By Tina Chi
All images by author unless otherwise stated.
Before arriving in Rome, I knew I wanted to study an active construction project. I had some background knowledge from taking construction management classes and previously working on one of the Bellevue Light Rail Projects. Luckily, the Metro Line C extension was under our itinerary to visit. On Thursday, September 12, our class visited one of the Metro Line C extension’s project at the 3.2 shaft Celimontana and the Fori Imperiali near the Colosseum.
The Metro Line C extension is publicly funded by the Italian government, Region Lazio, and the City of Rome. This project is overlooked by Seattle’s version of Sound Transit, Roma Metropolitane, who took us on a site tour of their project. However after the project is completed, Roma Metropolitane will not be running and maintaining the line. They will turned over to ATAC Roma who concurrently runs all the Metro Buses in Rome. The general contractor, Metro C S.c.p.A., is a joint venture made up of five entities: Astaldi, who has a 34.5% stake, Vianini Lavori S.p.A. who has a 34.5% stake, Hitachi Rail STS who has a 14% stake, Consorzio Cooperative Costruzioni who has a 7% stake, and cmb who has a 10% stake. All of these entities are necessary to ensure the project runs successfully and with minimal errors.
Metro Line C Project Details
The Metro Line C is a 25.5 km extension with 30 planned stations. 30 new trains with 6 cars each will be assigned to this line that holds 1,200 passengers. Maximum speed allowed will be 80 km/h and headway will range from 90 – 180 secs. This contract features Automatic Train Control (ATC) which allows the train to be driverless and allows supervision from the Graniti Depot Operating Control Center (Depot-OCC). Operating and controlling from station lights to elevators uses a system called Scada which is also used in the United States. There is a platform screen door system, which has not been introduced in the United States, that provided a guard for riders between the platform and the guideway. Engineers need to ensure that train doors perfectly line up with the platform doors and that they open at the same time. To get power for the trains, there is a Power Supply system that obtains electricity and distributes it throughout the extension for the optic fiber ethernet. The estimated cost for the construction and administration of the extension is 3,739,863,806 billion euros. The Metro Line C extension is expected to be completed in 2022.
3.2 Shaft Celimontana Project Details
The first site we explored was the 3.2 Shaft Celimontana near the site office. This shaft had a circular diameter of 34.80 meters and a depth of 59 meters. At the site, I observed several pieces of construction equipment such as a gantry crane with chain linked rigging over the shaft and a hydraulic excavator. According to the Metro C SPA website, they used the “down-up” to excavate and construct the shaft. I assumed down-up is the equivalent of the bottom up method. After further research from article comparing the bottom up and top down method, (Narotama University) I learned that the excavation starts from the ground floor downwards to the planned basement elevation. Meanwhile, the top down method can allow work simultaneously in two directions (upwards or downwards). The bottom up method in the end would take more time than the top down method. The reasoning behind choosing the bottom up method could be differing site conditions or carefulness of historical artifacts in a certain layer.
Fori Imperiali Station Project Details
The second site we went to was the Fori Imperiali near the Colosseum. This station had an approximate excavation of 30-50 meters for the width, a length of 240 meters, and a depth of 32 meters. One of the problems that the contractor encountered was site conditions. The Fori Imperiali Station was right next to the Colosseum so Constractors had to be extremely cautious with archaeological and historical artifacts and buildings in the surrounding area. Contractors also did not have a lot of space for material staging. This area has heavy pedestrian foot traffic so shipments from vehicles in and out of site had to be planned at less busy times of the day. With this station, designers planned on using top-down construction which included casting slabs against the ground that bisect the diaphragm walls. Some of the construction equipment I have observed on site and will be further discussing such as the gantry crane with a clamshell attachment and a hydraulic backhoe excavator.
Gantry Cranes and Clamshell
Out of the two gantry cranes I saw on the tour, one was at the 3.2 Shaft Celimontana. A gantry crane is a crane that moves horizontally on rail to lift vertically and move equipment and materials. What struck me was that, I rarely see gantry cranes in the United States let alone one used for rigging wooden planks to lower down into the shaft. As for the clamshell attachment at the Fori Imperiali station, it is typically used for marine construction in the United States. Clamshells are used for soft and loose soils because it has less of a digging force compared to backhoe excavator. For instance at Fori Imperiali, two excavators were used to excavate the hard soil and the gantry crane with a clamshell attachment moved it to the ground level where another excavator moved the soil into a truck to be hauled off. There are many things to consider when calculating productivity such as the type of soil, depth of cut, and the bucket size of the clamshell. One of the effecting figures would be the cycle time which was exceptionally long. With more information of the bucket rate and exact cycle time, would be able to calculate a ideal productivity rate.
