Shear stiffness from in-situ field vane testing: Theory, results, and reservations

Mark A. Styler, John A. Howie, James T. Sharp

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

Session: Soil Mechanics

ABSTRACT: fness from in-situ field vane testing: Theory, results, and reservations Mark A. Styler ConeTec Investigations, Ltd. and the University of British Columbia, Vancouver, BC, Canada John A. Howie Department of Civil Engineering University of British Columbia, Vancouver, BC, Canada James T. Sharp ConeTec Investigations, Ltd., Richmond, BC, Canada ABSTRACT This paper applies a theoretical equation to calculate the shear strain at the periphery of an in-situ vane. This is a new approach to interpreting pre-failure geotechnical vane data. We performed a suite of vane tests at a well characterized site in Surrey, BC to investigate the limitations of this equation. We found that the vane system, with a down-hole torque cell, has sufficient accuracy. We also found that the underlying assumptions behind the shear strain equation are reasonable. Our calculated shear stiffness degradation curves were comparable to empirically estimated curves. With this technique, high quality in-situ vane tests can be combined with shear wave velocities to calculate in-situ shear stiffness degradation curves in fine grained soil. RÉSUMÉ Cet article utilise une équation théorique pour calculer la déformation en périphérie des essais in situ au scissomètre. Ceci constitue une nouvelle approche qui permet de calculer la déformation de la rupture des essais in situ au scissomètre. Afin de déterminer les limites de cette équation, des tests au scissomètre ont été effectués sur un site à Surrey en C.-B. Nous avons remarqué que la mesure des capteurs de torque dans le trou de sondage est suffisamment courbes de la dégradation dtechniques empiriques. Avec cette technique, des essais in situ au scissomètre de grandes qualités peuvent être combinés à la vitesse de propagation des ondes de cisaillement afin de calculer des courbes de la dégradation du module de cisaillement dans des sols à grains fins. 1 INTRODUCTION Field vane shear (FVST) testing is used to measure undrained shear strength (su,fv). The strength is calculated from the peak torque measured to rotate the vane. The torque versus rotation data measured before the peak torque is not used in practice. Cadling & Odenstad (1950) derived a solution allowing the estimation of the elastic shear stiffness using pre-failure vane torque-rotation measurements, but Osterberg (1957) found no published experimental support for this approach. We reviewed the subsequent published literature and also found no published interpretations using this approach. Osterberg concluded that the uncertainty in estimating the down-hole rotation angle and down-hole torque from up-hole surface measurements prevented a reasonable calculation of the soil stiffness. However, FVST equipment has improved over the past 60 years. While our literature review found no published interpretations using the pre-failure data, we did find an interest in obtaining the stiffness using the in-situ vane (Young et al. 1988, Becker et al. 1988, and De Alencar 1988). In this paper, we present a closed form theoretical solution for the shear strain at the vane periphery based on similar derivations in rheology. Unlike Cadling & Odenstad (1950), this derivation requires no prior assumption of a constitutive model other than that the soil is a continuum and that the shear stress and strain are distributed uniformly over the vertical edges of the cylindrical surface circumscribed by the vane. We used the derived equation to interpret four FVST tests performed in a lightly overconsolidated, highly sensitive, marine silt and clay at a site at the intersection of Colebrook Road and King George Boulevard in Surrey, British Columbia. This well characterized site has been the subject of previous research (Rivera Cruz 2009, Weemees et al. 2006, Weech 2002, Crawford and Campanella 1991, Crawford and DeBoer 1987). We addressed Osterberguncertainty of the down-hole rotation and torque by estimating the uncertainty in the derived shear stress, shear strain, and shear stiffness. We used a down-hole torque cell which reduced the uncertainty in the calculated shear stress. We calculated the uncertainty in the down-hole rotation by accounting for the twisting of the deployment rods. The estimated uncertainties in the shear stress and shear strain results are insignificant. The FVST equipment we used is sufficient. We compared our calculated shear stiffness results to expected values estimated following the empirical

RÉSUMÉ: stiffness from in-situ field vane testing: Theory, results, and reservations Mark A. Styler ConeTec Investigations, Ltd. and the University of British Columbia, Vancouver, BC, Canada John A. Howie Department of Civil Engineering

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Mark A. Styler; John A. Howie; James T. Sharp (2014) Shear stiffness from in-situ field vane testing: Theory, results, and reservations in GEO2014. Ottawa, Ontario: Canadian Geotechnical Society.

@article{GeoRegina14Paper304,author = Mark A. Styler; John A. Howie; James T. Sharp,title = Shear stiffness from in-situ field vane testing: Theory, results, and reservations ,year = 2014}