At 44.6 km (27.7 mile), Malaysia’s Pahang Selangor Raw Water Tunnel will be the longest tunnel in Southeast Asia when complete. The three 5.23 m (17.2 ft) diameter Robbins Main Beam machines excavating the tunnel are advancing well with a novel type of shotcrete used as primary ground support under high cover.
The shotcrete is applied manually and consists of a continuous mixing and charging pump system, customized dry mortar mix with polypropylene fibers and a fast-acting accelerator. The shotcrete, developed by MCM Co. Ltd. and DENKA of Japan, has been used on numerous hard rock TBM and NATM projects in Japan, though this is the first time the system has been used internationally.
“The dry mix system is advantageous because it can be started and stopped whenever necessary, and can be mounted in the TBM back-up. Wet systems, by comparison, generate unused mortar if frequently started and stopped, requiring downtime for cleaning,” said Nobuo Suematsu, Marketing Director for MCM Co. Ltd.
MCM and DENKA worked with Robbins to create a near-zero rebound L1 shotcrete system to be used in the granitic hard rock of the tunnel, with rock strengths of up to 200 MPa (29,000 psi). The shotcrete is applied to the top 180 degrees of the tunnel following each TBM stroke in a 2 cm (0.8 in.) layer directly behind the cutterhead support. This layer is accompanied by ring beams depending on the ground conditions. Compressive strength of more than 15 MPa (2,200 psi) is achieved within 24 hours of application.
“The shotcrete is very quick setting with no rebound, and the mortar does not require high pressure application,” said Yoshikuni Nakano, Deputy Project Manager for contractor SNUI JV (a consortium of Shimizu Corp., Nishimatsu Construction, UEM Builders Bhd, and IJM Corp.).
The shotcrete system has so far resulted in less downtime compared to conventional ground support, as the shotcrete can be applied during excavation. In the somewhat fractured ground conditions currently being encountered, the machines would have required an estimated two hours of ground support work per excavated meter with conventional methods. This would have added up to 210 days to excavate and support the ground. With the fiber mortar system, this time has been reduced to 158 days — a time savings of 52 days.
With the three TBMs now between 4 and 6 km (3 and 4 miles) into their respective 11 km (6.8 mile) long sections, benefits such as dust reduction are also becoming clear. “The shotcrete offers environmental benefits, because it is transported in sealed cars without exposure to the surroundings,” said Andy Birch, Robbins Site Manager. Bonding, according to Birch, has been good and rebound has been minimized compared with standard shotcrete mixtures.
The Pahang Selangor Raw Water Tunnel, for the Malaysian Ministry of Energy, Green Technology and Water (KeTTHA), will transfer water from the Semantan River in Pahang State to the Selangor/Kuala Lumpur region, traveling as far as 1,200 m (3,900 ft) beneath the Titiwangsa mountain range in varying rock conditions with some ground water. TBM excavation is expected to be complete in 2013.