Tunnels and Underground Cities: Engineering and Innovation Meet Archaeology, Architecture and Art

Volume 4: Ground Improvement in Underground Constructions

Daniele Peila, Giulia Viggiani, Tarcisio Celestino, Daniele Peila, Giulia Viggiani, Tarcisio Celestino

• Publisher: CRC Press
• Year: 2020
• Print ISBN:
  9780367468682
  9780367468682
• Edition: 1
• eBook ISBN:
  9781000175530
• Topic: Technology & Engineering
• Subtopic: Civil Engineering
• Index: Technology & Engineering

Tunnels and Underground Cities: Engineering and Innovation meet Archaeology, Architecture and Art. Volume 4: Ground Improvement in Underground Constructions contains the contributions presented in the eponymous Technical Session during the World Tunnel Congress 2019 (Naples, Italy, 3-9 May 2019).

The use of underground space is continuing to grow, due to global urbanization, public demand for efficient transportation, and energy saving, production and distribution. The growing need for space at ground level, along with its continuous value increase and the challenges of energy saving and achieving sustainable development objectives, demand greater and better use of the underground space to ensure that it supports sustainable, resilient and more liveable cities.

The contributions cover a wide range of topics, from permeation grouting and consolidation works, safety operations, artificial ground freezing to frost heave control.

The book is a valuable reference text for tunnelling specialists, owners, engineers, archaeologists, architects, artists and others involved in underground planning, design and building around the world, and for academics who are interested in underground constructions and geotechnics.

Ground improvement in underground constructions

Permeation grouting and consolidation works to solve flooding problems in an old tunnel

A. Antonelli

MM S.p.A.

A. Balossi Restelli & E. Rovetto

Studio ingegneria Balossi Restelli e Associati

ABSTRACT: The raising water table in the Milan area affects some of the Metro lines constructed in the past without waterproofing. The problem should be faced by acting on the Metro lines in operation before water inflow could compromise the regular Metro service. The paper describes the two phases intervention studied for subway line M2 between the Piola and Lambrate stations. The first phase, was aimed at stopping water inflow and to repairing the invert strongly compromised. The second phase concerns water tightness and long-term stability of tunnel, through the creation of a shell of consolidated ground. Grouting operations will be executed both from inside the tunnel and from the surface, the last one requiring particular attention for the protection of the tunnel lining. Consequently a dedicated real-time monitoring system has been studied to avoid any damages to the lining in order to guarantee the metro service in safety.

1 THE AQUIFER AND ITS EVOLUTION IN MILAN AREA

1.1 Hydrogeological characteristics

The municipal territory of Milan occupies an area of sq. Km. 182, in central position with respect to the two important tributaries of the Po river: the “Ticino” at West and the “Adda” at East. It develops in an area with a very low slope of 2–3‰ with altitudes in m a.s.l. between 143–145 in the North and 102–103 in the South.

A thick alluvial and fluvio-glacial deposit characterizes the subsoil up to about 40 m from the ground level, with gravel and pebbles with particle size from medium to coarse sometimes intercalated with lenses of silts and clays, variable in size and depth. The Wurmian “gravelly-sandy” sediments outcrop on the surface. They are characterized by the proximity of the main water courses Olona, Seveso and Lambro whose contribution is represented by a continuous coarse sedimentation. At an average depth of 40 m there is instead a clayey level of some meters, which separates the most superficial coarse horizon, from the sandy one below and figures out diverse ways of water circulation of groundwater.

A dense network characterizes surface hydrography, only partly of natural origin (Lambro and Olona rivers, the Navigli and various irrigation ditches) which ensures drainage of the superficial waters and, at the same time, supplies the underground aquifer.

The superficial alluvial deposit have high permeability (about 5E-04 m/s), which decreases both with the depth and with the transition from North to South.

The first aquifer consisting of coarse deposits of the first 40 meters contains free groundwater; the name “second aquifer” identifies the sandy-gravelly succession with frequent clayey diaphragms between 40 and 110 m, almost always containing groundwater in pressure. Between the free groundwater circulation of the first aquifer and the water in pressure of the “second aquifer” there could be local intercommunications. The metropolitan lines of the city are in the range from the ground surface to about 35 m depth.

1.2 Historical data and sources of water table levels

The first data structured on the characteristics of the water table in Milan date back to 1889 with the construction of the first wells of the Milan aqueduct; to this day, the values of water depth in each of the 37 Drinking Water Plants are measured and recorded.

Since the 1950 two additional monitoring networks have been developed: - the first by MM, the Municipal institution dedicated since 1955 to metro lines implementation, along the directions of the metro lines that achieved n. 110 piezometers; - the second due to the sewerage Municipality of Milan that, starting from the 85 points of measurement in 1954, today has approximately 30 efficient piezometers.

Since 1998 the SIF (Aquifer Information System), that manages all the data related to the levels of the aquifer in the Milan area, has been set up at the Central Environment Management of the Province of Milan.

1.3 Historical evolution of the aquifer in Milan territory

Figure 1 summarizes the trend of the water depth in Milan from 1915 to today.

In the progress, the following phases could be found:

From 1915 to 1950: the groundwater level remained constant and very superficial for a long time even after the construction of the first Milan Aqueduct stations (the first pumping plant dates to 1889, in 1910 there were already 10 plants with 87 wells); the first signs of lowering date back to the years 1930–1940 and are linked to the intensification of industrial activity.

From 1950 to 1970 the massive industrial and urban infrastructures development and the increasing waters withdrawal accelerates the decreasing of the aquifer at a rate of one meter per year with tips of 2.5 m causing relevant subsidence phenomena, up to 22 cm in the Duomo area.

From 1975 to 1980 the trend abruptly inverts because many industries move to the North Milan area and many private wells close.

The following period 1980–1990 is characterized by the water table equilibrium until a new sudden increase, between 1990 and 1998, for the dismission of large plants (Pirelli, Breda, Falk, Montedison), responsible for the depression of the post-war years. Until 2007 the levels are still stable.

The levels return to increase from 2007 to 2015: a first and only partial interpretation may be due to the abandonment of several water extraction wells in the North of Milan due to the increasing concentration of contaminating agents.

Figure 1. Average value of water depth in Milan area from 1915 to 2017.

2 UNDERGROUND LINES AND RELATIONSHIP WITH THE AQUIFER

2.1 Aquifer levels in the projects of underground lines

In the design practice since the beginning of its activities, MM has always referred to two conventional aquifer levels.

• The first level named “reference aquifer” that refers to the water table during the construction phase which affects the executive methodologies
• The second level named “project aquifer”, the estimated “long term” level during the life cycle of the opera, that influences the sizing and the necessity of waterproofing

The choice of these levels, especially in the case of the project water table, is delicate since any long-term forecast is uncertain and full of consequences on the realization and maintenance costs of the opera.

During the design o...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Table of Contents
• Preface
• Acknowledgements
• WTC 2019 Congress Organization
• Ground improvement in underground constructions
• Author Index