LAUSR

Pre-excavation grouting (PEG) was developed in early 1980s to reduce water inflow in hard rock tunnelling. Over the years, positive experiences have been made with this method, particularly during the construction of subsea tunnels and tunnelling in urban environments which are sensitive to groundwater drawdown. The technical procedure of PEG has also been improved in order to satisfy the increasingly strict technical and environmental requirements. A common criterion for PEG application is based on the observational approach. The water inflow from probe holes might be used to predict the potential water inflow into the tunnel for the execution of grouting based on the predefined measures in terms of grouting pressure and grout take. A prerequisite for PEG application is the availability of a calculation method to obtain a quick estimation of potential water inflow into tunnels based on-site data. To meet this prerequisite, several relevant problems have been involved, such as the determinations of the limit of residual inflow, the geological and hydrogeological conditions, the grout mix and the grouting pressure, etc. (Stille 2012; Stille 2015; Shi et al. 2016; Shang 2020; Fan et al. 2020). The limit of residual inflow, Qlim, specifies the project requirements of groundwater control during tunneling as shown in Table 1. Qlim is one of the measures to assess the likely groundwater inflow that may occur during tunnelling. For tunneling in an environment which are sensitive to the groundwater drawdown, systematic grouting is usually adopted. If the tunnel is in a rural area without specific requirements on the inflow rate, a PEG trigger value might be adopted for the determination of the execution of grouting. The PEG trigger value, QA, is usually defined using the observation of water inflow from the probe hole. Generally, a lower PEG trigger value indicates a lower residual water inflow into tunnel, and the PEG trigger values vary with rock conditions and the limit of residual inflow. By empirical estimation, the range of upper limits of targeted residual inflow for different projects and situations can be in the range from 5 to 50 L/min per 100 m, as shown in Table 2. The ratio of Qlim/QA is about 12.5–15, except the case in Qingdao subsea tunnel. The empirical relationships shown in Tables 1 and 2 are case-specific and may not be applicable to tunnels under other geological and hydrogeological conditions. This study aims to establish the general relationships among the water inflow rates observed or estimated from the probe holes, the tunnel and the grouted tunnel, based on the analytical solutions derived by Park et al. (2008) and Li et al. (2018).