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Application Example of Ground Penetrating Radar in a Tunnel Engineering Inspection
4.1 Overview of Tunnel Engineering
A certain tunnel belongs to a double track separated type tunnel, with four lanes distributed on the left and right sides. The entire tunnel includes an entrance and an exit, and the construction method is used to construct both sides simultaneously. The total length of the tunnel is 5527 meters. The tunnel includes a large number of abandoned coal mines at the entrance. Due to the long-term overexploitation of these coal mines, they have become multiple goaf areas. Therefore, during specific construction, it is impossible to accurately measure the internal space size and extension form of these goaf areas. In the specific construction process, in order to improve the quality of tunnel construction and enhance the safety factor of tunnel construction, the geological radar detection method is used to detect the quality of tunnel lining in this construction. The geological radar prediction equipment for this time is the RAMAC/CUIII type ground penetrating radar, which uses a shielded antenna with a size of 100MHz. Using point measurement method for data collection, with a sampling frequency of 1196MHz and 512 sampling points; Stacking 128 times, measuring point spacing of 0.1m, and sampling time window of 620ns. The lining quality inspection adopts an 800MHz shielded antenna, and the continuous profile scanning method is used along the side line. The sampling points are 512, and the sampling time window is 50ns.
4.2 Testing of the thickness of the concrete spraying layer
Firstly, due to the certain compressibility of the surrounding rock inside the tunnel, a comprehensive inspection and analysis of the compressibility of the surrounding rock itself is required during the construction of lining concrete. At the same time, the thickness of the lining concrete is calculated based on scientific calculation methods to ensure that the thickness of the lining concrete can be used to support the surrounding rock. Minimize the compression value of the surrounding rock by utilizing the bearing capacity of the lining concrete. According to the instructions in the "Technical Specification for Tunnel Construction" in China, when determining and testing the average thickness of the concrete spraying layer, it is only considered a standard when 90% of its thickness is greater than the design thickness, and its thickness should also be 1.5 times the design thickness. In order to detect the thickness of the sprayed layer using geological radar without damaging the lining concrete, and through the analysis of the detection map, the final conclusion is that the thickness of the detection point and the minimum thickness of the wall have reached the standard level.
4.3 Detection of concrete defects
In tunnel engineering in China, due to the low quality of concrete and other issues, lining concrete mainly has three types of defects: low density, large void area, and voids. It is possible that during the tunnel blasting construction process, for example, when the tunnel engineering unit is carrying out secondary lining, if there are situations such as improper laying of waterproof boards, uneven sealing of concrete, and shrinkage of the concrete itself, it is easy to cause defects in the concrete. If blasting technology is used to detect the concrete, there may be situations where the blasting personnel are not proficient enough and the concrete quality does not meet the standards. Therefore, after blasting, there may be situations where the blasting degree is too high, the blasting degree is not enough, and the uneven surface of the blasting is uneven. Therefore, this inspection uses geological radar to detect concrete defects. The cavity of this tunnel often contains air. When electromagnetic waves pass through the air through the lining in the concrete cavity, there is a significant difference in dielectric constant, resulting in a reflected wave. The attenuation of electromagnetic waves in the air is relatively small, so strong multiple reflections are easily generated in the cavity, forming a banded or triangular waveform. The same phase axis is curved and discontinuous with adjacent axes. See Table 1 for waveform characteristics of GPR image corresponding to tunnel concrete that is not dense, void and cavity.
4.4 Steel support position and quantity
When detecting the distribution, support position, and quantity of steel bars inside the construction concrete, geological radar detection can be used. The principle is that the steel bars themselves have a high density, and the electromagnetic waves emitted by geological radar during survey will form a bending echo when encountering the steel bars. This process can immediately form a reflection image through data calculation software, etc. For example, when there is steel bars in concrete, the electromagnetic waves will present a reflection image when encountering the steel bars
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