that that of the cavity itself.
Several methods can be used to locate cavities. These include gravity, resistivity (and conductivity), seismic methods, and Ground Penetrating Radar (GPR). The methods that should be used depend significantly on the host geology, as discussed previously, and the depth and configuration of the voids. Each of these methods is discussed below.Gravity Methods
Ground Penetrating Radar
Shear Wave Reflection Surveys
Rayleigh Waves Recorded with a Common Offset Array
Cross-borehole Seismic Tomography
Roadbed Clay Problems
In general, as described in section Estimating Clay Content, estimating the clay content using geophysical methods is difficult. However, mapping clay underneath roadways can easily be performed as described in section Indentifying Roadbed Underlain by Expansive Clays.Estimating Clay Content
Identifying Roadbed Underlain by Expansive Clays (Determine Clay Content in Swelling Soils)