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 Dewatering tests results for underground C Line stations construction Massimo Grisolia & Giuseppe Iorio Department of Civil, Architectural and Environmental Engineering, University of Rome “Sapienza”, Italy Antonio Zechini RomaMetropolitane S.r.l., Rome, Italy ABSTRACT: The new C Line of Rome Underground -T4, T5 and T6A lots- mainly runs into volcanic deposits deriving from the Colli Albani apparatus, widely extended in south-eastern areas of Rome. The local hydrogeological framework is very complex due to the large variations of permeability in function of granulometry, cementation processes and secondary fracturing of pyroclastic strata. The bottom of the excavation of the underground stations lies 25/30 meters below the groundwater level. In order to allow dry conditions and to prevent bottom heave during excavation, the groundwater level is lowered by deep wells systems. Some dewatering field tests showed how the local stratigraphy strongly affects the dewatering efficiency. Steady state filtration FEM analyses allowed to compare permeability of soil with Lefranc in situ tests results and to verify the efficiency of grouted plugs. 1 INTRODUCTION stiff overconsolidated clay of Pliocene age (APL unit) covered by a layer of fluvio-palustrine very The new C Line of Rome Underground runs NW dense sandy gravels of Pleistocene age (SG) fol- to SE across the city centre, with a total length of lowed by either medium stiff clayey silts and dense more than 25 km and 30 stations. At present, the sandy silts (Paleotevere units AR, ST) (Figs. 1 T4, T5 and T6 A lots, about 10 km twin running and 2). tunnels and eleven stations, are in advanced work- These deposits are covered by a Middle to Upper ing state of progress. Pleistocene piroclastics volcanic soils deriving from Due to the presence of a densely built urban the Colli Albani apparatus, widely extended into environment, the selected solution for the construc- the southeast of Rome. tion of the stations consists of a “cut and cover” The pyroclastic sequence, linked to the past excavation retained by multi-propped diaphragm explosive volcanic activities, is characterized by old walls to minimise settlements and prevent damage altered tuffs (lithoid and pseudo-lithoid tuff T1-T2 to the structures. and clayely tuff TA), “pozzolane” (Pozzolane The excavation depth of the stations are about Rosse PR and Pozzolane Nere PN) and lithoid 25 to 30 m, with a maximum of 20–25 m below tuff (Tufo Lionato). Frequent and relatively deep the water table. In order to assure dry conditions ancient ditches filled by alluvial silty-clayey and and hydraulic heave stability, deep wells systems sandy deposits (Alluvioni dei fossi unit, LSO) cut have been designed to lower groundwater level and into the pyroclastic deposits. piezometric head during excavation. A layer of made ground (R) of varying thick- Dewatering systems have been preliminary ness covers the stratigraphic sequence and the nat- tested by long term pumping field tests. ural soil profile everywhere. The paper shows the result of numerical analy- The local hydrogeological framework is very ses carried out to interpret dewatering field tests complex and characterized by a double groundwa- made for three different stations of T5 lot, namely ter system. The upper main aquifer is mainly repre- Teano, Mirti and Gardenie. sented by the “pozzolane sequences” (PR/PN). The lower aquifer consists of the Pleistocene deposits (SG) which overlain Pliocene marine claystone 2 HYDRO GEOLOGICAL AND bedrock (APL). The pyroclastic deposits (poz- GEOTECHNICAL CONDITIONS zolane, lithoid and pseudo-lithoid tuff and clayely tuff) show large variations in permeability in func- The geological sequence along the new C Line tion of primary porosity, compaction processes, under construction consists of a base deposit of sealing, and secondary fracturing. Permeability 619 Figure 1. Geological profile from S. Giovanni to Malatesta stations (adapted from Metro C, Progetto costruttivo). Figure 2. Geological profile from Teano to Alessandrino stations (adapted from Metro C, Progetto costruttivo). Middle/Upper Pleistocene VS - Silty sand, sandy silt VS Pleistocene TL - Lithoid tuff ST ST - Silty sand, sandy silt TL Holocene R - Man-made ground TT TT - Pyroclastic Silty sand, sandy AR AR -Clayey silt and silty clay R silt SG SG - Sandy gravel LSO - Alluvial clayey silty and PN/PR PN / PR - Pyroclastic Sandy silt LSO sandy silt deposits black or red Old althered tuff TA Pliocene T2 TA - clayey tuff TA T1/T2 - lithoid and pseudo-lithoid APL - Silty clay T1 tuff APL T3 T3 - Lithoid tuff Lefranc Tests Lefranc Tests Permeability (m/s) Test number -9 -8 -7 -6 -5 -4 -3 -2 10 10 10 10 10 10 10 10 0 20406080100 R R LSO LSO PN-PR PN-PR TT TT TA TA T1-T2 T1-T2 ST ST AR AR SG SG APL APL Figure 3. Lefranc permeability tests results. 