Stability Analysis for Roads, Bridges, Dams and Slopes
Focusing on county and local roads, we evaluate the thickness of and materials within the base course, sub-base layer, and the upper subgrade zones principally using road-based GPR. Layer thickness and material types are needed to evaluate settlement and washout risk. Using GPR, we measure relative permittivity, a measure of water occurrence, within the layers which indicates strength and deformation properties of road aggregates. We look at the depth of the frost table, clarity of its reflection, and the effect of the frost action on the road structures.
We evaluate the degradation of bridges and small dams with ERI and GPR. Most GPR work on bridges involves collecting reflection amplitude data from bridge decks and preparing maps of damaged areas in the bridge structures. The primary cause of deterioration in bridge decks is corrosion of steel reinforcements which induces fracturing and voids in concrete which are the principal GPR targets. GPR is also sensitive to electrical and chemical changes (precipitation of secondary minerals) present in early phase corrosion and concrete disintegration. Our highest resolution antenna broadcasts at 900 MHz which penetrates to ~40” but provides low-resolution in concrete to 20 “.
While GPR and ERI are excellent methods for initial mapping of deteriorated locations on highways, bridges, and dams, geophysics alone is not proof of the deterioration. Other non-destructive evaluation methods and efficiently gathered ground truth are needed to verify the problems.
On slopes, we use ERI and GPR to identify features that control slope stability including: 1) thickness of surficial deposits, 2) the state of sediment and bedrock weathering, 3) fractures and sliding surfaces that might enable landslides and correlate to landslide scars, 4) faults, and 5) high-permeability pathways for groundwater flow (open fractures within which low resistivity fluids circulate). Cores are needed to characterize lithologies as well as geomechanical and hydrogeological parameters of the sediments and/or rocks. Geomorphic mapping can identify drainage, landslides, landslide scars.
Our deliverables include maps, cross-sections, 3D models of the features of interest, geophysical data, core descriptions, pertinent analyses of core samples, and summary interpretations.