Satellite accuracy

24 April 2008

Blasting 225000 M3 of Rock, Moving 350000 m3 of earth, constructing a 950 m tunnel and four bridges in just over three years is a tough challenge. But it is one which contractor Mesta is undertaking to upgrade a section of the E39 at Romarheimsdalen, near Bergen, Norway.

The E39 is the main north-south route on Norway's west coast and carries an average of 1500 vehicles a day. This volume may not sound high but the majority of the traffic using the narrow, twisting route are heavy goods vehicles. In winter the route is often covered with ice and snow, making the bends dangerous, while in summer tourists add to the traffic volume.

Mesta's work on the NOK 148 million (€ 15,5 million) project started in January this year with drill and blast work to widen and straighten the existing carriageway. Part of the 7,2 km upgraded route will follow the existing road but the mid section, with the new tunnel, has been realigned to the east.

“Around 80% of the construction activity on this project is on the middle 3,2 km section of the route which is away from the live traffic,” said Mesta project Manager Ola Kvammen. “Although the road improvement work only accounts for a small portion of the overall scheme, it also represents one of the biggest challenges of the project.

“Keeping the traffic flowing while we carry out the upgrade makes this part of the scheme quite complex. A lot of the blasting is being carried out very close to the existing road, so we have to stop the traffic before we blast and we cannot reopen the road until we have safety checked the area and cleaned up.”

Mesta has permission from the Norwegian road authority, Statens Vegvesen, to stop traffic on the E39 for up to an hour to carry out rock excavation. The need to reopen the road quickly after each blasting session meant that Mesta had to carry out the drilling to a high level of accuracy.

“Over break or excessive fracturing in the finished surface could delay reopening while safety checks are carried out. Carrying out additional bolting to stabilise the rock could also add to the traffic disruption. Payment on this project is also partly related to the accuracy and finish on the cutting walls, so getting the blasting right, first time is essential,” said Mr Kvammen. “The rock we are blasting through is a Gneiss that is quite hard - around 2000 MPa - and is solid below the surface, which has helped us to produce a good finished surface.”

Nonetheless, Mesta is also using the first two production models of a new surface drilling rig from Atlas Copco to help it achieve maximum accuracy. The two SmartRig ROC D7C rigs are fitted with a Global Positioning System (GPS) technology which is not only helping to speed up collaring but is also enabling Mesta to achieve a good finished surface.

“The SmartRigs are fitted with two Trimble MS750 receivers which communicate with an on-site base station to enable the rigs to use Real Time Kinetic (RTK) GPS to give accuracy to plus or minus 50 mm,” said Atlas Copco GPS product manager Jean Lindroos. “The rigs are aimed at the civil engineering construction market where GPS control is becoming commonplace for earthmoving operations but they can also be used for quarry applications.”

On Mesta's rigs, the GPS data is fed into an onboard computer, which uses a program called Anmask, which was jointly developed by Mesta and a Swedish software company. Anmask feeds the GPS data into its vector-based drill plan, which is displayed on an in-cab LCD screen that the operator uses to guide the drill feed to the location of the next hole.

Benefits

“The Anmask system enables holes to be drilled slightly off-line if there is an obstruction at the predetermined location and recalculates the drill angle and depth to achieved the finished face,” said Mr Kvammen. “The combination of the rig and Anmask enables us to drill parallel holes and achieve flat benches. The other main benefit is that the position of each drill hole does not need to be marked out, so we only need two engineers on site, rather than the six or seven that a project of this scale would normally need.”

The GPS system also allows the drilling record of each rig to be automatically logged and, according to Mr Lindroos, the improved drilling accuracy means less explosives are needed.

When CE visited the E39 site, the average drill depth was 7 m but holes up to 16 m deep have been successfully completed. The holes are being drilled using a 3 in (76 mm) diameter drill bit. “The rigs have been achieving a drill rate of around 1,5 m per min and between the two rigs we have been drilling up to 1000 m a week and blasting between 1000 and 3000 m3 of rock in each session,” said Mr Kvammen.

Elsewhere on site, work is underway on the road realignment, which also involves drilling and blasting work but the main focus is reusing the blasted material as fill. “The rock fill embankments will even out the gradient of the road on the realigned section. Some sections of fill will be up to 30 m deep when finished and stretch over 300 m,” said Mr Kvammen.

Construction work on the tunnel is due to start in December this year but Mr Kvammen said that the equipment for that element of the project has not yet been selected. The finished scheme is due to open to traffic in September 2008.

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