# Influence of Soil Parameters on Detecting Voids behind a Tunnel Lining Using an Impact Echo Method
## 1 Introduction  
With the development and large-scale construction of high-speed railways and urban mass transit in China in recent years, different types of lining quality defects have appeared as a result of various adverse factors, such as geological and topographical conditions, natural disasters and related problems during design, construction and operation management. The ineffective bonding state between the lining and ground is a significant factor that can directly induce numerous tunnel issues and affect the tunnel structure safety [1]. Accordingly, it is essential to evaluate the void conditions behind the tunnel lining. Existing detection methods include core drilling, geological radar, the impact echo (IE) method and other non-destructive test (NDT) approaches. The IE method has been used for pile and other plate-like concrete NDTs [2; 3]. During a tunnel NDT, excitation was conducted using an impact hammer on the tunnel lining surface and reflectionwas acquired using a vibration sensor. Due to the existence of structural defects, the tested signals differ. Therefore, one of the key techniques is recognising the existence and location of defects by means of different signal analysis approaches; for example, in the time domain [2; 4] or frequency domain [5; 6], by using dynamic stiffness or mobility [7; 8] and with comprehensive time-frequency approaches [9; 10]. [PERSON] et al. [11] evaluated the conditions of concrete-lined tunnels and shafts by means of IEs, impulse response and spectral analysis of surface waves. [PERSON] et al. [12] presented tests performed using IEs for shotcrete thickness measurement. The NDT thickness measurements exhibited a strong agreement with the results obtained by mine personnel using conventional methods. [PERSON] et al. [13] compared four NDT techniques to detect delaminations/flaws in a reinforced concrete tunnel liner, and the results showed that the IE method provided maximum flaw detection. [PERSON] et al. [14] combined the use of time domain characteristics, spectral content and wavelet transforms to reveal the effectiveness of grouting by using IEs, and their study indicated that the IE method was valuable for a quick and reliable grouting assessment. [PERSON] [15] inspected a section of a power tunnel concrete liner in Nepal using IEs and cores were drawn from selected locations for calibration purposes. [PERSON] and [PERSON] [16; 17; 18] investigated IEs for evaluating tunnel shotcrete by using both an experimental study and a numerical simulation. [PERSON] et al. [19] tested a three-layer model concrete tunnel lining using a combined stress wave propagation method. The results indicated that a combination of the spectral analysis of surface waves and IE methods provided a more efficient procedure. [PERSON] [20] summarised research progress on evaluating tunnel shotcrete using IEs. The waveform attenuation characteristic was suggested for evaluating the contact condition between the tunnel lining and surrounding rocks, and the P-wave velocity was proposed as a method for evaluating the early strength of shotcrete. [PERSON] et al. [21] proposed a method based on the IE amplification factor (Q-factor) for the quality control of backfill material in segmental-lined tunnels. The results indicate that the measured IE Q-factor can be directly related to the backfill material quality. [PERSON] et al. [22] tested the Chesapeake Channel Tunnel in Virginia using ultrasonic surface waves (USW) and IEs. The results indicated that in intact areas, the IE method could provide the thickness of the concrete lining and roadway wall, while in defective areas, the IE method exhibited frequency peaks that differed from the expected thickness frequency. [PERSON] et al. [23] used IEs to detect the voids behind the tunnel lining using different signal analysis approaches. The research studies mentioned above demonstrate that the IE method is a useful NDT method.