As some of our trainees very well know, it takes tremendous effort and time to plan and build an MRT line, 13 to 14 years to be exact. During this period, feasibility studies, soil investigation and final tests need to be completed. The bulk of this Herculean task involves going underground to build tunnels, which is a challenge in itself. Here’s a look at the work that went on behind the scenes.
70m-deep holes are bored every 15m to 20m along the planned train track to get soil samples such as alluvium, marine clay or soft soil, to assess if the soil in the tunneling area must be further strengthened to support the excavation work.
A close study of the foundations of nearby buildings (within 200m of the expected tunneling area) is done to determine the most suitable construction method.
Decision-making time! At this stage,
several important decisions are made on:
Size of MRT stations
Road diversions required according to Traffic impact assessment
Depth of tunnel
Existing utilities diversion required
These are followed by:
Announcement of number of stations and their locations near the end of this phase; and
Calling for tenders (6 months) for the various services such as civil works after this vphase.
Excavation, concreting and foundation works
The diversion of the roads takes place in various stages to make way for the construction. The time taken for the excavation and concreting works will depend on the factors which were assessed in the 2nd stage, for example, if the soil is assessed to be marine clay, they will be able to dig up to 3 m to an underground structure.
Tunnel-boring machines (TBMs) costing 20 million each do the boring . Thrust Hydraulic cylinders are used to push against concrete segments as the TBM moves forward.
Average tunneling rates
-Soft soil : 8.4m/day
-Rock : 3.5m/day
-Mixed ground : 2.8m/day
Round-the-clock boring operations
-One boring machine bores a roughly 1km-long tunnel.
-Each boring machine can move 1m to 7m per day.
-Downtown line 2 (16.6km) : 19 TBMs.
Stations are then installed with the interior design features as well as the fare machines and gates.
Operation essentials such as the electrical systems for train signalling and the platforms are fitted out.