## 1 Introduction  
Due to the influence and limitations of geographical location and surrounding environment, problems such as poor geological conditions, shallow burial depth, tight engineering land, complex surrounding environment, and strict control standards are common [1, 2]. To overcome these challenges, double-arch tunnels are widely used in urban tunnels because of their advantages of convenient portal selection, smooth route, land conservation, beautiful section shape, and relatively small impact on the surrounding environment [3, 4]. However, urban double-arch tunnels also have limitations, such as long construction period, high cost, various procedures, complex supporting structure, more intersection of excavation and supporting construction surfaces, and difficult quality control [5; 6; 7]. Additionally, with the construction of special underground projects involving environmental protection and surface cultural relics, an increasing number of ultra-shallow tunnels have been constructed. Hence, during the excavation and support process of an urban ultra-shallow buried double-arch tunnel with complex geological conditions, the stability of tunnel surrounding rock and the deformation control of surrounding underground pipelines have become issues that need special attention [8; 9]. According to modern tunnel theory, the surrounding rock is the main body to bear the unloading stress caused by excavation. In the process of tunnel construction, effective technical means are necessary to adopt to maximize the self-supporting capacity of the surrounding rock. Such means improve the stability of the surrounding rock and tunnel structure, which can not only improve the safety of a project during construction, but also improves the quality and service life of the tunnel structure during operation [10; 11]. To strengthen the bearing capacity of the rock mass and avoid possible risks in construction, various methods and measures are often adopted in the project, such as advance support, small pipe reinforcement, and grouting reinforcement to reinforce the surrounding rock [12; 13; 14; 15; 16; 17]. Compared with other treatment methods, grouting reinforcement technology has the advantages of high efficiency, convenience, low cost, wide treatment range, and good reinforcement effect, especially in rock and soil with high fragmentation degree [18; 19; 20]. The principle of grouting reinforcement technology is to use cement slurry, water glass, cement mortar, chemical grout, and other materials to fill the voids of broken rock masses effectively to improve the compactness, integrity, tension, compression, shear, and creep of the rock mass, so as to maintain the stability of rock mass [21]. The strength and stability of grouted surrounding rock formed after grouting reinforcement of broken rock mass is not only an important index to evaluate the effect of grouting reinforcement, but also a key parameter to determine the deformation and the self-supporting capacity of the surrounding rock. Extensive research has been carried out on grouting and reinforcement of surrounding rocks and remarkable results have been achieved. [PERSON] et al. [22; 23] found that the stiffness, roughness, peak shear strength, residual strength, and shear strength parameters of structural surface after grouting reinforcement were significantly improved. [PERSON] et al. [14] studied the grouting diffusion law, splitting grouting reinforcement mechanisms and grouting reinforcement effects using 3D simulated grouting test system. The uniaxial compressive strength of filled soil after grouting increased by 186%, the permeability coefficient decreased by 47 times, and the cohesion and internal friction angle increased by 45.3% and 44.9%, respectively. [PERSON] et al. [24] compared and analyzed the shear strength parameters of fractured rock before and after grouting consolidation, and considered that grouting significantly increased the shear strength of fractured rock. [PERSON] et al.