Hydrogeological prognosis with regard to realisation of the new Colle di Tenda road tunnel.

Author:Banzato, Cinzia
 
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INTRODUCTION

With the prospect of doubling the road tunnel that, via Colle di Tenda, joins Italy and France, the National Roads Authority (ANAS) based in Rome asked the Dipartimento di Ingegneria del Territorio, dell'Ambiente e delle Geotecnologie (DITAG) of Politecnico di Torino to make the preliminary hydrogeological prognosis, based on existing information and on information gathered during the research project itself. The geographical location of the area under assessment is on the border between Italy (southern Piedmont) and France (Provence-Alpes-Cote d'Azur), at the Colle di Tenda pass, where there is an existing road tunnel and a railway tunnel (Fig. 1).

In agreement with its French counterpart, ANAS considered two initial approaches to the project:

* Widening of the existing road tunnel and excavation of a new tunnel (high altitude solution)

* Excavation of a new twin-tube tunnel (low altitude solution)

Table 1 shows the relevant data for the existing and planned tunnels.

Table 1: Specifications for existing and planned tunnels Altitude of Altitude entrance on of entrance Italian on French side Section Length Tunnel side (m a.s.l.) (m a.s.l.) ([m.sup.2]) (m) Rail tunnel 1037 1010 8.099 Road tunnel 1320 1276 34 3.195 Planned 1320 1276 63 3.268 road tunnel Prior to consideration of problems of a purely geomechanical and specifically planning nature, choice of the best solution immediately appeared to be conditioned by problems of an applied hydrogeology and environmental nature, which, by the way, had already cropped up when the rail tunnel was excavated. Whatever the final choice, the new tunnel will in any case intercept a carbonate aquifer that feeds a series of important flows of groundwater, named "Sorgente di Tenda (Tenda Spring)" (Fig. 2), which were discovered during excavation of the railway tunnel (1889-1898).

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

In 1990 the spring was tapped in the tunnel by the Langhe and Alpi Cuneesi Aqueduct; this infrastructure is very important for the social-economic development of a vast area, which has few other water resources suitable for human consumption and which supplies, along with some other sources, a population of over 100,000.

The purpose of the prognosis includes gaining an understanding of the hydrogeological situation of the entire rock mass covered by the works, in order to foresee both problems that may arise during construction due to the presence of water and also the possible effects that excavations may have on the important water resource that is tapped from the railway tunnel below. With this in mind, a series of operations have been planned, set up and carried out, the most important of which are as follows:

* General hydrogeological survey to identify the structure of the aquifer and the area supplying it

* Census of internal and external groundwater points with qualitative-quantitative monitoring

* Positioning of a number of well loggings and interpretation of resulting data as regards the water-bearing structure

* Assessment of the effects of excavation on the water resource, from the quantitative point of view and also because of the materials used for lining, waterproofing and whatever may be necessary due to the karst nature of the rock formations

This study summarises over three years' work and the results achieved, with completion of the prognosis.

MATERIALS AND METHODS

Hydrogeology and water-bearing structure of the area of interest: The principal stratigraphic units forming the area of interest fall between the upper Mesozoic and lower Cenozoic eras. To summarise as far as possible, ignoring losing any of the geological and hydrogeological information, Table 2 has been drawn up for use in comparison of the maps to be presented later on.

