The Indonesian capital of Jakarta is the largest city in southeast Asia with more than 10.5 million inhabitants, and the second-largest conurbation worldwide with more than 34 million inhabitants in the metropolitan area. In order to accommodate the continually increasing traffic volume due to this population, construction began on Indonesia's first subway line in 2013, the MRT Jakarta.

Currently, Kota Station is the last section of the current construction phase II of the South-North MRT line. Near the old Jakarta Kota train station in the north of the city, the new station Kota will enable better access to the historical old town and business centre as a traffic hub.

To carry out necessary diaphragm wall works for the construction of the subway station, P.T. Bauer Pratama Indonesia, the Indonesian subsidiary of Bauer Spezialtiefbau, was commissioned by client Sumitomo Mitsui Construction Co. joint operation with P.T. Hutama Karya (Persero) to install the supporting walls for the lower level.

"Due to the challenging soil conditions, demanding technical requirements and strict location restrictions, we were awarded the contract for this project, which makes us very proud," Hemanth Narayanan, director at Bauer Indonesia, said.

Demanding challenges

One of the special challenges in this project was that the work had to be carried out on a narrow stretch of land between two busy main roads and among historical structures and residential buildings.

In addition, two bus lanes had to remain open for traffic during the entire construction period, which meant that work could only proceed on one side of the station at a time.

"The Pantjoran Tea House, a protected cultural building, was less than 3m away from the site and the diaphragm wall elements had to be installed directly on the property line," Narayanan noted.

In total, 136 diaphragm wall elements were constructed with a thickness of 1200mm or 1400mm over a length of 867mm. Due to the challenging subsoil consisting of soft to moderately firm clay at a depth of 7m to 8m, the panels were installed between 33m and 43m deep. The total area of the diaphragm walls constructed was nearly 29,500sq.m. The work was carried out using a Bauer GB 50 hydraulic grab and a Bauer MC 86 duty-cycle crane.

Polymer slurry and reinforcement cages

During the work, a polymer slurry was used to stabilise the excavated trenches. To inspect the quality of the slurry continually and comply with the requirements concerning verticality, regular tests were conducted. Because of the extremely limited site area, an adjacent property was also rented to set up the slurry plant and the stabilising slurry was transported onto the site via pipelines up to 360m in length.

The reinforcement cages required to stabilise the completed diaphragm wall elements were constructed in the company's workshop located 40km from the site. "A heavy haulage transport with police escort was organised every night after 10 pm for just-in-time delivery of the completed reinforcement cages onto the site," Narayanan explained. "In addition, special reinforcement cages made of fibreglass-reinforced plastic had to be installed on the two curved sides of the station, since a tunnel boring machine will be used to construct the subway tunnels." The construction and installation of the special reinforcement cages required particular caution, as the material is extremely brittle and rigid.

Successful completion

The work began in December 2022 and was successfully completed in June 2023. During the entire execution period, work sequences had to be followed strictly in order to keep from jeopardising the production schedule of the reinforcement cages. The particular location amid buildings with protected monument status also required extremely cautious and low-vibration work methods in a very tight space. Because the work was carried out in the historical city centre of Batavia, objects such as ceramics, wood stakes and mussels were occasionally found.

"Although we were faced with some challenges during this project, we were able to complete the work on time and without incident," Narayanan concluded.

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