Client: Bowmer & Kirkland
Duration: March – June 2019
As part of the Part of the development of HMRC Unity Square in Nottingham City Centre involved a lift pit core area, with a nominal 7.2m x 12m footprint with an excavation upto 5.2m BGL.
Groundwater level was in the region of 2m BGL. Ground conditions consisted of highly permeable River Terrace Deposits (Loose silty, sandy f-c GRAVEL c/w cobbles) overlaying the Nottingham Castle Formation.
There was an issue in that no conventional or suitable discharge point for abstracted groundwater, such as local utility manhole or water course could be utilised, without incurring excessive unit costs to discharge to utility outfall or having project programme issues by going down the EA permiting route to discharge to adjacent water canal.
Stuart Wells proposed a dewatering system, with all abstracted water being recharged back to ground, to comply with EA conditions to abstract and discharge groundwater.
A conceptual model was established and developed with the undertaking of 2no x 72 hour constant rate pumping tests during which groundwater was discharged back to ground via recharge wells. During the test period groundwater levels and flow was monitored togther with groundwater chemical analysis.
Further to this a deep well dewtering system was designed to achieve the obectives allowing excavation work to be carried out in dry and stable conditions. With a perimeter 7no x 10m deep well dewatering system installed around the lift core area.
Total abstraction flows ~ 60 l/s were successfully discharge via a total of 14no x 10m deep recharge wells located ~ 200m from the abstraction wells and ‘downstream’ of natural groundwater flow.