## 1 Introduction  
Recent developments in China's economy have remarkably expanded the construction scale of railways and highway networks in the western region of the country. However, several engineering disasters were also caused by extremely complex geological conditions and continuously deepening excavating tunnels. The main problems are the large deformation of the surrounding rock caused by the high ground stress and the weak rock layers. To these, creep compression deformation during operation can also be added. These factors have been discussed as seriously affecting the safety of tunnel construction and operation. This is while, under a high-stress and weak rock environment, the traditional'strong support and hard roof' method fails to be applied as a support. As a consequence, numerous theoretical and practical studies have been conducted internally and internationally to tackle the problem. For example, [PERSON] et al. [1] developed an improved creep model that considers the effects of stress state and time on the creep parameters. [PERSON] et al. [2], based on the kinetic energy theorem, Perzyna viscoplastic theory and the Nishihara model, established a unified creep constitutive model that can describe the whole process of decaying creep, stable creep and accelerated creep. Like [PERSON] et al. [3], in this paper, triaxial creep tests of rock salt under multi-stage temperatures were carried out to study the influence of temperature on the creep behaviors of rock salt. Based on viscoelastic mechanics, [PERSON] et al. [4] proposed an analytical solution for tunnel support systems. In another study, performance indicators were suggested by [PERSON] et al. [5] to be utilized for high-stress soft-rock tunnel lining. A multiple regression function superposition method was applied by [PERSON] et al. [6] to analyze the in situ stress and support of surrounding tunnel rocks under complex geological conditions. [PERSON] et al. [7] and [PERSON] et al. [8] analyzed the deformation mechanism and control methods of high ground stress and weak surrounding rock. The deformation mechanism of high ground stress soft rock tunnels was also studied by [PERSON] et al. [9] and [PERSON] et al. [10]. Accordingly, they proposed using compressible materials to absorb the deformation energy of the surrounding rock. So, as a result, a reduction in the effect of high ground stress on the secondary lining structure was discussed, which could ultimately prevent the large deformation caused by tunnel compression. However, while the above studies mainly resolved the problem of controlling large deformations during construction, no systematic theory or method has been set forth for controlling creep deformation caused by the high ground stress, i.e., the stress that is produced during and after completing tunnel lining construction. In this regard, numerous research studies have utilized foam concrete as the buffer layer of the supporting structure. For example, [PERSON] et al. [11] investigated the parameters influencing the cush-ioning performance of EPSF foam. In another study, [PERSON] et al. [12] examined the surface activity and the characteristics of foam concrete. The dynamic mechanical properties of porous concrete were also studied by [PERSON] et al. [13]. In addition, an innovative lightweight concrete and the dynamic impact characteristics of lightweight aggregate concrete were investigated by [PERSON] et al. [14] and [PERSON] et al. [15], respectively. Also, using foamed polystyrene beads as the coarse aggregate, [PERSON] et al. [16] proposed the possibility of producing self-compacting lightweight concrete. [PERSON] et al. [17] also studied the influence of mixture composition on the performance of foam materials. Nevertheless, the above research works all adopted theoretical or experimental approaches to the composition and performance of lightweight concrete foam.