Hydrodemolition techniques are being used for the refurbishment contract on Sweden’s Jäpströmmen hydropower station dam wall and three spillway gates using Aquajet’s Aqua Cutter robotic hydrodemolition equipment; including underwater operations and night-time temperatures of -25ºC.

Swedish hydrodemolition specialist E-Schakt has started work on the major refurbishment contract for the concrete dam and three spillways of the Jäpströmmen hydropower station, some 80 km west of Ostersund, mid-Sweden; to meet increased water volumes.

Built in 1944 and located on Lake Kallsjön, the dam walls and concrete structures of the three spillway gates have badly deteriorated as a result of poor initial construction methods and materials and subsequent maintenance.

The region experiences severe sub-zero temperatures throughout the winter creating considerable ‘frost and ice’ damage across the concrete faces with extensive cracking.

Major moss growth on the concrete walls in the summer months has further aggravated the situation resulting in further water seepage into the concrete fissures.

A two year contract has been awarded to Skanska for the complete refurbishment of the dam walls, gate structures and three spillway gates, by Fortum, the private company operator of the power station supplying power into the country’s national grid.

Start of Phase 1

E-Schakt was awarded a 3 phase sub-contract with work on Phase 1 starting in September 2009 for the concrete removal on the top slab and down-side wall of the dam, plus the gate 3 structure; all demanding hydrodemolition techniques.

Skanska will then replace the treated areas with new concrete and where necessary, new rebar, ready for E-Schakt to start Phase 2.

This is scheduled to start in May 2010, again using hydrodemolition on the lake-side dam wall, including underwater operations and gateway 2.

The third 12 week contract is planned for November 2010 and will include gate 1 plus the removal of a small fourth gate and replaced by a new dam wall structure.

Use of hydrodemolition techniques ensures no rebar damage, minimised risk of good concrete removal, eliminates dust and crystalline silica pollution and leaves a superior bonding surface. It is also substantially faster than mechanical removing methods and is also considerably less labour intensive.

Hydrodemolition

E-Schakt is using hydrodemolition equipment from Sweden’s Aquajet Systems and includes an HVD Evolution Aqua Cutter robot, an Aqua Frame for underwater operations and a PP-700 Ecosilence power pack.

Dam wall

Featuring a total length of 41.8 m, the all-concrete dam structure has a maximum height of 14 m, a 2.5 m wide top slab and 6 m wide base.

In Phase 1, E-Schakt has removed 500 mm depth of concrete from the top slab and within a five week period virtually completed the hydrodemolition of the dam’s dam-side wall to a depth of 200 mm; using both the Aqua Cutter robot and Aqua Frame for the angled face.

A key feature of the Aquajet robot has been, according to E-Schakt’s site foreman, Andreas Nordström, the ability to undertake geometric triangular cuts – a unique feature of Aquajet Systems.

With three triangular sections of the wall ‘face’, each measuring 4 m x 2 m, the system eliminated the need for time consuming alternative methods.

“Another advantage has been the 12 m high mast which has allowed us to work at the maximum height.“

A 1 m deep channel excavated along the dam ‘face’ will provide access for the Aqua Cutter robot to continue removing deteriorated concrete below the ground level.

During the second phase contract, E-Schakt will remove 5.5 m of concrete on the vertical element of the upstream side of the dam to a depth of 200 mm; including at least 1 m underwater.

For this operation, the Aqua Frame robot will be positioned on the vertical upstream damwall and be secured to the wall by divers. Then, in close collaboration with the Skanska divers, the E-Schakt operator can continue the under water hydrodemolition operation remotely from above the surface.

The Aqua Frame includes 8 meter side rails with a 4 meter traverse rail to support the oscillating lance in the traversing power head with its nozzle to continue operating at 1000 bar at a rate of 260 l/min – 330 l/min depending on the removal depth.

Spillway gates

The Järpströmmen installation features three steel spillway gates, one with a gate well and two without a well, featuring concrete slab spillways.

The three gates provide a total length of almost 60 m and a maximum height of 16 m.

Throughout each of the three phases of refurbishment, a single gate will be removed in turn for blasting and retreatment. Hydrodemolition techniques on all concrete structures to the gates will follow removal of the gate.

This aspect would normally provide problems for contractors with the numerous angles to be treated but, says Andreas Nordström, “It is the trickiest element but we are confident that the Aquajet’s unique control system will help us to remain on schedule.”

In total, the contract calls for the removal of more than 162 m3 of concrete from the dam and 439 m3 for all three spillways.

Freezing temperatures

The probability of night-time temperatures plummeting to below -20ºC is the principal concern for the sub-contractor throughout the winter months’ operations.

“Currently, in mid-October we are experiencing daytime temperatures of -9ºC,” confirmed Andreas.

“With the downstream river already frozen for the winter, the project could become a giant ice cube. Today is only -7ºC but by the night it is getting much colder.”

To prevent the hoses freezing overnight, E-Schakt has resorted to continually pass water through the high pressure hoses throughout the night and disconnecting the power pack, pass the water into the downstream river.

A heated circuit is also maintained in the water pumps 24/7 operation and antifreeze is also applied to the hosesA heated circuit is also maintained in the water pumps 24/7 operation and antifreeze is also applied to the hoses.

“It’s about preparing for the night-time temperature drop, otherwise we will have a real problem next morning,” he added.

First thing each morning, high pressure air is flushed through the hoses to clear the antifreeze before reconnecting then to the power pack.

Similarly during daytime breaks for meals and meetings, equipment is kept in ‘idle’ rather than shutting down.

Throughout each 12 week contract, E-Schakt has a team of three operators working a 12 hour shift, 7 days a week on a 7 day changeover cycle.

Järpströmmen hydropower station

The power station was built in 1944 and features three 118 MW tubines providing 415 GWh/year of electricity for the national grid.

Featuring a fall height of 66 m to the underground plant, the turbines are fed by a waterflow of 220 m3/sec from the 156 km2 lake Kallsjön, Sweden’s sixth largest lake with a depth of 134 km.