Effectiveness of osmotic tensiometer for suction measurements in residual soils

• Main Author: Sasitharan Purushothman
• Other Authors: Harianto Rahardjo
• Format: Final Year Project
• Language: English
• Published: Nanyang Technological University 2021
• Subjects: Engineering::Civil engineering::Geotechnical
• Online Access: https://hdl.handle.net/10356/153890
• Institution: Nanyang Technological University

The influence of matric suction on unsaturated soils is significant as it is a factor within the quantification of the shear strength of soils which directly contributes to the factor of safety of slopes. Slope stability in Singapore, a substantial portion of which are unsaturated residual soil slopes, has increasingly been threatened by the effects of climate change. Therefore, the monitoring of such slopes is important in order to have a clearer understanding of the relevant factors that contribute to its stability. The relationship between water content and matric suction is reflected in the soil-water characteristic curve (SWCC). The SWCC is a vital precept within unsaturated soil mechanics representing the interactivity between the incumbent water content of the soil and the corresponding matric suction. A wide range of techniques have been used to derive the SWCC, some of which are purely laboratory-based such as using pressure plates and the WP4C Dewpoint Potentiameter. The other instrument for suction measurement, that is applicable to field environments, is known as a tensiometer. This project focuses on examining the viability of employing the osmotic tensiometer as a technique in measuring suctions within residual soils. The scope of the project involves obtaining suction measurements within a laboratory environment to derive an SWCC using the conventional methods, which then serves as a comparative baseline to assess the accuracy and reliability of measurements conducted using the osmotic tensiometer. Numerical analyses to assess the potential applicability of the osmotic tensiometer in field environments were also conducted. The behaviour of the osmotic tensiometer was monitored and compensated for by theoretical equations derived based on prolonged observations of the pressure and temperature variations of the polymer hydrogels utilized within the osmotic tensiometer. The usage of these polymer hydrogels has allowed the osmotic tensiometer to have a wider range of measurements and not be affected by cavitation which causes conventional water-based tensiometers to have a limited range of measurement. The osmotic tensiometer(s) within this project were found to have a measurement range of up to 2000kPa, whilst yielding suctions measurements that are closely aligned to those measured using the established laboratory-based methods. The osmotic tensiometer was also able to maintain the accuracy of its readings across that range of suction values, indicating its suitability for use even during the periods of erratic weather which resulted in drastic changes in the soil suction. These findings validate the ability of the osmotic tensiometer to measure high suctions and demonstrate the advantage of using it to perform suction measurements within slopes.