The isarco river underpass is part of the southern approach to the Brenner Base Tunnel (BBT). The value of the contract is EUR 303M (USD 367M) for the 2.1km stretch, which involves the excavation of 6km of tunnels in total. The contractor is a joint venture named Isarco scarl (a JV of Salini Impregilo; Mandatee: Strabag, Collini Lavori and Consorzio Integra). The Isarco Underpass is, as it sounds, situated entirely in the Isarco river area and the ground conditions are heavily influenced by the presence of the river. Raffele Zurlo, CEO for the Italy section of BBT SE, explains that the project runs through a narrow valley.
“We have granite at the beginning and end of the drive, with soils (alluvial soil and debris) in the central section around the River Isarco,” Zurlo says.
“A number of site and geological investigations have been performed in different stages throughout the design process (since 2005) up to a detailed investigation that has taken place during excavation, in order to have an accurate take on any geological changes and or minor faults.
“At present we are performing two horizontal boreholes (with DAC tests) in order to investigate the exact position of the contact rock-debris and to be able to define the right position of consolidation interventions along the alignment.”
GROUND FREEZING AND TREATMENT
Ground freezing will be used for a limited stretch of the alignment, approximately 60m. The freezing process is basically divided into four steps (all steps will be performed from the shafts at the two edges of each drive):
¦ Consolidation intervention at the edges of the tunnel footprint by means of low pressure injections. The injections will be realised at controlled volumes and pressures and will involve two stages: the first is a cement injection as fill, and the second is a series of chemical injections in order to further reduce the permeability of the soil.
¦ Consolidation intervention ahead of excavation by means of low pressure cement injections reinforced with fiberglass.
¦ In this step freezing operations are performed. Freezing probe holes are drilled, and then steel pipes (diameter 114mm, thickness 10mm) installed. Inside the steel pipes, two coaxial pipes used to carry the nitrogen and brine will be installed: the external one will be in AISI inox steel 76mm in diameter and 5.6mm thick, the inner one will be copper, 1in diameter and 1.5mm thick. The design expects 64 freezing probes along the edges of the tunnel and additional 24 freezing in the crown area. The freezing will be realised by nitrogen in an open circulation and will be maintained by brine in a closed circuit. In order for the ground to be considered frozen, temperature has to be -10°C or lower. Proper instrumentation will be installed all along the outline of the tunnel in order to constantly check temperatures.
¦ Excavation and then construction of the final lining will be performed: the reinforced concrete lining and waterproofing will be very similar to that adopted in debris/alluvial soil sections (any differences dependent on different loading conditions)
Zurlo explains the freezing, “The nitrogen is circulated as a liquid in an open circuit, it evaporates along the path extracting heat from the ground. Nitrogen used for freezing, in gaseous form, will be released and dispersed with a temperature between -100°C and -60°C. Freezing conditions are maintained by the circulation of brine.”
He adds that the excavation will be mechanical, and the main difference with the excavation in ‘normal’ consolidated soil will be the temperature of the ground and environment.
LINING
All the tunnels in this lot are lined with cast in situ concrete. Linings vary according to rock/soil and loading conditions (eight sections have been identified).
Aside from the lightest section, A0, all sections will require steel reinforcement throughout. A0 only calls for the lower part of the section to be reinforced, while the top heading is unreinforced concrete. As a general rule, sections excavated in soil feature an invert arch while for the section excavated in rock the invert is lined with a flat slab. In both cases the invert is in reinforced concrete.
In order to improve the performance of the lining of the crown in a fire, the concrete also has synthetic fibres added the mix and an additional layer of galvanised steel mesh is added, if the lining is in reinforced concrete section.
WATER ISSUE
“Our tunnels excavated in alluvial soil and under the Isarco river will have a full waterproofing system consisting of a 3-4mm thick PVC membrane installed over a 900g/m2 geotextile,” says Zurlo. “The system is installed in compartmentalised sections in which it will be possible to perform, if necessary, injections in order to repair any damage that the PVC lining might experience during the life of the tunnel.
Sections of the tunnel in rock will be built without waterproofing, and instead a drainage system will be present in order to collect the limited water infiltration expected. A water column acting at the invert level runs from 0m, for limited areas in the southern part, to approximately 25m in the northern part of the lot.
The portion of the tunnel alignment under the Isarco River has cover measuring 6m to the crown of the excavation, in the area of the underpass, this is from 0.5m to 2.0m.
