MGF Assist with Upgrading the Existing River Intake at Witches Oak Water

  • Project Witches Oak Water, Derby
  • Client Severn Trent
  • Principal Contractor Mott MacDonald Bentley (MMB)
  • Project Lead Rob Eagles, Regional Engineering Manager
  • Key Benefit A bespoke temporary works solution in a restricted environmental location.
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Introduction

Witches Oak Water is a series of gravel pit lakes south-east of Derby, near the River Trent.

Severn Trent are the client for the recovery project which aims to safeguard future water supplies. The works include recommissioning of the existing infrastructure to increase the raw water supplies by around 90 million litres a day.   Principal contractor, Mott MacDonald Bentley (MMB) – the integrated design and build company between JN Bentley and Mott MacDonald was appointed to remove the existing pipework that connected the inlet and outlet structures.

Through sustainable design they retained the existing headwalls, and formed a new in-situ open channel that will connect the River Trent on one side to the lake on the other. The aim of the new channel is to increase flow capacity and promote biodiversity.

To support MMB and Severn Trent to upgrade the existing river intake, MGF were appointed to provide a design solution and supply the temporary works equipment.


The Solution

Having already worked on a similar scale cofferdam for Severn Trent , MMB called upon the expertise at MGF to assist with the project at Witches Oak.

An initial site meeting was undertaken in January 2023, to meet the project team, and understand requirements to enable the construction of the permanent works. A limiting factor from this meeting was the access to the works location, which would influence the temporary works solution. Optioneering, identifying hazards, understanding the environment, and discussing the possible construction sequence, including the proposed dimensions, were elements of information taken from the meeting. This insight was then used by MGF to develop a highly informed conceptual design that incorporated all these key factors.

Collaborative working from the outset was crucial
to reviewing and refining the design concepts and making sure they did not clash with any existing structures or proposed permanent work.

Using SketchUp, MGF proposed the temporary works sequence, which enabled MMB to clearly explain to the client (Severn Trent) their approach to carrying out the work. In addition, MGF created a short animation that detailed the installation sequence in 3D and provided a digital rehearsal of the construction. Proprietary shoring equipment was delivered by MGF in July 2023 to construct the cofferdam, which spanned 46.5 metres in length and was almost 13 metres wide. They included: a combination of 9.5 metre and 11 metre-long GU16N sheet piles; BS20 corner piles; a 406UC perimeter frame and 400 series struts and knee braces.

MGF UniShore was also used within the existing headwalls to transfer loadings from one side of the cofferdam to the other during the construction sequencing.

                                                                                                                      


The Verdict

Thanks to MGF’s experience and ability to develop highly tailored solutions for the project at hand, the temporary works were designed, and the equipment delivered by MGF from the local depot in Rugeley. MMB completed the installation to a high standard meeting project timescales. MGF also leveraged its wealth of company-wide expertise to overcome several complex project challenges with ease.

The proposed cofferdam was part installed within the River Trent to one side and an existing lake on the other, a risk of flooding was therefore an issue. MGF overcame this by designing for flood conditions with a weir wall at either end of the cofferdam. In keeping with sustainable construction, the existing inlet and outlet was to be re-used as part of the permanent works. MGF designed the cofferdam to wrap around the structures to establish a safe and dry working environment. The cofferdam was constructed using clutched sheet and corner piles that were driven into the lower gravel layers to help maintain groundwater control.

MGF designed a lower frame that enabled the central section of the cofferdam to be removed once the base had been cast, permitting full height pours for the concrete walls to take place. To prevent concrete being used in the footprint of the river or lake, MGF designed a shear wall with bespoke plates and a fixing detail into the headwalls to support either end of the cofferdam when the central section was removed.

Furthermore, due to the existing structures being retained and reused, the temporary propping needing to be positioned around or into the structures. MGF expertly overcame this issue by accurately modelling and checking the location of the props, as well as providing the MMB design team with the details they needed to complete an accurate load transfer assessment of the load being transferred into the existing structures.                                          

“The MGF team from design through to on-site assistance on the installation have been great to work with”.

Mike Lewis – Contracts Manager

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