Visual inspection and defects of unlined and lined water transfer tunnel

Visual inspection was conducted at a water transfer tunnel with 780 m length situated in Perak, Malaysia. The water tunnel consists of unlined rock tunnel with concrete lining, shotcrete lining and rock bolts as support system. Visual observation together with Terrestrial Laser Scanning (TLS) was used to conduct the visual inspection. During the inspection, the defects and condition of the tunnel support system, as well as the location of the defects were recorded. The defects were rated with the condition state rating in accordance to the Specification of National Tunnel Inventory. It was found that the major defects of the water tunnel are water seepage or water leakage, efflorescence, and corrosion, which mostly occurred at shotcrete lining with or without rock bolts. Localised rock fall was also observed in some of the area of the natural rock wall. The water tunnel is rated as defective which the defects have significant impacts on the tunnel safety and required repair works in medium term. Based on the comparison with previous case studies, it is suggested that the surrounding geology zone of the unlined tunnel and the periodic rainfall are the absolute factors to defects of tunnel. Reinforced shotcrete, grouting, rock bolt replacement and drainage system are the recommended measures to strengthen the tunnel stability. Advanced technological tools such as GPR, LiDAR, high resolution camera or even machine learning should be fully utilised in tunnel inspection to obtain a more comprehensive data and understanding on the tunnel condition.