A14 C2H Borrow Pit Dewatering Assessment

Case Study

As part of the £1.5bn project to upgrade 21 miles of the A14 between Cambridge and Huntingdon, ESI has developed a groundwater flow model of the shallow groundwater system to assess the effect on key receptors of dewatering via the proposed borrow pit and recharge lagoon system.

 

The A14 Independent Delivery Team, a joint venture formed by Skanska, Costain, Balfour Beatty and Carillion, was awarded the contract by Highways England to upgrade the existing A14 between Ellington, west of Huntingdon and the Milton junction on the Cambridge Northern Bypass. The work includes widening of the A1 between Brampton and Alconbury.

 

A14 Dewatering_Simulated groundwater levels showing groundwater level targetsAs part of this project the IDT requires a series of borrow pits, from which shallow sand and gravel and underlying Oxford Clay will be utilised for the road construction.

 

To construct the borrow pits the shallow groundwater must be dewatered and it is then proposed to recharge the discharge water back to ground, via a series of recharge lagoons.

 

The shallow groundwater discharges to a number of surface water features, including the River Great Ouse, brooks, ditches and fishing lakes, as well as supporting some private groundwater abstractions.

A14 Dewatering_EA Gauging Station Alconbury Brook

ESI was commissioned by the A14 IDT to develop a groundwater flow model of the shallow groundwater system and to assess the effect on key receptors of dewatering via the borrow pit and recharge lagoon system.

 

ESI collated available data for the area. A large amount of Ground Investigation data was available for the road upgrade; data for the wider area was more sparse and several data sources were utilised.

 

The data was used to develop a groundwater flow model using MODFLOW-NWT. This variant of MODFLOW was selected to assist with the difficulties often experienced in simulating groundwater flow in very thin aquifers and where dewatering will locally dry out parts of the groundwater system.

A14 Dewatering_Simulated change in groundwater levels from base case modelA14 borrow pit aerial view

The model was calibrated to available groundwater level and river flows. It was also supported by data from two short duration pumping tests undertaken as part of the project.

 

The calibrated model was used to assess the likely zone of drawdown around the borrow pits.

 

The model showed how groundwater discharges to surface water would be reduced and even reversed along some reaches.

 

 

Using this information likely compensation flows were derived to ensure that the nearby brooks and fishing lakes could be maintained during the period of dewatering.

 

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