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 Modelling and monitoring of a urban underground excavation A. Cantone Department of Civil Engineering, II University of Naples, Italy R. Fico Structural & Geotechnical Designer Collaborator, Naples, Italy F. Cavuoto Structural & Geotechnical Designer, Naples, Italy A. Mandolini Department of Civil Engineering, II University of Naples, Italy ABSTRACT: The highly urbanization of the area where Montesanto Station (Naples, Italy) is located and the need to guarantee the transportation service to travellers during the works gave birth to the plan of the combined use of 3D numerical analyses and the real time monitoring of significant parameters (displacements, strain, stresses and temperature) to confirm the set of design criteria assumed and calibrate the design parameters affecting the problem faced. The 3D analyses simulated the step by step excavation predicting the stress-strain behaviour; hence the comparison of the analytical predictions with the corresponding values derived through the monitoring (238 points being monitored) allowed to calibrate the model as the excavation advanced, thus refining the analysis itself and improving the safety level. 1 INTRODUCTION adopted, an intensive monitoring activity was planned under the supervision of the Civil The Montesanto Station of Cumana Railway Line, Engineering Department (DIC) of the Second owned by S.E.P.S.A. (“Limited Public Service University of Naples: a monitoring system based Company”), is located in the very historical hearth on the observation method was thus contrived; of Naples (Italy) and can be deemed a critical junction of the city transportation network, given the closeness both to the Montesanto Station of the cable railway line and to the Montesanto Station of Metro Line 2, a few meters far on the West side (see Fig. 1). Since 2004 expansion works are in progress at the Station in order to build, what is more, a pedestrian tunnel providing an alternative exit to travellers from the two lines currently in service (Cumana and Circumflegrea Lines). The highly urbanization of the area where the opera is located, the nature of soils crossed, and mostly the need to guarantee the transportation service to travellers during the works, gave birth to the plan of different measures to protect the existing tunnels and to soil improvements in the excavation area. The design of the technical interventions that could minimize the effects of the excavation on the existing tunnels was carried out by means of both 2D and 3D numerical analyses. In order to control the proper execution of the ongoing works and to verify the adequacy of the design solutions Figure 1. Location of Montesanto Station. 231 starting from the data collected through the been already executed to sustain the top tunnels monitoring (sidewalls displacements, stress state sidewalls, by means of 10 m long micro-piles; in the liners, etc), the numerical model selected for a 0.60 m deep parterre on piles has been also the design analyses was controlled or calibrated, constructed to sustain the rails. if necessary; in the latter case, a new verification The excavation of the descending tunnel started of the design solutions adopted would have been on Oct. 4, 2005 and was completed by Sept. 2006 achieved and more appropriate auxiliary solutions (see Fig. 3), whereas the big service tunnel was would have been set. completed by Dec. 2006. In the early 2007 the first connection portion with Metro Line 1 was built; the concrete casting of the as built tunnels was 2 DESCRIPTION OF THE OPERA & WORK carried out before the excavation of the pedestrian CHRONOLOGY tunnel. However, many works slowdowns occurred mainly due to the complexity of the specific yard The 40 m long, 7 m large pedestrian tunnel will be position. perpendicular to the two upper tunnels in service In April 2008 the construction of the pedestrian (see Fig. 2a); the distance between the upper rails tunnel below the two railways started. According and the design tunnel crown will be of just 3.5 m to the design, 7 advancement steps had to be (see Fig. 2b); the connection of the pedestrian performed: a first 6 m advancing step, five further tunnel with the station outside will be guaranteed 4.5 m steps, and a final 6 m step. At each step, by escalators, therefore a 30° sloped 8 m diameter before cutting and placing the pre-support liner, the tunnel has been already constructed (see Fig. 2c), soil improvements were accomplished, while the turning into a big service tunnel (11 m diameter) permanent reinforced concrete liner was planned when going down at the design tunnel depth. to be cast in place backwards after reaching the