# Bearing Capacity of a Concrete Grouting Pad on the Working Surface of a Highway Tunnel Shaft
## 5 Design Method and Engineering Application of Grouting Pad Thickness
### Discussion  
This study investigates two problems of the bearing capacity and thickness calculation of grouting pads for shafts. We have derived a unified solution for the plastic ultimate bearing capacity of grouting pads based on the analysis of the punching shear failure mode of grouting pads, considering the current lack of calculation methods for the bearing capacity of large-diameter shaft grouting pads in highway tunnels. Secondly, we propose a new plastic limit design method for grouting pad thickness in response to the problem of weak bearing capacity failure caused by previous elastic design methods for coal mine shaft grouting pads. The research results were applied to the 2-2 shaft project of the Tianshan Shengli Tunnel, which solved the problem of frequent cracking caused by the failure of the grouting pad during construction. The research findings were implemented in the Tianshan Shengli Tunnel's 2-2 shaft project, resolving the issue of frequent cracking due to grouting pad failure during construction. Compared with existing research [32; 33], this study analyses the punching shear failure mode of the grouting pad by simplifying its mechanical model as a simply supported circular plate. The grouting pressure and water pressure are equivalent to the edge load of the circular plate. The plastic structural limit analysis method is used to obtain a unified solution for the bearing capacity, which is compared with existing research. After examining the factors that influence the bearing capacity of the grouting pad, we propose a reasonable understanding that can aid in grouting construction. We clarify the bearing capacity of the grouting pad and establish a plastic limit design method for the thickness of large-diameter vertical shaft grouting pads. This method improves the defect of weak bearing capacity in the traditional elastic design method applied to the thickness design of large-diameter shaft grouting pads [29]. There are two reasons for the deviation caused by the two methods of elastic and plastic limit design. Firstly, the failure of concrete materials generally undergoes an \"elastic plastic\" process when failing [34]. Elastic failure occurs before plastic failure, and the required ultimate failure load is small [35]. The bearing capacity is directly proportional to the thickness \(B_{0}\), according to Equation (14), so the design thickness is relatively small. Secondly, the elastic design method is based on experience and is intended for coal mine shafts with a diameter of less than 7 m. However, for highway tunnel shafts with a diameter of 7.5-10.5 m, the \"diameter thickness ratio\" of the grouting pad is relatively large when using the elastic design method. This poses a thin plate problem in mechanics, as thin plates are more prone to damage than thick plates. However, the use of plastic limit analysis methods is only suitable for simple problems. The second method assumes that the material is rigid plastic and studies the behaviour of the structure when it reaches the plastic limit state based on the plastic defor  
Figure 17: The effect after the construction of the grouting pad. mation law, which is also the method used in this article to calculate the capacity. Based on this study, there are two limitations:  
1. When analysing the plastic ultimate bearing capacity of the grouting pad in the paper, it is assumed that the material is rigid plastic and elastic deformation is not considered, so that the internal stress-strain distribution before the plastic ultimate state of the grouting pad cannot be obtained. 2. When establishing the calculation model in this paper, it is assumed that the grouting pad corresponds to an axisymmetric circular plate, the bottom of the shaft working face is flat, and no settlement occurs before the grouting pad is poured.