Slope strain monitoring using soil-embedded distributed optical fibre sensor

Abstract

Slope monitoring is essential in periodical geotechnical monitoring exercise as slope behaviour would change over a period of time subjected to the surrounding environment. The conventional instrumentations for slope displacement monitoring are inclinometers, tiltmeters and extensometers, but main drawbacks of using available instrumentation are the difficulties in handling as well as high cost of equipment installation at a complex geological terrain and massive size of slope. The downside of these equipment can be overcome by alternatively employing a distributed optical sensing fibre technology for slope monitoring programme. However, current applications of distributed fibre optic sensor were limited by attaching the sensor onto the geo-structure surfaces such as soil nailing, anchor bolts or geotextile but the arrangement of sensors on soil-embedded soil slope is still under uncertain evaluation due to the non-linear soil behaviour. Therefore, this study focuses on efficiency of the soil-embedded distributed fibre optic sensing system based on Brillouin Optical Time Domain Analysis (BOTDA) technology as an innovative instrumentation apparatus to monitor deformation event of an unsaturated soil slope. BOTDA could be attractively employed for soil slope monitoring since it allows a continual measurement of strain along its interrupted length of fibre optic cables. In this study, a soil-embedded strain sensor placement approach was proposed which was achieved via the horizontal planting of a three-layered optical fibre cable in S-curve forming in the physical laboratory soil slope. The residual soil slope model was also instrumented with tensiometers to measure suction distribution and subjected to different simulated rainfall intensities and surcharge loading until failure. At the same time, the progressive failure images were also captured using DSLR camera which then analysed using Particle Image Velocimetry (PIV) method to compare with the new optical fibre sensor instrumentation set up. A total of ten laboratory schemes was performed including four preliminary infiltration tests subjected to three different rainfall intensities of no-rainfall, 1-hour and 24-hour infiltration and slope inclination of 27° and 45°. Before the infiltration tests, calibration experimental work on optical fibre was performed due to the non-linear soil behaviour which influences the true strain deformation of the soil slope. From the preliminary laboratory tests, the results show the soil-embedded sensing fibre arrangement has efficiently detected and measured the strain deformation due to both rainfall and loading. The captured strain data indicated a progressive deformation behaviour of a soil slope when there were changes in suction distribution and loading-induced activity on a soil slope as rainwater infiltration has gradually weakened the unsaturated shear strength of the soil by reducing soil suction, and rapid surcharge loading has also caused the development of excess pore pressure. This phenomenon had resulted in declining of soil effective stress that led to undrained bearing capacity failure. A series of numerical simulations were later conducted by employing the commercial software of SEEP/W and SIGMA/W to analyse further the deformation behaviour which also acted as a comparative case against to the PIV images and the experimental results. In comparison to the PIV measurements, the optical fibre sensor was found to be capable to exhibit overall deformation of the soil slope when placed under an optimum configuration layout of the sensor. The optical fibre sensor has effectively captured the deformation behaviour of the unsaturated soil slope model in the presence of rainwater infiltration and imposed surcharge loads. The outcomes from this study had contributed to important results in terms of the design of field deployment of soil-embedded optical sensing fibre in unsaturated natural soil slope and embankment subjected to rainfall infiltration or any similarities to the saturated and unsaturated condition.