The Six Stages to building an elevated MRT line - Structural works (3)
Previously on our mini series on constructing an elevated MRT
line, we explored the first two stages – preparatory works & earthworks. The
works in these 2 stages paved the way for our third stage of structural works
to commence. Join us as we discover more about the third phase of works in
constructing an elevated MRT line.
Fair warning: This post is quite lengthy due to its detailed nature.
Stage 3: Structural works Structural works are where it starts getting exciting. It is
also where the work starts bearing fruit that can be seen in the form of
viaduct piers and stations taking shape. Unlike an underground line, the works
at this stage become very real as they are highly visible as opposed to being
tucked away, mostly out of sight.
The aim of this stage is to complete the structure that will
eventually allow the line to operate. These include the viaduct components such
as piers and spans, station components such as station buildings and entrances
as well as other support facilities such as the depot buildings and electrical
substations. It does not, even though you might see these works being carried
out concurrently, include any equipment installation as this falls under
systems works which is the next stage of construction – that will be a topic
for next week.
Types of Viaduct Piers
We start off with viaduct pier construction as there are a few elements that
can be explored within this category of infrastructure for the line. Did you
know that there are different types of viaduct piers?
Normal/hammerhead piers – sub-type of movement joint pier
Portal piers
Cantilever piers
Normal/hammerhead JRL viaduct pier
The most common would be the normal or hammerhead pier. On the Jurong Region
Line, these are piers which support a crosshead allowing for the connection of
one to three segmental box girder spans. Typically, the spans are jointed to
the pier itself and the span hangs off the pier itself.
6 August 2022 - A view of normal/hammerhead piers for single trackways near Tengah Park station along contract J108.
5 February 2023 - A completed hammerhead pier near Jurong West station on contract J105.
Within the normal or hammerhead pier category, is a sub-category
which is designed to allow the viaduct span to expand & contract known as
the movement joint pier. These are constructed with stubs to support bearings
upon which the viaduct spans sit. One easily observed example is within
contract J108 along Bukit Batok Road, near Tengah Park station.
12 March 2023 - A movement joint pier, whose crosshead design is markedly different from other hammerhead piers. This movement joint pier supports a double trackway.
12 March 2023 - A close up view of the double trackway movement joint pier.
12 March 2023 - A pair of single trackway movement joint piers. One can note that the SBG utilizes a different design to sit on the bearings that connect it to the pier.
Portal Piers
Besides normal or typical piers, portal piers are another category of piers
that will be used extensively on the JRL. Portal piers are piers that use two
columns to support a beam upon which the viaduct span is supported. Do have a
look at this video for better explanation.
12 March 2023 - A portal pier under construction near Tengah Park station under contract J108.
25 February 2023 - Several portal piers will be built along Choa Chu Kang Avenue 3.
25 February 2023 - Work is underway for the construction of the remaining column in the road median for the portal pier across the junction.
Cantilever Piers
A third type of pier is the cantilever pier, although we are not certain that
this design will be used on the JRL. Nonetheless, a few exist along the Tuas
West Extension on the East-West Line (EWL). A cantilever pier supports the
viaduct span off-center and to the side of a typical pier.
25 February 2023 - A cantilever pier along the EWL.
25 February 2023 - Another cantilever pier near Tuas Link station.
Viaduct pier construction
Construction of the pier involves installing rebar, which acts as the skeleton
to provide rigidity, on top of the completed pile cap (read about pile caps in
our previous post here). Thereafter, a formwork is used to encase the structure
before concrete is poured in to cast the pier column. Depending on whether the
pier supports a single trackway or multiple trackways, a crosshead may be built
to support the viaduct spans. In the case of movement joint piers, studs for
bearings will be built.
30 October 2022 - With the completion of the pile cap under the earthworks stage, construction of the pier is able to commence.
30 October 2022 - Rebar is installed atop the pile cap.
2 December 2022 - Formwork is then wrapped around the rebar to give the pier its shape. The formwork also contains some detailing, such as grooves for the creation of designs on the sides of the pier. Have you noticed that the piers sport several vertical lines?
2 February 2022 - A pier partially cast along Bukit Batok Road.
12 March 2023 - Towards the top, the design flares out a bit to support the future crosshead that will be built to complete the pier. Once the formwork for the pier shaft is completed, casting begins.
12 March 2023 - The formwork for the shaft is removed and construction of the crosshead begins. A scaffold structure is used to support the formwork & rebar while casting takes place.
Viaduct span construction –
segmental box girders (SBG)
Viaduct construction typically uses SBGs which are essentially a viaduct span
that has been cut up into smaller segments. These are pre-fabricated in a
casting yard and brought over to the launch site for installation into place to
form a viaduct span. A span is a section of viaduct that sits between 2 piers,
spanning the length in between.
