The Triassic Basin soil and rock formations in the Durham, North Carolina, region provide challenges for geotechnical design and construction. These formations consist of highly heterogenous stratigraphy including sands, silts and clays overlying a weathered rock unit that can contain boulders, fractures and an inconsistent competent bedrock surface. A proposed light rail system was under design to extend through downtown Durham, and a geotechnical investigation was performed to better understand the behavior of the rock formation and its integrity.
Pyramid Geophysical Services was contracted to perform a Multi-Channel Analysis of Surface Waves (MASW) seismic survey across specific portions of the proposed rail alignment where geotechnical borings either could not be performed, or where significant variability in the depth of competent rock was observed. The goal of the seismic survey was to “connect the dots” between the geotechnical borings and provide a more comprehensive understanding of bedrock behavior along the alignment. Pyramid performed a total of 542 separate MASW shots resulting in 20 individual 2-Dimensional cross sections of velocity that were used to analyze the overburden, weathered rock thickness and depth to competent rock across the site.
The MASW results portrayed a highly variable bedrock unit in terms of both depth and competency. Isolated boulders were interpreted at various locations, and well as rock pinnacles and fractures. Pyramid analyzed the geotechnical boring data at locations where the MASW tests intersected boring locations, and overlaid these borings onto the seismic results (see above image).
The results of the seismic testing were combined with geotechnical boring data to generate plan-view contour maps of competent bedrock depths across the site (see example below). Correlations between the geophysical interpretations and boring data were exceptionally high, and the resultant data set provided engineers with a clear understanding of geologic conditions with which to finalize design plans. This particular project is no longer moving forward into the construction phase due to other factors; however, it provides an excellent example of using both physical sampling and interpretive geophysics to accurately understand local complex geology.
Contact us today to find out more about the benefits seismic surveys can provide for geotechnical investigations!