Procedia Engineering

Volume 143, 2016, Pages 1503–1510

Advances in Transportation Geotechnics 3 . The 3rd International Conference on Transportation Geotechnics (ICTG 2016)

San Pasquale Station of Line 6 in Naples: Measurements and Numerical Analyses

Gianpiero Russo1, Marco Valerio Nicotera1, and Silvia Autuori1

1Università di Napoli Federico II, Napoli, Italy [email protected], [email protected], [email protected]

Abstract The paper reports some geotechnical aspects of the design and construction of the San Pasquale Station, intermediate along Line 6 and located close to the sea and at the same time within one of the most fascinating district of the city known as Chiaia borough. The station required an excavation deeper than 25 m, almost entirely located in pyroclastic sand below the groundwater table. The main shaft is 85.5 m long and 24.1 m large, containing also the whole length of the pedestrian platform while a single large section tunnel, built before the excavation of the station shaft, accommodates the two operating rail tracks. In the paper monitoring data will be presented and discussed. Settlements and horizontal displacements represent certainly very significant outcomes among the observed data. Their variations along with time and main construction steps are first of all presented in the paper. The monitoring data have also been submitted to a process of careful interpretation based on the use of numerical analyses to better understand the interaction of deep excavation with urban area. The FEM code Plaxis has been adopted for such a purpose adopting advanced constitutive soil models which are available in the code library.

Keywords: Deep excavation, Monitoring, Numerical analyses, Sandy soil, Groundwater control

1 Introduction Napoli is one of the largest city of Italy. The density of population is nearly 2,000 inhabitants per square kilometre, the highest in Italy and among the highest in Europe. At the end of 90s Napoli has become notorious for its horrendous traffic problems with increasing air pollution, unacceptably long travel times in rush hours and negative effects on the public health and economy (Viggiani et al.; 2011).

Selection and peer-review under responsibility of the Scientific Programme Committee of ICTG 2016 1503 c The Authors. Published by Elsevier B.V. doi: 10.1016/j.proeng.2016.06.177 San Pasquale Station of Line 6 in Naples: Measurements and Numerical Analyses Russo et al.

In 1997 the Municipality of Napoli approved a new City Transportation Plan, that has led to a significant pressure for the construction of new underground train lines, stations and car parks. Metropolitana di Napoli, or Napoli Underground, is the metro system serving the city, including at present six underground rapid transit railway lines, a commuter rail network and four funicular lines, with planned upgrading and expansion works underway. Among the six already operating lines some are experiencing a substantial development with new stretches under construction. One out of these is the Line 6. When completed according to the current design state Line 6 (Figure 1) will connect the Western borough of Bagnoli to the city centre at Municipio station, with a total length of 8 km and 12 stations. The full line can be broken down into 3 stretches: • the stretch between Mostra and Mergellina stations, partially connecting the borough of Fuorigrotta to the city centre, is already operating; • the stretch between Mergellina and Municipio stations is presently being constructed; • the stretch between Mostra and Porta del Parco stations is under design. In the following, attention will be focused on the design and the construction problems of San Pasquale station of Line 6.

2 San Pasquale Station The station is intermediate along the new stretch of the line under construction and is located in a crowded area which is a sort of fashion district (Figure 1). The main body of San Pasquale station has a rectangular shape in plan of 85.50 m × 24.10 m and the maximum excavation depth is approximately 27 m (26 m underground water table) (Figure 2). The longer side of the station is parallel to the longitudinal tunnel axis and the closest buildings are all located on the north side keeping approximately a unique alignment which is again parallel to the longer side of the station. The main shaft contains the passenger platforms and eliminates the necessity of excavating platform tunnels underground. The excavation is supported by T-section diaphragm walls made by reinforced concrete and built using huge hydromill equipped with a 90° rotating head. Each panel of the diaphragm walls were built by intersecting two separate rectangular excavations. The total depth of each panel is about 50 m in order to obtain a substantial embedment in the Neapolitan Yellow Tuff formation (NYT). The first stretch of the panel was excavated under the protection of Cutter Soil Mixing (L’Amante et al, 2012). A fully top-down construction process was chosen for the station in order to avoid the complexity of drilling ground anchors at large depth below groundwater table. The diaphragms have been executed first, leaving soft eyes with fiberglass reinforcement bars to be drilled by TBM. The passage of the TBM was the second main step while the excavation of the station was executed as the final step and required the demolition of the tunnel lining within the station area. Geotechnical investigations were carried out at the design stage in the area occupied by the station which is approximately 2000 m2. The site is inserted in an urban area bounded by historical buildings on the north side and by the sea with the interposition of a public garden on the South side. The area is relatively flat with the ground level located between +2 and +2.30 m a.s.l.. The groundwater table lays at +1.30 m a.s.l.. Geotechnical tests were programmed and executed both in situ and in lab. Boreholes with continuous coring and Standard Penetration Tests were initially carried out as usual. Samples to be submitted to laboratory tests were retrieved. Further site investigations consisting in CPTs, Dilatometers test and Cross Hole tests were subsequently carried out. The plan view of site investigations is shown In Figure 3 and the boreholes with the main information retrieved are summarized in Table 1.

