3D Analysis of Settlement Trough Induced by Tunnelling

Abstract

Over the past decades, the speedy development of huge cities has given rise to many demands aimed at achieving more use of underground space. Tunnels are needed to accommodate several utilities such as transportation, electricity, communication systems, water supply and sewerage. Currently, many mega tunnelling projects are undertaken for the construction and extension of underground network in big overpopulated cities all over the world. This type of tunnel projects is expected to become the most important in the near future due to the badly need for reducing air pollution and traffic jams. Cairo is considered now as one of the most congested cities in the world; it's population has increased in the recent decades from 3.5 millions in 1960 to more than 20 millions today. Cairo began its rapid population explosion in the 1960's and 1970's so; the need for a new mass transportation system was inevitable. The construction of the Greater Cairo Metro was suggested by the Studies carried out between 1970 and 1974 to be the major mass transportation system. In some cities, tunnels are located at shallow depth under densely populated zones through soils or soft rocksand high groundwater table level.In these conditions, pressurized shielded TBMs must be used in tunnel construction.Many engineering challenges are imposed by soft ground tunnellingsuch as stability of the constructed tunnels and theireffect on existing facilities and structures. Therefore, it is highly essential to evaluate in detail the expected ground settlement hazards before starting the tunnel construction. In order to make sufficient assessment of settlement hazard, many factors should be considered in the calculations. For instance, tunnel depth and diameter, construction methods and details, 3D effect of tunnel construction, initial state of stress and the stress-strain behaviour of the surrounding soil. 3D numerical modelling should be used to simulate the construction of the tunnels as tunnel construction is a three-dimensional process. Also, 3D modelling is important to ensure proper analysis of ground surface settlement, and stress-strain situation in the soil and the lining.During tunnel construction, large number of processes takes place such asthe details of segmental concrete lining, the applied face pressure, shield overcutting and the grout pressure. All these processescan be taken into consideration by performing a three-dimensional (3D) finite element model of a mechanized tunnelling (using TBMs). The tunnel constructionprocess can be divided in many construction stages; each stage represents 1.5 m advance through soil which is equal to a length of a tunnel lining ring.Whileconstructing lining segments,the tunnel boring machine (TBM) remainsstationary. After the full erectingof a lining segment, the TBM continues soil excavatingto construct the next lining ring.In each stage of these stages, the same steps are repeated over and over again. In order to model this in 3D, a geometry consisting of slices (each 1.5m long) can be used.In each stage, the input for thecalculation is identical, except for its location, which is shifted by 1.5 m for each stage. Thesame partsof the excavation process are modeled by each calculation stage which includes: a) The excavation of the soil and removal of pore water within the TBM. b) The slight cone shape of the TBM. c) The installation of the new tunnel lining segment. d) The grouting of the gap between the soil and the newly installedlining segment. e) The bentonite slurry face pressure.