Monitoring of soil movement due to deep excavation works using Time Lapse Ground Penetrating Radar (TLGPR)
Creators: name: Pak, Chun
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 400502
subject: Civil geotechnical engineering

description: The aim of this thesis is to deploy and develop the Time Lapse Ground Penetrating Radar (TLGPR) for monitoring the change in soil mass during deep excavation works as well as the experimental works, so as to characterize the crucial parameters and equipment setting in execution of the TLGPR. This research is the first validation of TLGPR in Hong Kong, by comparing radargrams in frequency domain as a monitoring tools for deep excavation and lateral support construction works. 

The traditional monitoring measures for surrounding roads and pavement in deep excavation works only focus on the change of ground surface, including ground settlement, vibration and tiling of adjacent buildings. With the penetration capability of ground penetrating radar (GPR), the TLGPR provides a new perspective of monitoring tools for engineers to understand the changes in the retained soil mass. By comparing the differences in radargram before the commencement of construction and during the course of it, the changes in soil mass can be identified. However, comparing radargrams through visual inspection is no longer a scientific approach. This research reports the methodology using Wavelet Transform (WT) to convert radargram into peak centre frequency distribution with suitable spatial resolution (time-depth intervals). Comparing radargrams in frequency domain can eliminate human subjection and enhance the detectability.

The TLGPR sounds feasible in theory as dominating parameters and settings in real case validation of the method are shown, including the sensitivity of Wavelet Transform on converting radargram into peak centre frequency distribution, the balance between spatial resolution and processing time, the display of total error (detection threshold)  to prevent false alarm and its measures to eliminate the influence from positioning error as well as the applicability of common signal processing filter for radargram. 

The above-mentioned challenges tackled by a series of experiments are reported in this thesis. First of all, the conversion of radargram to peak frequency is executed by the means of  an In-house LabView programme developed by Hong Kong Polytechnic University. In addition, a series of sensitivity analysis were carried out by the programme to determine a suitable value of threshold amplitude (result is 0.4) in computation of a representative peak frequency distribution for TLGPR. Moreover, the spatial resolution of the TLGPR dominated by the sampling frequency during data acquisition and the time-depth interval in computing the peak centre frequency distribution in radargram is illustrated. Through experiments, the minimum sampling frequency (256 samples per trace) and time-depth interval (7 ns) are recommended in this thesis. Signal processing is a common procedure to enhance the readability of radargram. Two basic signal processing operations, Dewow and Subtract-DC-shift, are being studied and analysed by WT. Experiments to prove that signal processing does introduce artifacts into radargram that would subsequently change the peak centre frequency distribution. For TLGPR, hence, raw radargram shall be used for comparison. To prevent false alarm arising from the TLGPR, an experiment focuses on the error from data acquisition is reported in this thesis and conclusion is made that the detection threshold in percentage difference in peak centre frequency is 30% (with 95% confidence level). As the drifting of survey wheel and positioning error contributes to a significant magnitude of error into TLGPR, a method named Frequency Analysis on Artifacts in Radargram (FAAR) is then proposed and validated in this thesis. On one hand, it shows that the method of TLGPR is capable of eliminating the positioning error. On the other hand, it is concluded that the error from drifting of survey wheel is negligible. 

Lastly, it should be addressed that the findings were obtained in field with well-controlled conditions. It is believed that the findings have already provided the fundamental and indispensable understanding of the TLGPR and will be able to pave the ways for future full-scale application. This research is believed to have advanced the fronts of both material characterization research and geophysical applications.