The contents of nondestructive testing of building quality mainly include: strength, thickness and defect detection of building concrete; Detection of missing steel mesh and buildings; Corrosion detection of steel bars in buildings; Resistivity detection of building concrete: building leakage detection; Stress detection of building structure; Detection of bearing capacity of bridge structure. Applicable geophysical methods mainly include acoustic wave, ground penetrating radar, electromagnetic method, cross-well CT, electrical method, reflected wave method, buried pipe transmission method and so on.

After tunnel lining, due to many factors, the thickness of lining concrete may not meet the design requirements or there are quality problems such as void. In order to find out the lining quality problems in time, it is necessary to detect the lining quality of tunnel quickly and with high resolution, so as to provide a basis for the scientific management of tunnel engineering. Ground penetrating radar is the most commonly used geophysical method in tunnel quality detection.

The main contents of detecting tunnel lining quality by ground penetrating radar are concrete compactness, porosity and lining thickness. Generally, 500MHz or 900MHz high-frequency antenna is used for detection, and the detection thickness can reach several tens of centimeters. The survey lines are generally arranged at the vault, arch waist and side wall of the tunnel. The vault is near the top of the tunnel, the arch waist is about 1m above the arching line of the tunnel, and the side wall is about 1.5m above the drainage cover plate. The measuring method is contour method, the distance between measuring points is generally tens of centimeters, and the mileage is tracked by the measuring wheel.

See table 10.5 for the physical parameters of related media in the detection of tunnel lining thickness.

Table 10.5 Table of Physical Parameters of Related Media for Tunnel Lining Thickness Detection

Evaluation of lining thickness: firstly, determine the in-phase axis of the reflected wave at the interface between concrete and rock on the ground penetrating radar profile, read the bidirectional travel time of the reflected wave, and calculate the thickness of concrete lining according to the formula H=v×t/2. Velocity v can be calibrated by open hole interval or drilling data; According to the amplitude, phase and frequency characteristics of the reflected wave of GPR profile, the evaluation of compactness can be divided into two types: dense and non-dense. The waveform of non-compact concrete on radar profile is chaotic, and the in-phase axis is staggered. On the radar profile, the in-phase axis of the reflected wave at the interface between concrete and surrounding rock is arc-shaped and does not coincide with the adjacent road. According to this characteristic, the cavity range can be calculated. Due to the uneven surface of surrounding rock caused by blasting, the abnormality on the section should be carefully analyzed and confirmed when determining the gap.

The total length of a highway tunnel is about 1.6km. In order to fully understand the lining quality, ground penetrating radar detection was carried out before the tunnel was completed. The tunnel lining type is: SM3- designed lining thickness of 40 cmm4-designed lining thickness of 35 cm; Sm5—— The design lining thickness is 30cm. Figure 10. 18 is the GPR profile of the side wall survey line with mileage number K2 1+390 ~ K2 1+430. The lining type of this section is Sm5. In the figure, the wave event axis near 10ns is the event axis of the reflected wave at the interface of surrounding rock. Figure 10. 19 shows the calculated thickness curve of concrete lining.

Fig.10.18k21+390 ~ k21+430 GPR profile of side wall survey line.

Fig.10.19k21+390 ~ k21+430 Interpretation curve of concrete lining thickness of side wall survey line.