Xiongyao Xie

Evaluation of grout behind the lining of shield tunnels using ground-penetrating radar in the Shanghai Metro Line, China

For shield tunnelling construction in soft soil areas, the coverage uniformity and quality of consolidation of the injected grout mortar behind the prefabricated tunnel segment is the main concern for tunnel safety and ground settlement. In this paper, ground-penetrating radar (GPR) was applied to evaluate the grout behind the tunnel lining segments in Shanghai, China. The dielectric permittivity of the grout material in Shanghai Metro tunnelling construction was measured in the laboratory. Combining physical modelling results with finite different time domain numerical modelling results, we suggest that the antenna with frequency 200 MHz is well suited to penetrate the reinforced steel bar network of the tunnel lining segment and testing grout patterns behind the segment. The electromagnetic velocity of the grout behind the segment of the tunnel is 0.1 m ns−1 by the analysis of field common-middle point data. A wave-translated method was put forward to process the GPR images. Furthermore, combining the information acquired by GPR with experience data, a GPR non-destructive test standard for the grout mortar evaluation in Shanghai Metro tunnel construction was brought forward. The grout behind the tunnel lining segment is classified into three types: uncompensated grout mortar with a thickness less than 10 cm, normal grout mortar with a thickness between 10 cm and 30 cm and overcompensated grout mortar, which is more than 30 cm thick. The classified method is easily put into practice.

Image enhancement with wave-equation redatuming: application to GPR data collected at public transportation sites

In this paper, we describe the application of wave-equation redatuming through downward and upward continuations of surface ground penetrating radar (GPR) data to image the target beneath a surface layer with geometric or material property heterogeneities. We first tested this technique with the use of a synthetic radar profile generated by the finite difference time domain method to show its effectiveness. We then applied it to the surface GPR data collected in several urban public transportation infrastructure sites including a project to use GPR to assess the quality of mortar consolidation behind a tunnel wall in the Xiangyin highway tunnel, Shanghai, China. The purpose of this redatuming processing is to enhance target images in later times of a GPR profile, by eliminating the contamination in the GPR profile caused by strong diffractive scattering from the steel rebars in the surface concrete slab or tunnel wall. The results for synthetic and observed GPR data show that the wave-equation redatuming technique is an effective way to eliminate unwanted diffraction signatures and ease GPR image interpretation for data collected at a site with a strongly heterogeneous surface layer.

GPR identification of voids inside concrete based on the support vector machine algorithm

Voids inside reinforced concrete, which affect structural safety, are identified from ground penetrating radar (GPR) images using a completely automatic method based on the support vector machine (SVM) algorithm. The entire process can be characterized into four steps: (1) the original SVM model is built by training synthetic GPR data generated by finite difference time domain simulation and after data preprocessing, segmentation and feature extraction. (2) The classification accuracy of different kernel functions is compared with the cross-validation method and the penalty factor (c) of the SVM and the coefficient (σ2) of kernel functions are optimized by using the grid algorithm and the genetic algorithm. (3) To test the success of classification, this model is then verified and validated by applying it to another set of synthetic GPR data. The result shows a high success rate for classification. (4) This original classifier model is finally applied to a set of real GPR data to identify and classify voids. The result is less than ideal when compared with its application to synthetic data before the original model is improved. In general, this study shows that the SVM exhibits promising performance in the GPR identification of voids inside reinforced concrete. Nevertheless, the recognition of shape and distribution of voids may need further improvement.

Groundwater Level Monitoring for Hydraulic Characterization of an Unconfined Aquifer by Common Mid-point Measurements using GPR

Because of its limited bandwidth, ground penetrating radar (GPR) signal has an oscillating character that degrades the ability to distinguish reflection events and introduces an error in the velocity analysis. In this paper, we use the analytic signal to calculate the envelope velocity spectrum, which is void of the oscillating nature of a GPR wavelet. A comparison study on a synthetic common mid-point (CMP) gather demonstrates that the envelope velocity spectrum gives a more robust velocity estimation. We applied this technique to explore the potential of GPR for monitoring the pumping-induced groundwater level change, then estimating the hydraulic properties of an aquifer in Ulaanbaatar, the capital of Mongolia. A field survey was conducted along three different directions when a production well was under pumping and static conditions. In addition, a simple velocity picking scheme was suggested based on the maximum coherency curve. From the velocity picks, the vertical profiles of interval electromagnetic velocity and water content were estimated. The groundwater table was easily identified in these profiles. The groundwater level changes under the CMP sounding locations along the northeast and north survey lines were estimated to be 27 and 23 cm, respectively, whereas it failed to detect the groundwater level change on the northwest survey line because of the effect of strong scatterers near the groundwater table. The estimated groundwater level change was input into the unconfined aquifer model for estimating the hydraulic conductivity, which yields comparable results with a previous study. We conclude that the hydraulic parameter estimation using GPR is much easier and effective than the conventional hydraulic test methods.

