WITH RESERVOIR levels low, work is being carried out to increase spillway capacity at Yorkshire Water's Leeming Reservoir.
Framework contractor Bentley is raising the 130 year old impounding dam's spillway walls from 3m to 6m, and enlarging the tumblebay immediately behind the weir at the crest of the dam.
More intense winter rainfall has increased the water flow into the reservoir, driving the need to upgrade the 31m tall dam.
Spillway capacity will be increased from 55m 3/second to 90m 3/second, equivalent to a 1:10,000 year ow.
Enlarging the tumblebay pool at the top of the spillway is the more technically challenging part of the contract, requiring Bentley to cut into the dam's original puddle clay structure.
To ensure that the dam's watertightness is restored, a reinforced concrete cut-off wall is being installed beneath the tumblebay floor that will itself be of mechanically compacted puddle clay.
Tumblebay enlargement involves creating an excavation measuring 44m long by 8m to 14.5m wide and up to 6.7m deep.
Vertical Larssen sheet piles are working as propped cantilevers, with raking hydraulic struts supporting waling beams near the top of the excavation.
The base of the piles need no lateral support, enabling easier removal of material from the bottom of the excavation.
Concrete pouring for the new structure's base and walls has just started.
Designer for the £1.8M works is framework consultant Mott MacDonald. Work is due for completion in early August.