Pre-cast push

In October last year Cranes Today reported on the North East Line of Singapore’s mass rapid transit (MRT) rail network. Now construction is at an advanced stage on a new twin spur, from the North East Line to the housing estates of Sengkang and Ponggol.

The Sengkang light rapid transit (LRT) line has 14 stations and is 11km long. The Ponggol line has 19 stations and stretches for 13km. Both lines are elevated on viaducts and the average height of the roadway is approximately 15m above ground level with one portion elevated to 40m. For the lines to be delivered and operational by the year 2002 and 2004 respectively, special consideration in the purchase of machinery was necessary.

When BBR (S) Pte Ltd, a specialist subcontractor was awarded the contract for the construction of the foundation, columns, the precasting segments and the installation of concrete segments for Ponggol and Sengkang light rapid transit (LRT) stations, it needed to set up the complete pre-casting facilities at the allocated Sengkang and Hougang junction.

Contracted at a price of S$140m (US$77m) by the consortium of Semb-Corp and Mitsubishi Heavy Industry, BBR’s role is to construct the divider roadway which involves the construction of the foundations, columns, pre-casting of runway segments and installation works complete with all necessary finishing works along the guide way.

BBR’s task was to design, manufacture and install the complete pre-cast processing facility, which must meet stringent checks from the Ministry of Environment, Land and Transport Authority and the Ministry of Manpower of Singapore. A major part of the project was the crane systems for the handling of concrete segments during the manufacturing stage within the processing yard and at the installation stage.

The variety of these pre-casting works included columns, foundations, roadways, dividers, parapets and other finishing segments. With these tasks ahead, they had only three objectives in mind:

• to finish the project on time by improving productivity of casting segments and reducing the time needed for launching the concrete segments

• to minimise accidents at site

• to minimise machinery breakdowns.

More than S$10m was spent in purchasing ‘proven’ equipment which included eight units of 5t overhead travelling cranes, two units of 60t portal gantry cranes, two units of tailor-made 60t portal gantry, three crawler cranes, five 50t and three 25t mobile cranes. This equipment was employed to produce and install approximately 10,000 concrete segments.

BBR project manager John Faulkner explains: ‘There are cheaper alternatives around, that is for sure, but when we are considering what else is available, the prime importance we wanted was to have a piece of machinery that could work and do what we required.’

New equipment

Most of the cranes have standard two-speed hoisting, travelling and traversing motions and are used for the common handling applications. These include lifting steel reinforcing bars and cages to the steel moulds, moving the concrete segments after curing, and placing and storing the completed segments for installation.

BBR considered that Demag was the equipment supplier that could best assure the quality and features needed for the operation. Eight standard single girder overhead travelling cranes, rated at 5t, and two 60t portal cranes are being used, but Demag hoist and crane products are also used in other ingenious ways. For launching concrete segments, BBR employs two 50t customised portal crane of 24m span with a cantilever of 16.75m on one side that allows the cranes to ‘counter all sorts of alignment requirements on the site which the gantries have to be opened up to due to changes in track spacing,’ according to Faulkner.

At a cost of approximately S$700,000 per crane, these specially designed cranes are able to navigate in all sorts of alignment requirements. One side of the portal leg is powered by four Demag RS wheel blocks making it adjustable along the span to counter the changes and requirements of alignment on site. A slew ring is also attached to this portal leg to cope with the tight bending radii of up to 75m. Bogie wheels complete with vertical geared drives are used to accommodate the variation in the slope on the terrain. The vertical type bevel helical gearboxes were used to allow greater usage of the span of the cranes.

Powered by a 400kVA generator mounted on the main leg of the portal, the crane is self-driven and so does not need cable reeling drums, festoon systems or busbar conductors.

The two cranes operate separately and are identical and the cantilever section is colour coded and restricted at 30t capacity. The concrete segments are transported from the storage yard via low-bed transporters and picked up by these cranes. The concrete segments are assembled on site using precise hoisting and travelling motions.

As the segments are assembled, the runways are moved by hydraulic jacks, which are mounted on the sides of the divider roadway, thus enabling the launching portal cranes to move along each column. Faulkner explains: ‘It is a practical and efficient way to launch the segments without employing heavy capacity mobile cranes.’

Improving productivity

Faulkner says that the addition of the crane systems, particularly the portal cranes for the launching beams, have yielded a significant increase in productivity, when compared with other projects that he has managed. ‘We had a project requirement for our production. Our production requirement was that we are expected to erect four beams per week out of these launching gantries, and we have actually managed two beams in a day, so in terms of the overall operations, we don’t have any problems.’

In addition to the reduction of the overall time needed for segment installations, these cranes helped minimise the overall breakdown times. ‘In 18 months of construction work we have had very few problems in terms of breakdowns,’ Faulkner says. As far as BBR was concerned, the minor breakdowns that needed attention were deemed acceptable.

As most of the project is within the housing and residential estates of both Sengkang and Ponggol, work is carried out only during office hours. It is important, therefore, that these cranes are both reliable and productive, as any failure would have knock-on effects on the completion of the project.

As of June 2001, BBR had completed 100% of all works for the Sengkang LRT and was proceeding with work on the Ponggol section involving piling works, segment casting, segment launching and column construction. It is now expected that the Ponggol LRT will be fully completed by the end of August 2002, well ahead of the project schedule.