Tunneling under high overburden and high in-situ stress may lead to tunnel instability due to rock overstressing in its surrounding. Overstressing of the hard rock tunnel can cause brittle fractures in the form of rock burst or spalling, which may cause safety hazards, damage of equipment and production disturbances. Therefore, the evaluation of overstressing in deep hard rock is crucial prior to tunnel construction to minimize risks during excavation. In this study, the potential of overstressing at the Pahang-Selangor Raw Water Transfer tunnel was evaluated between Chainage of 6.8 to 41.5 km beneath the Titiwangsa Main Range, with high overburden of up to 1200 m. In addition, field observations were compared with predictions from empirical and analytical methods. The empirical assessment approach to the observation of actual overstressing appeared to be valid for the prediction of overstressing. The analytical method was used to analyze the tunnel's behavior under high overburden stress based on factors of strength and tangential stress. Analyzed results have shown that certain tunnel sections with high overburden have the possibility of moderate spalling. These approaches allow reasonable predictions of tunnel behavior under different rock conditions, support systems and overburden stress which serves as a useful tool in the observational design and construction method for long and deep tunnels.

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