Table 2: Hydrogeologic formations in Colle di Tenda Hydrogeologic Geological Lithological and deposition complex era characteristics Detritic-moraine Quaternary Silt, sandy silt, gravel, slope-type drift deposits and headers. Upper Flysch Eocene Alternations of prevalent shale with local arenaceous intercalations. Upper limestone Jurassic Stratified marly limestone, -Eocene calcarenites, cemented polygenic conglomerates (Eocene); limestone and dolomite limestone (Jurassic) Shale-evaporite Triassic Polychrome shale, breccias, Carniola, local chalky lenses, limestone breccias and dolomite limestone layers Lower Flysch Eocene Pelite and shale with sandstone, with dense stratification. Lower carbonate Triassic Limestone, marly limestone, -Jurassic dolomitic limestone and dolomite. Evaporite Triassic Thick masses of gypsum, anhydrite and Carniole with highly variable geometriescterised by relatively developed karst, with respect to thickness and extent. Hydrogeologic Hydrogeological Role of water complex characteristics -bearing structure Detritic-moraine Relatively variable Secondary aquifer that permeability, from interferes with the future low values in silty tunnel but not with the lithotypes to high Sorgente di Tenda in detritic. Upper Flysch Generally very low Above the upper Carbonate permeability. In formation, this is an highly fractured important overlaid zones permeability permeability threshold is distinctly higher. that gives rise to a thick karst saturated zone that is part of the water-bearing structure that feeds the Sorgente di Tenda. Upper limestone Relatively high Main aquifer feeding permeability related the Sorgente di Tenda to heavy fracturing In Eocene lithotypes, in general, fracturing is lower but karstification of the discontinuities is greater. Shale-evaporite Generally very low Aquifer linked to the permeability with main one conditioning the exception of the chemical properties the more fractured of the water from the horizons where Sorgente di Tenda permeability is greater. Lower Flysch Generally low Below the upper permeability with Limestone formation the exception of the and the Shale-evaporite fractured and formation, this is an cataclastic horizons important definite which often show a permeability limit degree of water that conditions water circulation. circulation in the main aquifer. Lower carbonate High permeability Secondary aquifer feeding due to karst and a series of water inflows fracturing. involving the French section of the rail tunnel. Evaporite High permeability in Secondary aquifer feeding sectors chara cterised a series of water inflows by relatively developed with relatively low karst, while in flow rates and intercepted relatively compact and by the current and scarcely fractured planned road tunnels sectors permeability is relatively low. servations made in the existing road tunnel show the chalk-anhydrite mass to be a very low yielding aquifer while the Carniole horizons show a significant water circulation setting in the main discontinuities and in the karst network The water-bearing structure of the area is conditioned by the presence of a main thrust surface, dipping to the NE, which divides the autochthonous sedimentary series of the Dominio Esterno Delfinese from the Falda Sub-Brianzonese. This surface is displaced by numerous sub-vertical tectonic discontinuities that significantly complicate the geometry of this contact. Moreover, within the Dominio Esterno Delfinese there are several discontinuity surfaces, also dipping towards NE, between the Flysch formation and the Evaporitic formation.

In the Falda Sub-Brianzonese succession the upper Carbonate formation, which contains the main aquifer, is in contact with the underlying Argillite-Evaporite formation concerned with the thrust plane and hence, characterised by considerable deformations with variations in thickness. This horizon conditions groundwater circulation towards the Sorgente di Tenda.

The upper Flysch formation, above the upper Carbonate formation, acts as an important overlaid permeability threshold that gives rise to a thick karst saturated zone that is part of the water-bearing structure that feeds the main spring. Figure 3 and 4 show the plan view and section of the mountainous massif, as taken from observations made in the current road tunnel.

The area feeding the spring is a narrow, straight NW-SE band, which goes from the deep Cabanaira valley to the Val Grande di Palanfre, covering a total of about 8 [km.sup.2]. GIS processing (Civita et al., 1999) was applied to the HYDRAC simulation model (Civita et al., 1982) to calculate the potential balance. The infiltration contribution of the overall area was found to be 0.24 [m.sup.3] sec1 and hence only a little lower than the average flow rate for the spring, as measured between 01/01/2003 and 25/11/2003 and amounting to 0.25 [m.sup.3] [sec.sup.-1].

The subject under examination: the Sorgente di Tenda: In order to better understand the characteristics of the water-bearing structure that govern birth of the spring and the general hydrodynamics of the system, a number of continuous well logging cores were planned and drilled and subsequently fitted as monitoring wells (Table 3).

Table 3: Properties of the core drillings Drilling Altitude Depth(m) Core sample Posizione tratto (m a.s.l.) filtrato (m) SB 1453 150 Continuous core from 100-150 sample SC 1565 110 Double samples from 85-100 SA 1385 280 Continuous core from 240-275 sample SA2 1385 75 from 50-75 SI3 1507 232 Continuous core sample [FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

Due to violent inrushes (Fig. 6), study on summarises the information shown by the core drillings, while their positions are shown in Fig. 5.

Between the set points at 2489 and 3058 m from the northern entrance to the railway tunnel, a thick carbonate series was found, consisting of highly permeable Jurassic and Eocene limestone and giving flow rates of the order of 0.4 [m.sup.3] [sec.sup.-1], on average. The aquifer is intercepted in the zone where the contact (permeability threshold) is situated, between the upper limestone formation and the upper Flysch formation. The rail tunnel was delayed...

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