07 avril 2016
A TF 3000 excavates a pedestrian tunnel
In theory, constructing a pedestrian tunnel 80 m long, 4 m wide with an excavation face of about 20 sq. m shouldn't have presented too many complexities, especially in Italy, where the sheer number and type of tunnels is in itself proof of the expertise in designing and building tunnels the country is known for. However, this small underground job in Pontremoli Magra, a town in the province of Massa-Carrara on the Tuscan side of the Apennines, had a concentration of problems that forced a tailor-designed solution.
First among the challenges were the characteristics of the rock masses that the short tunnel would have to cut through. The excavation would be performed within a rock sequence containing predominantly siltstone and fine-grained quartz-feldspar sandstone with intervals of medium to coarse-grained massive sandstone.
The entire sequence in the literature is given the name "Macigno" and represents the main element of the non-metaphoric "Tuscan Nappe" present in the upper basin of the Magra River.
Direct tests on the rock core and laboratory tests on integral samples taken during the continuous core surveys revealed that the rock mass fell into classes with an unconfined compressive strength of R=50-100 MPa and 100-200 MPa, whereas the RQD (Rock Quality Designation) was between 80% and 90% in the portions of relatively undisturbed rock.
These characteristics lent themselves to full-section excavation using a traditional blasting method, but given the proximity to the medieval walls, the solution was discarded since the volleys would have induced vibrations on buildings in the historical centre just metres from the entrance, as well as on the castle itself sitting practically on the vertical axis of the tunnel. Despite the high compressive strength of the rock mass, a mechanical excavation using a hydraulic breaker was chosen, a solution that would nonetheless deliver positive results thanks to the high degree of fracturing.
Works began by constructing a box-shaped tunnel entrance in reinforced concrete about 9 m in length inserted into the superficial layers for purposes of stabilisation and to allow the full restoration of privately-owned land on the slope. At that point, excavation works got started, but after over two months the extreme hardness of the rock and the small size of the breaker (a factor conditioned by an excavator sized to fit the tunnel section) made it clear that this method would not produce a satisfactory output.
Production was further slowed by the removal and transport of the excavated rock; this mucking process involved the excavator to exit, the loader to enter and the material to be loaded on a truck positioned in the tight space outside the tunnel entrance leading out to a parking area, which had continued accessibility for the duration of the project, as it was the only one that side of the town.
Advantages of the cutter head
With blasting not an option and taking into account the low productivity of the breaker, the contractor then evaluated using a roadheader, which had to be small due to the tunnel section. A solution was found by renting a GTF 400 RC with MultiRadius Pro technology, fitted with a Simex TF 3000 cutter head. Designed and built by Grotti Srl, the machine is much smaller than an electric-hydraulic powered roadheader of similar productivity and was also more economical to rent.
The strong points of the system are the robust telescopic boom, hydraulics with a flow rate of 540 l/min, dedicated software to control the hydraulic system and all machine functions, as well as the possibility of remote control, a feature that clearly improves safety conditions and makes it possible to work in difficult situations such as the Pontremoli tunnel.
The system 's efficiency had been earlier demonstrated by many projects undertaken by Grotti Srl using the GTF 200 RC, a similar machine but with less power, used successfully for the scarification of concrete surfaces inside tunnels mounted with other Simex attachments, like the MP 1000 profiling cutter head or the PLB 600 planer.
From the first tests performed on the excavation face with the more powerful GTF 400 RC fitted with TF 3000 cutter head it was clear that the system ensured higher productivity compared to the breaker and, what's more, performed more precise profiling of the section. The machine was also installed with self-cleaning cameras that allowed the operator to make checks outside the tunnel from a van equipped with large screens.
Without this system the substantial production of dust owing to the type of rock would have made the job impossible. Even if equipped with remote control and positioned far from the excavation face, the operator would have inevitably been exposed to the effects of dust and after a few seconds unable to determine the cutter head's position with respect to the section to be excavated.
To prevent the machine's engine from undergoing damage due to the dusty environment a conduit was installed to allow air intake from the outside. Smoke and dust, which could have poured out of the tunnel during the excavation phases and invade the centre of town, were intercepted and removed efficiently by nebulised water jets positioned around the tunnel entrance.
An excavation method with excellent results
Using the GTF 400 RC MultiRadius Pro proved an optimal solution because it delivered an output that was otherwise unattainable. What's more, the Simex TF 3000 cutter head turned out to be very productive also thanks to its patented coaxial hydraulic motor, a feature maximised by the high flow rate of the GTF 400 RC's hydraulic system. This made the TF 3000 more efficient compared to other types of cutter heads fitted with gear reduction units. Furthermore, the TF 3000 assured exceptional reliability; after over 300 hours of excavating rocks having a hardness of 180 MPa (with peaks of almost 200 MPa) in conditions of abrasive dust, the attachment delivered trouble-free operation.
The output of the GTF 400 RC was established at about 45 cu m of milled material (a linear metre of excavated section) every 8-9 hours of work, including the mucking process. As pointed out earlier, every two hours of excavation, the machine had to exit the tunnel to allow the loader to enter and haul away the material piled at the excavation face outside the tunnel, averaging four trips per work day.