LVDT Based System for the Measurement of the Prefailure Behavior of Geomaterials

This paper describes a device that can accurately measure the small strain behavior of geomaterials in the triaxial apparatus. It makes use of two miniature submersible linear variable differential transformers (LVDTs) mounted on a pair of yokes which clamp onto the soil specimen. A highly stable, low noise, custom designed signal conditioning system completes the design. The result is a device capable of consistently resolving displacements of less than 0.1 microns corresponding to strains of less than 0.0002% for standard sized (3.6 cm × 7.6 cm) triaxial specimens. The system is simple in design, can be easily implemented in existing triaxial equipment, and thus may represent a commercially viable solution for the investigation of the small strain behavior of soils in the triaxial apparatus. Along with a description of the mechanical and electrical system, the paper presents the results of proof testing performed to assess the stability and linearity characteristics of the system under a wide range of temperature and pressure conditions. These tests demonstrate the device's versatility and its ability to measure true axial strains. Recent modifications to the existing triaxial equipment, introduced to improve the investigation of small strain behavior in geomaterials, are also discussed. Finally, the performance of the device is illustrated with results of tests on Resedimented Boston Blue Clay (RBBC) and frozen Manchester Fine Sand (MFS). These geomaterials differ in stiffness by more than two orders of magnitude and establish a range in behavior that encompasses that of most soils. Test results confirm that the measuring system can provide a detailed characterization of the pre-failure behavior of soils in triaxial compression.