A new highway system is set to overcome the road congestion in Genoa.

The Genoa bypass, known in Italy as “Gronda di Genova”, will be constructed almost entirely underground, comprising 25 new tunnels for a total 50km length, with cross sections up to 500sq.m in the diversion and connection caverns. Autostrade per l’Italia, was awarded the concession to build, construct and operate the road by the Italian Ministry of Transport in 2016 with the expectation of starting works by the beginning of 2019 and to complete them in 10 years. The cost of the whole project is EUR 4.2bn (USD 5.24bn).

The highway system around Genoa is currently also used as a city orbital for urban traffic.

The new highway system will comprise two bi-directional carriageways and will double the capacity of the existing A10 Genoa- Ventimiglia route in the Genoa urban area, and will also enhance sections of the A7 Milan-Genoa and of the A12 Genoa-Rome routes, between East Genoa, West Genoa and Genoa Bolzaneto junctions. It will take a flow of traffic that includes vehicles travelling from Italy, on to France and Spain.

The most congested section in the regional network has been the Genoa bypass highway, which also included the Morandi Bridge, which crossed the Polcevera river, and took over 25 million vehicles per year. This bridge tragically collapsed on 14 August causing the deaths of 43 people. The Genoa bypass is now at centre of media attention because it was designed 30 years ago but has faced several delays for various different reasons. The Genoa bypass is intended to relieve pressure on the existing network by separating freight from urban traffic.

Since the 1990s there have been a number of tweaks to the alignment. Then, following a public debate between the local authorities and Genoa’s citizens that was held between 2008 and 2009, the bypass project was eventually approved in September 2017 after an Environmental Impact assessment and a formal agreement put in place. Activities were expected to start in early 2019.

Autostrade per l’Italia general manager responsible for the new works is Roberto Tomasi, who explains that the bypass project is a “strategic infrastructure project in Genoa as this city is one of few Italian cities that does not currently have a highway ringroad. So existing highways pass through residential areas, and amongst other urban buildings and areas. The A7 and A10 date back to the 1930’s and are quite close to the urban development.”

EXCAVATION METHODS

Monterosso and Amandola are to be the two longest tunnels, together running for 6km and coming from the west towards Savona. The Borgonuovo is 3km-long, which reaches the existing connection with A10 in proximity of Vesima.

Autostrade per l’Italia director of investment and infrastructure Alberto Selleri explains that these tunnels need to be excavated with two hydro-shield TBMs due to high hydraulic loads and due to the presence of asbestos along 70 per cent of the drives.

“The hydro-shield allows for the transportation of the spoil through pipes with the obvious safety improvements, both for the workers involved and the population living next to the works area,” Selleri says. “The two 14.75m-diameter TBMs will be launched from portals equipped with a sealing ring and concrete tunnel eye that will allow the TBM to pass-through.

Each TBM will excavate 14km of tunnel in total on the project. Following the completion of the first bore, each TBM will be translated to the second, crossing a viaduct for 100m that will be in place at a temporarily low height and later on will be lifted to the final height for commissioning.

In order to reach the portal of the second tunnel, each TBM will be partially disassembled after the completion of the tunnel then the TBM component will be transported along two viaducts and through a short tunnel (the Voltri Tunnel).

Selleri adds, “On the left side of the Polcevera river, there are other tunnels, excavated by [non-mechanised] methods, some of them reaching 4.5km. There are several interconnection chambers to allow diversions from highways towards the other direction. These tunnels allow the connection of the east-west direction to the highway arriving from the North (Milano) and the connection to the city. “We don’t know yet who will supply these two TBMs. Preparatory works have been assigned to Pavimental, which is a construction company that belongs to the group of Autostrade per l’Italia.

DIVERSION AND JUNCTION CAVERNS

A technical challenge of the tunnels’ construction is the excavation of diversion and connection caverns that allow the connection between different sections of the new highway system and the existing road network.

The highway tunnel progressively enlarges to accommodate the diversion and connection ramps aside the two lanes carriageway, thus the cavern cross sections area gradually increases up to 500sq.m.

The cavern excavation sequence is extremely complex, to guarantee safe conditions during all phases of the construction.

It entails different excavation stages and requires the performance of rock mass improvement treatments, dependent on the local geotechnical conditions.

According to the working plan, the cavern excavation will start from the main tunnel on the wider section of the cavern towards its narrow end.

The whole process includes 11 stages: 1. Excavation and lining of the main tunnel up to the cavern and then, excavation and lining of the approach tunnel section;

2. Excavation of the crown of the tunnel and rock mass improvement treatment;

3. Excavation of the right side of the tunnel; rock mass improvement treatment; casting of the right cavern abutment (reinforced concrete);

4. Excavation and lining of the diversion/ connection tunnel up to the cavern approach section; excavation and lining of the approach tunnel section;

5. Excavation of the left side of the tunnel; rock mass improvement treatment; cast of the left cavern abutment (reinforced concrete);

6. Excavation of the left side of the tunnel; Rock mass improvement treatment; cast of the left cavern abutment (reinforced concrete);

7. Excavation of the right side of the tunnel in the cavern narrow section; Rock mass improvement treatment; casting of the right cavern abutment (reinforced concrete); excavation and cast of the invert of the wider cavern section (reinforced concrete);

8. Excavation of the left side tunnel in the cavern narrow section; rock mass improvement treatment; cast of the left cavern abutment (reinforced concrete); completion of the invert of the wider cavern section.

9. Partial refilling of the side tunnels; crown excavation in the narrow cavern section; cast of the cavern crown lining (reinforced concrete);

10. Excavation and cast of the invert of the narrow cavern section (reinforced concrete); cast of the cavern end and transition walls (reinforced concrete);

11. Restart of the main tunnel excavation and lining.

GEOLOGY

As mentioned previously, several alignments have been studied over the last 30 years, so a great many geological investigations have been carried out for a number of possible routes. The Liguria region’s geology is primarily characterised by being increasingly rocky as you head towards the coast.

