Groundwater chemistry affected by underground construction activities
A leaking underground construction increases the groundwater recharge and decreases the residence time of the groundwater. If groundwater of a certain chemical composition dominated the location inside the bedrock of a planned tunnel prior to the construction phase, this water is likely to be replaced by water of a different chemical type because of leakage of water into the tunnel during the construction and operational phases. The changes in hydrological and hydrochemical conditions may in turn result in changed chemical processes and interaction between the water and the bedrock/fracture minerals. The processes that may be altered include acid base, redox and ion exchange processes. The changes in the chemical composition of the groundwater may be of importance to the degradation of the construction materials.
This thesis presents experience from one field study, three case studies and numerous studies mentioned in the literature. The field study and the three case studies show that underground constructions are likely to cause changes in the water chemistry. The hydrochemical changes have largely been seen to be dependent on geological conditions.
Common changes in water chemistry often include acidification and decreased alkalinity as a result of the oxidation of sulphide in wetlands or in fracture minerals that form sulphuric acid. This hydrochemical process has been seen to be triggered, thereby causing more aggressive hydrochemistry during the construction phases of the Hallandsåsen and Romeriksporten rail tunnels. Changes in the concentration of calcium and magnesium through cation exchange following acidification have been observed, primarily in the field experiment at Lake Gårdsjön.
Underground constructions often cause an increase in organic matter in the bedrock groundwater, which is measured as organic carbon. Decomposition of organic matter may lower the redox potential. Furthermore, the use of cementitious grout is likely to increase the pH near an underground facility. Changes in redox potential and pH are likely to have an impact on the redox state of iron and manganese. A more reducing environment may cause iron and manganese to become dissolved while a rise in the pH may lead to precipitation. The precipitation of iron and manganese is the most common cause of clogging of the drainage system in underground facilities.
Underground constructions have been seen to cause changes in the chloride concentrations. These observed concentration changes include an increase through the upconing of deep marine or brine waters. However, a reduced chloride concentration through deeper penetration of shallow meteoric water has also been observed.
The hydrochemical changes may have implications for construction materials, such as steel bolts, waterproofing grout and shotcrete, used in the underground constructions. However, hydrochemical changes that may shorten the lifespan of the construction materials can be limited through grouting and thereby the hydrological impact of the construction activities in sensitive locations. There is a need to establish and implement models to predict the prevailing hydrochemical conditions in the construction and operational phase of a facility based on information from the pre-construction phase.