With the rapid development of the social economy, traditional double-hole four-lane highway tunnels and double-hole six-lane highway tunnels can no longer meet the needs of traffic growth and have even become bottlenecks that restrict social and economic development. Consequently, there has been a wave of building double-hole eight-lane highway tunnels in various regions [
1,
2,
3]. The excavation span of a single-hole four-lane highway tunnel is generally greater than 18 m, and the tunnel structure is characterized by flat section and large span [
4]. At present, the support structure of large-span tunnels adopts a composite lining structure of primary support and secondary lining, among which the secondary lining mostly uses formwork reinforced concrete [
5]. Under the influence of periodic temperature changes, the secondary lining concrete of the tunnel will undergo thermal expansion (contraction) deformation, and the primary support and waterproof materials in contact with the secondary lining will constrain the deformation of the secondary lining, which will lead to temperature stress inside the secondary lining [
6]. Concrete, as a type of cementitious material, has a tensile strength much lower than its compressive strength. Therefore, when subjected to tensile stress caused by a decrease in temperature, concrete is prone to cracking. Once cracks appear in the secondary lining of the tunnel, it will not only affect the stress of the lining structure, but also become a channel for water leakage [
7]. Especially for single-hole four-lane highway tunnels, the section size and concrete volume of the secondary lining are larger than those of single-hole two-lane and three-lane tunnels. Under the action of temperature stress, cracks are more likely to occur in the secondary lining of a super large-span tunnel with four lanes in a single hole, and the harm of cracks is also more serious.
Due to the late start of single-hole four-lane highway tunnels, research in the field of tunnel engineering on large-span highway tunnels mainly focuses on single-hole three-lane highway tunnels and double-track railway tunnels [
8,
9,
10], while there are limited studies on single-hole four-lane super large-span highway tunnels. Previous research on large-span highway tunnels has mainly focused on construction methods [
11,
12], support parameters [
13,
14,
15], and stability control of surrounding rock [
16,
17,
18] during construction. Nevertheless, there has been less attention paid to the stress properties of the secondary lining of large-span highway tunnels, and related research has mainly focused on load effects. Xu et al. [
10] conducted model experiments and numerical simulations and found that due to the anisotropy of the rock mass and geo-stress field, the distribution of internal force and deformation of the secondary lining is uneven, and the axial forces and bending moments in the areas where the tangent line of the tunnel contour is parallel or perpendicular to the weak surface are more significant. Li et al. [
19] conducted theoretical calculations and on-site monitoring and revealed that the measured internal force of the secondary lining was less than the values calculated by the load-structure model. The smaller the elastic resistance coefficient of the surrounding rock, the greater the bending moment on each section of the secondary lining, and the smaller the axial force and sectional safety factor. Fang et al. [
20] conducted a load model test on the secondary lining of a large-span tunnel and found that under the combined action of surrounding rock pressure and external water pressure, the axial force of the secondary lining shows a conical distribution, and the axial force at the arch springs is greater than that at the invert and arch; the bending moment is distributed in a butterfly shape, with the arch springs bearing a positive bending moment and the inverted arch and tunnel vault bearing a negative bending moment; the arch springs has the maximum eccentricity, which is the most unfavorable position for the stress on the secondary lining of the tunnel. Wu et al. [
21] studied the stress properties of the secondary lining of a three-lane highway tunnel through model experiments, and the results indicated that the internal force of the secondary lining increased slowly and then rapidly with the change of surrounding rock pressure, while the eccentricity gradually decreased. Moreover, the internal force at the arch springs, sidewall, and inverted arch was significant and reached failure first. Xu et al. [
7] studied the deformation, internal force, and cracking properties of secondary lining under load−temperature coupling through physical model experiments, and the experimental results showed that the ultimate bearing capacity of the secondary lining decreased by 4% under repeated temperature cycling. The research on temperature stress in the field of tunnel engineering mainly focuses on tunnels in cold regions [
22]. Ling et al. [
23] and Zhang et al. [
24] studied the mechanical response of the lining of tunnels in cold regions under frost heave through theoretical analysis and numerical simulation. Li et al. [
25,
26] analyzed and discussed the mechanical behavior of the interaction between composite lining and surrounding rock in cold region tunnels under isotropic and anisotropic frost heave conditions and proposed the interaction behavior equation. Xu et al. [
27] obtained the progressive degradation law of tunnel lining in cold regions through model experiments and numerical simulations, and the degradation of lining will promote crack development. Sutoh et al. [
28,
29] investigated the damage distribution characteristics and deterioration model of tunnel lining in cold regions to provide a basis for tunnel maintenance. However, previous studies have indicated that in non-cold regions, even in areas with distinct four seasons, the stress of the secondary lining of tunnels is affected by temperature stress [
30]. Furthermore, previous research on the secondary lining of large-span highway tunnels has limited attention to temperature stress, especially in terms of the stress properties of the secondary lining of a single-hole four-lane super large-span highway tunnel under the coupling effect of load and temperature stress. As a result, the mechanical behavior and properties of the secondary lining under the coupling of load and temperature stress are not yet clear. Therefore, it is necessary to investigate the mechanical behavior and properties of the secondary lining of super large-span tunnels under the coupling effects of load and temperature stress.
The issue of the influence of temperature stress on general span tunnels is considered unimportant and often overlooked, but this influence cannot be ignored in super large-span tunnels. This paper presents a case study of the Letuan Tunnel, a super large-span highway tunnel with four lanes in a single hole, of the Binzhou-Laiwu expressway reconstruction and expansion project and focuses on the long-term mechanical response of secondary lining under load and temperature stress. Through detailed on-site stress monitoring, the variations of the concrete and steel bars of secondary lining with temperature were analyzed. Based on this, the effects of load and temperature on the service performance of the secondary lining were evaluated through numerical simulation, and finally the service performance of the secondary lining of super large-span tunnels under the combined effects of load and temperature was revealed. The present study could provide an important reference and basis for the research on the long-term stability and in-service durability of lining structures in super large-span tunnels.