In terms of pumping required during construction, Zurlo says that the pumping of water during excavation should be approximately 15 l/s, including water needed for works, such as dust control, while the pumping of water during excavation of shafts close to the Isarco river is approximately 25 l/s.
When the tunnel opens it will not have active pumping, as any water infiltration will be managed by gravity drainage systems.
The permanent drainage system comprises two different lines: one for water coming from platform/trains (in this line a minimum continuous flow of water is required in order to avoid any deposit) and another separate one for water infiltrations in drained sections of rock tunnel. All water will be managed by gravity, no permanently installed pumping systems are to be built.
JET GROUTING
Jet grouting is to be adopted under two different conditions:
¦ Underground for consolidation during excavation in unconsolidated material. In this situation the contractor is using a single-component grout. The column has a 650mm diameter with a length of 15-18m. In this application, low pressure cement and chemical injections are used in order to have proper stabilisation and waterproofing of the excavation face as it is often below the water table
¦ From the surface, in order to consolidate the ground ahead of excavation of the tunnel. Basically a number of vertical columns, penetrated among them, are executed some time before excavation reach the chainage in order to allow mechanical excavation of debris/alluvial soil (consolidation and waterproofing). In this application the jet grouting adopted is a two-component grout with column of 200mm diameter
THIRD PARTY IMPACT
“Our site is located relatively far from residential areas; our trucks usually do not use the smaller roads, but are able to use the A22 highway–we have a dedicated access route to the A22–in order to limit the disturbance to the general public,” Zurlo says. “Works are organized on a 24h/7 basis, we do not have specific restrictions beyond the usual limits set in place under Italian laws and regulations.”
In any case the BBT, since 1 February, is monitoring actual vibrations induced by the drill and blast excavation of tunnels in some of the buildings closest to the tunnels under excavation.
“Vibration induced in buildings (all monitored buildings are residential buildings) by tunnelling operations are well below safety limits foreseen by DIN 4150-3, the international reference standard.”
VENTILATION
In the Isarco lot, the tunnels are not incredibly long, and so a traditional ventilation system is enough to guarantee adequate flow and quality of air all along the tunnels.
At the very beginning of works, the system comprised a single ventilator, this has been supplemented with a second ventilator as excavation has progressed. The whole system has an installed power of 300kW and an air flow of 60m3/s. By the end of 2018 we’re expecting to have the break through on the southbound drive and so we will have also natural flow of air inside the tunnel.
HEALTH AND SAFETY CONSIDERATIONS
The works related to the Isarco lot are subject to Italian regulations. The Italian health and safety regulations are updated to be kept up with European and international standards, and can be considered in line with international best practice.
The amount of money spent in relation to health and safety costs is EUR 14M (USD 16M).
According to Italian regulations, health and safety spending has to be separated from the main construction budget in a detailed bill of quantities.
At the tender stage the bidder can offer a reduction in construction costs but health and safety expense cannot be subjected to any kind of reduction.
CHALLENGES
Zurlo explains, “In this project we have had, and we will have, a number of challenging issues as in any tunnelling project. Among the most significant, we can consider the underpassing of Isarco river that will be realised with the ground freezing techniques previously described, and the preparatory works we needed to implement in order to be able to perform said ground freezing.
“The ground freezing interventions will be realised from four shafts (two in the northern side of the Isarco river and two in the southern side): each shaft is 25m depth, with an area running from 800m2 to 1,600m2 and the bottom of the shaft located at 20m below water table.” The shafts are currently under construction, excavation of the first one have been started in January and will be completed by the end of summer.
“In order to facilitate excavation, consolidation interventions by means of jet grouting has been performed that can be summarised in about 800 jet grouting columns per shaft (the number of columns varies per each shaft from 600 to 1,000, the length varies from 8m, for the bottom plug, to 32m).
“The shafts are, of course, very close to Isarco river. It is easy to understand how challenging the excavation of these structures can be, either for the dimensions of the shaft themselves and with regards to the level of the water table. “Each shaft needs to be continuously monitored during construction with inclinometers, piezometers and topographical target in order to verify if the actual performance and tolerance of the structure can be considered in accordance with the design.”
The BBT is scheduled to be fully operational by 2027, while civil works are scheduled to be completed by 2022. Main works started in August 2016 while preparatory works, including the shifting of the SS12 road and the construction of a new road network, construction of loading/unloading areas and dedicated access to the A22 highway were carried out between October 2014 and December 2016.
