Groundwater ingress can cause a wide range of problems in an underground rock space throughout the construction phase and after, during the usage of the space. Rock grouting is one of the most common methods in waterproofing underground spaces (Sievänen & Hagros, 2002). The grouting design of a space includes the selection of the grouting materials to be used. Traditionally, the material selection is based on previous experiences in similar projects and environments. The main objective of this paper was to experiment the suitability of a new systematic method for the grouting material selection. The method tested for this purpose is called the Analytic Hierarchy Process (AHP), which belongs to the group of multi-criteria decision-making (MCDM) tools. This paper is a short description of the full study conducted for the master's thesis (Lehtonen, 2019) by the same title.
This study consists of two main parts, an introduction to the AHP and a case study in which the AHP was implemented into a shaft excavation project at Kittilä mine. In the first part, a short theory behind the AHP method is presented and an example is provided for further understanding of the method. This theory was then used to implement the AHP methodology into grouting material selection to determine the most suitable grouting material for the Kittilä shaft project. The three phases - decomposition, comparison and synthesis - of the AHP analysis were conducted for the case study. In the decomposition phase the hierarchy was created according to Kittilä shaft project needs and requirements and the main criteria, i.e. execution of grouting work, costs and schedule, and environmental conditions affecting the grouting material selection, were determined. In the comparison phase the material options were compared pairwise in comparison matrices at the lowest level of the hierarchy and the relative importances of the selected criteria were established. Material alternatives in the analysis were microcement and standard cement, and modern-day chemical grouts: polyurethane, colloidal silica and acrylics (Halls, 2016, cited in Hänninen, 2017). As a result, in the synthesis phase of the analysis, the alternative grouting materials were ranked numerically by their suitability to the underground rock construction project in question. The most suitable material according to the AHP analysis for Kittilä shaft project was determined to be micro-cement.
In conclusion, the grouting material selection could be successfully implemented into the AHP methodology and a reliable result was obtained. However, the analysis can only suggest for the most suitable material for grouting, and as such cannot guarantee a successful grouting outcome. The AHP analysis can only compare the selected material options according to the criteria used in the analysis. The results from the analysis must be critically analyzed, since simplifications are often needed in the analysis process. It is suggested that the AHP analysis can be used as a supporting tool in design to systematically compare grouting material alternatives and to provide transparency in a complex decision-making process.