A system for measuring local radial strains in triaxial apparatus

Abstract

The importance of local measurement of small strains is well recognised. Without such measurements, accurate stiffness determination is not possible and can result in overestimations of strains resulting from stress changes in the ground. Axial strain is often considered to be the primary strain to be measured under triaxial conditions and there has been a focus on its measurement. Radial strain is also important, for example for determining Poisson's ratio and bulk modulus, or maintaining K0 conditions during consolidation. There are a number of devices available that enable its determination, using both direct and indirect measurement approaches. This note describes a system for measuring radial strains directly using linear variable differential transformers. It is shown to be simple, reliable, accurate and robust.

Measurement of small-strain stiffness of soil in a triaxial setup: Review of local instrumentation

“…Based on studies conducted by Bonal et al (2012), Wu et al (2014), Nishimura and Abdiel (2017), Ackerley et al (2016), Brosse et al (2017), and etc. The following basic properties of small strain measuring device are extracted:…”

“…LVDT that adopts radial belt mechanism which can work in water was successively invented by Kuwano et al (2000). Recently, a fixed system of measuring radial strain made of three LVDTs which is robust and accurate was elucidated by Ackerley et al (2016). The resolution of radial LVDT can be measured up to 0.0001%.…”

“…In order to minimize the inevitable error related to conventional triaxial testing, many researchers contributed in advancing the triaxial setup by developing local transducers and sensors that can assess soil response over diverse engineering application. These transducers and sensors are normally placed on the specimen inside triaxial cell to evaluate soil parameters typically unattainable using the conventional triaxial apparatus for example (Nishimura, 2014;Wu et al, 2014;Ackerley et al, 2016;Shankar Kumar et al, 2016;Roshan et al, 2017). Measurement of strain on specimen is referred to as local measurement.…”

Anisotropic Stiffness and Shear Strength Characteristics of a Stiff Glacial Till

“…The Ehʹ and Eh u expressions are evidently more complex and sensitive to experimental errors. Recognising the difficulties in undertaking reliable radial strain measurements (Ackerley et al, 2016), Nishimura (2014aNishimura ( , 2014b proposed a combined drained and undrained probing approach that relies only on the generally higher quality axial strain data.…”

Local measurements of axial and radial strains using magnetic encoders in triaxial apparatus

“…1). The first one is based on the calliper principle proposed by Bishop and Henkel [5] and the second one is based on direct measurements [12], [13]. Due to hardware limitations, currently the maximum number of supported encoder-based sensors can be up to four transducers.…”