Case Study: Band Drain Removal Cofferdam, Mantrunk WwTW

Band Drain Removal Cofferdam, Mantrunk WwTW

Sheet Piling (UK) Ltd were employed by a Galliford Try, Costain & Atkins Joint Venture (GCA) for the ‘Supply, Installation & Extraction’ of sheet piles to form a 30.0m x 5.0m temporary cofferdam to facilitate removal of existing band drains.

This was an extremely challenging project in terms of technical and methodology restrictions imposed by the Design Team.

The cofferdam was required to remove existing band drains which were located on the proposed tunnelling alignment for the Tunnel Boring Machine (TBM) forming the outfall tunnel into the Manchester Ship Canal as part of the Mantrunk UIDS project.

The cofferdam was located in very close proximity to the existing foundations for Barton Bridge which carries the M60 motorway over the Manchester Ship Canal.

As such the Highways Agency Consulting Engineer imposed the following restrictions on the sheet piling works:-

• No impact driving of the sheet piles was permitted.
• Upon extraction no voids were to remain in the ground.

GCA’s temporary works design for the cofferdam required 19.0m long Arcelor Mittal GU32N sheet piles to be installed into predominately ‘Very Stiff Glacial CLAYS’ which under normal circumstances would prove very problematical using only traditional vibratory methods.

Consequently Sheet Piling (UK) Ltd proposed to utilise high pressure water jetting during installation of the sheet piles which was delivered via a heavy duty steel tube fixed to the sheet pile and jetting nozzle at the toe of the sheet pile. This is a technique which is commonly adopted in Europe but very rarely used in the UK.

Using a standard Bauer RTG19 Telescopic Leader Rig complete with a powerful Vibratory Hammer, the sheet piles refused approximately 5.0m above pile toe level. However once the high pressure water jetting was introduced at the toe of the pile, it was possible to drive the sheet piles to the required pile toe level in extremely competent cohesive material.

Upon completion of pile installation, the water jetting lances were capped off at ground level and remained in-situ fixed to the pile.

Following successful excavation of the cofferdam and removal of the band drains, the sheet piles were then extracted using the same vibratory plant & equipment.

To ensure that no voids were left in the ground following extraction of the sheet piles which may otherwise cause potential for settlement of the bridge foundations; during the extraction process a suitable weak grout was injected down the steel tubes continuously during the extraction process.

Whilst this ensured no voids remained following extraction of the sheet piles, it was essential to ensure the injected grout was of a suitably low crushing strength to ensure the Tunnel Boring Machine (TBM) could subsequent bore through any grouted voids.

Consequently during the complete sheet pile extraction and grouting process it was essential that all necessary Quality Control measures were in place, such as ensuring that all the grout was certified prior to installation, formed test cubes to ensure the grouts strength and monitored the volume of grout installed into the void as each pile was extracted to ensure all voids were completely filled.

The works were successful completed between January 2016 and August 2016 to the satisfaction of all parties involved with the project.