10.2478/sggw-2018-0012 Annals of Warsaw University of Life Sciences – SGGW Land Reclamation No 50 (2), 2018: 139–157 (Ann. Warsaw Univ. of Life Sci. – SGGW, Land Reclam. 50 (2), 2018) Long-term settlement induced by EPB tunnelling studied with numerical simulations MACIEJ OCHMAŃSKI Faculty of Civil Engineering, Silesian University of Technology, Gliwice, Poland Abstract: Long-term settlement induced by EPB 2002, Mair 2008, Wu et al. 2011, Ng tunnelling studied with numerical simulations. et al. 2013). This aspect of tunnelling Long-term settlement induced by tunnelling in becomes particularly important when soft cohesive subsoil is rarely considered as a sub- considering that long-term settlement, ject for research despite substantial evidence that it increases considerably after construction. The pa- which is generally caused by consolida- per aims to investigate the time effects of tunnel- tion, may constitute 30–90% of total ling with an earth pressure balance (EPB) shield settlement (Shirlaw 1995). There are two by means of analysing subsoil deformation with factors leading to tunnelling-induced the most up-to-date numerical tool. A three-dimen- consolidation, with the first one related to sional numerical model with a detailed description the technological processes significantly of the geometrical and mechanical characteristics disturbing subsoil around the cavity. of the tunnelling process and the various materials involved was built. The computational model vali- Mechanized tunnelling in cohesive soils dated on a real-case was then used to simulate tun- is mainly carried out using earth pressure nelling in different groundwater conditions prior balance (EPB) shields that excavate soil to construction. That was followed by a simulation with a rotary cutterhead and are pushed of groundwater drawdown conditions after tunnel- forward against previously installed seg- ling with impermeable and permeable tunnel lin- mental lining by a set of hydraulic jacks. ings. The calculations show the development of During this process, a grout backfilling is the settlement over time in different groundwater conditions together with the significant influence injected at the tail of the shield to fill the of the permeability of the tunnel. circumferential void between the lining extrados and cavity boundary. Immedi- Key words: numerical simulations, EPB shield, ately after injection, grout behaves as a long-term settlement liquid and with the progress of hydration turns into a relatively strong and stiff ma- INTRODUCTION terial preventing further deformation of the cavity. Subsoil consolidation caused There is substantial evidence that the by the imbalance between stress state in settlement continues to increase up to the soil and the pressure of the grout is several years after the construction of limited to relatively small portions of tunnels in soft cohesive subsoil (O’Reilly soil around the cavity and the intensity et al. 1991, Bowers et al. 1996, Harris of the induced effect is determined by 140 M. Ochmański the pressure difference between these more significant effects may be induced two materials. The second factor lead- by a groundwater drawdown after tunnel- ing to the consolidation settlement is ling, this effect is manifested not only by that tunnels act as drains introducing additional long-term settlement but also new boundary conditions at the cavity by the rotation of the lining segments contour. This results in a negative excess and the change in the structural forces in pore water pressure induced in the sur- that elements. rounding soil, thus an increase in the The analyses of the tunnelling proc- effective stress in the soil leads to the ess mainly refer to the almost immedi- consolidation process (Ward and Thomas ate short-term effects and are mainly 1965, Palmer and Belshaw 1980, Harris carried out using empirical approaches 2002). Additionally, the existence of the (Peck 1969) derived from a wide data- thin grouting layer around the segmental base of evidence. On the other hand, lining has a rather limited effect on its analytical closed-form solutions are not water-tightness, especially over the frequently used in view of their rather long-term (Mair 2008). Water flows into limited capabilities which are reduced to the tunnel through the grouting layer and elasticity and rarely plasticity with ques- the joints between the lining segments. It tionable assumptions that ignore much is often observed that the internal surface of the modern knowledge concerning of the lining is visibly wet even though soil behaviour. The most sophisticated auxiliary measures are applied to make tools are numerical simulations that have it watertight. The final long-term set- recently gained popularity due to their tlement