Use of Grouting to Reduce Deformations of an Existing Tunnel Underpassed by Another Tunnel

The Greater Cairo metre and El-Azhar road tunnels, the major project of underground structures in Cairo city, Egypt, have been constructed. During the construction of the Greater Metro Line 2 (Shubra El-kheima- Mubarak) and El-Azhar road tunnels, which are installed by the tunneling boring machine (TBM), Geotechnical challenges are expected to occur related to soil stability around the tunnels. One of these problems arises when parts of the metro and the road tunnels cross under an existing sewage tunnel during the construction. To control the potential problem, the National Authority for Tunnels (NAT) has applied grouting to the soil around the sewage, the metro and the road tunnels. In the present study, one of these problems is highlighted and a model is proposed to provide a prediction of the soil structure interaction using a 3-D model of the multi-crossing tunnel incorporating the effect of grouting. The cement-bentonite grouting is considered in this study. The objective of this study is to evaluate the usefulness of permeation grouting in reducing deformation around the sewage tunnel when the metro and the road tunnels pass underneath it at different crossing zones. To assess and understand effect of changing the soil stiffness by grouting on the behavior of the two tunnels, a parametric study has been performed. The study is conducted using a finite element method that models the three-dimensional behavior under both the effects of the tunnel crossings and effects of the grouting. A nonlinear stress-strain constitutive model is adopted to represent the soil surrounding the tunnel and a linear constitutive model is employed to represent the sewage tunnel liner. The effects are expressed in terms of the settlement and radial deformation of the sewage tunnel as the metro or the road tunnels pass underneath it for the cases with and without grouting. The 3-D model of predicting the behavior of the tunnel system under multi-crossing tunnels and grouting effect is described and the predictions are compared with the field measurements. The comparison reveals a good agreement between the computed and observed values.