Belly of the beast: FAST's Riyadh Metro TBMs
A journey to the centre of the earth-trembling equipment eating away at the bedrock beneath the slumbering citizens of Saudi’s citadel
The political capital of the Kingdom of Saudi Arabia, Riyadh, has long fallen short of many of its international peers in terms of infrastructure — its traffic issues crippling the commute of its six million inhabitants for want of public transport.
However, all of that is set to change with the construction of the Riyadh Metro — to date the largest mass transit system to leave the drawing board in the Middle East and a project more befitting of the largest city in the GCC.
The project, which is being facilitated by the Arriyadh Development Authority (ADA), comprises a network of six rail lines with a total length of 176km and some 85 stations — larger in both degrees than the three-line section of the Doha Metro that is currently under construction, though more is planned.
Engineer Ahmed Aldrees, ADA assistant project director for Package 3, explains: “Perhaps 12 years ago the traffic plan for Riyadh showed that the problem of the traffic could not be solved only by building new roads or extending existing ones.
“Today, we are working on one of the biggest projects in the world, in a crowded city, surrounded by people, traffic and utilities.”
The six lines of the project involve a mosaic of track situated at grade, overhead on viaducts and underground, combining cut-and-cover excavation and, of course, the deployment of tunnel-boring machines (TBMs).
At tendering, these six lines were split across three packages. The third package comprises Lines 4, 5 and 6, and is worth $7.82bn. It was secured 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.
Aldrees explains: “At 63km, Package 3 is the biggest in terms of line length — so keeping everyone calm and explaining the importance of the project is one of the biggest challenges.”
Enter the project’s seven TBMs, which are being deployed on the sections of the line where there is either no space at grade or the layout of the city is unconducive to either viaducts or cut-and-cover excavation.
Six of the seven TBMs are manufactured by Germany’s Herrenknecht, the leader in the segment since 1975, while the final machine was supplied by France’s NFM Technologies.
Aldrees details: “Across the whole project — 51km of which is underground — 37km will be executed by TBM and the other 14km by regular cut-and-cover excavation.”
At 13km, the subterranean Line 5 accounts for approximately a third of the TBM work on the project, running along King Abdulaziz Street between transfer stations with Lines 1 and 2, interspersed with 11 additional stations.
It is being bored by two 2,000t Herrenknecht TBMs, each 105m-long and 19m in diameter and tuned to slice through the bedrock beneath Riyadh based on studies of the soil conditions.
Oscar Ruiz, FAST’s lead TBM engineer on the project, notes: “The soil of Riyadh is consistent limestone, which means it is a very stable material for excavation. The only issue that we have found is that some areas have cavities, but we can manage this as these can be detected by the TBM.”
TBMs excavate hard rock using disc cutters mounted in the cutter head. The disc cutters create compressive stress fractures in the rock, causing it to chip away from the tunnel face. The excavated rock, known as muck, then passes through openings in the cutter head to be carried out of the tunnel by a system of belt conveyors and muck cars.
For the Riyadh Metro project, the TBMs are laying a segmented concrete tunnel lining as they progress, ensuring the stability of the tunnel walls behind the cutting heads. Hydraulic jacks then brace against the tunnel segments behind the machine, keeping the high thrust force from impacting the bedrock.
FAST’s engineers and technicians have capitalised on the stability of the rock by gradually picking up the pace of their excavation and accompanying logistics.
Ruiz notes: “We changed the original design and increased the opening rate — the percentage of open holes in the cutting wheel — to 70%. We also increased the capacity of the water and power lines to treat the material, and in our case, it is working very well.”
So far, the average pace of excavation by the two TBMs working on Line 5 is 25m/day, which has recently increased to 35m/day — far outstripping the pace of the 21 Guinness record-breaking TBMs of the Doha Metro, which are progressing at an average speed of between 12m/day and 21m/day.
Aldrees adds a caveat: “This is within Package 3, but in Package 1 and Package 2, where they are using the other TBMs, I think the production is between 15m/day and 20m/day — because their TBMs have only recently started work, there is still a learning curve.”
FAST began excavations on the first section of Line 5 from the location of the Ministry of Education on 30 June, and finished the 1.2km tunnel on 4 October, when the TBM dubbed San’ah broke through at the heroically named Salah Al-Din station. The re-emergence of the TBMs at each pre-excavated station allows for critical maintenance to be carried out on the machines. On 11 February, the San’ah machine completed its second, 2.5km length of tunnel.
At the back end, the logistics supporting the operation are a serious undertaking.
Aldrees explains: “TBMs are a suitable methodology for executing the project in Riyadh, precisely because of the narrow roads, traffic and utilities in the city centre. But TBM machines entail intensive logistics. We use approximately 200 to 250 trucks a day to ferry in the fuel, water and pre-cast elements, and to take away the excavated material. This has caused obstruction for traffic and caused a problem with cleanliness of the roads, because the excavated material is like mud, and there is a lot of slippage from the trucks.”
At this stage of the project, most of the initial teething problems in terms of logistics have now been cleared up by conveying resources to the sites at times that avoid the peak traffic flows specific to the locations.
Ruiz notes: “In the morning, because of the [rush hour] between 6am and 8am, these hours are bad for the traffic. Several times we had problems, but finally we organised better and improved movements of the trucks, and for the moment, we have fixed this problem.
“In our case, the logistics is pretty close. Our disposal area for the mud is around 30km away, and in the case of the water, the national water company has provided us with a connection, and only at certain times of the day do we have to request trucks.”
A second layer of logistical complexity is entailed in the delivery of the pre-cast tunnel lining elements, seven of which go into each complete 1.6m-long ring laid down by the TBMs. Across the entire Riyadh Metro project, some 161,875 precast tunnel-lining elements will be placed underground.
The facility producing precast elements for Package 3 is conveniently situated just 5km away from the project site — making it easier to coordinate the transport of the segments.
Aldrees comments: “We had an ideal case on Package 3, because the pre-cast factory is very close to both TBM machines, but if we talk about Package 2, it has used a local precast facility based in Dammam, so each journey is approximately 350km.”
Finally, each operation is supported by a team of 23 technicians working inside the tunnel with the TBM, replicated across three separate shifts, and around 100 personnel supporting the TBM from the outside.
The resource-intensive nature of this process was brought into sharp focus when the initial decision was made to opt for the use of TBMs on the Riyadh Metro over simpler cut-and-cover operations.
Aldrees notes: “It depends on the work. If you look at King Abdulaziz Road, it is efficient to do it by the TBM because of the length of the tunnel, but if you have a tunnel that is less than 2km, I don’t think it would be efficient to use this machine, because it needs a lot of logistics and a lot of setup before it starts work, so cut-and-cover might be more feasible.”
As it stands, the smooth operation of the TBMs is something for the consortium to celebrate. The FAST team hopes to complete its tunnelling work by June, while the entire project will be executed within 60 months.
Alstom has now also begun production of the 69 trainsets for the FAST Consortium’s three lines at its plant in Katowice, Poland. The 36m-long driverless trains are scheduled to be delivered to ADA for testing in 2017.