INTERNATIONAL SOCIETY FOR SOIL MECHANICS AND GEOTECHNICAL ENGINEERING This paper was downloaded from the Online Library of the International Society for Soil Mechanics and Geotechnical Engineering (ISSMGE). The library is available here: https://www.issmge.org/publications/online-library This is an open-access database that archives thousands of papers published under the Auspices of the ISSMGE and maintained by the Innovation and Development Committee of ISSMGE. Geotechnical Aspects of Underground Construction in Soft Ground – Viggiani (ed) © 2012 Taylor & Francis Group, London, ISBN 978-0-415-68367-8 3D numerical modelling and settlement monitoring during excavation of the Metro-Torino South extension G. Barla, M. Barla, & G. Leuzzi Department of Structural and Geotechnical Engineering, Politecnico di Torino, Italy ABSTRACT: The Metro-Torino South extension is under construction. The excavation of a 3.9 km long tunnel was recently completed by a 7.8 m diameter EPB Tunnel Boring Machine. A critical section of the line is represented by the underpass of Corso Spezia, in the Lingotto area, where the main difficulties are associated with the high water table and the poor ground conditions encountered. Concern was also raised due to the influence of the excavation on two adjacent masonry buildings of 1930. This required an intensive monitoring system to be designed and installed. Monitoring took place continuously during excavation and allowed one to infer a time dependent function which describes the settlement history sat- isfactorily. A 3D finite element model was also created to simulate the excavation process and its effects on the adjacent buildings. This model was validated through a back analysis procedure in order to assess the soil parameters and simulate the tunnel excavation in stages. This paper will describe the case study and the 3D numerical modelling work by comparing the results obtained with the monitoring data. 1 INTRODUCTION 2 METRO TORINO LINE 1 Tunnelling in Torino took place extensively in the Construction of the completely automatic under- last decades involving a number of interferences ground Metro Torino Line 1 represented a dramatic with the built environment that needed high engi- innovation of the city transportation system. The neering expertise to be dealt with. This paper will system started to play its role in the early 2006 just focus on one of these interferences occurred dur- a week prior to the Winter Olympic Games that ing the construction of the South extension of the took place in the city. Metro Torino Line 1. The total length of the line in service is about In the Corso Spezia area, the excavation of the 9.6 km and includes 15 stations connecting the tunnel is adjacent to two sensible buildings and city centre with the western suburbs. A new South below an underground car pass. The tunnel axis extension of the line is now under construction is located at about 15–20 m depth while the water between Porta Nuova and Lingotto (Figure 1). table is 12 m below the surface. A poor quality Construction works started in 2006 and are soil layer characterises the stratigraphic sequence expected to be completed by 2011. Works include in this area and ground improvement techniques a 3.9 km long tunnel and 6 stations. An extension were used in the attempt to limit settlements. of the line (1.9 km and 2 stations) towards Piazza With the great number of monitoring data Bengasi is expected to be constructed starting with recorded during and after excavation it was pos- 2011. sible to describe successfully the time dependent The main tunnel was excavated at an average displacements occurred at the site. Given the com- depth of 15–20 m using three EPB Tunnel Boring plexity of the case study, it is of interest to create Machines, 7.8 m in diameter and 80 m in length. a three dimensional model and back analyse the The tunnel lining is formed with precast con- problem, although by using a simple constitutive crete segments (7 sections a day were completed law. for an average progress rate of 10 m/d). Another This paper will describe the results obtained. EPB machine has just completed the excavation In situ monitoring data will be compared with the of the new extension between Porta Nuova and results of modelling. Lingotto. 911 Figure 1. Plan view and geological longitudinal cross section of the Porta Nuova–Lingotto extension. 