HMC Analysis of a Tunnel in Swelling Rock

EURO:TUN 2009 2nd International Conference on Computational Methods in Tunnelling Ruhr University Bochum, 9-11 September 2009 Aedificatio Publishers, 477-484

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HMC Analysis of a Tunnel in Swelling Rock

Ivan Berdugo1, Leonardo do N. Guimarães 2, Antonio Gens3 and Eduardo E. Alonso3 1Department of Civil Engineering, PUJ, Bogotá, Colombia 2Universidade Federal de Pernambuco, Recife, Brazil 3Department of Geotechnical Engineering and Geosciences, Technical University of Catalonia, Barcelona, Spain

Abstract

The phenomenon of swelling in tunnels excavated through sulphate-bearing rock is quite frequent in some geological formations. The paper describes the case of the Lilla tunnel excavated in anhydritic-gypsiferous of Eocene age. Very large swelling displacements were observed affecting the first few meters below the tunnel inverts. In the paper, a fully coupled hydro-mechanical and chemical analysis is presented that takes into account the dissolution and precipitation of the anhydrite-gypsum chemical system and their associated hydraulic and mechanical effects. The analysis provide a better understanding of the swelling ground phenomena and offer a firmer foundation for taking engineering decisions concerning mitigating measures.

Keywords: Tunnelling in sulphate bearing rock, swelling rock, hydro-mechanical and chemical (HMC) formulation, numerical analysis, geochemistry

Ivan Berdugo, Leonardo do N. Guimarães, Antonio Gens and Eduardo E. Alonso

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1 INTRODUCTION

The phenomenon of swelling in tunnels excavated through sulphate-bearing rock (normally anhydritic-gypsiferous claystones) is well attested. Swelling appears preferentially at floor level, without noticeable movements in abutments and in the crown. In any case, ground heave and total radial pressures evolve at high rates without signs of stabilization in some cases. Recent key contributions to the study of swelling in hard gypsiferous-anhydritic clayey and marly rocks have been reported in [1], [3], [4]. Recently, the authors were involved in the analysis of swelling phenomena which affected three tunnels of the new high speed railway Madrid-Zaragoza-Barcelona during construction. Those tunnels were excavated through Tertiary moderately soft gypsiferous-anhydritic claystones from the Lower Ebro Basin (Northern Spain). In the paper, the case of Lilla tunnel, the longest of the three tunnels is briefly summarised. Afterwards, a hydro-mechanical and chemical (HMC) analysis is presented. Examination of the analysis results leads to an enhanced understanding of the phenomenon and its evolution.

2 GEOLOGY AND GENERAL FEATURES OF THE LILLA TUNNEL

The Lilla tunnel runs through Early Eocenic argillaceous rocks containing anhydrite and a complex system of cross-shaped moderately dipping fibrous gypsum veins. The excavated material consists basically of a horizontally-oriented monotonic series of gypsum-bearing brown argillaceous rocks. Expansive clays were only occasionally detected in isolated points of the matrix. An important aspect is the existence of a persistent system of open low friction-angle slickensided surfaces. They are related to strong kneeling folds produced by the high curvature radius of the regional tectonic. Groundwater in the tunnel is highly mineralized, with important contents of both sulphates (1783 mg/l) and calcium (500 mg/l) The tunnel has a length of 2 km and it was excavated with a horseshoe cross-section of 117.3 m2 (radius of the vault: 6.76 m). Overburden varies between 10 m and 110 m. Excavation was performed by drill and blast from the two portals, dividing the section into head and bench. Temporal supports consisted in sprayed concrete and rock bolts; steel arch ribs (HEB 160) were only installed in zones of low quality rock. Lining consisted in 300 mm thick mass concrete (25 MPa).


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Figure 1: Geological longitudinal section of the Lilla tunnel

A 300 mm thick flat-slab constructed in mass concrete (20 MPa) was placed on the tunnel floor, but it was only concreted after the total excavation of the bench. Therefore, the floor of the tunnel was exposed to the action of environmental agents during the construction period. Due to the low permeability of the massif, waterproofing of the excavated section was restricted to portals.

3 HEAVE OBSERVATIONS

First expansions were detected in the flat-slab just after the full tunnel excavation was completed. Large expansive phenomena occurred in a generalized way at floor level, but movements were only slight in the unlined vault. Heave was followed by damage to the longitudinal drainage system and, finally, by local failures of flat-slabs. Figure 2 illustrate the evolution of heave in a number of sections. It can be noted that the movements are quite large and do not show any indication of a decreasing rate of growth. After the ground heave was observed, a length of tunnel was provided with an invert arch. Instrumentation was also installed including total pressure cells on the underside and sliding micrometers to measure displacements under the floor. Figure 3a shows the swelling stress developing on the invert arch; very large stress values, typical of this type of phenonena, were measured. The distribution of heave movements in the ground is presented in Figure 3b. Only the first few meters of rock below the invert appear to be involved.

411+100 412+000 413+000

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m a.s.l. South Portal(Martorell)

North Portal(Lleida)

Middle EoceneLegend

Early EoceneQuaternaryLimestoneMarl

Claystone & SiltstoneGypsiferous-anhydriticClaystone

Colluvion


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0 100 200 300 400 500Time (days)

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Figure 2: Heave evolution measured in some sections of the Lilla tunnel

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(a) (b) Figure 3: a) Evolution of total radial pressure in sections with invert-arch. b) Distribution of

heave displacements in the ground below the invert (section 412+150)

4 FORMULATION OF THE ANALYSIS

The observed heave phenomenon results from the interaction between chemical processes (dissolution/precipitation) and hydromechanical behaviour. Hence, a proper analysis requires the performance of coupled THC computations. Recently


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such a formulation has been developed [2] and incorporated in the computer programme CODE_BRIGHT. For space reasons, only some basic features are outlined here. A chemical system containing anhydrite and gypsum is considered with their corresponding solubility constants:

[ ] [ ]4AnK Ca SO= ⋅ [ ] [ ] 24Gy wK Ca SO a= ⋅ ⋅ (1)

where aw is the activity of water defined as

exp exp(273.15 ) (273.15 )

o w m ww

l l

s M s MaR T R T − −

= ⋅ + ρ + ρ (2)

where the first term depends on the osmotic suction, so, and the second one is related to the matric suction, sm. If the following inequality holds: 2/An Gy wK K a< , anhydrite precipitates. If, on the

other hand, 2/Gy w AnK a K< , then gypsum precipitates.

The variation of porosity of the medium, φ, is affected by chemical and mechanical phenomena as:

(1 )(1 )

r

r

DMDDt M Dtφ −φ= + −φ ∇

+u (3)

where the first term expresses the chemically-induced porosity changes and the second term is the classical variation of porosity due to volume change. Mr is the sum of reactive minerals that is obtained from the geochemical model. Finally, the volumetric deformation associated with precipitation/dissolution is assumed to be linearly related to the change of reactive mineral.

5 RESULTS OF THE ANALYSIS

Two phenomena are believed to underlie the observed heave phenomena: infiltration of water into the rock and evaporation of the groundwater into the tunnel. Both have been analyzed in one and two dimensions with the HMC formulation, only the results of the 1-D modelling of the water infiltration case are shown here. Figure 4 presents the evolution of the observed heave with time. As shown in Figure 5a, movements take place in the first 4 metres or so, consistent with observations. Those displacemnts are naturally related to the amount of precipitated mineral that increases with time (Figure 5b).


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Figure 4: Computed heave evolution due to water infiltration and mineral precipitation

(a) (b)

Figure 5: a) Distribution of heave displacements b) Distribution of precipitated mineral. 1-D analysis of water infiltration case

6 CONCLUSIONS

A coupled hydromechanical and chemical (HMC) analysis has been performed to simulate the phenomenon of swelling ground driven by processes of dissolution and precipitation of gypsum and anhydrite. A real case, concerning a tunnel of excavated in Eocene claystone has been used as reference. This type of analysis can provide a more rational basis for the understanding of the swelling ground phenomena and offer a firmer foundation for taking engineering decisions regarding mitigating measures.

ACKNOWLEDGEMENTS

The support from ADIF to this work is gratefully acknowledged.


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REFERENCES

. [1] Amstad, C. & Kovári, K. Untertagbau in quellfähigem fels. Eidgenössisches

Departement für Umwelt, Verkehr, Energie und Kommunikation (UVEK) & Bundesamt für Strassen (ASTRA), Zürich, 2001

[2] Guimarães, L. do N., Gens, A. & Olivella, S. Coupled thermo-hydro-mechanical and chemical analysis of expansive clay subjected to heating and hydration, Transport in porous media, 66, (2007), 341-372

[3] Kovári K. & Descoeudres, F. Tunnelling Switzerland. Swiss Tunnelling Society, 2001

[4] Wittke, M. Design, construction, supervision and long-term behaviour of tunnels in swelling rocks. In Van Cotthen et al. (ed.), Metaphysics coupling and long term behaviour in rock mechanics, London: Taylor & Francis, 2006

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HMC Analysis of a Tunnel in Swelling Rock