The HCTA is equipped with a complex strain measurement system that incorporates six Micro-Epsilon eddyNCDT 3010, high-resolution non-contact displacement sensors. The test rig experiments are designed to enable the study of the pre-failure deformation characteristics and the large strains behaviour via continuous tests on a single soil sample (drained or undrained). This allows soil stiffness to be analysed under different strain and stress conditions.

Dr Erdin Ibraim, reader in Geomechanics at the Department of Civil Engineering at University of Bristol commented: “We have been using eddy current displacement sensors from Micro-Epsilon for several years now. Six eddyNCDT 3010 sensors are installed on our recently developed HCTA, which we are using to test granular soils in multi-axial loading conditions. The HCTA at Bristol is one of only a handful developed in the world. It incorporates a very complex measurement system and so the eddy current sensors were selected for their excellent technical qualities, including high resolution, linearity and long-term stability.”

 

Non-contact

Six eddyNCDT 3010 sensors are mounted to the HCTA’s small strain measurement system. The sensors have a measuring range of 2mm and are mounted in three pairs to measure the axial, circumferential and radial displacement of the soil samples. Sensor resolution is 0.1µm and linearity is +/- 0.25 per cent FSO. The axial and circumferential displacements are measured in the central part of the specimen using two pairs of non-contact sensors fixed on stainless steel rods. The corresponding rectangular aluminium plate targets are fixed at different locations on the outer side of the sample. The outer radial sample displacements are deduced by the average of the measurements given by two non-contact sensors pointing aluminium foil targets placed on the sample’s side of the outer membrane – in direct contact with the soil.

In order to take advantage of the sensors’ high resolution over a complete test and up to large strains, the non-contact transducers have to be re-located during a complete test, so that the best accuracy for strains is maintained at each investigation point. “The rotational movement of the drive shaft, which passes through the bottom of the cell plate, is transformed by a system of bevel gears into horizontal, vertical or circumferential transducer displacements,” explained Dr Ibraim. “In large civil engineering projects such as the construction of motorways, embankments, dams, foundations for bridges and buildings, soil displacements are absolutely critical to the design process. Advanced numerical analysis can be used to predict the displacements of these systems. However, owing to the complexity of soil behaviour, realistic prediction of ground deformations is only possible if the models of soil stiffness are supported by data generated by relatively sophisticated soil laboratory testing.

 

High resolution

Dr Ibraim continued: “While the ground deformations can be related to the displacement measurements on the HCTA, the stiffness of the soil is related to its elasticity but the region of elasticity for soils is very small. This means that you need to develop very complex test rigs and displacement measurement systems that utilise high-resolution sensors, which are able to measure very small displacements down to 0.1 microns typically. We therefore required non-contact displacement sensors that could operate at high resolution, and Micro-Epsilon was able to offer a solution via its eddyNCDT 3010 measurement system.”

Micro-Epsilon’s eddyNCDT 3010 non-contact displacement sensor is a compact, single channel system comprising an eddy current sensor, a sensor connecting cable and amplifier electronics (signal conditioning unit). The application at Bristol University is a multi-channel (six sensors) set up, with synchronisation of separate channels.

The eddyNCDT 3010 operates on the eddy current measuring principle. This principle is used for measuring targets made from electrically-conductive (ferromagnetic or non-ferromagnetic) materials. A high frequency alternating current flows through a coil in the sensor housing. The electromagnetic field from the coil induces eddy currents in the conductive target, which alters the AC resistance of the coil. This change in impedance produces a linear electrical signal that is proportional to the distance of the target from the sensor.

 

Thermal stability

Temperature-dependent measuring errors are minimised using Micro-Epsilon’s electronic (active) compensation method, which ensures thermal stability of less than 150 ppm/F. The sensor is maintenance and wear-free and offers flexible field calibration for different target materials. Dr Ibraim concluded: “My experience of working with Micro-Epsilon sensors has been a very good one. The eddyNCDT 3010 sensor performance is excellent. In addition, when we needed technical support, the response from Micro-Epsilon was both swift and professional. In fact, I recently purchased a further 12 eddy current sensors from Micro-Epsilon for similar test rig (Cubical Cell Apparatus) applications.”