Evaluation of grouting design considering favourable and unfavourable geological conditions

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In Scandinavia, continuous pregrouting is performed to seal the rock fractures and lower down the leakage of water after the rock excavation. The requirements for leakage are based on the assessment of adverse effect on environment due to the lowering of ground water level and decided by the environmental court. The requirements for leakage in urban areas can be strict, e.g., 2-5 l/min for 100 m tunnel. This makes pregrouting a very significant part of tunnelling in hard rock conditions considering the time and budget of the construction work. In Sweden, pregrouting design is performed based on geological and hydrogeological field investigations and the difficulty level of grouting to reach the requirement for leakage. The steering parameters for the grouting stop criteria are the pumping time under constant pressure and the grout volume. The design of the stop criteria is performed based on scientific research and experience from previous projects. In this paper, the evaluation of the grouting data from certain access tunnels of the extension of the Blue metro line in Stockholm are presented. The extension of the Blue line of the Stockholm Metro project involves six underground metro stations, which are approximately 11 km of tunnelling in total. The aim of this work was to come up with recommendations for the optimization of grouting stop criteria, which can be used in future projects with similar geological conditions. For this purpose, evaluations were performed for different parameters of the stop criteria, e.g., grouting time, volume, flow etc. The whole evaluation was based on the grouting data from 115 grouting fans which were compared with the design work. Data from 50 fans are presented in this paper for two access tunnels Londonviadukten AT1 (31 fans) and Järla Östra (19 fans). The achieved sealing efficiency based on the measured leakage into the tunnel was estimated due to many inaccuracy and external factors. The estimated achieved sealing efficiency was then compared with required sealing efficiency set to reach the requirement for leakage according to the leakage prognosis. The geological and hydrogeological field investigations during the design phase were summarized and later been compared to the actual geological field mapping that was performed during the construction work. Finally, recommendations were made to optimize the stop criteria for grouting design, considering favourable and unfavourable geological conditions.

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