1504 San Pasquale Station of Line 6 in Naples: Measurements and Numerical Analyses Russo et al.

Figure 4 shows the geological soil profile along the longitudinal cross-section located in the middle of the station. The profile includes an upper layer of marine sand (layer A) underlying the hand made soil up to a depth of 17 m from the ground surface; this layer rests on a layer of pyroclastic products consisting of silty sands, or ashes and pumices up to a depth of 41 m (layer B). A thin layer of yellow tuff pozzolana (layer YTP) separates the layer B from the formation of the NYT (Autuori et al. 2013). In Figure 5, the values of the cone resistance qc of CPT are plotted versus depth. The trend of the results is typical of cohesionless soils, including the relatively high scatter. Taking into account the in situ effective stress, the soils near the surface can be classified as very dense; the density slightly decreases with increasing deptth. In Figure 6 are reported shear wave velocity profiles determined by both SDMT and CH. The agreement is quite satisfactory, even though the values from SDMT are on average slightly larger than those obtained by the CH. This could be related to the different local effect of the two in situ tests: in fact, with SDMT, the soil is locally displaced (and compressed) by the dilatometer, while in the CH, the soil expands after drilling the holes. The main differences between the two tests are concentrated at a depth of about 7 and 11 m, where it seems that the dilatometer has detected two very dense or slightly cemented layers that the cross-hole has not intercepted. The shear wave velocity increases in the sand from 150 m/s near the ground surface to 500 m/s at a depth of 20 m; in the pyyroclastic soils below the sand, it keeps constant with depth at around 350 m/s and increases at the bottom of the deposit to reach almost 1000 m/s at the top of the underlying tuff.

Figure 1: Line 6 of Napoli Underground.

Figure 2: Plan view and transverse vertical section of the San Pasquale Station.

1505 San Pasquale Station of Line 6 in Naples: Measurements and Numerical Analyses Russo et al.

Figure 3: Plan view of site investigations. Figure 4: Stratigraphy of San Pasquale site.

I.D. Groundwater level Depth Sample SPT CPT Cross-Hole SDMT m. a.s.l. m number number B87 3,4 25 1 5 S196 2,3 39,5 2 4 S197 2,3 39,5 5 S198 2,3 45 1 5 S199 2,3 45 1 5 S227 31 1 8 P3 2,6 35 6 x P4 2,6 35 6 x P5 2,6 43,5 x S1 2,23 44,5 8 S2 2,06 45 9 x SG1 2,27 45 9 SG2 2,27 49 9 SG3 2,27 52,5 9 SG4 2,29 43 8 SG5 2,03 41 8 SG6 2,37 45 SG7 2,29 41,5 8 SG8 2,01 50 9 x SASP1 2,3 x x SASP2 2,4 x x SASP3 2,2 SASP4 2,5 SASP5 2,4 SASP6 2,3 Table 1: Boreholes and information retrieved.

1506 San Pasquale Station of Line 6 in Naples: Measurements and Numerical Analyses Russo et al.

Figure 5: Cone resistance qc Figure 6: Shear wave velocity Vs obtained from 5 CPT. obtained in CH tests and SDMT.

3 Monitoring As already mentioned in the introduction, San Pasquale station is very close to some historical and valuable buildings belonging to the downtown. It is excavated in granular soils sitting above a rather homogenous and not altered NYT layer with a rather superficial grouundwater table governed by the nearby sea level. Of course the design first and the construction later had to cope with sevveral difficulties arising by the above mentioned features. The method of construction and the stages of execution were revised several times in order to minimize such effects. In the end it was preferred the excavation with top-down technique using the intermediate slabs as temporary supports and the association of deep dewatering wells to depress the ground water table inside the main shaft (Russo et al., 2012). It is clear thhat the problem is a very complex one with the settlement induced by subsidence increasing the already critical settlement induced by the deformations of the diaphragm walls. Table 2 summarizes the main construction’s steps. The plan view of the monitoring system is reported in Figure 7. Figure 8 shows the evolution along the time of the excavation depth and of the groundwater level. The subsidence for each step of the excavation as measured on the ground bbenchmarks along the buildings and on the buildings themselves are shown in Figures 9 and 10 respectively. Both sequence of data show that the maximum settlement occurs approximately in correspondence of the middle of the shaft. Figures 11 and 12 show the horizontal displacement measured via the inclinometers inside the panels P13 and P74 located by the two opposite sides of the shaft. Each Figure is related to the steps summarized in Table 2.

