Dewatering Tests for Underground Stations in Rome
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SPECIAL DEWATERING AND FEATURE ARTICLE ISSUE: GROUNDWATER CONTROL Installed dewatering system at Teano Station Dewatering Tests for Underground Stations in Rome The new C Line of the Rome Metro runs Hydrogeological and Geotechnical cementation processes and secondary northwest to southeast across the city Conditions fracturing of pyroclastic strata. The new C center, with 30 new stations and a total Line runs mainly through volcanic deposits length of more than 15.5 mi (25 km). Due The local hydrogeological framework is very deriving from the Colli Albani apparatus, to the presence of a dense urban complex due to the large variations of per- prevalent in the southeastern areas of Rome. environment, the selected solution for the meability resulting from granulometry, construction of the stations consists of a cut- and-cover excavation, which is retained by multipropped diaphragm walls to minimize settlement and prevent damage to the nearby structures. The bottom of the excavation of the underground stations are about 82 ft to 98 ft (25 m to 30 m) below ground surface, and about 66 ft to 82 ft (20 m to 25 m) below the groundwater table. To ensure dry conditions and to prevent hydraulic base heave instability, deep well systems were designed and used to lower groundwater level and piezometric head during excavation. The dewatering field tests showed how the local stratigraphy strongly affects the dewatering efficiency. Finite element modeling (FEM) of steady state filtration Geological profile and were used to compare the soil permeability legend from Teano to Alessandrino stations with Lefranc in-situ test results and to verify (adapted from Metro C, the efficiency of the grouted plugs. Progetto costruttivo) Massimo Grisolia, Dept. of Civil, Architectural and Environmental Engineering, University of Rome (Sapienza), AUTHORS Giuseppe Iorio, AGiS Ingengeria S.r.l., and Antonio Zechini, Roma Metropolitane S.r.l. DEEP FOUNDATIONS • MAY/JUNE 2017 • 107 The geological sequence along the new Dewatering Field Tests (26 m) into the volcanic deposits (“TA” and for each well) indicated that the deeper C Line consists of a very heterogeneously “T1-T2”), and each was fitted with a slotted aquifer is fed by the sandy gravels of the Dewatering field tests were conducted layered deposit. The base deposit of stiff screen that extended from the groundwater Pleistocene deposits (SG), and the flow is systematically and consisted of the overconsolidated clay (“APL,” Pliocene age) table to the bottom of the well. The total semi-confined by the low permeable following general steps: is overlain by fluvio-palustrine very dense pumping rate was approximately 270 gpm altered pseudo-lithoid tuff (T2) and the sandy gravels (“SG,” Pleistocene age), 1. Perform step drawdown and constant (17 liter/s), which resulted in a lowering of clayey tuff (TA) layers. The hydraulic which are then overlain by medium stiff rate discharge tests (24 hr) in the first the groundwater table by less than about disconnection between the ST stratum and clayey silts or dense sandy silts (“AR” or installed well 8 in (0.2 m), maximum, on the outside of the overlying pozzolane (PR) caused no “ST,” Paleotevere units). These deposits are 2. Perform a long-term dewatering test (8 the excavation. The dewatering operation relevant effects on the shallow aquifer. overlain by covered by pyroclastic volcanic to 21 days) by pumping all of the wells allowed dry conditions during excavation. soils, tuff, silty sand and sandy silt (Middle installed in each station Mirti Station — The excavation for to Upper Pleistocene) deriving from the Mirti Station consisted of a rectangular box 3. Perform measurements inside each Colli Albani apparatus. A layer of made shape that was about 213 ft (65 m) in station using electrical piezometers and ground (“R”) of varying thickness covers length by about 121 ft (37 m) in width, and outside each station using Casagrande the stratigraphic sequence and the natural was excavated to a maximum depth of piezometers, which were installed soil profile everywhere. about 112 ft (34 m) from the ground along the perimeter of the excavation The local hydrogeological framework is surface. At this station, the required Dewatering field test results at Mirti Station very complex and characterized by a double groundwater system. The upper main aquifer is mainly represented by the “pozzolane sequences” (“PR/PN”), while the lower aquifer consists of a deposit of sandy gravel (“SG,” Pleistocene), which is underlain by marine claystone bedrock (“APL,” Pliocene). The pyroclastic deposits (e.g., pozzolane, lithoid and pseudo- lithoid tuff and clayely tuff) show large variations in permeability due to porosity, compaction processes, sealing and secondary fracturing. Permeability values for each soil type