Crack Detection for Dikes using Distributed Temperature Sensing

Abstract

One-third of the Dutch dike system consists of peat dikes. Drought causes these dikes to crack and fail more easily. Visual dike inspections are therefore inefficient, especially considering the increasing climate changes of the future. Research has shown that a fiber optic sensor cable (FOS), used in distributed temperature sensing (DTS), can help measure soil thermal responses, but the question remains whether it is also suitable as a replacement for visual inspections of dikes. This study helps to answer this question by coupling a finite element method (FEM) model with measured DTS information collected at Flood Proof Holland (Delft). In addition, (thermal) images are used to calibrate the FEM model. The measurement period was 20 summer days. The measured data consisted of webcam images, thermal images, and temperature time series of a FOS cable. The meteorological data was obtained via a weather station located in Rotterdam. The FEM model, calibrated and validated with the measured data, helped to find the thermal response of the system in situations of which no data was available, for example having various crack dimensions, cable positions or climatic conditions. The more distinguished the material properties of air and soil are (days with high water content and/or high radiation), the better the crack detection via DTS. After correction for the overestimation influence of radiation on the black FOS cable on sunny afternoons, the thermal response of the crack is corresponding to the air temperature more compared to the thermal response of grass. Crack detection via DTS turned out to be possible by x,T- and t,T-plots (diurnal temperature variation) and a regression plot with the daily peak-to-peak amplitude of the air temperature on the one hand and the daily peak-to-peak amplitude of the cable on the other. The advantage of the regression plot is that only one DTS thermal time series is needed to determine if the segment is most likely cracked or not. Furthermore, the peak-to-peak axes allow for diurnal climatic condition indications, and with it the heat storage and release of the system: rainy, cloudy days are in lower axis regions whereas sunny, clear days are in higher axis regions. However, due to a small measurement period, this method is only fully proven for sunny afternoons. Future studies have to map the thermal processes for other situations too.