1. Introduction
With the rapid development of China’s economy, the role of heavy-haul railway transportation in promoting national economic development has become increasingly more important [
1]. Compared with traditional railways, heavy-haul railways have the characteristics of large shafts, high opening density, and greater vibration loads on the base of a tunnel. Therefore, the bottom structure of a tunnel is more prone to damage [
2,
3]. According to the research of relevant references, the main contradiction in the substrate quality of heavy-haul railway tunnels in operation in China lies in the contradiction between the increasing weight of the train axle and the resulting substrate damage. The main manifestations are that with the long-term repeated vibration of heavy-haul trains and groundwater seepage, the small defects in the tunnel substrate have evolved into structural and softening between surrounding rocks, and even damage such as emptying, arching, and filling layer cracking or staggered platforms, slurry, and mud.
Softening and the void between the tunnel invert and surrounding rock is a major form of basement damage. The first reason for this is that the early tunnels in China lacked forward-looking design, and insufficient attention was paid to the design of the tunnel drainage structure. Over time, the only drainage pipes become blocked by impurities, and the groundwater that cannot be discharged smoothly forms a “scouring” effect on the surrounding rock. In addition to the poor construction quality of some tunnels, the gap between the basement structure and the surrounding rock creates “congenital defects”. The second reason is that the repeated action of the groundwater and train load causes the enhancement of the surrounding rock liquefaction liquidity. The rock mass is constantly “emptied” with the action of seepage, and the created base voids increase. Once the tunnel base voids appear, it greatly weakens the stress performance of the structure, induces inverted arch cracking and damage, and even causes a large-scale fracture, staggered platform, or collapse of the structure at the bottom of the tunnel, which will worsen driving conditions and seriously affect transportation efficiency and driving safety. Base voids are still some of the important factors causing damage to the upper structure of a tunnel. Therefore, it is of great significance to study the vibration response of heavy-haul train railway tunnels with base voids.
For the dynamic response and fatigue damage life analysis of the substrate structure of heavy-haul railway tunnels, researchers studying this topic in China have conducted in-depth research. Zou Wenhao [
4] combined on-site measured data and a numerical simulation, and the dynamic response law of the substrate structure of a tunnel with the action of a load of a 30 t axle heavy train was studied. It was proven that the numerical simulation results could be generally consistent with the actual measurement trend in the field. The dynamic stress response of the base structure on the side of the heavy load line of the tunnel structure was significantly greater than that on the side of the empty train line, and the stress performances of different forms of tunnel inverts were analyzed. Xue Jilian [
5] analyzed the cause of the tunnel bottom structure, and the dynamic response of the tunnel bottom structure reinforced by polyurethane with the action of 30 t axle heavy trains was studied using finite element software. Yin Chengfei et al. [
6] combined the composite lining tunnel of the Shuohuang Railway, studied the dynamic stress changes in different positions of the filling layer and inverts of the heavy-haul railway tunnel. Ding Zude [
7] applied a concrete plastic damage model to analyze the stress level and damage value of a tunnel invert with different base voids for a heavy-haul railway tunnel. When the lateral base void reached 120 cm, the damage value of the bottom structure reached 1.0. Deng Bin [
8] established a numerical analysis model and analyzed the dynamic response characteristics of the laying structure of a heavy-haul railway tunnel with different laying thicknesses with the action of a 30 t axis heavy train load. Xu Xinli [
9] established a three-dimensional dynamic analysis model for a tunnel, and the stress state and fatigue life of the substrate structure of the full-frame and half-frame heavy-haul train tunnel were studied. Liu Ning [
10] applied the Miner linear fatigue accumulation damage criterion and calculated the service life law of the substrate structure of heavy-haul tunnels for different substrate softening and base void conditions. It was proven that the degree of base void significantly affected the fatigue life of the tunnel structure. Lihui Xu et al. [
11] proposed a novel coupled periodic tunnel–soil analytical model for predicting ground-borne vibrations caused by vibration sources in tunnels. Xuming Li et al. [
12] proposed a hybrid approach that combined the Bayesian Neural Network and the impedance model to predict vibrations inside a building by considering the coupling loss of soil and building structure. Zou, C. et al. [
13] developed an efficient computational model that characterizes the dominant mode of vibration transmission through each structural element including those in transfer structures in building designs where ground and building columns are not aligned.
