Underground works, rehabilitation of tunnels and shafts, implementing security requirements

A little background

Underground works occupy an important place in this section of the website, which is dedicated to "technical explanations". Although it is not properly speaking a technique, the question is worth addressing as these works require a particular combination of methods and techniques which sets them apart.

Underground works comprise:   
– the construction of shafts,  
– the treatment of existing shafts,  
– the shutting down of shafts.

Combined techniques

Underground works require a number of techniques to be applied in difficult-to-access, confined spaces (a constraining environment):  
– Blasting works;   
– retaining structures for frames and vaulted galleries (cintres TH); 
– nailing (or pinning);  
– concreting methods (projected concrete, casing);  
– injection (soil reinforcement);  
– temporary and permanent shoring;  
– drainage (pumping, draining…);  
– reconnaissance drilling;  
– environmental treatment.  

Retaining structures

When dealing with altered soils or altered rocky terrains, several retaining techniques can be considered:  
– precast concrete elements (chiefly used with tunnel-boring machines),   
– metal structures for vaulted tunnels  
– projected reinforced or fiber concrete, sometimes associated with bolting.

Excavation techniques

Several parameters need to be taken into account when boring shafts:  
– their geometry (size, verticality or horizontality, length…)  
– nature of the existing terrains (rock, soil…)  
– hydrogeological conditions.

In rocky environments, Blasting and techniques involving TBM are the most common.

In the case of soil two approaches are possible :
– Blasting or excavation with BRH (hydraulic rock splitter) after injection or prior nailing reinforcement;  
– tunnel-boring machines and retaining structures as work progresses.

Water

Underground works are generally expoed to significant hydraulic strains. Several operations may then be necessary:   
– putting in place a drainage system in order to lower the high water table (pumping associated with drilling),  
– making unnels watertight as retaining structures are put in,  
– setting up a draining device associated with several environmental treatment devices within the tunnel itself.

Rehabilitation and tunnel equipment

Underground works must be regularly maintained. Provisions have to be made for consolidation and drainage operations.

Tunnels involve connected operations:  
– shutting off the access to unused shafs
– taking measures to provide vital space for the fauna (e.g. providing means of access for chiropterans and slithering animals);
– installing outlet gates to remove external hydrostatic pressure (watter supply tunnels); 
– installing stone traps ahead of pressure pipes;
– setting up a water-treatment system (notably in the case of mine shafts).

Gilles's advice about underground works  :

"In the case of rehabilitation or maintenance of wastewater draining tunnels, it is necessary to provide proper ventilation as well as individual lighting and an MX2 or Multigaz Minitor gas detector."

Caution measures

It is necessary to exercise caution as underground works may have a major impact on the environment. Operating at urban sites requires careful monitoring of the land movements induced by excavating and by the long-term nature of the works.

Maintaining urban works such as sewer tunnels also requires paying attention to the effluent gases emanating from the waste water.

We recommend to use of a type of concrete suited to the more or less aggressive nature of the environment.