MRTA Purple Line, Thailand

Costing an estimated THB61 billion, Bangkok’s MRT Purple Line was conceived as a solution to the travel and traffic woes faced by commuters in Nonthaburi Province, a densely packed urban center northwest of the Thai capital, Bangkok.

When completed, it is estimated the new metro rail line will take some 50 000 passengers off saturated roads during rush hour.

As a leading designer of rail infrastructure, Aurecon has for the past eight years worked closely with the Mass Rapid Transit Authority of Thailand (MRTA) and Italian-Thai Development (ITD), to oversee the design, planning and construction of the track works for the Purple Line.

“Once operational, the Purple Line will help ease traffic conditions and provide residents in Nonthaburi Province with a quick and convenient means to travel to and from the central and southern parts of Bangkok. Aurecon is proud to be associated with this landmark project that improves the lives of Thai residents. In the coming years there is further economic, transit-oriented development scheduled in Thailand and this project positions us well to be able to support the many upcoming projects in this area,” said Sakorn Kruemai, Country Manager of Aurecon in Thailand.

Design and construction work has been completed for the 23 km of elevated tracks and 16 stations, as well as for a 330 m bridge across the Chao Phraya River and a 3-storey depot building. The Purple Line’s headquarters is home to a train depot, train stabling building, workshop, car park and office space. The project is currently in a testing and commissioning phase and will be ready to start operations in mid-2016.

Aurecon’s commission with ITD started in November 2012, when the company was called in to consult on trackwork design services, including the track plinth and slab design, the selection of track components (the fastening and third rail system), and stray current protection design. The company also contributed its expertise to the rail structure interaction analysis.

As part of its work on the design, Aurecon designed in complying with specification of project specifying ballastless tracks throughout. Although this track structure is a higher capital cost when compared with traditional rail track design, it has the advantage of significantly reducing the need for frequent maintenance, which in turn reduces overall lifetime costs and operational disruptions.

Unlike conventional tracks, ballastless tracks are supported on a continuous slab of reinforced concrete with robust stress resistant properties. By comparison, traditional track designs require frequent maintenance because of the wear and tear that the welded rails and supporting sleepers experience. 

During the construction, one of the most notable engineering achievements was the construction of the 330 m Phra Nang Klao Bridge over the Chao Phraya River.

The bridge was designed by MRTA’s project consultant and constructed by a local civil works contractor. For Aurecon, its work on the bridge included the analyses of the trackwork over the bridge, plus the design of the ballastless track, which was then constructed by ITD. Aurecon’s analyses made up a crucial part of the bridge project, by determining the need for, and the location of, the rail expansion joints.

The bridge’s design consists of a five-span continuous rigid frame connected by single cell pre-stressed concrete box girders. Using pre-stressed concrete in the design meant the bridge’s spans could be longer because of the higher tensile strength pre-stressed concrete has versus traditional reinforced concrete. The bridge was constructed using a balanced cantilever erection method with the assistance of form travelers.

At approximately 15 m, the above-average depth of the Chao Phraya River also represented a significant foundation challenge for the Phra Nang Klao Bridge. To overcome this, the design team selected groups of 2 m diameter piles arranged under pile heads as the most efficient solution. The elevation of pile caps was determined by taking reference from an existing road bridge located 42 m downstream.

To support the bridge on the river bed, substructure wall type piers with Convex-Lens cross-sections were used for their aesthetic appeal. The bridge’s superstructure comprised 1 800 mm round bottoms, selected to achieve consistency in structural design. Keeping in mind that round bottoms are inefficient for bridge structures, a structural depth increase of 400 mm was then required to boost the dead load and compensate.

Construction work on the Purple Line is ongoing and passenger operations are on track to begin in August 2016.

Initial services will be with a three-car train, which will increase to six-car trains in the future. As of this moment, a Monitoring & Evaluation (M&E) system has been set up to oversee various aspects of the Purple Line’s final phase of construction, which will lead into the final stages of getting the train line operational.

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