Construction

Construction Overview

The project works include tunneling and construction works in main stations’ locations, controlling and operation centers, stabling and maintenance depots, casting and reinforcements works for many lines and viaducts, transfer services, providing the project with electricity and deep tunneling works of the lines.

The construction works are divided into 85 stations: Qasr Al Hokom station, King Abdullah Financial District Station, Western Station, and fifth lounge station in King Khaled International Airport. Plus, implementation of an operations and control center, stabling and maintenance depots, and construction works for the projects’ power plants.

Bridge and Viaduct

There are 6 different construction methods being used to build bridges and viaducts for the Riyadh Metro project. Each method is being deployed to optimize the construction process taking into account speed, length of span and complexity of the construction locations.

Full Span Launching Method"

On the Lines 4&6 of Riyadh Metro project, 450 ton full span girders are pre-cast and then lifted onto a girder carrier. The girder carrier moves over the existing spans to carry the pre-cast girder to the rear of the girder launcher. The girder beam launcher lifts and places the pre-cast girder into its final position. The girder carrier then returns to collect the next span, in the meantime the beam launcher moves forward ready to install the next span.

Pre-Cast Beam Method

In this case a span consists of two full length pre-cast beams and a cover, which is placed on the top of the beams once both beams are in place. The beams, each weighing about 200 tons, and the cover are pre-cast and then transported from the pre-cast yard to the installation point. The beams are then lifted into position and the cover placed on the top to provide the necessary track width. This method, used mainly on Line 6, was selected to enable transportability of the beams across the city from the pre-cast yard to the installation points.

Segmental Method

A full span is assembled from a number of segments which are pre-cast, transported to the installation point and then lifted into place. Each segment weighs 35 – 40 tons. When all the segments for a span have been lifted into position the segments are then tensioned and locked into position. This method is being used on all lines.

The Balanced Cantilever Method

This method is normally used where longer spans are required. Pre-cast segments are assembled at the installation point. The first segment is lifted on to the top of a pier. The next segment is attached on one side and the following segment attached on the opposite side. Each time, a segment is installed first on one side and then the other. This allows the installation to be balanced similar to a weighing scale and the weight distribution takes advantage of the cantilever forces to remain in position. This method is not as fast as the other methods because of a longer set up time.

Scaffolding Method

This is an alternative method used where building particularly long spans is required. Scaffolding is erected where span segments are to be installed and then the segments or beams are lifted into place onto the scaffolding, and eventually secured. Once the full span is completed the scaffolding is removed and put in place for the next span. This method is rather slow because erection and dismantling of the scaffolding is time consuming.

Cast in Situ Method

This method is not particularly common, but it is being used at some stations and a few locations where the track curves. The process involves setting up scaffolding around the installation point, then constructing the reinforcing steel framework, installing the shuttering and finally pouring the cement. Once the cement has set the shuttering is removed and your span is in place. This method is also very time consuming because there are many stages from the beginning to the end of the process each dependent on completion of the earlier process.

Tunneling

There are three methods of tunnel construction being used on the Riyadh Metro project, each adapted to optimize the construction process of the whole network taking into account the speed, geology and above ground buildings in areas where the lines run.

Tunnel Boring Machine (TBM)

A tunnel boring machine is a very large scale rock drill. The whole TBM equipment is over a hundred metres in length consisting of the cutting head, the electric motor which drives the head, the tunnel lining installation machine and the conveyor unit which takes away the excavated material. The TBM is controlled remotely by computers and skilled tunneling engineers from an operations room which monitors and adjusts the TBM equipment direction, speed and operations. In the case of the Riyadh Metro the TBM cutting heads are 9m in diameter. Seven TBMs have been deployed across the whole construction project cutting a total of 39km of tunnel. Two TBM manufacturers were selected for the project: Herrenknecht from Germany and NFM Technologies from France.

New Austrian Tunneling Method (NATM)

The New Austrian Tunnelling method (NATM), is also known as Sequential Excavation Method (SEM), The fundamental difference between this new method of tunneling, as opposed to earlier methods, comes from the economic advantages made available by taking advantage of the inherent geological strength available in the surrounding rock mass to stabilize the tunnel. NATM integrates the principles of the behaviour of rock masses under load and monitoring the performance of underground construction during construction. It relies on exploitation of the strength of native rock mass; minimization of loosening and excessive rock deformation, which is achieved by applying a thin layer of shotcrete immediately after face advance and measurement and monitoring – NATM requires installation of sophisticated measurement instrumentation. It is embedded in lining, ground, and boreholes. In the event of observed movements, additional supports are installed only when needed, with a resultant overall economy to the total cost of the project.

Cut and Cover Method

The cut and cover method consists of excavating a large trench along the line of the tunnel, installing the base and sides tunnel lining in the trench, then putting a cover over the base and sides and finally refilling the trench around the constructed tunnel to reinstate the land over the tunnel. This method is used for shallow underground tunnels.