Case Study: Sheet Piling and Associated Temporary Bracing Works at Ambleside WwTW

Sheet Piling and Associated Temporary Bracing Works at Ambleside WwTW

Contract No:C1218
Location:Ambleside WwTW
Value:Approximately £270,000.00
Summary:Design, supply and installation of sheet piles and temporary circular waling beams to form a 17.2m diameter temporary cofferdam.

Sheet Piling (UK) Ltd have recently completed sheet piling and associated temporary bracing works at Ambleside WwTW as part of a £40million investment by United Utilities.

The project involves laying a new 6.5km sewer and upgrade of two wastewater treatment works to reduce the amount of phosphorous in Lake Windermere which can be detrimental to aquatic life.

Sheet Piling (UK) Ltd were employed by Advance, the Joint Venture between Balfour Beatty and MWH, who are one of the Construction Delivery Parties for United Utilities AMP6 Capital Investment Programme.

The works involved the design, supply and installation of sheet piles and associated temporary steelwork bracing frames to form a 17.2m diameter circular cofferdam to facilitate an excavation depth of 8.4m.

The sheet piling works were required to facilitate construction of a new reinforced concrete Final Settlement Tank structure in the existing ‘live’ treatment works.

In addition, 2no ‘blister’ cofferdams were installed to allow construction of RAS and Distribution Chambers to the main Final Settlement Tank.

The Temporary Works were not only designed to provide lateral earthworks stability for the excavation works, which were adjacent to existing structures, tanks and services; but also, to provide a cut off into the underlying cohesive strata.

Temporary Steel Sheet Piles at Ambleside

The prevailing ground conditions on site were made ground to a depth of 2.0m underlying which was a loose granular alluvium being more cohesive with depth. Furthermore, the natural groundwater table was approximately 2.0m below existing site level.

As such, in order to provide a suitable Temporary Works solution to support the excavation; a sheet pile cofferdam with 3no levels of circular steelwork waling beams together with an extensive dewatering system was required.

In total, 148no VL605 16.0m long sheet piles were installed together with approximately 50Te of temporary steelwork bracing.

Each of the 3no levels of circular steelwork waling beams were pre-bent to the required diameter off-site in 5no equal segments prior to being fabricated on-site.

To ensure the exact tolerances required on-site for the cofferdam could be achieved, an off-site mock-up of the completed circular waling was undertaken in Sheet Piling (UK) Ltd.’s Preston Yard.

It was important to consider the construction sequence for the excavation works of the main circular cofferdam and ‘blister’ cofferdams to ensure the Temporary Works were not subjected to any out-of-balance forces. This information and sequence was clearly transferred to the Advance site team.

The sheet piles were installed using Sheet Piling (UK) Ltd in-house ABI TM14/17SL Telescopic Leader Rig complete with Vibratory Hammer and serviced by a Sennebogen 643R Telescopic Crawler Crane.

The installation works commenced in mid-July, and were completed in 2 weeks to the mutual benefit of all parties.