Characterizing Geologic Interfaces in Sand, Gravel, and Cobble Deposits

The Phoenix Sky Harbor International Airport Automated People Mover project will require tunneling in coarse riverbed sand gravel and cobble (SGC) deposits. Significant areas of unstable, running ground, primarily in Holocene deposits of SGC, have been encountered on previous tunneling projects in the area. Several investigation techniques, including the use of state-of-the-art seismic refraction microtremor surveys, and the rotosonic drilling method were implemented in order to accurately locate the Holocene-Pleistocene contact, and define the extent and frequency of clean and clayey lenses and boulders within the project geologic profile. The complete geotechnical investigation consisted of seismic surveys, rotosonic drilling, lab testing, and various downhole geophysical tools including dual induction logging, neutron and gamma logging, acoustic televiewing, and boring diameter measurements with a three arm caliper. The combination of seismic survey, laboratory testing, and field logging provided a good estimate of the Holocene/Pleistocene contact and general assessment of the clay content of each geologic unit. Seismic survey results indicated an abrupt increase in shear wave velocity at depth, and laboratory test results indicated increased clay content at depth within the Pleistocene deposits. Field logging of the continuous rotosonic cuttings verified the increased clay content at depth, but also indicated embedded clean cobble and boulder lenses. The downhole geophysical measurements offered an excellent compromise in scale between the relatively small laboratory test points and the very large scale seismic refraction microtremor surveys. The induction resistivity tool provided an excellent correlation with the clay content identified in the formation, and the neutron logging provided confirmation of the clay content increase at depth.