Accurate thickness estimation of a backfill grouting layer behind shield tunnel lining by CMP measurement using GPR

The distribution of a backfill grouting layer behind concrete lining of a shield tunnel is important for evaluation of its stability. In this paper, we proposed to use Common Mid-Point (CMP) method by Ground Penetrating Radar (GPR) to estimate the dielectric permittivity and thickness of the backfill grouting layer. We developed a CMP measurement system with two designed bowtie antennas. Field experiments were carried out in two shield tunnels in Shanghai, China in January 2012. One is the Yangzi River tunnel under operation and the other is the Changjiang Road tunnel under construction. Two datasets were acquired in each tunnel. Frequency wavenumber (FK) filter was applied to suppress the coherent clutter and enhance the hyperbolic reflection signal. A new algorithm of envelope velocity spectrum analysis was proposed for accurate estimation of dielectric permittivity and thickness of the grouting layer from processed CMP profiles. Both the grouting layers behind the tunnel under operation and that under construction can be clearly estimated. The thickness of grouting layer behind the investigated two segments in Yangzi River tunnel were respectively estimated to be 16 cm and 17 cm, a little less than the designed thickness of 20cm. In addition, air void was discovered inside the grouting layer behind one of the lining in Changjiang Road tunnel. This technique provides important information for evaluation of the safety of a tunnel, mitigation of the risk of ground settlement.

Non-destructive evaluation of shield tunnel condition using GPR and 3D laser scanning

Shield tunnel requires routine inspections for construction safety and quality control. The rapidness, non-destructiveness and high precision of GPR and 3D laser scanning technology enable them to be widely and increasingly used in tunnel detection. The evaluation of Yingbin No.3 Road Tunnel is presented in this paper to illustrate the application of GPR in the inspection on thickness of grouting layer behind the segment and 3D laser scanning technology in the testing on surface situation of tunnel liner. The advantages and limitations of the above-mentioned two methods are discussed in the paper and suggestions are provided.

SIMULATION OF WAVE PROPAGATION OF FLOATING SLAB TRACK-TUNNEL-SOIL SYSTEM BY 2D THEORETICAL MODEL

Presented herein is a computationally efficient 2D theoretical model for simulating the steady response of a floating slab track-tunnel-soil system. The track-tunnel coupled system is simplified as a beam-spring system and embedded in soil layers. The tunnel is modeled by a Timoshenko beam with its interaction with the soil layers accounted for by two transfer matrices, with each derived for the soil layer above and beneath the tunnel. The approach as proposed herein has been referred to as the Timoshenko beam-transfer matrix method (TTMM), that allows one to analyze the response of the coupled system, including the tunnel motion and soil stresses. The results obtained were compared with those furnished by the pipe-in-pipe (PIP) approach, and were found to be consistent for exciting frequencies smaller than the tunnel second-mode cut-on frequency. The origin of discrepancies was investigated by the dispersion characteristic analysis, which is attributed to the absence of several in-plane modes when the tunnel is simplified as a Timoshenko beam.

GPR identification of voids inside concrete based on support vector machine (SVM) algorithm

In this paper, voids inside concrete is identified and classified from ground penetrating radar (GPR) image in a completely automatic way basing on the support vector machine (SVM) algorithm. The entire process can be characterized into four steps: 1), the original SVM classifier model is built up by training the synthetic GPR data obtained by Finite Difference Time Domain (FDTD) simulation, after data preprocessing, segmentation and feature extraction. 2), the classification accuracy of different kernel functions is compared with cross-validation method and the optimization of penalty factor (c) of SVM and the Coefficient (σ2) of kernel functions are obtained by the grid algorithm and the genetic algorithm (GA). 3), this model is then verified and validated by applying to another set of synthetic GPR data, also obtained by FDTD simulation, to test the effect of classification of voids inside concrete. The result shows a high success rate for classification. 4), this original classifier model is finally applied to real GPR data to identify and classify voids. The result is less than ideal when compared with the application to the synthetic data before this original model is improved. In general, this study shows that SVM exhibits promising performance in GPR identification of the voids inside Concrete. Nevertheless, the recognition of the shape and distribution of voids may need further improvement.

Design and optimization of the antenna applied for detecting the voids in tunnel lining by GPR

In this paper, a new Antipodal Vivaldi antenna has been designed to be used for detection of the voids in tunnel lining by Ground penetrating radar (GPR). In order to obtain a desirable antenna performance, the wider impedance bandwidth, the higher radiation efficiency and gain, three parameters of the antenna structure, including a) length of microstrip line L, b) tapered slot opening rate R, c) elliptical axis radius Re have been chosen to optimize the qualities of antenna based on FDID method. Moreover, the optimal Antipodal Vivaldi antenna has been used to detect the void in tunnel lining, and its proved that the new antenna has a good level of matching with the tunnel lining.

Design and Test of an Improved Dipole Antenna for Detecting Enclosure Structure Defects by Cross-Hole GPR

In underground engineering, the enclosure structure is a crucial part to guarantee the safety of excavation in soft ground regions. To inspect defects in the enclosure structure and assess its integrity, the cross-hole ground-penetrating radar (GPR) is considered as a promising tool. To make this technique feasible for underground structure applications, suitable antennas should be devised. In this paper, we designed an improved dipole antenna for a stepped frequency cross-hole GPR system to detect enclosure structure defects. Numerical simulations were adopted to optimize antenna design parameters including the half-cone angle θ, the feeding gap g, the cylinder radius r, and the cylinder length l. Then the optimized antenna was fabricated, and the performance of the antenna was evaluated by measuring the return loss, the voltage standing wave ratio (VSWR), the phase response, and the coupling. The results indicated that the proposed antenna operated well in the frequency range from 0.45 to 1.1 GHz and had the best performance at 0.52 GHz. Moreover, a physical model experiment was carried out and proved the antennas feasibility for enclosure structure defects detection.