Liquefaction potential of silts from CPTu

Abstract

Silt tailings (slimes) are difficult materials to test in that, like sands, it is extremely difficult to obtain undisturbed samples and subsequently re-establish them in a triaxial cell for element testing in a laboratory in anything approaching their in situ condition. Evaluation of silt tailing behaviour has to depend on in situ tests, and the piezocone (CPTu) in particular. However, CPTs in silt generate substantial excess pore pressure and there is no established methodology to evaluate the measured responses in terms of soil properties, as drained sand-based CPT interpretation is inapplicable. A case history of particularly loose silt tailings is reported in which the National Center for Earthquake Engineering Research (NCEER) liquefaction assessment method would lead to uncertainty in the liquefaction potential. However, the extremely high CPTu excess pore pressure ratio, Bq, and low dimensionless CPT resistance, Qp, at this site indicates liquefaction is likely occurring during pushing of the CPT. Detailed finite element simulations of the CPT using a critical state model provided an effective stress framework to evaluate the in situ state parameter of the silt from the measured CPT data. This framework shows that the group of dimensionless CPT variables Q(1 – Bq) + 1 is fundamental for the evaluation of undrained response during CPT sounding. And, despite the high silt content, the interpretation indicates that the tailings are indeed liquefiable.Key words: liquefaction, CPT, silt, finite element, critical state.

Review on Liquefaction Potential of Silts from CPT Data

“…Following the studies of Robertson and Wride [1], which evaluated liquefaction potential without pore water pressure measurements from the CPT, Shuttle and Cunning [7] studied the liquefaction potential of silts using piezocone data. Silt tailings (slimes) were studied in their research.…”

“…Shuttle and Cunning [7] focused their study on the Rose Creek Tailings Impoundment located at the Anvil Range Mining Complex near Faro, in the Yukon Territory of Canada. Between 1969 and 1992, tailings from an adjacent mining complex were deposited in the Rose Creek Tailings Impoundment.…”

“…The soil type index (I c ) was produced from the Q, B q , and F values. The high B q values indicated that the silt classified on a soil behavior type basis as "soft clay" to "organic soil" [7]. The CPT used at the site recorded shear wave velocities as well as pore water pressure measurements (CPTu).…”

“…Also, these estimates of void ratio were approximate, which is another reason why the CPT was performed. After the investigation, the silts were considered non-plastic, with about 66% of the soil being silt-sized particles and the remainder consisting of fine sand [7].…”

“…However, their evaluation is only acceptable if the sandy material has less than 35% fines. Shuttle and Cunning [7] used the same approach evaluating soils for liquefaction potential based on the CPT, but they focused their attention on silt tailings rather than sands, and used a piezocone for their evaluations. This article seeks to build upon the work of Robertson and Wride [1] and Shuttle and Cunning [7] for evaluating liquefaction potential of silts using CPT data.…”