were determined using Lefranc tests at variable head and using pumping tests. The Lefranc tests indicated Dewatering field test results at Teano Station a wide range of permeability. Dewatering field test results at Gardenie Station Seepage flow estimated for Teano Station Permeability drawdown of the groundwater table in the Soil layer / type Low High volcanic soils was about 59 ft (18 m). The Gardenie Station — The excavation for diaphragm wall, the groundwater table was At Mirti and Gardenie Stations, the dewatering system consisted of 14 wells, -7 -2 Gardenie Station consisted of a rectangular drawn down only about 3.3 ft (1 m). hydraulic seepage scheme was very Pozzolane (PR/PN) 7.5x10 cm/s 3.6x10 cm/s each having a diameter of about 15.75 in box shape that was about 374 ft (114 m) in -6 -2 Similar to the other two stations, the different from the Teano configuration, and Lithoid and pseudo-lithoid tuff (T1-T2) 4.9x10 cm/s 1.0x10 cm/s (400 mm), and were installed within the length by about 92 ft (28 m) in width, and dewatering operation allowed dry con- far from the design forecasts. At Mirti -6 -3 Pleistocene deposits (ST) 1.3x10 cm/s 1.7x10 cm/s excavation. The wells penetrated into and was excavated to a maximum depth of ditions during excavation. Station, seepage flow is mainly horizontal pumped water from the volcanic deposits about 89 ft (27 m) from the ground surface. and derives from the aquifer located in the The main geotechnical and environ- Teano Station — The excavation for Teano (“TA” and “T1-T2”). The total pumping At this station, the required drawdown of Seepage Flow Analysis old altered tuffs (TA-T1), which are mental problems related to water seepage Station consisted of essentially an elon- rate was approximately 1,110 gpm (70 the groundwater table in the volcanic soils At Teano Station, water flow mainly characterized by a high permeability. The are summarized into four main items: gated box shape that was about 460 ft liter/s), which was significantly more than was about 59 ft (18 m). The dewatering derived from the deeper aquifer located in overlying low permeability pseudo-lithoid (140 m) in length by about 46 ft to 92 ft the design assumption and resulted in a 1. Lowering the groundwater level to allow system consisted of 14 wells, each having a the silty sand/sandy silt stratum (ST, fluvio- tuff (T2), in which the piezometric (14 m to 28 m) in width, and was excavated pore water pressure profile different from a excavation in dry conditions diameter of about 15.75 in (400 mm), and palustrine). The upper volcanic deposits pressure drop is concentrated, creates a to a maximum depth of about 95 ft (29 m) hydrostatic trend. The groundwater table were installed within the excavation. The (PR) and the ST stratum were not acting as hydraulic disconnection between the 2. Ensuring base stability during from the ground surface. The natural was lowered by about 2.3 ft (0.7 m), wells penetrated about 112 ft (34 m) into a single hydrogeological unit because the clayey tuff (TA) and the overlying excavation groundwater level was about 46 ft (14 m) maximum, on the outside of the excavation the volcanic deposits (“TA” and “T1-T2”). piezometric level recorded in the ST pozzolane (PR). At Gardenie Station, 3. Mitigating the seepage effect on the above the bottom of the excavation. The and was quite limited in drawdown away During the long-term pumping tests, the stratum was about 39 ft to 46 ft (12 m to 14 m) seepage flow derives directly from the stability of the diaphragm walls dewatering system consisted of 12 wells, from the diaphragm wall. The dewatering groundwater table was lowered between lower than the level measured into the shallow aquifer located in the volcanic each having a diameter of about 15.75 in operation allowed dry conditions during 6.6 ft to 19.7 ft (2 m to 6 m) using a total 4. Minimizing ground settlements upper volcanic (PR) deposits. The low deposits. Piezometric measurements in the (400 mm), and were installed within the excavation. pumping rate of about 1,110 gpm (70 induced by the lowering of the pumping rate (less than 16 gpm [1.0 liter/s] upper level of the old altered tuffs (TA) excavation. The wells penetrated about 85 ft liter/s). External to the excavation and groundwater table 108 • DEEP FOUNDATIONS • MAY/JUNE 2017 DEEP FOUNDATIONS • MAY/JUNE 2017 • 109 The geological sequence along the new Dewatering Field Tests (26 m) into the volcanic deposits (“TA” and for each well) indicated that the deeper C Line consists of a very heterogeneously “T1-T2”), and each was fitted with a slotted aquifer is fed by the sandy gravels of the Dewatering field tests were conducted layered deposit. The base deposit of stiff screen that extended from the groundwater Pleistocene deposits (SG), and the flow is systematically and consisted of the overconsolidated clay (“APL,” Pliocene age) table to the bottom of the well.