In summary, a certain amount of scientific research has been carried out on the vibration response and damage development law of the base structure of a heavy-haul railway. However, numerical simulation analysis is mostly used in the study of heavy-haul railway tunnel damage, and the accuracy of the calculation results needs to be tested. There are very few existing tests on the dynamic response of a heavy-haul railway tunnel structure and the surrounding soil layers. Based on the statistical results of a heavy-haul railway tunnel, a 1:20 scale tunnel model test is established in this research. Single-point excitation vibration is used to simulate the overloaded train load. The dynamic response characteristics of the tunnel structure and the surrounding soil layer are studied under the conditions of substrate health and a base void using an acceleration sensor, strain gauge, and soil pressure box.
2. Detection of Tunnel Base Voids
At the time of this research, all of the 20 single-hole and double-line tunnels with the detected prominent base damage problems are from a special heavy-haul railway for coal transport that has been in service for more than 30 years. The tunnels are mainly of short–medium length. The cumulative length of the measured tunnel is 41,102 m. The tunnel length accounts for the largest proportion in the grade V surrounding rock environment, approximately 48.0%.
Geological radar is used to detect the voids and loose areas of a tunnel basement, and this radar mainly detects the scope, position, and scale. A typical base void radar effect diagram is shown in
Figure 1. The statistical plots of the tunnel base voids are shown in
Figure 2 and
Figure 3.
From
Figure 2, the average longitudinal length of the base voids below the tunnel’s heavy-haul line is approximately 2.63 times that below the empty cabin line. The continuous base void length below the heavy-haul line is approximately 65.1% within the range of 0–8 m, and the ultra-long distance voids above 12 m also account for a considerable proportion.
Figure 3 shows the cumulative length statistical diagram of different widths corresponding to different surrounding rock levels. D is the effective width of the tunnel invert, that is, the widest transverse distance of the invert. From the general trend, the transverse void width of the base is mainly concentrated in the range of 0–1/2 D. The worse the tunnel damage interval is with the surrounding rock mass, the larger the base void width is. In grade IV surrounding rock, the base void widths of 1/2 D–3/4 D and 3/4 D–D account for 21.9% and 11.4%, respectively, while in grade V surrounding rock, the base void widths of 1/2 D–3/4 D and 3/4 D–D account for 22.9% and 12.1%, respectively.
In summary, the base voids of heavy-haul railway tunnels mostly occur in heavy-haul lines. The base void length is mostly concentrated in the range of 0–8 m, and the width is mostly concentrated in the range of 0–1/2 D. From grade II to grade V surrounding rocks, the cumulative value of the average base voids width increases by approximately 31.1%, 47.7%, and 52.3%.
6. Discussion
Based on the statistical results of base damage detection of a heavy-haul railway tunnel, a scale test of tunnel has been established. The dynamic response characteristics of a heavy-haul railway tunnel structure and the surrounding soil under the conditions of healthy base and base void have been studied. The base voids change the tunnel dynamic response, deteriorate the stress state of the structure, and shorten the fatigue damage life of the tunnel. In particular, the base voids have a great impact on the structure within the void range. The research results have certain reference value for similar tunnel tests. However, only one form of base void has been discussed in this test; in future studies, it can simulate the heavy-haul trains with different axial weights or speeds by changing values or frequencies of the output load of the vibrator, or it can change the width or length of the base voids to further enrich the research results. To facilitate the installation of the sensors, the scale model did not consider the inverted arch filling layer and the rail plate. Thus, there is a difference between the test results by using the present scale model and the actual situation. In the future, it is planned to assess one or one set of reduction coefficients through the field test data, so that the test results of this paper can been applied in real life.