Role of Practical Geophysics in In-Situ Characterization for Underground Construction in Phoenix, Arizona

• Published in: Proceedings of the Fourth International Conference on Unsaturated Soils 2006; Carefree, AZ; USA; 2-6 Apr. 2006 pp. 577 - 588
• Main Authors: Ackerman, A. F, Rucker, M. L, Smith, D. E, Durkee, D. B
• Format: Book Chapter Conference Proceeding
• Language: English
• Published: American Society of Civil Engineers (ASCE) 2006
• Subjects: Civil Engineering & Construction Materials; Environment & Environmental Engineering; General References; Geophysical surveys; Underground construction; Arizona; Site evaluation
• ISBN: 9780784408025; 0784408025
• DOI: 10.1061/40802(189)44

The application of surface and downhole geophysical methods for site characterization can provide data, information and insight into the subsurface that often cannot be achieved by traditional drilling and sampling techniques. Geophysical measurements applied over a broad area can complement geotechnical sampling methods applied at discrete locations. From a practical perspective, access to areas that cannot be drilled provides subsurface coverage that would otherwise be missing or could only be achieved at extreme cost. This paper presents the results of a comprehensive investigation and characterization program that included the use of state-of-the-art surface seismic microtremor surveys and downhole geophysical methods to complement rotosonic drilling and sampling methods and an innovative method of pressuremeter testing. Refraction microtremor surveys provided continuous simplified vertical shear wave profiles and associated low strain modulus profiles in 36-m (120-foot) increments along an 1,800-m (6,000-foot) alignment profile. Downhole geophysical methods included resistivity logging using an induction electrical sonde to provide vertical subsurface profiles to high resolution (about 0.5 m) that were then correlated back to estimates of clay content in the coarse sand-gravel-cobble (SGC) formation. Geophysical measurements were correlated to downhole pressuremeter test results to verify the relationships between the surface and downhole geophysical methods and in-situ high-strain borehole modulus results. Results from these methods were used to estimate the depth of the Holocene-Pleistocene contact, estimate strength and modulus parameters, and identify the extent and frequency of clean lenses and boulders within the geologic profile. The results of the investigation provided a detailed geologic characterization that was used to optimize tunneling depth, perform engineering analyses for support of excavation and underpinning, and evaluate standup time and tunneling methods.