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INNOVATIVE FOREPOLING IN TURIN’S GRAVELY SOIL FOR T HE VAL METRO LINE 1 EXTENSION

Pelizza Sebastiano1, Roberto Crova2, Di Bella Rosario3, Blandino Luigi3, Alessio Carlo4

1Politecnico di Torino – University of Technology - Turin 2G.T.T. S.p.A.- Turin

3Co.ge.fa. S.p.A. - Turin 4A&K Geotechnical Engineering Company - Turin

Keywords: Ground reinforcement, self drilling rock bolts, silicate injection foam resin short reaction time LAYOUT OF THE WORK The work concerns the Turin automatic underground system – Southern extension of Line 1 – Lot 1 between the Porta Nuova station (chainage 0+013) and the Marconi station (chainage 0+594). Lot 1, which was built by A.T.I. Lauro S.p.A. – Co.GE.Fa S.p.A., is characterised by three macro types of intervention: - excavation of a natural tunnel (named GN1) with two tracks (useful internal radius of 4.00 m)

dead hole excavated with traditional method, prior to roof reinforcement from above with cement injections, which develops along a curve for about 94 m from the Porta Nuova station and crosses under the Corso Vittorio Emanuele II and Via Nizza crossroads;

- construction of an artificial tunnel – a three track stretch (with a useful internal width of 10.00 m) between micropile sidewalls excavated with the cut and cover method for an overall length of about 195 m (including two head shafts called A and PL1);

- excavation of natural tunnel (named GN2-3) with two tracks (useful internal radius of 3.65 m) dead hole excavated with traditional method, prior to roof reinforcement from above with cement injections, which extends for about 292 m until the entrance of the Marconi station situated in Marconi Square.

Lot 1 is a rather brief stretch of the infrastructure (Porta Nuova – Lingotto is 3.600 metres long and there are 5 intermediate stations) which is just 600 m long. However, this stretch is characterised by different subjects and by a particular complexity due to a series of external constraints that condition the execution of the work: intervention in a highly urbanised zone of the city, characterised by a series of buildings of high value; the presence of an important sub-service network and sewage networks; the geology of the Turin subsoil, which is characterised by gravels that require consolidation and reinforcement work before excavation; the necessity of not interrupting the public and private road traffic in a crucial point of the city and last, but not least, the layout of this stretch which contains a very tight bend between Corso Vittorio Emanuele II and Via Nizza (R=55 m). The intervention zone in fact passes under a highly urbanised area, with heavy road traffic flows between Corso Vittorio II and Via Nizza. The area under question, which is right in the centre of the city, is characterised by the presence, and extreme closeness, of inhabited, historical and monumental buildings dating back to the XIX century. Finally, but surely not the least important, the environmental and geological conditions of the Turin subsoil are well known to those who are responsible for work in the capital city of Piedmont: the so-

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called “Turin gravel”, loose, pebbly gravel with few fines, which is sometimes in more or less cemented lenses and sometimes with large pieces of sandy muddy layers or lenses.

The Turin Municipality required an intervention that would involve the minimum impact on surface road traffic while avoiding costly deviations of the sewage and sub-service systems. The critical nature of the aforementioned situations resulted to be particularly significant for the construction of the GN1 natural tunnel, which is in a bend, with a bend radius of just 55 m and an overburden of about 8 m, a lower measurement than the diameter of the excavation section. It is in fact in the ambit of the bend that 3 principal sewage systems, arranged longitudinally along Corso Vittorio Emanuele II, must be passed under. These sewage systems, of considerable dimensions and carrying capacity and whose service cannot be interrupted, lie on the roof of the tunnel with a very reduced clearance of about 1.00 – 1.50 m on the tunnel boundary. The presence of these sewage pipe systems screens the tunnel and strictly limits the possibility of conducting regular preventive interventions from the ground surface.

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In order to deal with all these constraints and work in such a way as to guarantee complete safety, it was decided to first of all pass under the sewage pipe system by tunnel. The dead hole excavation procedure was therefore reinforced with an integrated consolidation intervention of the ground on the roof which basically eliminated the impact on the surface road traffic, thus avoided the necessity of deviating the sewage system; in this way it was possible to carry out the work quickly. It was therefore necessary to set up an original reinforcement technique in order to pre-support the tunnel that would be able to satisfy all the requirements necessary to obtain an optimal result of the work.

