The FAST Lane: machines of the Riyadh Metro
On the trail of the FAST Consortium machines installing viaduct for the Riyadh Metro project
The Riyadh Metro is a project that involves the digging of some 51km of tunnel, and the progress of the tunnel-boring machines (TBMs) working on the project has been frequently celebrated. However, amid all this fanfare it is easy to forget that 125km of the 176km project is being installed above ground, either at grade or on elevated viaducts — project work that is also complex.
Engineer Ahmed Aldrees is the assistant project director at the Arriyadh Development Authority (ADA) for Package 3, which comprises Lines 4, 5 and 6 of the six-line Riyadh Metro and is worth $7.82bn.
Aldrees notes: “At 63km, Package 3 is the biggest in terms of line length. We are working on one of the biggest projects in the world, in a crowded city, surrounded by people, traffic and utilities — so keeping everyone calm and explaining the importance of the project is one of the biggest challenges.”
This package is being carried out by the FAST Consortium — led by the Spanish construction group FCC, and including Samsung C&T, Alstom, Strukton, Freyssinet Saudi Arabia, Atkins, Typsa and Setec.
In all, there are six different construction methods being used to construct the bridges and viaducts for the Riyadh Metro project: one being conventional in-situ casting and the other five involving the placement of pre-cast elements by cranes or specialised machinery — with each method being selected to optimise the construction process in a given situation based on a variety of factors, including the time and space constraints placed on the operations by the surrounding environment and traffic.
Full-Span Launching Method
However, Package 3 has the distinction of being the only package to use the Full-Span Launching Method (FSLM), which, as the name suggests, allows full-span girders weighing up to 450t to be pre-cast and lifted into a girder carrier. This device then travels over the existing spans to deliver the girder to the launching gantry, which then lifts the girder out over the empty space between the end of the existing viaduct by resting its weight on the next supporting pier.
The FAST Consortium is installing 256 full-span girders using two sets of FSLM girder carriers and launching gantries made by UK-based firm Dormanlong Technology. The pre-cast moulds are by South Korea’s Dondo.
According to Eng. Aldrees, the Riyadh Metro represents the very first instance in which the FLSM technique has been used in the Middle East. He notes that a three-month learning curve was required to get the teams involved up to speed with the mechanics of the two-day cycle of production for laying the girders.
Explaining the choice of the technique, he notes: “FSLM has the advantage of causing no interruptions for the traffic below and delivering a high degree of quality and safety, with the single casting and erection operations delivering a more consistent overall result.”
However, the technique can only be used for spans of up to 36m and only on completely straight alignments. In addition, “once this method starts it cannot stop”, notes Eng. Aldrees — so all supporting piers have to be completed in advance of the operation.
One way around this requirement is to use the Pre-Cast Beam (PCB) method, which exchanges the 450t full-span girders for two beams, each weighing about 200t, and a cover.
Pre-Cast Beam method
Eng. Aldrees explains: “PCB is very useful where there are interruptions — with certain piers not completed due to the presence of utilities or other issues — because cranes are used to lift it. So you can skip spans. It is the fastest method of all the techniques, as you can install one span a day without any issues. FSLM can do one span a day at its optimum.”
In all, some 322 spans are being installed on Package 3 by using the PCB method.
Eng. Aldrees adds: “FSLM and PCB are of limited use on sharply angled sections due to economic and technical issues. Accordingly, the Pre-cast Segment Method (PSM) should be adopted for those areas.”
Pre-Cast segment method
Like with FSLM, PSM involves the use of launching gantry machines to manoeuvre pre-cast elements into position. However, unlike FSLM, these smaller segments can be delivered to site by truck and lifted into place on site and do not require any separate girder carriers.
The individual segments slot together like toy blocks and are then bound in place by a combination of “epoxy material applied on the joint-surfaces and pre-stressed tendons that run through the spans”, explains Eng. Aldrees.
Like the Pre-Cast Beam method, PSM is being used to install spans at a rate of approximately one per day — though like the FSLM technique, a three-month lead time was required to teach the teams how to use the launching gantry system. A total of 302 spans are being installed as segments — 271 of them by launching gantry; the rest using a combination of cranes and scaffolding.
Eng Aldrees notes: “Each span is made up of several segments (usually 10 for a span of 36m). It is advantageous to use this technique to navigate sections with curvature — as the segments can be fabricated with a kink.”
The FAST Consortium deployed three PSM launching gantry machines made by Deal, an Italian company that was also responsible for supplying the Dubai Metro machines. Each individual segment weighs around 35t-40t.
A slight variation on the theme of PSM, is the Balanced Cantilever Methods (BCM), which is similar to PSM, but involving the use of a ‘lifting frame’ that sits on a single pier and installs segments one-on-either-side in sequence — thereby allowing it to maintain its balance like a set of weighing scales.
This technique is used where longer spans are required, and FSLM and PSM launching gantries are unable bridge the gap.
Due to the long set-up and complexity of the BCM process, FAST’s planned use of this technique is limited to just 23 spans.
FAST has three lifting frames supplied by Italy’s Cimolai and Comtec for this technique.
Looking at the two segment techniques together, Eng. Aldrees notes that PSM/BCM have several advantages, including the factory-controlled quality of the pre-cast segments, the safety of the installation and the ability to cover sharply angled and long-span sections.
He adds: “Each precast element for the full-span, pre-cast beam and segment techniques is produced from yards that have been purpose-built for the metro project.”
Across Package 3, the FSLM technique is being used on Lines 4 and 6, and the PCB method is also mainly being used on Line 6 — to enable the transport of the beams across the city from the pre-cast yard to the installation points.
Scaffolding is being used in some instances, as it is useful for installing long sections that are difficult to bridge with any of the other techniques. However, this method is rather slow because the dismantling of the scaffolding is time consuming, and because the scaffolding can only be moved to the next span once each full span is completed.
Cast-in-situ techniques using steel shuttering are being used at a few stations and where the track curves. This is also time consuming.
The FSLM and segment techniques are both being presided over by a master specialist and three superintendents. For the FSLM, these supervisors are all South Korean, and for the segment techniques there is an Italian master specialist with three Egyptian superintendents. The teams of workers are of Indian, Pakistani, Nepali, Filipino and Egyptian nationality.
However, FAST and ADA have also been working to ensure that there is a strong programme for the transfer of skills.
For the FSLM works, Eng. Aldrees explains: “A Saudi Arabian engineer joined from the start and worked together with the teams and he knows all the process and systems.”
While for the segment techniques, he adds: “Local subcontractor, Fressinet Saudi Arabia, under the FAST Consortium, was equipped with considerable skills, techniques and experiences associated with these works.”
Finally, FAST and ADA established an academic partnership, allowing PHD research students from the King Saud University to undertake research projects to develop further insight into all of the techniques involved.
Given the pioneering nature of the project in Saudi Arabia, it is a worthy subject.