Knowledge of rock types along with their competency and strength properties are important parameters when designing any construction project. Underground excavations may be vulnerable to water bearing shear zones, and geophysical methods can provide cost effective solutions for mapping these zones. Changes in soil lithology across a site may also cause differential settling, and a deeper understanding of site conditions can be achieved with the addition of a geophysical survey.
HGI has conducted many different types of geophysical methods to solve complex problems for the construction industry. Some examples include:
- Rock strength properties for excavation and dredging with electrical resistivity
- Mine tailings impoundment design with seismic refraction and gravity
- Soil lithology changes using electromagnetics
- Fissure investigation with resistivity and seismic
- Soil mapping for landfill expansion with resistivity
- Windmill site design with seismic refraction
- Road base integrity mapping with GPR
- Rippability studies with seismics
Example: Rock Properties for Excavation
Currently, the Panama Canal is undergoing expansion, including excavation of the canal sides and dredging the bottom to increase draft. Along the southern portion in the Gaillard (or Culebra) Cut, the material is all rock and the type of rock greatly influences the dredging method. HGI conducted a marine resistivity survey to understand the spatial distribution of rock properties, specifically rock hardness, which was shown to vary by type, age, and degree of weathering. Fortunately, a large number of boreholes existed which were used to calibrate the resistivity data. The scatterplot of resistivity vs. hardness below shows a robust goodness of fit when considering regression modeling. However, it was noticed that the co-located data clustered into three logical groups: tuffaceous, agglomeratic, and andesitic materials. These groups were used to convert the spatially continuous resistivity data to a spatially continuous rock hardness map for ease of interpretation.