Civil Engineering applications
Earth structures, such as embankments, canals, earth dams, sea walls and flood defenses require ongoing monitoring and maintenance to identify potential failure zones and to compensate for the effects of settlement. Extreme weather events leading to prolonged periods of desiccation or saturation threaten embankment stability.
The most important impact involves a compromise of deadlines; an expense for the repair of roads and railway tracks and damaged sites and the safety exposure for users. As a result, the minimization of geo-hazards impacts is an issue of great concern for transport administrations. National networks span thousands of kilometers of roads and railway lines, crossing different types of geographical terrain, soil and weather conditions. It is extremely difficult to protect and/or remediate every site under significant risk. At the same time, risks must be managed in affordable manner.
Several geophysical methods are useful to study embankment condition. Ground Penetrating Radar (GPR) can assist to the monitoring of river levees for detecting animal burrows, which may trigger levee failures by piping. The manageability and the non-invasiveness of GPR are particularly suitable for this application because it is an extensive investigation method that enables one to rapidly cover a wide area, locating voids that are difficult and costly to locate using other intrusive methods and because GPR returns detailed information about the possible presence of voids and discontinuities within embankments.
Resistivity imaging, or electrical resistivity tomography (ERT), is sensitive to lithological and mineralogical heterogeneity and changes in soil moisture content. In locations where lithology and mineralogy are unchanged, changes in successive ERT surveys will be due to ground water movement and subsequent moisture content variations. Resistivity has become an important engineering property because it can be used to derive the volumetric moisture content in the calculation of soil moisture deficit. Also, resistivity imaging can be used to map the spatial and temporal changes in moisture content, enabling real-time assessment of plasticity changes, for example in response to sustained drought or rainfall.
On the other hand, seismic methods provide very important information about the elastic properties within embankments and levees. Rapid, non-intrusive surface wave surveys produce depth profiles from which ground models can be generated for use in earthwork condition assessment. Stiffness throughout earthworks controls the behavior under static and dynamic loads, and characterizing heterogeneity is of interest in relation to the stability of engineered backfill and life-cycle deterioration in aged utility and transportation infrastructure. Continuous surface wave methods are routinely used to identify interfaces between fine- and coarse-grained fill in an end-tipped embankment. MASW is used to determine the low stiffness (Gmax) values of the embankment fill.