The geological sequence along the new C Line consists of a very heterogeneously layered deposit. The base deposit of stiff overconsolidated clay (“APL,” Pliocene age) is overlain by fluvio-palustrine very dense sandy gravels (“SG,” Pleistocene age), which are then overlain by medium stiff clayey silts or dense sandy silts (“AR” or “ST,” Paleotevere units). These deposits are overlain by covered by pyroclastic volcanic soils, tuff, silty sand and sandy silt (Middle to Upper Pleistocene) deriving from the Colli Albani apparatus. A layer of made ground (“R”) of varying thickness covers the stratigraphic sequence and the natural soil profile everywhere. The local hydrogeological framework is 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 a wide range of permeability. Soil layer / type Pozzolane (PR/PN) The main geotechnical and environ- mental problems related to water seepage are summarized into four main items: 1. Lowering the groundwater level to allow excavation in dry conditions 2. Ensuring base stabi l i ty during excavation 3. Mitigating the seepage effect on the stability of the diaphragm walls 4. Minimizing ground set t lements induced by the lowering of the groundwater table 108 • DEEP FOUNDATIONS • MAY/JUNE 2017 Permeability Low High 7.5x10-7 cm/s 3.6x10-2 cm/s Lithoid and pseudo-lithoid tuff (T1-T2) 4.9x10-6 cm/s 1.0x10-2 cm/s Pleistocene deposits (ST) 1.3x10-6 cm/s 1.7x10-3 cm/s Teano Station — The excavation for Teano Station consisted of essentially an elon- gated box shape that was about 460 ft (140 m) in length by about 46 ft to 92 ft (14 m to 28 m) in width, and was excavated to a maximum depth of about 95 ft (29 m) from the ground surface. The natural groundwater level was about 46 ft (14 m) above the bottom of the excavation. The dewatering system consisted of 12 wells, each having a diameter of about 15.75 in (400 mm), and were installed within the excavation. The wells penetrated about 85 ft Dewatering Field Tests Dewatering field tests were conducted systematically and consisted of the following general steps: 1. Perform step drawdown and constant rate discharge tests (24 hr) in the first installed well 2. Perform a long-term dewatering test (8 to 21 days) by pumping all of the wells installed in each station 3. Perform measurements inside each station using electrical piezometers and outside each station using Casagrande piezometers, which were installed along the perimeter of the excavation (26 m) into the volcanic deposits (“TA” and “T1-T2”), and each was fitted with a slotted screen that extended from the groundwater table to the bottom of the well. The total pumping rate was approximately 270 gpm (17 liter/s), which resulted in a lowering of the groundwater table by less than about 8 in (0.2 m), maximum, on the outside of the excavation. The dewatering operation allowed dry conditions during excavation. Mirti Station — The excavation for Mirti Station consisted of a rectangular box shape that was about 213 ft (65 m) in length by about 121 ft (37 m) in width, and was excavated to a maximum depth of about 112 ft (34 m) from the ground surface. At this station, the required Dewatering field test results at Teano Station drawdown of the groundwater table in the volcanic soils was about 59 ft (18 m). The dewatering system consisted of 14 wells, each having a diameter of about 15.75 in (400 mm), and were installed within the excavation. The wells penetrated into and pumped water from the volcanic deposits (“TA” and “T1-T2”). The total pumping rate was approximately 1,110 gpm (70 liter/s), which was significantly more than the design assumption and resulted in a pore water pressure profile different from a hydrostatic trend. The groundwater table was lowered by about 2.3 ft (0.7 m), maximum, on the outside of the excavation and was quite limited in drawdown away from the diaphragm wall. The dewatering operation allowed dry conditions during excavation.
