Geotechnical
Using geophysics to help solve geotechnical problems may not seem like a logical choice, but HGI has been increasingly applying a variety of techniques to enhance the state of knowledge of engineering and construction projects. Whether it is to understand concrete properties of roads, geologic materials for excavation, foundations, or hazards, geophysical mapping of electrical, seismic, or magnetic properties can provide a much greater level of confidence to your project.
Why Conduct Geophysics for Geotechnical Site Evaluations
A geotechnical geophysical survey is often the most cost-effective means for obtaining subsurface information. Additionally, these methods may be applied over large areas in a relatively short amount of time. Geophysics may be used to fill in the gaps between boreholes or find subsurface features that might not be detected by a standard drilling and CPT program. Other advantages of using geophysics include site accessibility, portability, and noninvasiveness. Geophysical equipment can often be deployed in austere areas inaccessible by drilling rigs (e.g., in heavily forested areas, on steeply dipping slopes, or in sensitive areas). Geotechnical geophysics is not a substitute for drilling, rather it complements a well-planned, cost-effective sampling program.
- Mapping lithology
- Locating sand and gravel deposits
- Determining water depths
- Mapping bedrock topography and structure
- Identifying regions of potential weakness (e.g., shear zones and faults);
- In-situ rock properties
- Mapping near-surface karstic sinkholes
- Finding clandestine air-filled cavities, tunnels
- Estimating rippability
- Mapping foundation integrity
- Investigating dam and levee integrity
- Evaluating landslide hazards
- Mapping concrete and asphalt properties
- Locating rebar and pipes
Example: Mapping Lithology
A geological mapping project was conducted in the Panama Canal to help understand material properties for dredging. Within the Gatun Lake region, loose sandy fill can easily be removed by suction dredging, while harder marine-based sedimentary rock must first be drilled and blasted before using a bucket dredge for removal. Understanding the subsurface properties can limit the use of the more expensive blasting method. The image below shows resistivity acquired in the canal, where high resistivity values appeared to correlate with the former route of the Chagres River. In this area, the river bed was primarily coarse sands and gravels that meandered through harder outcrops of rock (shown as lower resistivity). These data were given to the Panama Canal Authority to refine costs of construction.
Geologic mapping project in the Panama Canal.