The tunnel In the Clearwater Project site, Los Angeles, he suffered A collapse in the night of 9 July in a Wilmington construction site, several km from the only output point on the surface compared to the excavation front. The collapse, which involved 31 workers fortunately remained unharmedhas not currently a declared technical explanation: the causes that have entailed the collapse of the tunnel walls and for this reason the processes were not completely interrupted, in order to be not known, in order to ascertain structural and geotechnical stability of the system, still in the completion phase.
How the Los Angeles tunnel is done
The realization of this new tunnel counts a excavation length of about 11.3 km, that advances day after day to its completion (estimated in 2027). The realization of the excavation takes place through the use of a machine TBMalso called Mole: In practice, it is a mechanized element that is rotating remotely and at the same time has the ability to Shatter the ground in front of the excavation And progressively advance, automatically removing the resulting material also produced. In addition, during the excavation, All walls downstream of the front are protected by structural support elements type prefabricated in concrete, that is, not making the concrete jet on site but using elements pre-realized in special establishments. The implementation of these elements always takes place thanks to the help of the TBM. These prefabricated circular structures (which are actually made by many arches that have come together) are made for rings/subsequent councils as the mole advances: i pieces They are assembled one after the other, mechanically connected by the workers downstream of the installation operations.
The infrastructure in question is not a gallery for the passage of vehicles, but a sort of pipe made to guarantee the movement from the wastewater treated from the purification plant up to the outlet in the ocean. Although therefore the dimensions are contained with respect to those of a usual gallery, The geotechnical-structural operation of the system remains the same And therefore the stability of the excavation front, as well as the side walls, is always related to the mechanical characteristics of the soils that are crossed.
Clearwater Project tunnel: the causes of the collapse
Given also the non-availability of photographic material that makes the post-crocal condition understand, it is extremely difficult to express a judgment on the possible causes that have led to collapse. The official declarations of the bodies and companies involved declare the collapse of a portion of tunnel In a stretch between the entrance/output on the surface and the excavation front, almost 10 km from the entrance point. The entry/output point is the same used for rescue operations. Therefore, it seems that the crisis does not concern the excavation front but the side walls and, therefore, could probably have triggered:
- At the turn of one of the connections between the circular prefabricated elements, which are put in place during the progress of the excavation. Difficult to say if the failure can be due to the assembly or the materials with which this has been performed (for example, due to the presence of some defect of the mechanical connection organs);
- In an area where the coating assembled by the TBM had not yet been completed, therefore very close to the excavation front. In this case, the pressure imposed on the TBM can be crucial in the response of the soil of the walls that are not yet covered. Any outcrop against the excavation of material other than that they were tarate The pressure of the machine could lead to significant variations to the local stability conditions of the land.
The construction of a tunnel is a significantly complex activity, reason why the whole construction phase is constantly assisted by an extensive geotechnical monitoring and analysis of data recorded as the excavation proceeds. This complexity is partly associated with the difficult – practically impossible – characterization of the land crossed along the excavation path with absolute certainty. The test campaigns, in fact, are often confined to discreet investigation points and can hardly swing The whole path in detail. It follows that The geotechnical conditions of the land that you cross could changeeven for short stretches, but significant for static purposes. Monitoring allows, for example, at the same time as the excavation, one Reading movements of the campaign plan and in some cases the levels and any oscillations of the waterfall waters present inside the ground. This guarantees a control of the project hypotheses as the progress of the progress and the possibility of correcting the shot during construction.
Waiting for updated news about this, La Stampa has declared that the works will not resume until clarity has been made on the reason that triggered the collapse.
