Very deep seismic piezocone testing in geocharacterization

Paul W. Mayne, James Sharp, Mark A. Styler, David Woeller

In the proceedings of: GeoRegina 2014: 67th Canadian Geotechnical Conference

Session: Laboratory and Field Testing

ABSTRACT: seismic piezocone testing in geocharacterization Paul W. Mayne Georgia Institute of Technology, Atlanta, GA, USA James Sharp, Mark A. Styler, and David Woeller ConeTec Investigations, Richmond, BC, CANADA ABSTRACT As the demand for deeper site exploration continues to increase because of heavier structural loads and larger more critical projects, the utilization of seismic piezocone testing (SCPTu) is clearly an optimal approach for geotechnical site characterization, since up to five separate readings (qt, fs, u2, t50, and VsVH) on soil behavior are procured during a single sounding. The advancement of such soundings can routinely achieve depths of 30 to 60 m. With specialized equipment and procedures, penetration depths of 100 to over 160 m are now possible. RÉSUMÉ L'exploration profonde de sites suscite un intérêt certain, et ce, plus particulièrement en raison de la demande grandissante pour des infrastructures de plus grandes tailles et pouvant supporter des charges de plus en plus importantes. L'utilisation du piézocône sismique stable (SCPTu) est définitivement l'une des techniques privilégiées pour la caractérisation des sites géotechniques. Son utilisation permet d'obtenir, lors d'un même sondage, cinq lectures séparés (qt, fs, u2, t50 et VsVH) sur le comportement du sol. L'avancement de telles techniques de sondage permet d'atteindre des profondeurs de pénétration de 30 à 60 mètres. Ces profondeurs peuvent même atteindre de 100 à 160 mètres avec un équipement spécialisé. 1 INTRODUCTION The requisite exploration depths for geotechnical site investigations depends upon the particular project and ranges from only 1 to 2 m for pavement studies, up to 10 m for small buildings, and 30 m for many moderate size bridges. As larger structures and heavier loads are increasingly imposed by civil engineering designs, there have become demands for deeper site investigations related to deep foundation designs in supporting pile lengths of 60 m and up to 100 m, or deeper. Major structures that may be subject to earthquake loading require site-specific analysis, often requiring geotechnical data to great depths. While conventional drilling, sampling, and laboratory testing can accomplish these tasks, they do so at considerable expense, both in time and money. Cone penetration testing (CPT) involves continuous collection of geotechnical information in a vertical sounding that uses an instrumented electronic probe with load cells, pressure transducers, and geophones. This facilitates data procurement on the subsurface environment that is faster, efficient, and more economical when compared to traditional drilling methods. Several illustrative examples of data collection by seismic piezocone tests (SCPTu) are presented to show the increasing depths achieved using this technology. 2 SEISMIC PIEZOCONE TESTING Seismic piezocone penetration testing (SCPTu) is the hybrid evolution of three test methods (Campanella et al 1986): (a) cone penetrometer testing (CPT) that used two axially-oriented load cells to record cone tip resistance (qc) and sleeve resistance (fs) with depth; (b) piezoprobe testing (PPT) that measures porewater pressure (u1 or u2) responses locally in the vicinity of the perturbation; and (c) geophysical downhole testing (DHT) that obtains a vertically-propagating horizontally-polarized shear wave velocity (VsVH). Thus, SCPTu = CPT + PPT + DHT. The shear wave data can be collected either using a true-interval procedure which uses two levels of geophones, or via a pseudo-interval method that relies on a single elevation of geophone(s). Additionally, a biaxial inclinometer is included to track the wandering position of the penetrometer, primarily useful to warn against rod buckling and/or excessive deviations from vertical plumbness. Yet, in the case of deep soundings, a depth correction (z*) using the measured inclination angles is warranted. Tip resistance, sleeve friction, and dynamic pore pressure readings are recorded at 1 to 5 cm depth intervals, offering unparalleled detail in the geostratigraphic profiling of subsurface layers, lenses, or soil zones. The Vs is normally obtained at 1-m rod breaks, although new developments now allow continuous-interval seismic piezocone testing (CiSCPTu), as detailed by Styler & Mayne (2013). Perpetual vibratory source soundings (PS-SCPTu) are also showing promise for continuous shear wave measurements (Styler et al, 2014). The shoulder porewater pressure reading (u2) is needed for correcting the measured qc to the total cone tip resistance (qt). For some penetrometer systems, there can also be a correction on the measured sleeve friction (fs) due to porewater effects, and an approximate method to obtain fst is detailed elsewhere (Mayne 2007). Also, at specified depths, the penetration can be halted to monitor the decay of porewater pressures with time, termed dissipation testing. For convenience, the duration to reach

RÉSUMÉ: deep seismic piezocone testing in geocharacterization Paul W. Mayne Georgia Institute of Technology, Atlanta, GA, USA James Sharp, Mark A. Styler, and David Woeller ConeTec Investigations, Richmond, BC, CANADA ABSTRACT As the demand for deeper site exploration continues to increase because of heavier structural loads and larger more critical projects, the utilization of seismic piezocone testing (SCPTu) is clearly an optimal approach for geotechnical site characterization, since up to five separate readings (qt, fs, u2, t50, and VsVH) on soil behavior are procured during a single sounding. The advancement of such soundings can routinely achieve depths of 30 to 60 m. With specialized equipment and procedures, penetration depths of 100 to over 160 m are now possible. RÉSUMÉ L'exploration profonde de sites suscite un intérêt certain, et ce, plus particulièrement en raison de la demande grandissante pour des infrastructures de plus grandes tailles et pouvant supporter des charges de plus en plus importantes. L'utilisation du piézocône sismique stable (SCPTu) est définitivement l'une des techniques privilégiées pour la caractérisation des sites géotechniques. Son utilisation permet d'obtenir, lors d'un même sondage, cinq lectures séparés (qt, fs, u2, t50 et VsVH) sur le comportement du sol. L'avancement de telles techniques de sondage permet d'atteindre des profondeurs de pénétration de 30 à 60 mètres. Ces profondeurs peuvent même atteindre de 100 à 160

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Paul W. Mayne; James Sharp; Mark A. Styler; David Woeller (2014) Very deep seismic piezocone testing in geocharacterization in GEO2014. Ottawa, Ontario: Canadian Geotechnical Society.

@article{GeoRegina14Paper203,author = Paul W. Mayne; James Sharp; Mark A. Styler; David Woeller,title = Very deep seismic piezocone testing in geocharacterization,year = 2014}