12 February 2023 - A view of the casting yard for contract J103 with segmental box girders.
12 February 2023 - A view of the segmental box girders at the contract J107 casting yard.
Viaduct span construction – Span by Span technique (Launch
gantry)
One of the methods for construction using SBGs is the launch gantry using the
span by span technique. This is a specialized crane system that allows the
launcher to move along the course of the viaduct to install SBGs. Occasionally,
the launcher facilitates post-tensioning works as well which allow horizontal
rigidity within the span that allows it to remain suspended between piers.
Without this rigidity, the span is likely to collapse on itself, thus
multi-fibre cables are used to thread the inside of the SBGs to provide
strength to remain in place.
12 March 2023 - The contract J108 launch gantry constructing a viaduct span along Bukit Batok Road.
12 February 2023 - The contract J102 launch gantry 'Gaia' on site in Tengah.
12 March 2023 - Two launch gantries for contract J102 'Gaia' (to the back, on viaduct) and 'Mulan' (on the ground in the midst of assembly).
Viaduct span construction – Balanced cantilever technique (Lifting
frames) Another method for launching SBGs is the use of lifting frames using the
balanced cantilever technique. This method is similar to that for hanger beams
with the exception that the hanger beam relies on an external crane to lift the
SBG to viaduct level. In the case of lifting frames, the frame itself is
capable of lifting the SBG up without the need for use of external cranes.
12 February 2023 - Hanger beams being installed near Jurong West station along contract J105.
19 February 2023 - Hanger beams with the first SBG of each trackway on either side installed.
12 February 2023 - Hanger beams along Jurong West Street 64.
19 February 2023 - Segmental box girders installed using the balanced cantilever technique.
12 March 2023 - Lifting frame equipment on site at contract J106 along Jurong West Street 64.
12 March 2023 - Installation of segmental box girders using the balanced cantilever technique near Tawas station.
12 March 2023 - A close up view of the box girder installation.
Viaduct construction – Parapet walls
Upon the completion of the viaduct spans, parapet installation can begin.
Parapets are the low protective walls along the edge of the viaducts. They
serve to prevent objects from falling, allow for the installation of equipment
hanger racks and at the same time make the viaduct look neater overall. At
certain locations, noise barriers will be installed on top of the parapet wall.
27 December 2022 - Parapet wall installation on the East-West Line (EWL) near Tanah Merah.
27 December 2022 - Parapet installation works seen from street level.
16 December 2022
Station construction
Most of the stations along the JRL are located along road medians. As such, we
expect that the method of construction will be similar to those in other
Southeast Asian countries like Kuala Lumpur’s MRT or Bangkok’s BTS.
5 February 2023 - A view of piers for the station at Gek Poh in the median of Jurong West Street 75.
12 March 2023 - Station concourse level crossheads under construction at Gek Poh station.
5 February 2023 - Piers for Corporation station under construction in the former & eventual road median. The roadway is temporarily diverted to create space for foundation & structural works.
19 February 2023 - Corporation station site.
12 February 2023 - Piers for Jurong West station in the former & eventual road median. The roadway is anticipated to be diverted back by late-March.
Some stations however, will not be located above roads but on
empty plots of land or on the road table. Such stations can be found at Choa
Chu Kang, Tengah, Hong Kah, Jurong Pier, Tawas, Peng Kang Hill, Tengah Park,
Bukit Batok West, Jurong East, Jurong Town Hall & Pandan Reservoir. These
stations will undergo a typical building’s construction sequence where the
foundation is built and the structure above that thereafter.
2 January 2023 - A view of Choa Chu Kang station, being built on the site of the former bus interchange, next to the existing MRT station.
25 February 2023 - A view of the Choa Chu Kang station site.
13 January 2023 - A view of the site of future infill station JS02A, located between Choa Chu Kang West & Tengah stations.
13 January 2023 - Tengah station under construction.
12 March 2023 - Tengah Park station under construction. The pile caps for the station columns can be seen. The station should be progressing upwards once these earthworks are completed.
18 December 2022 - Works at Jurong Town Hall station.
25 February 2023 - Site progress at Jurong Town Hall station.
12 February 2023 - Hong Kah station site.
12 February 2023 - Hong Kah station site.
Depot & other support infrastructure construction
The depot and other supporting infrastructure such as electrical substations
are integral components in the operation of a new MRT line. Similar to station
buildings sited on empty plots of land, these structures will be built akin to
typical buildings with the foundations first and thereafter the rest of the
structure.
12 February 2023 - A view of the Tengah depot.
12 February 2023 - Tengah depot.
12 February 2023 - Tengah depot.
23 February 2023 - Electrical substation under construction along the Thomson-East Coast Line.
23 February 2023 - Electrical substation under construction.
Once the structural works are completed, work transitions to the
next phase of construction – systems works. Here, all other systems are
installed and we’ll go into greater depth next week on which components are
involved. By the way, have you heard about our giveaway? Participation is
simple – just like our Facebook page & comment on our weekly quiz in the
comments on our post. We hope to see your participation.
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