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Sat 27 Jan 2007 12:00 AM

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Axera drills seismic chamber in Sharjah

Gulf Rock Engineering Group, a Sharjah-based drilling and blasting specialist contractor has taken delivery of the region's first Sandvik face drilling jumbo rig, a Tamrock Axera T08-290

Gulf Rock Engineering Group, a Sharjah-based drilling and blasting specialist contractor has taken delivery of the region's first Sandvik face drilling jumbo rig, a Tamrock Axera T08-290.

Although purpose-ordered for a government tunnelling project in the UAE, early delivery has allowed the company to put the jumbo rig through its paces on a 25m long tunnel for a seismic chamber, according to Jonathan Cottam, general manager, Gulf Rock.

Located in the Hajjar range of mountains and 15km from the UAE gulf of Oman coastline, the seismic chamber will be connected to other seismic monitoring centres including one at the American University in Sharjah. This will allow triangulation of the region's earthquakes.

Neighbouring Iran is located on a highly active fault zone and the UAE has experienced recent tremors following these earthquakes.

In a joint venture between the University of Sharjah and the University of California in San Diego, the Sharjah Global Seismic Network stations will be jointly run to form part of a global network. The seismographic station will be able to identify the origin, depth and strength of earthquakes in the region to provide a meaningful academic database to guide, for example, future building specifications in Sharjah and the UAE.

With Halcrow International Partnership as consultant engineer for the government of Sharjah, Gulf Rock started excavations on the tunnel at Wadi Hilu, some 90km north east of the city of Sharjah, in a three-month contract.

Featuring a 3.9m wide x 3.9m high arch profile, drilling is currently some 6m beyond the portal. According to Cottam, the company is initially experiencing very poor Grade 3 and 4 gabbro at the portal but is optimistic this will improve to at least Grade 2.

Gulf Rock is drilling 1.2m deep holes with the Sandvik's drill jumbo but hopes to increase this to 2m once the rock improves, continuing with a 55 hole pattern.

Over the first 5m of tunnel, adjacent to the portal face the contractor has installed lattice girders at 1m longitudinal spacing and steel-fibre reinforced shotcrete sprayed to a thickness of 250mm. The shotcrete is built up over 3 - 4 layers, each mixed with steel fibres with a 40kg/m3 mix to enhance tensile strength.

Following the section of lattice girders a combination of systematic dowelling and shotcrete provide the structural lining of the tunnel. A primary layer of sprayed shotcrete at 60-70mm thickness is applied before drilling and installation of double-corrosion-protected rock dowels.

The 2m long rock dowels are inserted into the roof radially on 1m centres and thereafter at 1m longitudinally.

"As we penetrate deeper into the tunnel, a persistent sub-horizontal fault on the face is rising to intersect with the tunnel roof profile. This intersection may also require the installation of lattice girders," said Cottam.

Once excavated, an internal wall will isolate the final 5m to form the seismic chamber.

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