Laser Measurement of Radial Displacements in Triaxial Tests: ENSG

Created in 1908, the ENSG became ”École Nationale Supérieure d'Ingénieurs” in 1948. It is an engineering school funded by the French Ministry of Education, Research and Technology. The ENSG is one of the founders of the National Polytechnique Institute of Lorraine (INPL) along with six other engineering schools and a school of Architecture. Situated in Nancy, the ENSG has a central position in the European community. Over a period of three years the ENSG trains general engineers who acquire in depth knowledge in geosciences. It is the only engineering school in France to offer this type of training.

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THE PROBLEM

The measurement of strain in triaxial shear tests and the subsequent determination of the key soil parameters (shear modulus, elastic modulus, etc.) is critical to understanding the mechanical behaviour of soils.

Most currently available systems need to set the measuring system directly on the soil sample to be tested, which can disrupt the soil and therefore the results (Cole 1978, Clayton et al. 1989).

Thus, some authors have implemented contactless systems, including devices based on the use of a laser spot (eg Romero 1999; Dasari 2002; Messerklinger & Springmann 2007). These devices have the advantage of not requiring contact with the specimen and so do not disrupt the mechanical behaviour of soil.

THE SOLUTION

ENSG & GDS Instruments developed a device to monitor the radial profile of the specimen whilst under test using highly accurate laser precision. The system is based on the use of two laser profilers supplied by Keyence (The LJ-G80 lasers). The lasers monitored a 32mm length on the sample from a distance of 80 mm. The sensors were installed on to a modified conventional triaxial cell.

GDS Instruments integrated the laser heads and receivers into their GDSLAB software (see Figures 3 and 4), to accommodate ENSG testing requirements.

The software allowed the laser to measure a point measurement and also scan the sample profile, which could be used to give a measurement volume change. GDSLAB can record a profile scan at a maximum rate of every 5 seconds.

The laser profilers used to measure the distance between the samples and sensor have an accuracy of about ± 10μm in optimal conditions of measurement.

Alongside the lasers the GDSLAB software allowed complete automated control of the triaxial test to run a suite of tests on the sample.

Figure 1. Shows the specimen whilst under test using highly accurate laser precision. Note: The lasers are mounted onto an existing triaxial cell.

RESULTS

Figure 2 shows data from part of a profile obtained for a sample as it strains. The red line is the sample at the start of the test, the blue line is the sample at the end of the test stage. The top-cap can be seen at the end stage. From this profile it is possible to estimate the local lateral deformation of the sample at all heights, thus also being able to give information about the current volume of the sample and the way the sample is deforming.

Fig. 2 Laser profilers shown on sample

CONCLUSION

The ability to measure the sample diameter at a point without touching the sample was proved with this method. Further to this, the ability to measure the sample profile, and therefore the sample volume is extremely interesting, especially for applications such as the testing of unsaturated soil where sample volume measurement is extremely difficult to

Fig. 3 Screenshot of the laser driver connecting to the GDSLAB Software.

measure via more traditional measurements such as volume of fluid leaving or entering the sample. The ease at which these profiles can be measured using GDSLAB software makes this an extremely powerful tool. Fig’s 3 & 4 are screenshots taken when using the GDSLAB software.

Fig. 4 Screenshot of the lasers being controlled via the GDSLAB software

Website:

http://www.ensg.inpl-nancy.fr/

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