Search

Browse

• Communities
• Authors
• Year
• Titles
• Subjects

My Space

• Login

Stability and subsidence of underground openings constructed with the NATM method in urban areas

Saratsis Georgios

Full record

---

Summary

A major challenge of European cities is to control urban sprawl, which encourages car dependency and has a major impact on urban GHG production. The new model of sustainable city is a compact city growing within its current limits and irrigated by a dense public transport networks. Considering the current lack of surface space in European city centres, future trends call for more underground infrastructure to be built in a crowded urban context under severe environmental constraints: zero impact on surface structures.Tunnelling is quite a difficult activity to plan and to manage, with several recent collapses and incidents demonstrating its potential for sudden and catastrophic events. In normal conditions, settlements at the surface are the main source of impact. Minor damages on surrounding structures are frequent, and can reveal costly in many ways: cost of repairs, additional delays, loss of public image. Sudden incidents can occur from unexpected ground or groundwater conditions, or from inappropriate construction management. They remain exceptional, but can have far-reaching consequences: fatal events, buildings damaged beyond repair, project interruptions, unpredictable economic losses, and loss of confidence from the public. Ultimately, they make underground works difficult to insure and delicate to include into urban planning schemes.This thesis refers to the back – analysis of the conditions that have lead to the collapse of the shaft-cavern system in the densely populated Sao Paulo city of Brazil constructed with the Conventional Tunnelling Method (CTM) or New Austrian Tunnelling Method (NATM). More specifically the main aim is to propose a methodology for the prompt identification of weak points of underground constructions by the NATM method in urban areas. Τhis methodology consists of material modeling and data analysis, numerical modeling, simulation of the underground excavation process and monitoring guidelines.This system lies close to the Pinheiros river, in the SW sector of the city, and is a part of the new Line 4 (Yellow Line) of the expanding São Paulo Metro. The Yellow line is 12.5 km long, linking the city center with the western suburbs by four interchange stations.On the afternoon of Friday 12th January 2007, a dramatic accident occurred during the enlargement of the Eastern cavern (serving as a future metro station) with a bench advancing towards the shaft. Nearly the whole of the east cavern of the station of 40 m length suddenly collapsed, immediately followed by collapse of nearly half of the adjacent 40 m diameter and 35 m deep station shaft. This multiple accident occurred so fast that there was no time for warning to be given. The seven unfortunate victims died after falling from the surface and becoming deeply buried under the collapsed rock and soil.According to extensive investigations after the occurrence of this incident it was concluded that the collapse happened due to poor characterization of the geomaterials, of the design and of the in situ monitoring system during construction. Poor design has led to the decision of open support system during the excavation of the top heading and the bench, that in turn have led to extensive deformations until final collapse. Previews back – analysis attempts were based on the assumption of undetected large rock wedge (key – block at the roof) that overloaded the support arch and led to the failure of the foundation rock mass and ultimately to the final collapse. In this study a different path is followed. Assuming a continuous rock mass with equivalent mechanical properties and a kinematic cohesion – friction Mohr – Coulomb softening model and using a 3D numerical code which can simulate shear band propagation, it was found that according to the applied cavern construction procedure a massive collapse may be generated that exhibits similar features with those found in the field. These features refer to the time of the incident, the newly formed shear band normal to the longitudinal axis of the cavern far from the shaft, the amount of deformation recorded by the multiple – borehole extensometers and the mode of failure of the open arched support system.It was found that the failure is manifested with the evolution of shear bands starting from the corners of the top-heading of the cavern as this approaches the shaft, while retreating. The failure surface from the cavern extends upwards and forwards to connect with the shear band propagating from the shaft forming a rigid block of rock mass of weight of 20,000 t that finally caves in the tunnel and shaft; hence, the system behaves as a “trap-door mechanism.The elastic and strength properties of the intact rock identified in lab were upscaled by using a special theory suited for discontinuous rocks based on the Damage Mechanics theory. For this rock mass model the two decreasing curves of cohesion and friction angle with the accumulation of plastic shear strain in the rock mass have been prescribed as an input into the FLAC3D code. Hence, the plastic shear strain is chosen as the softening (aging) parameter of the rock adjacent to the shaft – cavern construction as it is loaded during excavation. Based on the strength of materials database StremaDB we have processed and finally specified the constitutive behaviour of the overlying rock material. Hence, we use this relational database together with the accompanying tools (like program for identification of material parameters etc) as an indispensable tool for such type of analyses.History points have been specified in the model to store vertical displacements exactly on the actual locations of the vertical extensometers Since these extensometers failed to give an early warning of the incipient collapse of the cavern’s roof and based on the collapse mechanism manifested by the propagation of shear bands, there have been put additional history points in the model along radial lines passing through the anticipated path of the shear bands. These history points have been placed at the wall of the shaft and around the cavern at a short distance from the junction with the shaft which is a region of high stress concentration. One radial extensometer and its history point located inside the caved volume of the rock masshas captured promptly the incipient collapse phenomenon. At this particular history point inside the caved rock mass the vertical displacement has suddenly increased 30% between two sub-sequent steps. Also, the model are in accordance with recorded displacements before and during collapse at the Eastern station.The axial force (N) – bending moment (M) interaction relations for the case of Factors of Safety (FS) equal to 1 and 2, respectively, were also assumed. The model’s predictions of the axial forces and bending moments acting on the composite shell support elements at the phase of the incipient collapse have been uperimposed with these diagrams demonstrating the failure of the temporary support at the sides and the crown of the cavern.

Available Files

Services