620 values for each soils type have been determined by level was 14 m above the bottom of the excavation. Lefranc tests at variable head (Fig. 3) and by some Dewatering system was constituted by n. 12, preliminary pumping tests. Lefranc tests indicated 400 mm diameter wells penetrated 26 m into the a wide range of permeability, with values from volcanics deposits (“TA” and “T1-T2”) with slot- 7.5 ⋅ 10−9 m/s up to 3.6 ⋅ 10−4 m/s for pozzolane PR/ ted screen extending from the water table surface PN, 4.9 ⋅ 10−8 m/s to 1.0 ⋅ 10−4 m/s for the lithoid and (assumed +23.5 m O.D.) to the bottom (Figure 4a). pseudo-lithoid tuff (T1-T2), and 1.3 ⋅ 10−8 m/s to The total pumping rate was approximately 17 l/s 1.7 ⋅ 10−5 m/s for the Pleistocene deposits ST. and the established pore water pressure distribu- tion inside the ground allowed dry conditions dur- ing excavation. Side effects on external water table 3 GEOTECHNICAL AND were very limited (maximum lowering of 0.20 m). ENVIRONMENTAL CONDITIONS 4.1.2 Mirti station The construction of C Line stations requires deep The station consists of a rectangular box retained excavations under high hydraulic head. The main by diaphragm walls 65 m long and 37 m wide. geotechnical and environmental problems related The maximum excavation depth is about 34 m to the water seepage are connected with: from the ground surface. In this case the required drawdown of the water table in the volcanic soils 1. lowering the groundwater level in order to allow is about 18 m. The dewatering system consists of the excavation in dry conditions; 14 400 mm diameter wells placed inside the station 2. stability of the bottom line during excavation; box, pumping from the volcanics complex “TA” 3. seepage effect on the stability of diaphragm and “T1-T2”, (Fig. 4b). During long term tests the walls; achieved drawdown was enough to allow dry con- 4. ground settlements induced by water table ditions during excavation. The total pumping rate, lowering. approximately 70 1/s, was much more than design prevision, and a water pressure distribution with depth, different from an hydrostatic trend (Fig. 4b) 3.1 Dewatering field tests was observed. The lowering of external water table Dewatering field tests were systematically carried in the volcanic deposits was quite limited (0.7 m). out, consisting of: 4.1.3 Gardenie station − step drawdown and constant rate discharge tests The station, retained by diaphragm walls, is about (24h) carried out in the first installed well; 114 m long and 28 m wide. The maximum exca- − long term (8–21 day) dewatering test by pump- vation depth is about 27 m from the ground level. ing all the wells installed into each station. Dewatering system was constituted by 14 400 mm Monitoring activities included the measure- diameter wells penetrated 34 m into the volcanic ments of daily pumping rate and the hydraulic deposits “TA” and “T1-T2” (Fig. 4c). During long head for each well. Measurements were taken term tests the water table drawdown was only of inside the stations by electrical piezometers and about 2.0–6.0 m at a total pumping rate of approx- outside by Casagrande piezometers installed along imately 70 1/s. The external groundwater level low- the perimeter of excavation. The instruments lay- ering was about of 1.0 m. out is schematically shown in Figure 4. 5 SEEPAGE FLOW ANALYSIS 4 DEWATERING FIELD TEST RESULTS Although the geological pattern of the three sites The results of the dewatering field tests, in terms were similar, the performances of the dewatering of total pumping rate, drawdown achieved and tests were very different. pore pressure distribution inside the excavation In the case of Teano station, water flow mainly areas, are presented for three deep stations (Teano, derived from the deeper aquifer located in ST Mirti and Gardenie. (Fig. 5). The upper volcanic deposits (PR) and fluvio-palustrine (ST) were not acting as a single 4.1.1 Teano station hydrogeological unit since the piezometric level The excavation consists of an elongated box about recorded in ST stratum was around 12–14 m lower 140 m long and from 14 to 28 m wide which accom- than the level measured into the upper volcanic modates the station and the “Manufatto Diramazi- deposits.