Problem solvers at the Doha Metro – a surface drilling solution to an underground dilemma

2016 Jun 14

Engineers working on the new super subway now being built in Doha, the capital of Qatar, are using surface drilling technology to solve a common underground dilemma.

The first construction phase of the new Doha Metro is progressing according to plan as three of the four tunnels in the network begin to take shape.

The Green Line, also known as the Education Line, was one of the first projects to get under way. It consists of twin tubes running into Doha from the south before bearing off westwards, through the university district, and on to the Al Rayyan sports stadium.

This is also the line that will transport many thousands of football fans to and from the stadium during the FIFA world Cup in 2022.

Like all of the tunnels in the system, the Green Line has an inner diameter of 6.17 m and a total underground length of 37 km (2 x 18.5 km). It is also being driven at a depth of about 20 m through soft rock – a mix of Simsima limestone, Midra shale and Rus formations.

Progress is on schedule, but the tunnels are constantly subjected to heavy groundwater ingress which presents a major challenge for the engineers, not least at the connection points for the
many emergency cross passages all along the alignment.

These passages, which are 10–15 m long and constructed using mini excavators, cannot be connected to the main tunnels until the surrounding ground has been thoroughly dewatered – a task that requires special drilling technology and expertise.

The work is difficult but is being successfully carried out by the project contractor PSH JV, a joint venture of Porr Bau GmbH of Austria, Saudi Binladin Group of Saudi Arabia and Hamad Bin Khalid Contracting (HBK) of Qatar.

Ferenc Lavicska, Plant Manager of the PSH JV, tells M&C: “The focus here is on quality, reliability, safety and sustainability, so when we were looking for a way to tackle the water problem for excavating the cross passages we needed the best solution we could find.”

The right stuff

Several different solutions were evaluated before the contractors settled on the one it considered was most likely to succeed – FlexiROC T20 R drill rigs together with the Symmetrix drilling system and round-the-clock technical support, all provided by Atlas Copco.

“During this selection process we had a very good technical discussion with Atlas Copco and they showed us that they could provide a solution that lived up to our demands,” says Lavicska. “Since then it has become obvious that we made the right decision.”

The FlexiROC T20 R drill rigs, which are mostly designed for use on the surface, proved to be well suited for this underground application. They are compact units designed with a short, customized feed, enabling them to work freely in the very confined space inside the tunnels.

Equipped with the powerful COP 1140 rock drill, which has a high torque rotation motor, they can drill at any angle. They can also be used together with permanent casing tubes (after a simple modification of
the drill steel support system) and are exceptionally easy to maintain.

Dewatering the ground

The FlexiROC T20 R rigs start drilling the dewatering holes immediately after the TBM’s last gantry has passed the cross passage installation points.
The holes are 76 mm in diameter and installed mainly in the tunnel walls to a depth of 12–14 m. Up to 10 holes are completed per day in two, 10 hour shifts.

During these essential dewatering operations the rigs have also proved to be very fuel efficient, consuming just 20–25 liters per hour. In addition, all rock drilling tools, including R32 drill
rods, shank adapters and drill bits, are proving to be perfectly matched to the application and are long lasting. The service life of the bits, for example, is reported to be 1–1.5 months after
continuous use.

Lavicska comments: “The whole arrangement is working out very well. The equipment is not just getting the job done it is also proving to be very robust which is necessary for this application. We are very satisfied with the progress.”

Meeting the challenge

Atlas Copco has provided the necessary training for the rig operators at the site, in this case sourcing expertise from several countries in the company’s global organization. Specialists from Sweden, Italy and the UK have conducted training sessions at the site on drilling, grouting and maintenance, often in the open air as access to the tunnels is severely restricted.

Besides this, Atlas Copco’s local distributor, Oriental Trading Co, is just one hour’s drive from the site and is on call 24/7 to make sure the equipment stays up and running.

So far, more than 350 dewatering holes and 600 grouting holes have been completed and all of the planned holes are expected to be in place by August 2016.

Vasanthalu Shivakumar, Sales Manager, Gulf Region, for Atlas Copco, concludes: “The solution to this challenge at the cross passages was the complete package we were able to provide – not just the
right equipment but the right training as well as good service support, which is of the utmost importance on this prestigious project.”

Construction of the Green Line is expected to be completed by 2019.