Contract tendering process begins

The Land Transport Authority (LTA) has recently released tenders for 2 contracts as part of the Cross Island Line (CRL) civil works, beginning the process for release of all 17 expected civil works contracts. Contract CR101 for the Design and Construction of Changi East depot was released on 16 August 2019, followed by contract CR105 for the Design and Construction of a large single bore tunnel from Aviation Park station to Loyang station on 23 August 2019.

This marks the first time that LTA has explored the use of a large-diameter tunnel boring machine to create a single bore tunnel between stations. All MRT projects prior have used twin bored tunnels which are essentially separate tubes for each track direction. A single large bore on the other hand will utilise a single tube tunnel with dividing wall to separate the two tracks.

Cross section of CRL single bore tunnel. Visible are the overhead rigid catenary supports, tunnel dividing wall and other
ancillary structures.
Source: Christian Arellano

There are a few benefits to utilizing a single large-diameter bore for MRT tracks. One such benefit is that the need for mining to construct mid-tunnel cross passages is negated. Cross passages are tunnels that connect the twin bore tunnels for use during emergency situations such as evacuation during a fire event. Such tunnels are dug using the New Austrian Tunneling Method (NATM) which can be a slow and dangerous process that involves stabilizing the earth through which the cross passage is constructed through.

Additionally, by eliminating the need for a cross passage, wastage can be reduced helping the project to become more environmentally friendly. During the construction of a cross passage, portions of a completed pre-cast tunnel need to be removed to make way for openings to the cross passages. The pre-cast tunnel segments need to be installed during tunnel construction to facilitate progress of the tunnel boring machines and cannot be left out to ensure the integrity of the tunnel and minimize and subsidence (movement of soil) to the ground through which the tunnel is being created. By using a single large-diameter bored tunnel, this waste is eliminated as the dividing wall allows access points to the opposite bound to be created without the need for demolition of tunnel linings.

Using a single large-diameter bored tunnel eliminates the need for cross-passages between twin bored tunnels.
Source: Christian Arellano

Another benefit is that more cross-over points can be provided for along the line. When using a twin bore tunnel setup, each track is isolated within its own tube tunnel. At points where the tracks allow trains to transit from one direction to another, a connecting track called a cross over or siding needs to be constructed. Due to limitations in the size of the single tube tunnels, it is impossible to house cross-overs or sidings within a single track tube tunnel and disruptive cut-and-cover trench tunnel works are required for the provision of cross-overs or sidings. However, with a large-diameter tunnel, as the tracks are placed side-by-side within a single large tube tunnel, it is easier to provide for more frequent cross-over points with minimal disruption to the surface.

More frequent cross-over points can be provided when using a single large-diameter bored tunnel as opposed to a twin bored tunnel arrangement.
Source: Christian Arellano

The release of the first two contracts for the CRL is a welcome milestone and we look forward to the release of further contracts in time to come.