###### Abstract
The numerical method is based on the empirical, analytical method to predict the water inflow, simulate the distribution of seepage field under the action of tunnel drainage, and then calculate the external water pressure of tunnel lining by the reduction coefficient method. The numerical method is based on the reduction coefficient method, and there are also problems, such as strong experience. It is also a common method to calculate the external water pressure of lining by theoretical formula. [PERSON] et al. [16] used Flac 3D to establish a numerical model to simulate the influence of grouting ring thickness and permeability coefficient, lining permeability coefficient and control tunnel displacement on lining water load and compared it with the analytical solution of lining water pressure. However, the theoretical formula for calculating the external water pressure is derived under ideal conditions. Most of them are for homogeneous surrounding rock, and the calculation results deviate greatly from the actual values. For a long time, China's tunnel engineering has basically followed the principle of 'combination of drainage and plugging, mainly drainage' in dealing with the external water pressure of the tunnel. However, a large amount of long-term drainage will cause the water layer to be drained, resulting in a decline in the regional groundwater level, resulting in settlement and deformation of the surface at the top of the cave, and destroying the ecological balance [17]. Therefore, when constructing tunnels in high-head areas, 'blocking water and limiting drainage' is undoubtedly a reasonable waterproof and drainage method. [PERSON] et al. [18] used a simplified model to investigate the variation of the external water pressure of the lining with the displacement and the thickness of the grouting ring under the condition of 'water plugging and limited drainage'. [PERSON] et al. [19] studied the variation law of lining water pressure under different drainage types and obtained the variation law of lining external water pressure with grouting thickness and permeability coefficient of grouting ring. [PERSON] [20] obtained the relationship between the external water pressure of the lining and the displacement of the drainage rate under different grouting ring parameters through theoretical calculation and analysis so that the external water pressure of the lining can be significantly reduced or even eliminated with a smaller displacement. [PERSON] et al. [21] used the indoor test method to study the distribution law of external water pressure underwater plugging and drainage restriction with the drainage segment as the research object. [PERSON] et al. [22] used the the 2020 version of ABAQUS numerical simulation method to study the variation law of tunnel seepage field and lining external water pressure under different drainage hole schemes. [PERSON] [23]used three-dimensional quasi-continuous medium seepage theory to study the distribution of water head fields in rock strata with uniform and non-uniform lithology under different drainage schemes by numerical simulation. [PERSON] et al. [PERSON] et al. (2018), combined with the actual project based on the three-dimensional finite element model, studied the variation of the infiltration surface of the tunnel surrounding rock and the distribution of the osmotic pressure on the lining under different schemes such as consolidation grouting and drainage holes. [PERSON] et al. [PERSON] et al. (2018) studied the reasonable external water pressure value and limited discharge standard of water-rich tunnel lining structure through fluid-solid coupling numerical analysis. In summary, the current research focuses on the variation law of tunnel external water pressure under different drainage schemes, grouting thickness, grouting ring parameters and other factors, mainly in the design and operation stage of the tunnel. However, dealing with the external water pressure is also one of the key problems to be solved in tunnel excavation.