Advancements in the interpretation of seismic piezocone tests in clays and other geomaterials

Abstract

Seismic piezocone testing (SCPTu) offers an economic and expedient means for the modern collection of geotechnical data during site investigation of soils. It is actually a hybrid test that has all the advantages and the merits of classic cone penetration testing (CPT) where it obtains three continuous readings with the depth: cone tip resistance (qt), sleeve friction (fs), and porewater pressure (u2); plus geophysical component involving downhole testing, where shear wave velocity (Vs) measurements are recorded at 1-m depth intervals. The conducted study aims at making full usage of the four main readings obtained from the seismic piezocone test: cone tip resistance, cone sleeve friction, porewater pressure and downhole shear wave velocity. New links between undrained shear strength and shear wave velocity are investigated and previous relationships between stress history and shear wave velocity are revisited and improved. A unified approach for estimating stress history of wide variety of geomaterials using net cone tip resistance is presented. Predictive equations for detecting sensitive clays from seismic shear wave velocity are introduced with a focus on sensitive and structured clays within North America. Modification of a hybrid spherical cavity expansion theory- critical state soil mechanics solution is introduced addressing the stress history of sensitive and structured clays. A new analytical approach allows the direct assessment of undrained rigidity index from CPTu data which finds value in assessing undrained shear strength, yield stress, and coefficient of consolidation, the latter from piezodissipation tests. The special nature and difficulties of organic clays are discussed and an indirect means of detecting their existence using piezocone testing is presented. Finally, an effort is undertaken to calibrate an analytical model for evaluating the undrained shear strength and stress history of clays using two large databases.