Project Dewatering Limited assisting the construction of a deep shaft in Middlesbrough

​Project Summary

The proposed work involved constructing an 8.3m diameter, 14.3m deep shaft at a wastewater treatment plant. A review of the site investigation report revealed the presence of an upper unconfined aquifer and a lower confined aquifer, with a layer of sandy clay acting as an aquitard. The shaft would penetrate this aquitard and extend into the lower confined aquifer. The client reported the presence of large boulders near the shaft location, which were not documented in the site investigation report.

Dewatering strategy

PDL implemented a deepwell dewatering system, with wells positioned equidistantly around the shaft at approximately 5m intervals and extending to a depth of 18m. The wells were drilled to a 310mm bore diameter and fitted with 165mm casing and screen. The annular space of the wells was filled with graded filter media, and centralisers were used to ensure the casing remained centred in the bore, allowing for an even distribution of the gravel pack. A Rotary Sonic drilling rig was used to mitigate delays or well relocations caused by encountered boulders.

The lower sand aquifer was under hydrostatic pressure, with groundwater first encountered at 12m and recovering to 6.5m after 20 minutes. To reduce hydrostatic pressure and achieve sufficient drawdown, the wells extended to 18m, terminating in the clay layer. The discharged groundwater was processed through a silt separator tank to minimise the amount of fines

entering the discharge point.Outcome

The project is ongoing, and PDL is recording good drawdown in the pumping wells. We are closely monitoring the drawdown and maintaining regular correspondence with the client to ensure the successful completion of the shaft excavation.