3 SUBSOIL CONDITIONS 4 CASE STUDY Torino metropolitan area, located at the west- Immediately out of the Spezia station, the tunnel ern edge of the Po valley, has an overall surface passes close to two 1930 sensible 5-storey masonry of about 130 km2, 80% of which are a level area buildings (named 973 and 974) and below an enclosed by the rivers Stura di Lanzo, Po and underpass (Figures 1 and 2). Ground improvement Sangone, while the remaining 20% are made up by measures were therefore commissioned as defen- a hilly area connected to the low reliefs of Mon- sive means for the buildings. ferrato. The city area is situated on the end sec- A comprehensive monitoring plan was set up tion of the great alluvium fan of the Dora Riparia in order to monitor the building response during river and, from the morphological point of view, the grouting process and tunnelling. Surface set- appears to be almost flat with a weak dip starting tlements were monitored using a number of incli- from West and going towards East, with elevation nometers, extensometers, as well as traditional and ranging from 260–270 m a.s.l. to about 220 m a.s.l. state of the art monitoring techniques. Monitor- (Bottino and Civita, 1986). ing included a computerised total station with The subsoil conditions in Torino are character- optical targets installed on the surrounding build- ized by sand and gravel deposits, with different ings in order to measure the induced settlements cementation degrees increasing with depth. Lenses continuously. of cemented soil or conglomerate are often present, Detailed site investigations including borehole down to a depth of 8–10 metres. The ground water drilling with core recovery (Figure 2a) and the level along the tunnel axis is well known based on retrieval of undisturbed samples of the cohesive piezometric measurements and predictions on long layers allowed one to obtain a stratigraphic section term conditions have also been carried out. as shown in Figure 2b. In addition to the usual With the available data from geotechnical inves- stratigraphic sequence of the Torino subsoil, with tigations and numerical analysis performed in the the GU1, GU2, and GU3 units, the presence of a past, a geotechnical model for the subsoil condi- silt layer (GU2a) containing a lens of even weaker tions has been derived. For the depth relevant to properties (GU2a mod) was found. tunnelling, four soil layers (Geothecnical Units, This unit (GU2a), based on the results of labo- GU) were defined (Barla & Vai 1999, Barla & ratory tests, exhibits a time dependent behaviour. Barla 2005, Barla et al. 2010): An additional feature to be underlined in the sec- − GU1: superficial layer (filling and clayey sandy tion is the presence of an ancient river bed along silt); the Corso Spezia underpass alignment, filled with − GU2: sand and gravel from loose to slightly poor quality landfill of anthropic origin. The cemented; ground water table is well known and located − GU3: sand and gravel from slightly to medium about 12 m below the ground surface. cemented; During excavation, the EPB-machine suddenly − GU4: sand and gravel from medium to heavily lost face pressure and mud when driven through the cemented. underpass. Cracking and collapse of a number of waterproof panels in the underground pass did take The stratigraphic sequence is variable along the place, in conjunction with floor heave just above the line. Figure 1 shows the longitudinal geological pro- tunnel. As a consequence car traffic was stopped for file of the South extension, of interest in this paper. some time to allow for repair work to take place. 912 Spezia 973 974 120 underpass ] 2 100 80 60 40 20 Specific energy [MJ/m 0 0 5000 10000 15000 20000 25000 Advancing step Figure 3. Specific energy versus chainage monitored by the TBM during face advancement along the Toino Metro Line 1 Extension from Porta Nuva Station to Lingotto. F NT E =+2 ××π (1) s A uA where: − F is the thrust force applied on the cutting wheel; − N is the number of revolutions per second; − T is the torque; − A is the tunnel cross sectional area; − u is the rate of penetration. It is shown that starting with advancing step 1000 (i.e. at chainage 1400) the specific energy drops dramatically to reach an average value of 20 MJ/m2 under the 973 and 974 buildings. As the underpass is being reached the specific energy drops again to its minimum of 10 MJ/m2 under it. Therefore, as anticipated, the poor ground condi- tions below buildings 973 and 974 and the under- pass itself is well underlined. Based on conventional topographic surveys of target points on the ground surface (1 target point Figure 2. Plan view (a), geological profile (b) and pho- above the tunnel crown and 2 additional ones on each tograph (c) of the area where the Metro Tunnel reaches side of the tunnel axis, to reach a total of 5 monitor- the Spezia underpass. ing points in every monitoring section) the settle- ment through above the tunnel could be obtained. The monitoring data were interpreted by using the well known settlement equation (Peck, 1969): 5 ANALYSIS OF SETTLEMENT MONITORING 22 ssvvs ,max exp(−yi/ i ) (2) TBM monitoring took place systematically dur- ing excavation in order to measure the machine where sv = settlement, sv,max = maximum settlement parameters such as mud pressure at the tunnel on the tunnel centre-line, y = horizontal distance face, rotational speed, penetration, advancement from the tunnel centre-line and the i horizontal rate, thrust, torque, etc.