STABILIZAION WORKS OF TUNNEL WITH STEEL FIBER INSTEAD OF WELDED WIRE MESH AS SHOTCRETE REINFORCEMENT, TRACK DOUBLING PROJECT FOR NORTHEASTERN LINE MAB KABAO - THANON CHIRA JUNCTION CONTRACT C3 - TUNNEL WORKS, THAILAND

摘要

Track Doubling Project for Northeastern Line Mab Kabao - Thanon Chira Junction Contract C3 - Tunnel Works is situated in Saraburi Province and Nakhon Ratchasima Province of Central and Northeastern Thailand. It comprises of three railway tunnels with a total distance of 12.5 km. It is one of the flagship track doubling projects of the State Railways of Thailand of which the contract was awarded to the Joint Venture of ITD - RT as an EPC contractor. The stabilization works of the tunnel during construction period and for its long-term use with utmost safety of the workers and the train passengers is of prior importance. One of those stabilization works is the shotcrete supporting work. The tunnels had been excavated using a drill and blast method through various kinds of rock conditions of sandstone, siltstone and shale. A typical tunnel section is designed for two single track tunnels of approx. 8×9 m (W × H), horse shoe-shaped cross section and one double track tunnel of approx. 12×9m (W × H). According to initial technical specification, the shotcrete support is required with welded wire mesh of 4mm and 150mm square reinforcement fixing to the specified tunnel crown and walls before shotcrete spraying. The compressive strength of the shotcrete at 28 days of 30 MPa is required. To date, the shotcrete spraying onto the fixed welded wire mesh had been carried out for a period of about six (6) months since beginning, using a minimum of eight (8) workers for a time consuming of about 3-4 hours for fixing the welded wire mesh of approx. 200m~2 whereas the total area of the mesh fixing is more than 200,000 m~2. Thus, the Contractor has proposed the steel fiber instead of the welded wire mesh as the shotcrete reinforcement to the Client. Its usage advantage is evidently the savings of the manhour of labor that is due to no fixing of the welded wire mesh. The ready-mixed shotcrete with steel fiber from concrete batching plant can be immediately sprayed to the tunnel surfaces. Meanwhile the initial requirement of steel fiber content of 60 kg per cubic meter of shotcrete was specified in the Technical Specification. However, the Contractor has proposed the deviating quantity of the steel fiber for the works of 25 and 30 kg per cubic meter of shotcrete. Further, the quality control of the shotcrete with steel fiber reinforcement is subject to the European standard of EN-14488-5 testing of which energy absorption of test panels shall not be less than 700 Joule.