Undercrossing live tunnels in parallel below the water table in an urban environment

Singapore Metro system has been developing in recent years to enhance the connectivity of existing network Mass Rapid Transit (MRT) system. Currently, there are 5 lines across the island in operation – North South Line (NSL), East West Line (EWL), Circle Line (CCL), North East Line (NEL) and Downtown Line (DTL). In addition, Thomson-East Coast Line (TEL) is under construction and 2 more lines – Jurong Region Line (JRL) and Cross Island Line (CRL) – are under planning. This increased connectivity has resulted in a more complex train network, which in turn poses greater construction risk of tunneling under or above existing lines. Contract T222 involves the construction of Outram Station and the TEL tunnels. The Outram Station will become an interchange connecting 2 existing lines in the Central Business District (CBD) located between Havelock Station (HVL) and Maxwell Station (MAX), overcrossing the existing NEL and undercrossing the EWL tunnels. One unique aspect of T222 is that the tunneling works undercrossing the EWL tunnels are in parallel for about 365m for both the tunnel drives with a clear distance ranging from 1.8m to about 8m.This is the first time such a tunnel drive is planned in a urban district with some complex geology under the ground water table. Being within the urban district, there were numerous challenges to be encountered. One key challenge was to manage the stakeholders and to control the excavation to ensure that there was no over/under excavation that would impact the live tunnels. One another key challenge was to mine under the water table which was the sensitive criteria as drawdown could also cause movements to the tunnels and in addition managing the same water during the interventions was another concern. This paper discusses the approach of tunneling for the undercrossing of the EWL tunnels, key tunnelling parameters used, stakeholder management and coordination, instrumentation arrangement, monitoring as well as the challenges faced and opportunities for future application.