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SGER: Geotechnical Site Characterization for Instrumented Excavation Sites

In this SGER, a geotechnical site characterization program will be performed by Georgia Tech (GT) using in-situ seismic piezocone (SCPTu) and seismic flat plate dilatometer tests (SDMT) to capture the geostratigraphy, soil properties, and small-strain stiffness at three sites. Two sites involve specially-instrumented large excavations at the Ford Design Center on the Northwestern University campus and the Prentice Women's Hospital in downtown Chicago. The third site is the National Geotechnical Experimentation Site (NGES) on the western shore of Lake Michigan. These projects are selected in collaboration with a joint NSF project by Northwestern University (NU) and the University of Illinois at Urbana-Champaign (UIUC) to develop new integrated tools to predict, monitor and control ground movements associated with construction of supported excavations. Since September 2002, the NU-UIUC activities have focused on collecting and analyzing detailed field performance data at several excavations in the Chicago area, developing and evaluating new methods to track excavation progress, and developing numerical techniques to automatically update predictions of performance of the excavation support system. Block and piston samples have been collected for laboratory testing using internally-mounted local strain sensors to capture the nonlinear stress-strain-strength behavior of the soft clay soils.<br/> Of particular merit is the inclusion of the small-strain shear modulus (Gmax = G0) in the numerical scheme as a key initial state parameter in order to realistically model the induced strains and stresses within the soil mass surrounding the excavations. Current available commercial packages (e.g., PLAXIS, CRISP, FLAC, SV-Solid) do not include any built-in algorithms to start the constitutive nonlinear stress-strain-strength curves of the soil regions from the initial fundamental G0 stiffness. Yet, natural soil deposits and formations all originate from an initial state condition specified by their in-place void ratio (e0), vertical stress (svo), hydrostatic porewater pressure (uo), lateral stress coefficient (K0), and small-strain stiffness (G0 = rT Vs2). While laboratory tests such as resonant column and bender elements can provide evaluations of G0, only discrete values are obtained and sample disturbance issues often arise. Therefore, in-situ soil parameters and properties are desired, particularly using geophysical tests via the planned sets of SCPTu and SDMT soundings and derived shear wave velocity profiles. <br/> Since the Evanston NGES is located nearby, the GT team will calibrate their new true-interval downhole seismic array in a SDMT deployment with standardized test data already available and documented at the NGES. At each of the three test sites, a series of 3 to 6 SCPTu and SDMT soundings will be performed to optimize data field collection. Based on prior information, the sites are underlain by sand and/or fill overlying soft clayey silts, stiffer clays, and tills (Finno, 1989 GSP 23; 2000, GSP 93). At select soundings, detailed shear wave profiles can be obtained by frequent true-interval surveys. Working with Professor Rich Finno of NU and Professor Yousef Hashash of UIUC, the interpretation of relevant parameters ?e.g., g??f', su, OCR, K0, E', Eu) will be evaluated for input into the numerical modeling. <br/> In terms of Intellectual Merit, the intended goals involve the optimized collection of site-specific data using hybrid tests (SCPTu and SDMT) which combine penetration type probes with geophysical methods to ascertain multiple measurements in a single sounding. In-situ shear wave profiles will allow the utilization of small-strain stiffness for the initial portion of the stress-strain-strength curves of soil layers within the updated FEM algorithms by NU-UIUC. <br/> In terms of Broader Impacts, this SGER seeks to establish a collaborative effort between NU-UIUC-GT on a major initiative towards a fully-integrated system of laboratory and in-situ testing, numerical finite element simulation, and continued feedback from embedded-wireless field sensors within the controlled excavations during construction. The utilization of small-strain stiffness is paramount to reflecting the true anticipated response of the construction operations. The complementary nature of the laboratory testing and cone/dilatometer/shear wave testing in calibration with full-scale field performance of walls and supports during excavation and detailed modeling can benefit future urban projects and infrastructure redevelopments in the US. Finlaly, during field visits to the NU and UIUC campuses, the GT team will offer demonstrations of in-situ techniques to graduate students and interested parties.

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