Predicting the grouting effect on leakage-induced tunnels and ground response in saturated soils

Highlights

• A new set of analytical solutions for predicting the effect of tunnel leakage is proposed.

• A simplified model is proposed to quantitatively describe the effect of grouting on tunnel leakage.

• The tunnel leakage state can be easily evaluated using the simplified model.

• The effect of grouting on tunnel leakage is affected by tunnel lining permeability.

• The leakage induces significant settlements rather than tunnel convergence and internal forces.

Abstract

This paper suggests a new set of analytical solutions for predicting the effect of grouting on ground and shield tunnel behaviour as a consequence of steady water inflow into a tunnel in saturated clay. These solutions allow the effects of the tunnel lining and grouting material to be considered on the basis of their relative permeability to the surrounding soil. Moreover, the equivalent permeability of the tunnel lining can be derived based on the water inflow rate, which is commonly assumed to be proportional to the soil permeability in a typical analysis. A comparison of the results of analytical and numerical simulations indicates that the solutions are sufficiently accurate for tunnel applications. The effect of grouting material on the tunnel-leakage-induced seepage field, ground and tunnel settlements, internal forces and tunnel convergence of the shield tunnel are examined with the verified solutions. Based on the analytical results, the effect of grouting material on the tunnel leakage appears to depend on the permeability of the tunnel lining. Thus, a dimensionless parameter RP_slg integrating the permeability and geometry of the tunnel lining and grouting material is proposed to quantitatively describe the effect of the grouting material on tunnel leakage. Using RP_slg, the effect of the grouting material on tunnel leakage can be easily derived. Indeed, the analytical results show that tunnel leakage produces significant ground and tunnel settlement. The grouting can clearly reduce the effect of tunnel leakage on tunnel and ground settlement when the relative permeability of the grouting material and soil k_g/k_s is less than 0.01. Moreover, the increase in the tunnel-leakage-induced bending moment and the decrease in hoop thrust are generally small but this may increase the permeability of the tunnel joints. The leakage-induced tunnel convergence is small and is therefore unlikely to degrade the serviceability or safety of a tunnel.

Introduction

The construction of tunnels in soft soils has become increasingly common in recent decades. The lining of a shield-driven tunnel is assembled by bolting together prefabricated concrete segments. The joints are very likely to open and to shear between segments when subjected to thrust jacking forces in the process of shield tunnelling and permanent earth and pore pressure. Meanwhile, concrete cracks may be generated. Furthermore, the tunnel segments usually contain preformed holes for synchronous grouting during tunnelling. For simplicity, for the abovementioned opening and shearing of segmental joints, the cracks and grouting holes are referred to as tunnel defects in this study. When a tunnel is built in saturated soils, ground water may flow into the tunnel through these tunnel defects, as shown in Fig. 1 for a tunnel in Shanghai, where shield tunnels are embedded in saturated clays.

In low-permeable clay, tunnel leakage will result in a continuous decrease in pore pressure around the tunnel and thereby induce ground and tunnel settlement. This leakage-induced tunnel settlement will deteriorate tunnel defects and threaten the safety of tunnel operation. To control tunnel leakage in practice, grouting is widely used as a water barrier. Indeed, grouting is always used to backfill the physical annular gap between the ground and tunnel lining, caused by shield machine driving, to reduce tunnelling-induced ground settlement during tunnelling, as illustrated in Fig. 2 (Youn and Breitenbücher, 2014); thus, the annular gap grouting material is also considered the primary water barrier for segmental tunnels. However, how to predict the effect of annular gap grouting material on leakage-induced ground and tunnel response is still not well understood even though it has been widely adopted in tunnel engineering practice.

Monitoring long-term pore pressure is very helpful in capturing the effect of grouting material on tunnel leakage, and this is important for maintaining existing tunnels. However, field observations of tunnel-leakage-induced pore pressure changes are very limited because of the tremendous cost of long-term monitoring. To determine the effect of grouting material on tunnel-leakage-induced ground and tunnel response, the effect of grouting material on leakage-induced pore pressure must first be predicted. However, because of the complexity of tunnel leakage in saturated clay, grouting material has long been excluded from studies of pore pressure changes caused by tunnel leakage. Many researchers have studied tunnel-leakage-induced ground and tunnel response using numerical solutions without considering the effect of grouting material (O'Reilly et al., 1991, Shin et al., 2002, Wongsaroj, 2005, Zhang et al., 2005, Wongsaroj et al., 2007, Mair, 2008). Moreover, as an important supplement to numerical simulation, analytical solutions are also valuable for predicting tunnel-leakage-induced ground and tunnel response because they are simple to use and have a clear physical meaning (Lei, 1999, El Tani, 2003, Kolymbas and Wagner, 2007, Park et al., 2008, Huangfu et al., 2010, Zhang et al., 2012). Notably, these analytical solutions are very useful for predicting different tunnel leakage problems. The common assumption behind these solutions is that the effect of grouting material is negligible.

This paper presents a new set of analytical solutions for predicting the effect of grouting material on leakage-induced ground and tunnel response. The novel feature of these solutions is that the effects of tunnel lining and grouting material have been rigorously considered. This paper is organized as follows. First, an analytical solution for predicting the pore pressure around a tunnel considering a tunnel lining and grouting material is developed. An analytical solution for leakage-induced ground and tunnel settlements and tunnel behaviour in terms of hoop thrust, bending moment and convergence is then derived by considering the effect of grouting material. The analytical solution is validated using the numerical solution because of limited field observations. The effect of grouting material on ground and tunnel response is then clarified by a parametric study. Finally, a shield tunnel in Shanghai is examined as a case study.