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Dome saves on tank costs

Novel design provides a cost effective solution to tank uplift problem in Lancashire.

AN INNOVATIVE design concept has greatly reduced construction costs of two underground storm- water storage tanks in Fleetwood, Lancashire.

Amec Civil Engineering and its foundation arm Amec Piling, together with designer Halcrow came up with a novel idea to overcome significant uplift forces on the tanks. The design not only reduced the amount of steel and concrete used in the structure but also cut down on the excavated spoil taken off site.

Groundwater is just 3m below ground level and with water pressure of up to 250kN/m2 on the tank bases, there is potentially around 35,000t uplift on each.

To provide sufficient additional dead-weight to resist this, a conventional concrete base slab would have been 6m thick, requiring around 7,500m3 of concrete with 200kg/m2 steel reinforcement for each tank.

The team's alternative design consists of a 600mm thick lightly reinforced inverted concrete dome, which is filled with 3,000m3 of compacted granular fill and topped with a 500mm thick horizontal concrete slab above.

The design relies upon the granular fill to provide the dead-weight and the dome geometry to provide structural integrity - in that a significant proportion of the horizontal component of dome thrust is carried by earth and water pressure on the 1m thick shaft walls.

Major savings are made because granular fill is about half the cost of structural concrete, and the dome configuration allows reinforcement in the slab to be reduced to 133kg/m2. Amec says the new design saved 3,000m2 of excavation, 6,000m3 of concrete and 82t of reinforcement.

However, the inverted dome only provided just over half of the dead- weight required, still giving an unacceptably low factor of safety against uplift. Consider- ation was given both to extending the shaft wall through some hard clay bands, which were unsuitable for digging with a conventional grab, and by installing 300 ground anchors.

But Halcrow suggested building a tank roof capable of carrying backfill to increase the dead-weight further, which also reduced the volume of excavated material to be dumped off site. This was of a similar design to the base and consists of a 500mm thick reinforced concrete shell with a heavily reinforced concrete ring beam.

The facility at Fleetwood wastewater treatment works is part of client North West Water's Fylde Coastal Waters Improve- ment Scheme, with Bechtel Water Technology as project manager and planning supervisor.

The tanks will have a combined storage capacity of 30,000m3 and comprise twin circular shafts of 40m internal diameter, 24m deep.

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