Abstract

The field Vane Shear Test (VST) is a widely used in-situ test method to measure undrained shear strength and sensitivity of saturated, fine-grained soils. The United States Bureau of Reclamation (Reclamation) commonly performs this test method to help inform numerical modeling of earth embankment dams when undergoing a risk analysis or design. Although the test method itself has been a geotechnical tool for quite some time, its primary use has traditionally been limited to sites with soft and/or relatively shallow clay soil. Typically, embankment and foundation materials of interest at Reclamation facilities are at greater depths, under higher effective stresses, and can be relatively stiff. Testing of stronger soils poses issues when performing the VST; typical commercially available equipment has a limited torque capacity to cause yielding of the soil. As a solution, modifying the dimensions and aspect ratio of the vane is an economic means of increasing the measurable range of undrained strength. Yet, the effects of these modifications are not well understood. Soil strength anisotropy is one of the primary components of this uncertainty. Testing on a sandy lean clay has been conducted to enable side by side comparisons of traditional aspect ratio vanes versus the proposed modified vanes to quantify the potential differences in measured undrained strength. In addition, measured undrained strengths from the various vanes are compared to results of laboratory testing on the same sandy lean clay (i.e., direct simple shear and triaxial compression) to provide a better understanding of the differences between the in-situ and laboratory test methods. This paper presents the apparatus developed to allow full scale vane shear tests to be conducted in the laboratory and summarizes the results of tests on a normally consolidated sandy lean clay.

Abstract

The evaluation of in-situ behaviour, strength and compressibility of a soil profile is routinely performed by geotechnical engineers through field tests, such as the seismic piezocone penetration test (SCPTu), the flat dilatometer test (DMT) and the field vane shear test (FVT). This paper aims to compare the results of a CPTu, DMT and FVT to evaluate an organic alluvium soil in terms of: i) in-situ soil behaviour classification, ii) undrained shear strength and iii) stress history. To compare and complement the in-situ results, laboratory tests were carried out to determine the grain-size distribution, the Atterberg Limits, the pre-consolidation pressure, the organic content and the undrained shear strength under isotropic consolidation triaxial test (CIUC). The results showed that the soil evaluated herein exhibited a clay-like behaviour based on the classification system of both tests (DMT and SCPTu), which agrees with the laboratory characterization. Furthermore, the OCR (overconsolidation ratio) calculated from the SCPTu and DMT also shows a convergence with the values determined from laboratory tests. The SCPTu performed in this soil was predominantly undrained and enabled the calculation of undrained shear strength. Based on this, the methodologies based on Nkt and N∆u (from SCPTu) were compared with the undrained shear strength from the FVT and that obtained from DMT, based on the KD parameter. Finally, a comparison is presented to discuss the influence of shear mode in the undrained shear strength and the applicability of the methodologies used to evaluate the soil behaviour and the stress history.