Development of novel and economical sensing techniques to measure volume change and suction in unsaturated soil specimens

Abstract

The measurement of suction and volume change during most of the geotechnical experiments is a challenging task due to a variety of limitations in current measurement techniques. Currently no single equipment can cover the entire range of total suction that may be encountered during the test. Also most of the volume measurement techniques for unsaturated soils are either providing insufficient accuracy or having destructive approach. In spite of having such a big importance of suction and specific volume on unsaturated soils, there is still no perfect technology available for the measurement of these two parameters. This study aims to develop a new economical, non-destructive and thorough technique to measure volume change and suction in unsaturated soils. Automated digital image processing technique was used for the accurate and precise measurements of the soil specimen volume while a force sensor was used to measure the soil suction. The proposed volume measurement technique involves capturing a 360 degree view of the specimen using a fixed camera. A structured light laser pointer was projected on the soil specimen while capturing the 360 degree view. The laser pointer provides the reference points on soil specimen for measurements. The high resolution images were then processed using functions developed within MATLAB which automatically calculates the radius and height over the entire lateral surface of specimen and hence the volume of specimen. The computations also allowed the reconstruction of a 3D mesh model of the specimen. A validation test on a dummy object showed that the error in radius and height measurements at more than 75% measurement points was less than ±0.05 mm and ±0.07 mm, respectively. A 99% accuracy was achieved in the volume measurement of the soil specimen. The measurement of suction was performed by measuring the effective stress within the soil specimen using a force sensor called “Flexiforce sensor”. The sensor was installed in an initially saturated soil specimen prepared at high water content. The specimen was then allowed to air dry, and the water content was measured along with the stresses in soil at different stages during drying. The validation for suction measurement was performed by comparing with the SWRC determined by other authors using the conventional procedure to the SWRC measured using our technique for the same soil. A strong resemblance in the behaviour has been observed between the measurement by conventional technique and our approach until the air entry value of this soil. But a drastic break in similarity was found near the air entry value, which might be occurring because the stresses in soil no longer increases with an increase in suction.

Date of Award	2015
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology