Abstract

Many of the large number of underground works constructed or under construction in recent years are in unfavorable terrains facing unusual situations and construction conditions. This is the case of the subject under study in this paper: a tunnel excavated in evaporitic rocks that experienced significant karstification problems very quickly over time. As a result of this situation, the causes that may underlie this rapid karstification are investigated and a novel methodology is presented in civil engineering where the use of saturation indices for the different mineral specimens present has been crucial. The drainage of the rock massif of El Regajal (Madrid-Toledo, Spain, in the Madrid-Valencia high-speed train line) was studied and permitted the in-situ study of the hydrogeochemical evolution of water flow in the Miocene evaporitic materials of the Tajo Basin as a full-scale testing laboratory, that are conforms as a whole, a single aquifer. The work provides a novel methodology based on the calculation of activities through the hydrogeochemical study of water samples in different piezometers, estimating the saturation index of different saline materials and the dissolution capacity of the brine, which is surprisingly very high despite the high electrical conductivity. The circulating brine appears unsaturated with respect to thenardite, mirabilite, epsomite, glauberite, and halite. The alteration of the underground flow and the consequent renewal of the water of the aquifer by the infiltration water of rain and irrigation is the cause of the hydrogeochemical imbalance and the modification of the characteristics of the massif. These modifications include very important loss of material by dissolution, altering the resistance of the terrain and the increase of the porosity. Simultaneously, different expansive and recrystallization processes that decrease the porosity of the massif were identified in the present work. The hydrogeochemical study allows the evolution of these phenomena to be followed over time, and this, in turn, may facilitate the implementation of preventive works in civil engineering.

Highlights

  • Introduction and ObjectivesKarst in evaporitic rocks presents a large variety of risks associated with any type of infrastructure

  • Since evaporitic rocks can be karstified in months and/or years, continuous monitoring and surveillance of the hydrogeological behavior of the massif near a tunnel is crucial during its useful life

  • The main objective of this work is to contribute to a greater knowledge in the gypsum and evaporitic rocks karstification field in those massifs crossed by tunnels by applying the saturation index of the mineral specimens present in them. To reach this main final objective, we address, in an orderly manner, the following specific objectives: (i) the definition of the geological and hydrogeochemical conceptual model of the site, characterizing the local petrology and mineralogy; (ii) the chemical characterization of the groundwater and its evolution over time; (iii) the study of the groundwater chemical stratification and the potential relationship between the stratification and the geology and mineralogy of the area; (iv) the determination of equilibrium states of the saturation reactions of the groundwater regarding the different evaporitic minerals; and (v) the identification of the modifying processes of mineral transformation as a consequence of the flow alteration caused by the drainage operations

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Summary

Introduction

Karst in evaporitic rocks presents a large variety of risks associated with any type of infrastructure. Risk can persist during the construction phase, and throughout the useful life of the infrastructure, since detecting in advance all problems of this nature is practically impossible [1]. Since evaporitic rocks can be karstified in months and/or years, continuous monitoring and surveillance of the hydrogeological behavior of the massif near a tunnel is crucial during its useful life. Regarding the risk reduction strategy during the previous phases of the investigation, the geological predisposition constitutes the most important factor in determining the karst conditions that could occur on site during construction. Of the set of factors that explains the Sustainability 2021, 13, 1505.

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