The Cesana tunnel, located close to the Italian-French border in Piedmont (North West Italy), is the main structure on the SS24 road improvement works that were planned for the XX Winter Olympic Games. The tunnel has a length of 1860m and is located between a 370m long artificial tunnel and the 1280m long Claviere tunnel along a mountainous road which leads to the picturesque resort. Construction by the Italian-Spanish consortium SIS (SACYR S.A., INC General Contractor S.p.A. and SIPAL) started in March 2005.
Due to the presence of asbestos-bearing rocks that were unexpectedly discovered during tunnelling, work was suspended at the two excavation fronts in autumn 2006, leaving a 105m stretch of tunnel unexcavated. When the asbestos-bearing rock masses were encountered, the tunnel had been excavated from four fronts simultaneously; two starting from the portals and two from the 290m long access adit (figure 1). In order to isolate the asbestos-bearing rock masses from the remaining parts of the tunnel, two confinement barriers were constructed out of HDPE membrane and fixed onto a frame made out of steel tubes and wooden planks. Access to the face has been provided through two decontaminating filters installed on these barriers.
Being the first time that a tunnel had to be excavated in asbestos-bearing rocks in Italy using conventional tunnelling, it was necessary to define an excavation method that would take into account the health of the workers during excavation and muck handling as well as find a site for storing the hazardous muck. Following the evaluation of all possible sites, it was decided to build an underground storage deposit. It is important to note that this will be the first underground deposit of hazardous materials in Italy. Generally speaking, the design of the completion of the Cesana tunnel, though the asbestos-bearing rock, had to take into account three distinct parameters that are closely connected to each other:
• the design of the tunnelling method and in particular, the support, final lining, and operational sequence to be employed for excavation in the asbestos-bearing rock mass
• the design of the underground storage cavern in which the asbestos-bearing muck will be deposited
• the environmental impact study of the tunnel stretch with asbestos and that of the underground storage cavern
In the following the two last topics are discussed and presented, highlighting the most important design aspects.
Geology
During the project design the excavation of the Cesana tunnel was foreseen to be entirely in the Dolomites of the Chaberton-Grand Hoche formation. The presence of serpentinite rocks was both unexpected and unforeseeable. The mountains slopes are covered by thick and unstable debris and are subject to rock-falls and avalanches, making the construction of roads for exploration rigs unfeasible.
Apart from a limited stretch at the start where the steel-pipe umbrella method was used to cross the debris, drill-and-blast was used in the good quality dolomites. In August 2006 unexpectedly bad rock mass conditions were encountered during the excavation, requiring intensive ground reinforcement with fibre-glass grouted elements at the face and around the tunnel. These conditions were due to the tectonic contact between the Chaberton-Grand Hoche dolomites and the calcaschists with “green stones” (ophiolites). The contact between the two units is rarely visible from the surface due to the presence of the debris cover. Both bibliographic and specific studies considered the contact to be located outside the tunnel alignment, dipping subvertically, therefore, only marginally interfering with the excavation. During tunnelling, it was actually discovered that this tectonic contact has a lower dip angle, a total length of 401m and is associated with an intensely fractured rock mass. serpentinitic ophiolitic rock of greenish colour was encountered at the tunnel face at chainage 3+059m. Taking into account the risk associated with the presence of asbestos minerals in such lithotypes, and in accordance with the Piedmont Environmental Protection Agency (ARPA Piemonte), air and rock samples from the tunnel were taken for mineralogical analyses. The analyses conducted on air and rock samples, using an electronic microscope, a scanner as well as microanalysis of dispersion energy, revealed the presence of tremolite fibres in high concentrations for the valley front (289-55,000mg/kg) and moderate concentrations for the adit front (<100mg/kg). This was therefore classified as a high-risk zone for asbestos contamination. From a geomechanical point of view, the tectonized ophiolitic rock mass is of very poor quality, due to the presence of well developed laminated foliation surfaces.
The need to develop a new design assuming that the remaining 105m would be excavated in an ophiolitic rock mass consisting of green schists of variable concentrations of amphibolite and tremolite became immediate. The new design would need to account for problems related to both the excavation procedure and equipment as well as the disposal of asbestos contaminated material.
