| Summary: | 博士 === 國立成功大學 === 土木工程學系碩博士班 === 101 === Nondestructive testing technologies are associated with a type of geophysical exploration. The main advantages are that the use of such technologies is economic and fast, and it can provide characteristics for continuity of objects that traditional underground boring cannot provide. Therefore, they are widely used for detection in geotechnical engineering. For the application of nondestructive testing techniques, as compared to the traditional single instrument used in such applications for investigation of old foundation structures, we use ground penetrating radar to implement the investigation of an old foundation for an ancient structure consisting of basically remnants. For efficient safe detection of dams after an earthquake, ground penetrating radar and resistivity image profiling can be used to detect the damage resulting in cracks in dams, and can within a short time also determine the depth of cracks and provide emergency detection for the depth of the seepage line in dams. It can be widely used in an Embankment Safety Assessment Mode in order to establish the correlation of the appearance and internal cavity grade of concrete dikes and to collect the factors of dike using a visual investigation method, using multiple linear regression analysis, cavity grade via mode predictions and compared with cavity grade defined by ground penetrating radar figure levy to verify the accuracy of mode. We can use an easier, more convenient, safer and quicker prediction method to assess the cavity grade situation in a dike located under concrete slab.
To enable NDT technologies to be effectively applied in geotechnical investigations, this research attempts to set a data judging standard for detection and analysis processes, and the Small East Gate section of Taiwan Fucheng, the Hutoupi Reservoir and Tsengwen coastal embankments are the objects of this research to be discussed. The results are as follows:
(1)This study applies two methods to estimate the wave velocity of the medium, which are described as follows: The known buried depth object: For field detection or the simulation test, the wave velocity of the medium could be estimated by the actual buried depth of the object and the travel time of the radar signal. A probing steel bar: We also placed a solid steel bar with a diameter of 1.3 cm vertically into the ground and utilized GPR to conduct the detection. The two-way travel time to the bottom of the solid steel bar is shown in the GPR detection results. Accordingly, the estimated velocity of the strata around the bottom of the solid steel bar could be obtained.
(2)Because GPR imaging is based on the difference in dielectric properties of a radar wave passing substances, and the interface between different materials will generate different reflection intensities, which can be indicated by setting different colors in the GPR system in which the various color levels in the image can correspond to the reflection intensities of the formation and remains under consideration. Based on this principle, this study suggests that the color levels in the radar image correspond to the conditions of the buried underground remains or cultural relics by comparing the radar image obtained in the detection with the results of the archeological trial excavation. According to these results, the outer diameter and the inner diameter of the outer wall of the Small Eastern Gate may be determined.
(3)Based on the difference in dielectric properties for materials, ground penetrating radar can be used to detect internal defects in a dam. The GPR detection results are usually interpreted using an image, and the image is produced based on reflected electric signals; hence, if the strata are original and not disturbed, no artificial disturbance or excavation has occurred, and the GPR images will be layered, continuous and smooth. Once the strata have been disturbed, the layered signal in the image will break off. Because an interpreter performs the GPR image analysis using his own vision, the GPR image analysis is very subjective. If the interpreter’s experience is insufficient, the GPR results will often be misinterpreted. Therefore, this study used a digital signal processing method, the Hilbert Transformation, to improve the accuracy of the GPR image interpretation.
(4)The management authority drilled boreholes at the study site in 2011 to detect the groundwater water table. The survey line was near the observational wells in order to validate the correctness of using RIP to detect the water table. Hence, the RIP response of the water table in the verification wells could serve as a reference during the detection of the water table at the study site. Therefore, according to the RIP survey results, the inverted resistivity around the water table is about 40-50 Ohm-m.
(5)Establishment of the statistical model for cavity detection: Since the outlook of the concrete faces is related to the cavity behind the concrete faces, the degree of cavity erosion of the dyke could be assessed using a linear equation. Because removing the concrete face of the dyke to check for a cavity is forbidden, the degree of cavity erosion was evaluated using a GPR image. In addition, the outlook factors must be easy to obtain and must also be sensitive to the cavity. The statistical model to correlate the degree of cavity erosion in the dyke to the outlook parameters must be of a simple form. Therefore, a multiple linear regression analysis method was used to create a simple multivariate linear equation.
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