THE “PELIZZA METHOD” This technique consists in reinforcing the ground, along the tunnel border, with short threaded steel bars (length 3 m) which are durable (nominal external diameter of 38 mm, thickness 5 mm, maximum tensile stress equal to 580 kN, yield point 480 kN), self drilling (diameter of the drilling head “bit” of 76 mm) in order to transmit an injection of expandable bi-component organomineral silicate, and able to permeate even the most sandy and least permeable portions of ground. The resins that were used are constituted by a first component made up of modified silicates (density 1.15 kg/l and viscosity at 25° of 50 mPas) and a second component, a modified MDI iso-cyanate (density 1.23 kg/l and viscosity at 25° of 180 mPas). This resin, due to the reaction of two distinct components, has a remarkable capacity to expand (with an increase in volume of up to 50 times in unconfined conditions), which allows it to penetrate by some tens of centimetres into the ground. The use of this resin as an injection medium has guaranteed the formation of a consolidated layer of ground around the tunnel (with a thickness equal to about 30 cm and strength and compression characteristics varying between 3 and 3.5 MPa) in a few minutes (the hardening cycle is complete in about 2 minutes), which is able to improve the geomechanical characteristics of the ground itself, thus allowing advancement to begin again immediately. The consolidation method was experimented and perfected in a test site (Photograph 1) on an excavation face for the emptying of the cut & cover material.

Photo 1

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The consequent construction method- considering the difficulty of developing a realistic calculation of the structure constituted by the strip of reinforced and resin consolidated ground – was also experimented through dead hole excavation of a stretch of tunnel that was not subject to any particular surface constraints and which was closely monitored. In this way, a kind of curved comb that is able to guarantee the stability of the free span is created inside the ground (Figures 1 and 2, Photograph 2).

Figure 1

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Figure 2

Photo 2

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The work can then progress at full section excavation of about 55m2 (Photograph 3).

Photo 3

The preliminary support is made up of steel arches (with coupled IPN 160 profiles), with intervals of 1 m, a “bullflex” type thickness compensating element which determines the immediate settlement of the steel arches and 20 cm of cast concrete and an interposed electro-welded mesh. The reinforcement and excavation methodology that was used made it possible to avoid ground release phenomena, and thus to completely avoid possible over-excavation and to keep the excavation profile practically coincident with the geometrical dimensions foreseen in the tunnel design stage; this aspect was of particular importance while passing under the sewage pipeline where even limited subsidence or instability phenomena could have compromised the functionality with possible, even serious, repercussions on both the surface and on the excavation advancement face. The system is very flexible since it allows the intensity of the intervention to be varied in function of the nature of the ground and it intervenes on the number of bars, on their interaxes, and if necessary, on the length and frequency of the intervention, which can be at each steel arch or with a longer step. In the work under examination, the self-drilling bars are placed below the last steel arch installed during advancement and with such an inclination that the subsequent steel arch can be installed in a “leaning” position. However, in more difficult stability conditions, it is possible, using steel arches with simple profiles or reticular steel arches, to make holes in the core of the section and insert the bars inside the steel arch, thus further increasing the support and reinforcement of the ground function carried out by the steel bars (which result to be completely constrained to the steel arches).

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EXCAVATION PERFORMANCE During the work, it was possible to observe that the entire work cycle to obtain an advancement of 1 m of excavation, lasted about 12 working hours (thus allowing a mean daily advancement of 2 m): - installation of the self-drilling bars (n° 37): 2 hours; - injection of the bi-component resin: 6 hours (with a quantity of resin that varied between 20

and 25 kg per metre of injected bar); - full section excavation (55 m3 with length of shot equal to 1 m): 2.5 hours; - installation of the preliminary lining (steel arches and cast concrete): 1.5 hours. The cost of the consolidation treatment is determined by a fixed factor (the number of self-drilling bars which are constant in number and length) and a variable factor (the quantity of injected resin which is a function of the characteristics of the ground encountered as the excavation proceeds). As a rough idea, a mean overall cost of the consolidation technique can be evaluated as about 12.000 € per metre, 90% of which is determined by the specific consumption of the resins. An estimation of the incidence on the cost of the finished tunnel, also including the cost of the lateral cement injections, is about 30%.