resulting from the combination impressive capabilities which allow for of both factors generally occurs over the introduction of complex geometry of a much larger area and the settlement the problem and include various contact trough profile is much deeper and wider behaviours between different elements with little curvature compared to the and sophisticated constitutive models short-term one (Mair and Taylor 1997). for natural and artificial materials. Fur- In densely urbanized cities, cyclic and thermore, the progressive development transient seasonal effects are drastically of contemporary IT technology allows, limited by the presence of buildings especially thanks to parallel comput- and concrete pavements, roads which ing, coupled problems to be solved with efficiently reduce the migration of water precisely described geometry discretized from rainfall and prevent the evaporation with hundreds of thousands of finite of moisture from the subsurface. There- elements. Moreover, according to the fore, the level of the groundwater table provisions contained in the current codes is influenced by the fluctuation of river of practice (e.g. in the European Union levels, leakage from water mains, deep- CEN, 2004), geotechnical design should -well pumping and the presence of nearby always consider the effects induced by tunnels. The change in the groundwater the changes in the groundwater table level prior to tunnelling may also lead as it leads to time-related effects, one to significant changes in the ground set- of the consequence is increased earth tlement as noted by Mair (2008). Even pressure acting on the lining (ASCE Long-term settlement induced by EPB tunnelling studied... 141 1984). Despite that, most of the analy- description of the computational model, ses of the tunnelling process even using a validation based on a real case study of sophisticated numerical tools still refer the Metropolitan Rapid Transit Authority to the immediate response and neglect to (MRTA) project in Bangkok (Suwansa- analyse the long-term effects related to wat 2002, Surarak 2010, Sirivachiraporn consolidation which are assumed to be and Phienwej 2012, Likitlersuang et al. rather negligible. 2013) is presented. Finally, the long- The aim of the paper is to analyse the -term effects related to the consolidation effects related to long-term consolida- of soil induced by the tunnelling process tion as a result of shield tunnelling and are analysed together with the effects of the groundwater table drawdown. The the groundwater drawdown by means of paper demonstrates the capabilities of the ground surface settlement. Analyses the most up-to-date and realistic repro- are performed introducing impermeable duction of the tunnelling process with and fully-permeable tunnel linings. EPB technology by means of numerical simulations. An automated numerical COMPUTATIONAL MODEL model introduced with a script prepared in python objected-oriented language The computational model is introduced for commercial AbaqusTM FEM code in the commercial AbaqusTM (Hibbitt developed by Ochmański et al. (2018) is and Sorensen 2001) FEM code as a used here to allow for fast solutions with three-dimensional symmetrical problem a strong theoretical basis. After a brief as depicted in Figure 1. Simulations are FIGURE 1. A three-dimensional symmetrical FEM model of tunnel drilled with the EPB shield 142 M. Ochmański fully automatized and controlled by a is calculated assuming hydraulic heads at script written in the Python program- the boundaries of the model and known ming language. The model introduced value of the permeability coefficient of presents an attempt to describe in as pre- each layer. The nonlinear, irreversible cise as possible all of the components of stress-displacement response of the sub- the EPB technology which allows for the soil in the presented model is described elimination of many subjective assump- by means of the hypoplastic constitutive tions which are typical of the simulation models (Gudehus et al. 2008) freely of this process, such as the fictitious available as a FORTRAN implementa- boundary conditions of prescribed tion at the website of Soilmodels project displacements at the cavity boundary to (www.soilmodels.com). Specifically, the reproduce volume loss. For the sake of von Wolffersdorff (1996) hypoplastic brevity, only a brief description of the model was used for the coarse-grained computational model characteristics is materials and the Mašín (2005) model given here, in order to learn about the for the fine-grained materials. To cali- fine details the reader is referred to the brate the former one, 13 parameters need work of Ochmański et al. (2018). to be assigned, while for the later one, 12 parameters are necessary. Further-