4 Numerical Anallysis The analysis and interpretation of the problem is highly complex due to the three-dimensional geometry, the soil heterogeneity and the presence of the groundwater lowered with intensive pumping

1507 San Pasquale Station of Line 6 in Naples: Measurements and Numerical Analyses Russo et al.

during the main excavation. The main purpose of this study is to obtain the best interpretative model that allows to capture the effects induced by the excavation inside and outside the station.

STEP DATA WORKS Depth of excavation (m asl) Dewatering (m asl) other

1 9/15/08 Diaphram wall 2 8/27/09 Archeological excavation -4 3 5/10/10 Dewatering test (-22) 4 7/11/10 Main excavation -7 -10 TBM and cover slab 5 3/23/11 Main excavation -9 -12 6 12/24/11 Main excavation -12 -25 7 1/21/12 Main excavation -15 -25 8 9/12/12 Main excavation -25 -29 9 9/29/12 Sea side excavation -9 -29 10 12/27/12 -25 Bottom slab 11 10/23/13 to 7/24/14 Building side excavation -13 End of pumping 23/10/2013 Table 2: Main construction’s steps

4 2 0 -2 -4 -6 -8 -10 -12 -14 -16 -18 -20 -22

Excavation level (m asl) -24 Groundwater level -26 Ecxavation level of main shaft -28 Excavation level of sea side shaft Water levelinside the mainshaft asl) (m -30 Excavation level of builing side shaft -32 Maximum water level inside the main shaft Minimum water level inside the main shaft -34 Fase 1 2 3 4 56 7 8 9 10 11 Figure 7: Plan view of monitoring system. -36 11 12 12 12 13 13 13 13 14 14 14 14 08 08 09 09 09 09 09 09 10 10 10 10 10 10 11 11 11 11 11 12 12 12 13 13 July July July July July July Mar Mar Mar Mar Mar Mar May May May May May May Juan Juan Juan Juan Juan Juan Nov Nov Nov Nov Nov Nov Sept Sept Sept Sept Sept Sept Figure 8: Excavation depth and groundwater.

via via civ. 202 civ. 1Bausan civ. 207 civ. 215 civ. 1 civ. 202 civ. 1Bausan civ. 207 civ. 215 4 4

2 STL 20 STL 19 STL 18 STL 17 STL 16 STL 15 STL 14 STL 11 STL 10 STL 9 STL 8 STL 7 STL 6 STL 5 STL 4 STL 3 STL 2 STL 1 CS 15 CS 14 CS 13 CS 12 CS 11 CS 08 CS 07 CS 06 CS 05 CS 04 CS 03 CS 02 CS 01 CS 16 CS 2 0 0 -2 -2 -4 -4 -6 -6 -8 -8 -10 -10 -12 -12 -14 -14 Step 1 Step 7 Step 1 Step 7 -16 -16 Step 2 Step 8 Step 2 Step 8 -18 -18 Step 3 Step 9 Step 3 Step 9 -20 -20 Step 4 Step 10 Ground benchmanrk displacement (mm) Step 4 Step 10 Building benchmanrk displacement (mm) Step 5 Step 11 -22 Step 5 Step 11 -22 Step 6 -24 Step 6 -24 -26 MAIN SHAFT -26 MAIN SHAFT -28 -28 4840526488 121620 24283236 444856 60 6872768084 92 96100 104 108 112 116 120 124 4820405264 1216242832364448606872768056 84 88 92 96100 104 108 112 116 120 124 128 132 136 140 144 148 152 156 160 164 -30 -30 0 6,520,2 26,8 37,9 49,7 56,6 67,3 78,6 85,3 91,5 102,5 112,4 120,2 13,2 19,7 33,4 40 51,1 62,9 69,8 80,5 91,8 98,6104,75115,75 125,65133,45 144,75 163,15 146,45 Distance (m) Distance (m)

Figure 9: Ground benchmarks data. Figure 10: Building benchmarks data. A FEM model was implemented referring to the transversal cross section of the station shaft. Figure 13 shows the cross sections adopted for the 2D parametric analyses. The geometry of the full box used for the fem calculation is such (250 m x 90 m) to minimize the edge effects. The subsoil layered model consists of 4 layers: the layers labelled A, B, C and D correspond to those already

