Hydrogeological Reference Conditions for Assessment of Environmental Impact and for Grouting Design

Merisalu, Johanna (Chalmers University of Technology) | Fransson, Ǻsa (Chalmers University of Technology)

OnePetro 

Abstract

Dewatering of groundwater resources induced by leakage into underground constructions can cause land subsidence, damage to constructions and their foundations and disturbances of groundwater dependent ecosystems. To reduce the environmental impacts, safety measures, e.g. sealing fractures by grouting to reduce inflow of groundwater or artificial recharge to maintain groundwater levels, must be implemented. Site investigations of the total geological and hydrogeological conditions at a site before construction is, due to financial aspects, most often not possible. To handle these uncertainties in the design- and construction process, it is suitable to use the observational method, which include the idea of identification, confirmation or rejection, and revision of the most probable and unfavorable conditions, and predefined technical design solutions for conditions that can reasonable be anticipated or foreseen. To assess the geological and hydrogeological conditions at an early stage of a project we suggest that geological and related hydrogeological reference conditions are used. Fundamental to our approach using reference conditions is the grouping of materials with similar geological and hydrogeological conditions and engineering characteristics. In this paper, we present conceptualizations of five reference conditions common in western Sweden and two examples of reference conditions in Singapore. The conceptualization of reference conditions includes a description of: the geological material; the hydrogeological properties and behavior within the environment; and the engineering characteristics related to water control and grouting. Examples of technical design solutions used to adopt to project specific requirements for inflow and drawdown for underground constructions constructed in environments representing one of the suggested reference conditions in western Sweden are also presented to exemplify the application of reference conditions for technical design.

1. Introduction

As demand for land above the ground surface increase, it becomes more attractive with underground facilities. Dewatering of groundwater resources induced by leakage into underground constructions is known from several underground projects around the world (Kvӕrner and Snilsberg, 2013; López-Fernández et al., 2012). A lowered water table may result in land subsidence and damage to buildings and their foundation (Roy and Robinson, 2009; Xue et al., 2005), impacts on groundwater dependent ecosystems such as peatlands, streams, springs and lakes (Kvӕrner and Snilsberg, 2008) and changes of the groundwater chemistry (Mossmark et al., 2008). Furthermore, large inflows of water into underground constructions can cause casualties, economic losses and adverse working conditions (Coli and Pinzani, 2014; Hou et al., 2016).

According to Chapter 11 in the Swedish Environmental Code, all water operations (change of water level, land drainage, groundwater drainage or groundwater infiltration) needs an environmental court ruling from the Land- and Environment court (Naturvårdsverket, 2008). The project specific court rulings often include requirements regarding maximum allowed groundwater drawdown or inflow into the underground space. To be able to state relevant requirements in the environmental court ruling, it must be ensured that the requirements are both technically relevant, achievable and measurable. Therefore, the hydrogeological descriptions must closely link to possible technical solutions for project specific requirements and to the monitoring set up.