Following the natural course of the Valcarce River from Villafranca, Spain’s north west motorway, in the El Bierzo district of the Province of Leon, begins a climb to the Piedrafita Pass. The highway, now in its final stage, will improve links between Galicia and La Meseta, opening the door to the Burbia River valley and to Ambasemestas.
The 16.7km long section has two 7m wide roadways, with 2.5m of exterior shoulder and 1m of interior shoulder in each direction. The roadways are separated by a central reserve with an average width of 11m.
The route includes the construction of 18 twin viaducts, as well as two sections of twin tunnels, the 500m long Villafranca and 250m Trabadelo tunnels.
The project was awarded to Bierzo joint venture, made up of NESCO, ACS and TECSA, with a construction project from AEPO. TYPSA has an active role as control and monitoring unit, permanently at the job site, as well as being technical assistant to professional staff.
Geology
The materials passed through are from the Los Cabos Series, integrated with quartzites and metamorphic sandstones from the Upper Cambrian and Ordovician periods. Schists insertions with predominately metamorphic sandstone correspond to a sandstone with a low level of metamorphosis and a quartz contents of around 50%.
Three lithological groups were found: alternation of quartzites and metamorphic sandstones, schists and alternation of schists and quartzites.
The quartzites and metamorphic sandstones appear in stratified layers with a varying thickness between others that can present thin insertions of schists. In compression tests, the values oscillate between 16 and 140MPa.
The alteration of this group is low – grades II-III and I. The fracturing, in RQD terms, is reduced (from 40-75% – 15% in the opening zones).
The schists appear stratified in packages of 0.2 to 0.5m and occasionally present quartzite insertions. Its strength varies between 18 and 90MPa. The alteration is low (grade II) and fracturing reduced (RQD of 75%).
Finally, the alternation of schists and quartzites is presented in layers with varying thickness whose resistance varies between 40MPa for the schists and 78MPa for the quartzites. Alteration is low – grade II – while fracturing is high, with RQD values of between 0 and 25%.
In the sections where these materials have been affected by fault zones, the alternation has logically increased, reaching grade IV, and the RQD values notably decreased.
Geometry and function
The tunnels, 14m wide and 9.5m high, are separated by more than 1.5 diameters, and thereby considered independent. They have a capacity for two roadways and can be expanded in the future to three.
Excavation plan
Dealing with rocks with a relatively high resistance, the planned construction system was the New Austrian Tunnelling Method (NATM) with excavation by blasting an advance heading of 5.5m height and excavation of the bench in a later phase.
The support is made up of sprayed concrete, with fibre reinforcement, or alternatively welded mesh, bolts and steel ribs. The length of the passes varies between 1m and 2m.
For primary support of the rock mass, three different sections were considered in the project; their characteristics are summarised in Table I.
In addition, for the Villafranca tunnels, a model section D was included in the project, which, starting from the characteristics of model section C, incorporates a row of 32mm bars, each 0.5m long.
To minimise the height of the face excavations in the opening zones, the attack face of the tunnel was advanced with respect to the initial plan. First the excavation was secured by a pre-support umbrella of 24m long micropiles, 0.3m apart and staggered, and reinforced with metal tubes of 90mm outside diameter and 6mm thick.
Blasting called for 45,695kg of gel dynamite, 53,367 electric detonators of high insensitivity, and 11,350m of detonation string weighing 100g/m.
Two types of detonators were used: micro timers with high insensitivity (AI) of 30 milliseconds (14 cycles) and timers with high insensitivity (AI) of 500 milliseconds (12 cycles). In the larger blastings, up to 100kg of explosives and 90 detonators were used.
The fine cut was also applied to the excavation blast pattern.
Mechanical or mixed breaking up
In areas of poorer geotechnical quality of very intense fracturing or weathered rock mass, mechanical excavation was used – shovel, backhoe and hammer-pick – with the occasional use of small amounts of explosives to fragment the more solid zones (snakeholing).
Progress of the excavation
The suitability of the support adopted for each advance comes from the determination of the rock mass quality indexes by RMR (Bieniawski, 1979) and Q (Barton, Grimstad et al 1993). These indexes, are summarised in Table 2.
From a geomechanical classification point of view, relatively low quality indexes were obtained, with RMR between 29 and 61 and Q between 0.10 and 3.64, which, along with the strong registered convergences, have conditioned the type of support selected in each advance.
With this information, support section C was adopted all along the length of the tunnels, except in the areas of the portals, where the section foreseen in the project plan was used. In both tunnels, the evolutions of the convergences were verified, in accordance with a string diagram in the verification sections established every 10m.
The following magnitudes were registered at the origin:
Villafranca tunnels – maximum convergences:
On string E (horizontal, during the excavation): 197 mm.
Convergence results
The main increase (more than 90% of the convergence total) was produced almost simultaneously with the carrying out of the excavation. The convergence speeds were attenuated until stability was reached.
In both cases, the registered magnitudes exceeded, by more than 15 times, the reference values established in the project, (for predominant conditions: horizontal convergence from 1 to 10 mm; for fault conditions: horizontal convergence from 4 to 15 mm).
The strong displacements registered in the first advances determined the reinforcement measures, (for example bolts in the trusses of the excavation zone, bracing of the section by means of extending the concreted slab, etc), which permits a decrease in the convergences during the excavation progress.
Average output
During construction of both tunnels, day and night shifts were worked, placing the supports in advance sections, with spacings of between 1m and 2m, achieving an average output of 2.25m/day at each face. Later on the zone was excavated and supported with an average output of 5m/day.
Support considerations
The actual adaptation of support to each particular case, derived from the empirical considerations or from complex numeric models, should be based on the observation (monitoring) of the real behaviour of the mass-support group in relation to the excavation until a balance is reached, ultimately following the NATM.
In this sense, the magnitude of the registered convergences and their evolution advised, from a safety point of view, the adoption of the model section most resistant, even reinforced. The adoption of a resistant lining was not considered until construction was well under way, for which the primary support should be definitive.
As for the sprayed concrete, the average thickness executed was around 400mm, much more than the 150mm that was theoretically established for model section C. This was due to the high fracturing of the mass noted above.
Indeed, this characteristic prevents strict adherence to theory, with the existing recesses and projections caused by the frequent loosening of blocks and plugs. A much greater thickness of gunite was generally needed than that deemed necessary in accordance with the section chosen. Besides conveniently covering the support elements, meshes and trusses ensured that the shotcrete works predominately in compression.
According to the real geometry of the excavation, buckling or tension stresses could call for locally determined shotcrete zones with thicknesses of less than 300mm of sprayed concrete and therefore produce a much less efficient resistance response.
Sealing and final lining
To prevent any possible filtrations at the joints or accumulations of humidity, a continuous PVC laminate with a thickness of 1.2mm was used in addition to a geotextile layer of 500g/m² to protect the laminate holding up the primary support.
The section has been finished with a lining in H-25 plain concrete, 300mm thick, continuous and concreted in situ with a travelling formwork along the arch. Initially, the construction project proposed a finishing with prefabricated slabs, but the continuous lining was considered convenient, due to the significant convergences detected in the primary support.