1508 San Pasquale Station of Line 6 in Naples: Measurements and Numerical Analyses Russo et al. identified in the simplified stratigraphy of Figure 4. As regards to the mechanical behavior of the soils, the Hardening Soil constitutive model was attributed to the upper layers of the loose soils and rigidity parameters were estimated on the base of the CPT shown in Figure 5. In particular the secant stiffness. (E50) was assumed to be 3 times the value of the average cone resistance qc within the layer. The unnloading/reloading stiffness Eur was assumed twice the module E50, that is equal to 6 times the average value of qc. For the underlying layers the linear elastic perfectly plasttic Mohr-Coulomb model was used (Russo et al., 2015a, b). In Table 3 the main soil properties are reported. The calculation consists of 24 steps in order to reproduce as realistically as possible the true construction sequence. In this paper, for obvious reasons of conciseness, only the results obtained at the end of the dewatering test (step 3) and at the closure of the dewatering and the subsequent sealing of the mmain shaft (step 11) are described. In particular only the horizontal displacement measured and calculated in panels P13 (sea side) and P74 (building side) are represented in Figures 14 and 15 respectively. The comparison of the results with the monitoring data confirms that the deformation behavior of the diaphragm walls is greatly influenced by the depth of the bedrock which acts as a rigid constraint to the base. The agreement is rather satisfactory and of course it is at least for engineering purposes. The difference between maximum calculated and measured horizontal settlement is about 20%. The maximum value of the calculated settlement close to the excavation shaft is 22mm at the end of step 11. This value compared with the 24 mm recorded by the optical survey on the ground benchmarks shows a better satisfactory agreement. In terms of difference the percentage of the disagreement is 10%.

Figure 11: Inclinometer P13. Figure 12: Inclinometer P74

Figure 13: Zoom view of the Mesh . SOIL Ȗ C ij’ LAYER k[m/s] n E ref[MPa] E [MPa] E [MPa] E[MPa] MODEL [kN/m3] [kPa] [°] ur 50 oed ur A HS 18 0 36 6.9x10-7 0,3 39 39 78 - B HS 16 0 36 1.46x10- 0,3 40 40 80 - 7 C MC 16 200 27 4.15x10- 0,3 - - - 2400 5 T MC 17 580 27 3x10-5 0.3 - - - 6700 Table 3: Soil parameters adopted.

1509 San Pasquale Station of Line 6 in Naples: Measurements and Numerical Analyses Russo et al.

Figure 14: Horizontal displacement calculated and Figure 15: Horizontal displacement calculated and measured respectively by inclinometer P13 and P74 at measured respectively by inclinometer P13 and P74 at the end of dewatering test for the step 3. the ennd of the excavation and pumping for the step11.

5 Conclusion Huge excavations are going on in the city of Napoli to build a new line of the underground network. Some of these excavations present among the other geotechnical difficulties in area which are closely surrounded by important buildings. The numerical analysis of the excavation of the San Pasquale Station described captures satisfactorily the behavior of the excavation process for all phases of execution. The calculation provides the results in terms of subsidence of the ground surface and horizontal movements of the diaphragm walls slightly smaller than those measured. . The difference between maximum calculated and measured horizontal settlement is about 20%. The maximum value of the calculated settlement close to the excavation shaft is 22mm at the end of step 11. This value compared with the 24 mm recorded by the optical survey on the ground benchmarks shows a better satisfactory agreement. In terms of difference the percentage of the disagreement is 10%.

References Autuori S., Russo G., Nicotera M. V. (2013), Studio preliminare degli effetti indotti dallo scavo della stazione San Pasquale, Incontro Annuale Ricerca-tori di Geotecnica, Perugia, ISBN 9788890642135. L’Amante D., Flora A., Russo G., Viggiani C. (2012). Displacements induced by the installation of dia-phragm panels. Acta Geotechnica, 7:203-218. Mormone L., Falconio G., Mandolini A. (2013). Groundwater management during excavation of S. Pasquale station in Naples. Geotechnical En-gineering for the preservation of monuments and historic sites, ISBN 978-138-00055-1. Russo G., Corbo A., Cavuoto F., Autuori S. (2015). Artificial Ground Freezing to excavate a tunnel in sandy soil. Measurements and back analysis. Tunnelling and Underground Space Technology 50, pp.226-238. Russo G., Autuori S.., Cavuoto F., Corbo A., Manassero V. (2015). Excavations in the Neapolitan subsoil: The experience of the Toledo station service tunnel. Proceedings of the International Workshop on Volcanic Rocks and Soils, pp 193-194. Russo G., Viggiani C., Viggiani G.M.B. (2012). Geotechnical design and construction iussues for Lines 1 and 6 of the Naples Underground. Geo-mechanics and Tunnelling 5, No. 3. Berlin.

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