Bologna – Florence A1 highway. Monte Mario tunnel. Firedamp risk during rehabilitation works of tunnels lining. Technical – scientific advisory support during design phase

Client: Tecne SpA
Project: Monte Mario tunnel (Bologna – Florence A1 highway)
Data: September 2021 – March 2024

Repeated uplifts of the roadway platform and cracks in the Monte Mario tunnel walls and roof ( Figure 1) required demolition works and the partial reconstruction of the final lining (Figure 2).
Interventions for lining restoration (radial reinforcements in the roof and the complete demolition of the invert with additional excavations in the rock mass) defined by the Construction Design (CD) may reactivate the hydraulic communication between the tunnel and the surrounding gassy rock masses.
Bibliographic research (Figure 3), past and recent surveys and investigations along the track of Monte Mario tunnel lead to the hypotesis of methane inflows induced by restoration works, which will reduce the impermeability of the lining or interfere with the rock mass crossed by the tunnel.
Tecne entrusted SERENGEO to define, during CD, the organizational, technical, technological and procedural solutions to be adopted during the lining restoration works, in order to ensure the maximum safety against detonation / deflagration of air – methane mixtures generated by hypotetical methane inflows into the tunnel.

Figure 1. Location of the Monte Mario twin tunnels

Figure 2. North tunnel: location of the pedestrian and vehicular bypass axes with respect to the section (in red) where lining restoration interventions will be carried out

Figure 3. Monte Mario tunnel route superimposed on the methane occurences map (sources: Scicli, 1972; Berry et al, 2000; SPEA 1998 – 2003)

Objectives

  • Define the methane emission model for the rock masses that will be involved by restoration works;
  • Develop safety solutions (organizational, technical, technological and procedural) to be adopted during restoration works in the twin tunnels (North and South) and in vehicular and pedestrian by-passes;
  • Define the safety procedures to be adopted during core drilling of boreholes from inside the Monte Mario tunnel in the surrounding gassy rock masses.

Activities performed

  1. Analysis of the technical-scientific literature on:
    a. geo-structural framework of the area surrounding Monte Mario tunnel route;
    b. hydrocarbons and methane evidences in the investigated area and in the geological formations involved by the tunnel route (Figure 3);
  2. Analysis of the documentation relating to the investigations conducted along the Monte Mario tunnel route before its construction (Figure 4) and of the measured methane concentrations;
  3. Examination of the knowledge elements acquired during Monte Mario tunnel construction (methane concentrations measured in prospecting and consolidation holes at the tunnel face);
  4. Geo-structural analysis of the rock masses surrounding the two tunnels: geostructural characteristics (fracturing degree, presence of carbonaceous layers, folds, tectonic alterations, etc.) detected at the tunnel faces during excavation;
  5. Attribution, based on the results of the Studies & Research of the previous bullet points, of the emission model to the rock masses surrounding the two tunnels, that will be involved by lining rehabilitation works;
  6. Definition of the safety solutions (Figure 5) to be adopted in the two tunnels (North and South) and in vehicular and pedestrian by-passes during execution of rehabilitation works to provide the construction site with maximum safety conditions against ignition of air – methane mixtures;
  7. Definition of the safety procedures to be adopted during core drilling of boreholes in the gassy rock masses surrounding the two tunnels.

Innovative aspects

  • Definition of innovative safety solutions and measures for the execution, under maximum safety conditions, of restoration interventions of the permanent lining in operating tunnels that pass through firedamp-prone rock masses.

Figure 4. Monte Mario geo-structural profile. Boreholes drilled along the tunnel route before its construction (1985 – 2001) and methane concentration measurements (green dots). Red line: section where the PE interventions will be carried out.

Figure 5. Localized ventilation and indicative position of methane monitoring sensors along the tunnel during lining restoration works