# Fatigue Performance of Tunnel Invert in Newly Designed Heavy Hauilway Tunnel
## 1 Introduction  
Researchers reported that fatigue would not occur in the tunnel basal structures. [PERSON] et al. [1] conducted a series of fatigue tests based on the measured vehicle loads in highway tunnels, and the results revealed that the floor was still intact after 6 million cycles. Through scale model tests, [PERSON] et al. [2] stated that fatigue failure of tunnel bottom structures would not occur under the long-term action of a high-speed train. [PERSON] et al. [3] and [PERSON] et al. [4] indicated that segments of the shield tunnel bottom would be stable for the designed service life (100 years) using fatigue tests. The increase in train axle load can considerably increase the railway's capacity and efficiency and reduce the transportation cost per unit freight [5]; however, tunnel basal degradation increases with increasing axle load and operation period [6]. For example, the Daqin heavy haul railway has the world's highest annual freight volume (445 million tons in 2013 [7]). After more than 20 years of operation, basal deterioration is increasing over time in many tunnels, and most basal deterioration is appearing on the side of the full load track [8], which decreases the operational safety. Serious foundation subsidence and base rock voids have been found below the full load track in the Jundushan Tunnel along the Daqin railway [9]. Severe basal deterioration has also emerged along another heavy haul railway, the Shuohuang railway, including basement subsidence and rising soil on the side of thefull load track, as observed in the Shuiquanwan and Daping tunnels, where the maximum settlement is 1.5 cm in the Shuiquanwan tunnel [8; 10]. To study the mechanism of basal degradation and the fatigue performance of the base structure of heavy haul railway tunnels, through numerical simulation, [PERSON] [11] indicated that a loose basement leads to a significant increase in the maximum tensile stress of the inverted arch filling (IAF) and fatigue damage would occur under the long-term operation of a 30 t/axle load heavy haul train. Based on a fatigue test, [PERSON] et al. [12] also found that the lower the stiffness of the basement, the faster the development of the fatigue damage of tunnel inverted arches. [PERSON] et al. [13] stated that when a void exists in the base, the displacement and maximum principal stress of the tunnel bottom structure caused by train load increase significantly; then, based on the existing 5-\(N\) curve of concrete tension fatigue, the calculation suggested that the fatigue life of the tunnel bottom structure is less than 100 years when the basal void is \(\geq\)2 m.  
The fatigue performance of a structure is not only affected by the dynamic load, but also by the static load [14]. Under the same dynamic load, the fatigue life decreases with increasing static load. Stresses of tunnel bottom structures include the dynamic stress caused by the train load and the static stress caused by surrounding rock pressure. Existing research on fatigue performance of a railway tunnel has mainly focused on the influence of dynamic stress [1; 2; 3; 4]; the influence of surrounding rock pressure on the fatigue performance of the tunnel bottom structure needs further study, especially in shallow buried weak surrounding rock with high surrounding rock pressure. The Haoji railway in China is newly designed and the longest heavy haul railway in the world, with 235 tunnels located along the 1837 km railway. According to a survey, basal deterioration has appeared in many existing heavy haul railway tunnels, so analyzing the fatigue of the new line's basal structures that are experiencing higher axle load and freight volume is essential. In the investigation described in this paper, three-dimensional (3D) numerical simulation and three-point bending fatigue tests were combined to investigate the fatigue performance of tunnel bottom structures along the new line.