Hydraulic Backhoe Excavator
Hydraulic backhoe excavators were very common around the two job sites we visited. On the job site, we saw the excavators work with the gantry cranes to dump soil into the nearby trucks. Hydraulic backhoe excavators are a type of excavating equipment with a bucket with cutting teeth on the end of a boom. Similarly to uses in the United States, excavators are used for trenches and large excavations. According to SAE international, operators must be wary of the rated lifting capacity of 75% of the load before the excavator tips. This is different from being on level ground where the rated lifting capacity is 87%. In order to calculate productivity, I would need the bucket capacity, fill factor of the material, operational efficiency, and the cycle time.
Trucking in and out of Fori Imperiali is a challenging task to plan. At Fori Imperiali, we saw trucks shipping out excavated soil. Looking at the site logistics next to the Colosseum, contractors would have to plan a route at a certain time of day to ship materials in and out. This plan would have to minimize travel, and ensure pedestrian safety. The trucks cannot be too large or it will damage the old fragile roads. Contractors will need to choose between bottom dump or end dump trucks when backfilling which have different load capacities. Contractors should be wary of some of the organizations who measured standards for rated heaped capacities such as The International Organization for Standards (ISO), Society of Automobile Engineers (SAE) international, and the Committee for European Construction Equipment. Other factors engineers will need to calculate productivity rates include the cycle time, load time, resisting forces, vehicle weight, rolling resistance, average speed, and operational efficiency. These choices will have an effect on productivity rates and therefore scheduling and the final cost of the project.
Tunnel Boring Machine
In order to cut from one station to the next for the trains to pass through, the team used two Tunnel Boring Machines from Germany. These circular tunnels are side-by-side and single track with a 5.8 meter diameter. The TBMs have a earth pressure balance shield which is specifically used for soft soil site conditions with water pressure. The diameter of the boring machine is 6.70 meters and rings that are 30 cm thick. The productivity of the TBM is estimated to be 5-10 meters a day. This source from Metro S.P.A compared to the days in between a similar segment of 8.3km for London’s Crossrail’s project proved to be viable. So for a segment like the Monte Compatri/Pantano–San Giovanni where the length one way is 9.5km, that means one of the TBMs would be onsite roughly 950 days or 136 weeks. There are some areas where the tunnel need to be enlarged such as the station platforms. In these areas, the method of sequential excavation was used. Other considerations when choosing to use a TBM include the price of shipping the machine from Germany, installation, additional equipment to lower the TBM, maintenance of the TBM, and standstill rates. Usually TBMs work 24/7 which means laborers and engineers will be working more hours to ensure everything is going smoothly.
Ground Freezing and Compensation Grouting
In the case of the underpassing, the contractor used ground freezing to stabilize the ground while conduction excavation of the line tunnels underpassing under Line A San Giovanni Station. Ground freezing is a technique where a subcontractor is hired to place pipes vertically deep into the ground and freeze between 2-4 weeks with coolant or liquid nitrogen. This will cutoff the groundwater to increase ground stabilization for tunnel excavation. Ground freezing can be used in any type of soil condition and weather. Trevi is the artificial ground freezing company that was contracted for the San Giovanni Station. They constantly monitored the soil conditions for temperature before freezing began, and during the soil freezing process. For this company to complete their work, the general contractor would have to figure out an area for them to set up their piping and generators as well as trucking the generator onsite.
Compensation Grouting is another important piece of this job in which Trevi was contracted to complete for the Metro Line C extension. The Aurelian Walls surrounding the shaft at Largo dell’Amba Aradam have unstable foundations. With tunneling, that could further sink or destroy the walls. Contractors planned to use compensation grounding to solidify the soil and ground around the walls and in areas affected by the TBMs. Trevi inserted tubes filled with a cement mixture to grout 1,530 cubic meters after drilling alongside the TBM.
The machinery chosen to complete the work on this project should be compared to the productivity rates of other potential machinery that could complete the same task. For instance instead of using a gantry crane, the contractor can choose to use a tower crane or a crawler crane. The contractor could use a front shovel excavator instead of a hydraulic backhoe excavator. Other considerations other than productivity when choosing equipment include the ability to ship from a nearby source and to put equipment the together. In the end, many pieces of machinery on the Metro C Line extension in Italy and the equipment in heavy civil projects in the United States are very similar with the equipment makes and models. This information helps translate comparable projects across continents to increase knowledge of construction techniques. Therefore, these similarities will increase productivity, save time, and decrease cost.
“3.2 Shaft Celimontana.” Metro C, metrocspa.it/en/stazione/pozzo-2/.
“Metro Roma ‘Line C’ – San Giovanni Station.” Trevi, www.trevispa.com/en/metro-roma-line-c—san-giovanni-station.
Noegroho, R. Randi Oktovan. Comparison of Work Between Bottom Up Method and Top Down Method: Execution and Timing. metrocspa.it/en/stazione/pozzo-2/.
“Underground and Special Engineering: Trevi Spa.” Underground and Special Engineering | Trevi Spa, www.trevispa.com/en/.
Schaufelberger, John, Migliaccio, Giovanni. “Construction Equipment Management.” Google Books, books.google.com/books/about/Construction_Equipment_Management.html?id=Yiu9PAAACAAJ.