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The FAST Lane: machines of the Riyadh Metro

by John Bambridge on Nov 8, 2016

A view of an FSLM launching gantry from the rear.
A view of an FSLM launching gantry from the rear.

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.”