The intervention area soil, upper than the water dig bottom, in order to speed up the advancement, table, is rather heterogeneous: the descending given the reduced room available. tunnel housing the escalators and the big connection service tunnel lie within a region made of Neapolitan yellow tuff, whereas the tunnel to build will lie in pyroclastic sand (except for a short first portion). The excavation, carried out by employing the traditional technique (by cutting and temporary supporting with steel arches and shotcrete, before the permanent concrete casting) will be executed after the accomplishment of different soil improvements: a ring of sub- horizontal metallic pipes in the crown (7° sloped, 10 m long), 5.5 m overlapped with those installed in the previous step; the excavation ground will be improved by glass-reinforced plastic nailing (20 mm diameter, 12 m long); jet-grouting sub- horizontal columns will support the pre-support bases. Moreover, consolidation interventions have (a) (b) 2 E IN L O T (c) Figure 2. Pedestrian tunnel views: (a) plan; (b) section 1-1; (c) section 2-2. Figure 3. Tunnel excavation snapshots. 232 The last excavation step was completed at Dic 1, the simulation; in particular, the elastic modulus 2008. The activities were very complex, due to both of the pyroclastic sand Es was assessed; the geo- the reduced room available (Fig. 3a) and the need technical investigation lead to assume a value of to manually carry out the cutting of the sub-verti- Es ranging between 50 and 100 MPa; within this cal micro-piles consolidating the ground from the range the displacement derived with the analyses > sidewalls and the foundation slab of the upper tun- varied of about 5 mm, whereas for Es 100 MPa nels, where these were interfering with the excava- the displacement variation highly reduced. tion front (Fig. 3b). The results that were finally accounted for cor- = respond to Es 100 MPa, considered the most reli- able value according to the experimental campaign 3 ANALYSIS previously performed; the displacement of the four sidewalls relating to the Circumflegrea and the The numerical analyses were based on both two- Cumana Lines tunnels were thus derived, together dimensional and three-dimensional simulations, with the corresponding stresses; moreover, a fur- performed by means of Plaxis2D software and ther inspection was provided by the stress analysis FLAC3D software, respectively; given the com- of the temporary support of the design tunnel. plexity and the significance of the project, the The displacements of the most relevant points two-dimensional-based design assumptions were on the existing tunnels (corresponding to some confirmed and refined with the 3D analyses. In par- of the points monitored, see next section) were ticular, the FLAC3D is an explicit finite-difference derived for each of the 7 advancing steps; program for engineering mechanics computation Table 1 reports the vertical (w) and the horizontal that can simulate the behavior of three-dimensional (u) components of the estimated displacements structures built of soil, rock or other materials that relating to the four sidewalls of the two existing undergo plastic flow when their yield limits are tunnels caused by the excavation occurring beneath reached. The full dynamic equations of motion are (the values are ordered by following the excavation used, even when modelling systems are essentially advancement direction). static; this enables FLAC3D to follow physically unstable processes without numerical distress. The model assumed in the FLAC3D is consti- Table 1. Displacement of sidewalls relating to the exist- × × 3 tuted by a 72 40 50 m parallelepiped, having ing tunnels: Circumflegrea (Circ.) and Cumana (Cum.) 8 different solid groups, each with different and specific mechanical properties (see Fig. 4); besides Circ. Cum. Circ. Cum. the modelling of the different types of soil and the structural elements of the existing tunnels (tuff), w (mm) u (mm) the step by step soil improvements foregoing the tunnel excavation have been integrated as well. Step d (m) 12341234 All the mentioned groups are characterized by the 00 00000000 Mohr-Coulomb constitutive model. 1 6,0 17,6 12,9 13,0 14,9 4,2 5,4 3,9 6,3 Several parametric analyses were performed 2 10,5 21,4 13,0 13,1 15,0 4,7 5,4 3,9 6,4 in order to weigh the main variables affecting 3 15,0 21,6 13,6 13,5 15,0 4,8 5,7 3,6 6,6 4 19,5 21,6 17,2 15,4 15,3 4,6 6,1 2,7 6,8 5 24,0 21,7 19,5 19,6 15,6 4,8 5,3 2,7 6,8 6 28,5 21,7 19,9 20,5 15,6 5,0 5,0 3,1 6,8 7 34,5 21,8 20,0 20,5 18,8 5,0 5,0 3,0 7,5 Wall 1 R3 V1 PEDESTRIANR2 TUNNEL 10 11 R1 9 1 UNDER CONSTRUCTION 3 2 MONTESANTO 4 3 bis 5 6 T A STATION 7 NMEN 8 ALLIG E.L.