Muck treatment and disposal
Three different options were considered for the handling of the excavated material:
• transportation to an existing waste disposal site for dangerous materials
• on site treatment of the muck to transform it into an inert material
• disposal of the muck in an underground deposit that would be built inside the Cesana Tunnel (in a good quality dolomitic rock mass)
The first option was rejected because of the lack of a suitable site at a reasonable distance were the 20,000m3 could be deposited. The second alternative refers to transforming the rock containing asbestos into an inert material using a new thermo-mechanical process developed by the National Research Council of Italy (CNR), which is however in a laboratory development stage. The waste is crushed, mixed with clay, and subjected to very high temperature (650 – 1200°C). A material of ceramic properties without free fibres of asbestos is obtained. This method was also discarded mainly due to the size and location (outside the tunnel) of the plant required. Also, being at an experimental stage, questions were raised about the volume of material that could actually be processed with this method. The best option considering worker safety and works planning, was to construct an underground repository, located in a position along the tunnel where good rock mass conditions had been encountered during tunnelling. Following the choice of the site, it was necessary to define how to store the muck in the underground repository. The first option considered was to simply unload the muck in the cavern. This option was discarded due to the muck exposure to the underground environment during the entire period of the works. The second option was to store the muck in large size bags but this solution was also discarded since the filling of the bags would require breaking up the muck underground with the consequent environmental problems. Placing the bags on top of each other would also be difficult. Finally, it was decided to use pre-cast reinforced concrete 2x1x1m containers which would ensure robustness, easy-handling, long-term durability, increased storing capacity and easy stacking (figure 2). Once filled, inside the contained asbestos contaminated excavation area, the containers will be sealed with a layer of early-strength shotcrete applied at a low pressure and transported to the repository. Up to six containers can be placed one on top of the another (figure 3). Each row will be separated from the previous one by a layer of reinforced concrete. The deposit will have a capacity of about 20,000m3 and can host more than 10,000 concrete containers. The safety equipment, used by workers during the excavation such as masks, overalls, filters, etc., will be stored inside bags that will be placed on top of the last row of containers.
In order to prevent and mitigate the risk of fibre dispersion inside the underground repository during the storing works, an emergency unit with encapsulating paint will be installed inside the cavern to permit quick operation if some containers should break during handling. Furthermore, a water barrier and an aspiration ventilation system with an air filtering system will be installed at the entrance in order to avoid any fibre dispersion outside the deposit.
Repository construction
The storage cavern section will be 21m wide and 13m high and the underground deposit will be 300m long. Taking into account the very good geomechanical conditions of the rock mass where the deposit will be located, support consists of systematic fully grouted bolts and a 15cm thick shotcrete layer. In order to eliminate any possible contact between underground water and the waste, the cavern will be completely waterproofed with a HDPE membrane that is protected by two layers of TNT. The final lining will be obtained by lattice girders and shotcrete (20-30cm thick) and is designed to ensure the long-term stability of the cavern. The muck storing containers will be placed on a reinforced 50cm thick concrete slab. The Piedmont Environmental Protection Agency requested that the excavation of the cavern shall be proceeded by the construction of an exploration drift. This exploration drift will have a 25m2 section and will be excavated by the drill-and-blast method. It will also contribute to reducing vibrations during the blasting operations of the cavern.
Integrated performance
According to Italian regulations, an integrated analysis of the performance of an underground deposit has to be performed considering the following aspects:
• Geologic evaluation; the deposit will be constructed inside a homogeneous dolomitic rock mass, under an overburden ranging from 250 to 455m. The bedding of the dolomitic formation will be crossed at a steep angle, orthogonally to the cavern axis. No faults or other tectonic structures are foreseen along the deposit
• Geomechanical evaluation; with reference to the Bieniawski classification, the rock mass belongs to classes II and III. Geomechanical observations during tunnel excavation and in-situ and laboratory tests suggest good geomechanical properties both in terms of shear strength and rock deformability. Additional testing and detailed monitoring during excavation of the exploratory drift will provide a complete geomechanical reference model for the cavern
• Hydrogeological evaluation; as observed from the tunnel excavation and from permeability tests, the rock mass can be considered as completely impervious (K=2-7×10-7m/s); localized water inflow in fractured zones is expected
• Geochemical evaluation; the rock surrounding the deposit will be dolomite off a very fine grain, a massive structure and an homogeneous texture. This rock mass does not show any sensitivity to chemical dissolution and is not susceptible to karstic phenomena
• Biosphere impact evaluation; the underground location of the deposit and the prevention and mitigation measures adopted in order to avoid any fibre dispersion outside the deposit (encapsulating paint, vacuum ventilation system, water barrier) are sufficient to prevent any impact on the biosphere during the disposal phase). In the long-term, the possible impact on the biosphere could only derive from the collapse of the containers which would subsequently lead to a dispersion of fibres inside the deposit and in the water. Despite the very low probability of these events occurring, the deposit will be isolated from the tunnel by a double concrete wall. A drainage system is also foreseen to collect the incidental water infiltration, which will be directed into a shaft equipped with an alert system and pumping facilities
• Operating phase evaluation; the storage operating phase includes transportation of the containers inside the deposit by tracks (3 containers per track). The containers will be put in place by a 60kN capacity forklift which is able to reach a height of 8m. A team of three trained workers equipped with masks (P3 filter) and tyvec overalls is foreseen during storage. The containers will be numbered, marked and registered daily
• Long-term phase evaluation; the stability of the deposit has been evaluated through geomechanical scenario analyses performed by numerical modelling taking into account long-term rock mass and lining properties
In order to obtain the authorisation necessary to operate the designed waste underground deposit, according to National Italian Regulations, the following management plans have been prepared: operational management plan, environmental reclamation plan, post-operational management plan, monitoring & control plan, and financial plan. The first four plans, in practical terms, are similar to instruction manuals for the several activities that have to be performed during the life of the deposit after it has been constructed while the financial plan evaluates the overall costs of construction and management of the deposit. Currently all authorizations have been obtained and construction is expected to start in Summer 2009. T&T
Fig 1 – The tunnel location plan Fig 3 – The storage area Fig 2 – The container for the contaminated muck Intensely weathered and foliated mica-schists