A study coordinated by CNR and Turin University was done to check the geology and hydrogeology in presence of asbestos in Alpine areas.

“Hydrogeology is important to us because we have high hydraulic load and we need to preserve all the sources,” Tomasi says. “We conducted a census to get a clear idea in advance.” The geological composition includes serpentinite, calcareous schist, metabasite before going to the fault area in Sestri- Voltaggio.

“On the Appennine area we have Flysch arenaceous or clayish,” Tomasi says. “We have a very good limestone on the south side of Genoa bypass so we aim to use it for aggregate. “The bypass crosses faults, which are delicate from an excavation point of view, such as the Sestri-Voltaggio fault, which is the passage between the Apennines and Alps.

“There are also other areas with heterogeneous formations where we expect some challenges as the material is altered with a large amount of water.”

Late and post-orogenic deposits are also present; they could be ascribed to the Tertiary Ligurian-Piedmontese basin and to marine and continental deposits of different ages, which have partially covered the bedrock units and filled the paleo-valleys. The Voltri Group and the Sestri-Voltaggio Zone are separated by a north-south structural lineament, the so called “Sestri- Voltaggio Line”, which is identified as the physical limit between the Alps and Apennines.

The geological sequences belonging to the Voltri Group are referable to the Piedmontese-Ligurian domain, such as an ocean basin, consisting of native intrusive and extrusive rocks and sediments of different types, deposited over the basaltic lava; in the literature, many tectonic-metamorphic units have been distinguished inside this unit, traditionally referred to two main groups:

– units consisting mainly of metagabbro and serpentine rocks (Beigua Unit, Ponzema Unit, S.Luca Colma Unit);

– units consisting mainly of calcareous schists and prasinites (metabasite), but also of subordinated mantle origin rocks such as serpentinite and peridotite (Alpicella Unit, Ortiglieto Unit, Palmaro- Caffarella Unit, Voltri-Rossiglione Unit).

The metamorphism grade reached the blue shale facies (up to eclogites for the Voltri unit), subsequently downgrading to the green shale one.

The Sestri-Voltaggio zone affects the mountain ridge that forms the upper part of the right hydrographic side of the Polcevera valley and includes tectonic-metamorphic units, highly different for lithology and degree of metamorphism: – Monte Gazzo tectonic unit, consisting of Triassic dolomite and limestone;

– Cravasco-Voltaggio tectonic unit, made up of serpentinite, meta-basalt, phyllitic limestone and shale;

– Monte Figogna tectonic unit, consisting of serpentinite, metabasite and shales, sometimes with thin carbonate and siliciclastic levels.

The metamorphism grade progressively decreases from the Triassic carbonate units to the ophiolitic series of the last two tectonic metamorphic units.

The Flysch Apennine Domain affects the Polcevera river valley and includes a series of tectonic and tectonic metamorphic units, which are very homogeneous from the lithological standpoint, with a metamorphism grade progressively decreasing proceeding from West to East; the Polcevera river left bank units may be considered as non-metamorphic.

SEA WORKS AND SPOIL

“We have already begun marine works, which are essential for the handling of asbestos-contaminated spoil,” Tomasi says. “Concrete caissons are being built to safely store the spoil; and they will be placed on the sea bed, which is at a depth of 10-12m. These caissons will look like an island, growing from the sea.

“These caissons are actually waterproofed tanks with high-density membrane (HDPE) used to safely store this kind of deposits. The excess of water will be processed to avoid contaminating the sea.

“Caissons are filled up to 1m under the sea level and above it will be placed a covering cap made of material without asbestos. This is a layer of capping, to isolates the asbestos.” LINING

Precast segments will be used with a variable thickness between 600 and 700mm, according to the loads. The length of each segment is 2m. “We are going to use the same type as for the Santa Lucia TBM in Florence that is 9+0,” Selleri says. “We also expect the launch of drainage pipes in advance in some areas to manage critical hydraulic loads.

“In the TBM supply tender we will ask the possibility to use an accessible cutterhead that will allow the replacement of most of the excavation tools without the need to expose the personnel to high hyperbaric condition. The TBM and the site logistic will be ready to execute high pressure hyperbaric intervention to cover all cases that are not covered by the accessible cutterhead and to cover any emergency situation.”

WATER

The TBM is designed to keep back a hydraulic load up to 16 bar. “We will control through surveys and advancement checks whether hydraulic loads in faults zone might be higher than 16 bar,” Selleri says. “If the hydraulic load is higher than this threshold, we will create drains to decrease pressure and to excavate without any problems. These drains will be created by directional drilling through the lining. As these drains will work on fracture areas, the material is already permeable so there is no need to use big drains. We expect to use drains of 101.6mm.”

VENTILATION

In terms of ventilation for operational tunnels, Selleri explains that they expect longitudinal ventilation with jet power, which will push smokes in the opposite direction than where people are evacuated.

“As all tunnels be twin-tube, they will be connected by refuges and escape routes every 300m so that people can evacuate through the other tunnel,” Selleri says. “Some tunnels without an adjacent tube at east of Polcevera will have an underground passage beneath the platform. While the tunnel, which connects Genoa east to Genoa west will have a side micro tunnel of escape. This microtunnel will be excavated with a micro TBM of 4.50m diameter. The micro tunnel is 4.2m of diameter and 4.5km long.”

MONITORING

Monitoring will be done with inclinometers, piezometers and jacks. “We will do checks with satellite to identify any subsidence near some buildings,” says Tomasi.