- $40,000 to replace a pre-tensioned concrete slab in a multistory building because a contactor cored through a tendon.
- Tens of thousands of dollars in loss and 3 days of downtime with multiple contractors onsite because a contractor damaged a building electrical service cable during concrete saw cutting.
- Lengthy delays and soaring costs due to removal and soil remediation associated with an underground storage tank discovered during excavating.
Could these be avoided? Yes, with the help of geophysics.
As discussed in the previous geophysics’ series post, geophysical tools can help at any project stage. Here are a few ways Geophysics can bring value to your project:
Non-destructive/non-intrusive. The investigations are performed at the surface and don’t require significant site disturbance, if any.
Fast. Data can be acquired quickly compared to traditional invasive methods, such as drilling and trenching. Preliminary results are often available in the field.
2- or 3-dimensional images to “fill the gaps”. Borings and soundings provide only 1D results, everything between testing locations is “assumed”, posing quantity and schedule risks. 2D or 3D maps can expose hidden risks and enable better estimates.
High productivity rate. Geophysical imaging can cover large areas and long distances. For example, several miles of Ground Penetrating Radar (GPR) profiles and an Electromagnetic (EM) survey of a several-acre site can be completed in one day.
Feasible with difficult site conditions. Geophysical investigations can be performed on steep slopes, inside buildings, on vertical surfaces, under bridges, and in other conditions where applications of invasive methods (drilling, trenching, etc.) are limited.
Can be used on land, on water, underwater, in air. The geophysical tools can be used in different environments.
Comprehensive site conditions model. Geophysical data sets can be remarkably accurate, especially when interpretations are constrained by other data sets, such as borehole control. Geophysical investigation can contribute to developing of a comprehensive subsurface conditions model.
Here are just a few applications of how geophysics can help with site investigation:
Subsurface utility locating. Often the information about utilities is incomplete, inaccurate, or unavailable. Multiple geophysical tools are available for this application, such as GPR, Radio Frequency (RF) locator, Electromagnetic EM, Traceable rodder, or Sewer camera, etc. The deliverables can range from simply marking utilities in the field to digital models showing horizontal and even vertical positions of utility lines.
Rebar/Tendon mapping. Tendons and reinforcing bars in concrete can be mapped non-destructively with GPR and EM tools prior to saw-cutting or coring to avoid structure or tool damage.
Underground storage tank (UST) mapping. The presence or absence of a suspected UST can be checked quickly by using GPR, EM and high-sensitivity metal detectors. The results often can be seen in real time in the field.
Top-of-rock/Soil thickness/Lithology. The top-of-rock can be effectively mapped with electrical resistivity and seismic methods. These deliverables are continuous 2-dimensional vertical cross sections or 3-dimensional data sets opposed to 1-dimensional results from drilling and sounding, which means number of boreholes can be reduced or more efficiently located. Anomalous zones identified by geophysical profiles can be further ground-truthed by drilling or trenching.
Sinkhole/Karst. The presence and extents of a sinkhole can be evaluated with Electrical Resistivity Tomography (ERT). Often, sinkholes at the surface are not just depressions in the ground, but are a part of a karst system, especially in areas where soluble rock (carbonates and evaporates) are present. These sinkholes can be connected to much deeper karst features, such as solution-widened joints, and can extend tens and hundreds of feet deep. These features can be missed with traditional invasive exploration methods.
Landfills. The extents of leach fields and landfill (including uncontrolled landfill) sites can be mapped with ERT, Spontaneous Potential (SP) and EM methods. The integrity of liners and landfill thickness can be mapped with ERT.
Dam seepage. Zones of seepage can be effectively mapped by ERT and SP methods. Preliminary results can be available within hours after the survey.
Soil resistivity. Soil resistivity measurements are performed in environmental, agricultural, electrical and corrosion engineering, and geotechnical investigations. ERT and Wenner 4-electrode methods can be performed in the field to cover large areas cost effectively.
Bridge deck deterioration. GPR can be used to assess bridge deck deterioration. This method is superior to traditional chain-dragging. Maps of bridge deck deterioration can be generated for further analysis. Only a few core samples are typically required for interpretation constraining and verification.
Pavement thickness/Debonding. Several GPR options are available for pavement investigation, such as traditional ground-coupled, air-launched systems and 3-dimensional arrays. Data can be quickly acquired and often interpreted in real-time, proving information on the number of pavement layers, layer thicknesses, rebar presence, rebar spacing and depth, debonding, etc. and 2D and 3D maps can be generated for further analysis.
Seismic site classification. Seismic surface wave methods, such as Multi-Channel Analysis of Surface Waves (MASW) and Refraction Microtremor (ReMi), can be used for seismic site studies and are an alternative to invasive Seismic Cone Penetration Testing (SCPT).
Borehole methods. Geophysical data can be acquired from boreholes and supplement invasive techniques. Down-hole seismic and cross-hole seismic methods can help to determine the in-situ conditions and determine the engineering properties of rocks (i.e., rippability and competency) and soils by measuring compressional (Vp) and shear wave (Vs) velocities.
