McAllister Bros team working at Kings Scholars' Pond Sewer Rehabilitation

SUMMARY

The objective of this project was to stabilise and remediate a section of the King’s Scholars’ Pond Sewer (KSP) that spans the crown of the London Underground tunnel. This required the construction of a 9.5m long Vierendeel truss and associated glass fibre reinforced plastic (GFRP) liner. The modular structure was designed to be installed through the aperture of an existing manhole and then bolted together to form a watertight bridge within the existing sewer and spanning the London Underground tunnel beneath.

The King’s Scholars’ Pond Sewer is a brick sewer constructed in the 1850’s and following the route of the River Tyburn. At the station it crosses approximately 3m below busy main arterial road and bridges the crown of the London Underground tunnel. At this location the sewer is supported by two primary cast iron beams, which span between pad-stones embedded within the brick wall of the railway tunnel. Between these two primary beams are profiled cast iron invert plates connected back to the beams by bolted connections. A brick crown completes the sewer profile.

During routine inspections by Thames Water, evidence was found of internal degradation and deterioration of the sewer that included significant cracking of the brickwork caused by settlement of the railway tunnel beneath. On identification of these defects, temporary strengthening in the form of steel ribs was installed while a permanent design solution could be developed.

Pre-installation

The existing sewer with temporary strengthening ribs prior to commencing the renovation

Being situated beneath a key arterial road into central London and above the London Underground railway lines would make any form of top down or bottom up solution particularly difficult and extremely costly. It was therefore decided to proceed with a design for a bridging structure that could be constructed entirely from within the existing sewer.

To achieve the required design life of 120 years the primary structure was fabricated using duplex stainless steel. The complete steel structure weighs approximately 9 tonnes, however each individual steel member had to be small enough to manoeuvre down a 600mm x 750mm manhole shaft and transported 30m along the sewer where the structure was then assembled. When completed the minimum clearance between the new and the existing structures would be only 40mm. The modular glass fibre reinforced plastic (GFRP) lining system was designed to be fully removable which will enable future inspections of the brickwork behind to be carried out.

Due to the sensitivity of the existing cast iron structure, monitoring arrays including strain gauges and inclinometers were used to check for movement during the site works. The existing brickwork structure was repaired using traditional crack stitching and grout injection. To maintain the stability of the overall structure the temporary strengthening ribs were then replaced sequentially with new low profile straps that would support the existing brickwork above and allow construction of the new permanent structure. These straps also had to be designed such that they could be removes from around the new structure once it had been completed.

Construction team assembling one of the Vierendeel trusses

Due to the extremely tight space restrictions, each side truss had to be assembled along the centre line of the sewer, lifted and slid into position on the bridge bearings as a complete unit, each weighing approximately 3 tonnes. Cross beams and bracing were then installed in the roof and floor to complete the main bridging structure.

Completed primary steel structure

Once complete the temporary works could be removed from around the new structure and GFRP panels installed to create a watertight pipe to carry the sewer flows.

The completed structure with GFRP liner panels installed

CRITERIA

Innovation

The adoption of a solution that could be constructed from within the existing sewer made it possible to eliminate the need for a 3 month road and rail closure that would have been required to carry out renovation of the existing structure by either top down or bottom up methods.

The permanent works design was carried out jointly by SMB and McAllister Group. Stantec carried out the design of the primary steel structure. McAllister Group were responsible for the design of the liner system. Close co-operation was required between the respective engineers who developed a co-ordinated computer model for the final design. This model was then further developed into a virtual reality animation of the planned construction sequence which was used as part of the briefings given to the site teams involved in the construction work.

Aside from the permanent works design the most critical factor in delivering this project lay with the management of confined space entries. The King’s Scholar Pond sewer is a storm overflow and it was therefore not possible to fully isolate flows within the system while site works were being carried out. To mitigate the risks of inundation a series of weir boards were established upstream of the works to minimise the flows. Residual dry weather flows could then be managed by over pumping through the work site. Flow monitoring devices and remote cameras were used to provide information to the site teams relating to the actual flows conditions within the upstream sewer network. Coupled with automatic alarm systems this gave the site teams the ability to assess the actual conditions within the surrounding sewer network in real time, allowing them to make informed decisions as to whether personnel could safely enter the sewer.

Environmental Management

The chosen design avoided the need to carry out excavation works in the highway with an estimated saving in carbon dioxide emissions of 24,500 tonnes.

To maximise the working time available and limit the overall duration of the project site works were planned to be carried out around the clock. The site was located in a busy area of London and surrounded by commercial and residential properties noise pollution during night-time working was of particular concern. The power supply for the site was achieved using a hybrid generator system that uses a diesel generator to provide power during the day and charge a series of battery units. These batteries would then provide silent power overnight.

Community Impact and Customer Care

The chosen design allowed the project to be completed with the little or no disruption to the general public, road traffic and rail passengers. A small site compound was established at the main sewer access point with a remote storage facility established close by. This meant the site footprint to be kept to the absolute minimum, with materials and equipment being brought in only when required.

Transport for London were carrying out significant roadworks in the vicinity of the site for the duration that site works were being undertaken. Close co-ordination was maintained between these two adjacent projects to minimise disruption to the public.

Communications with the surrounding residents and businesses was managed by the Principal Contractor and included open public forums and letter drops to keep the public informed of progress and any events which might impact them.

Project Management

Successful delivery of this project was achieved through the collaborative efforts from the various organisations involved. Working within the constraints of the Principal Contractor’s overall programme, McAllister Group were responsible for programming the sequence of works, developing and documenting the Safe Systems of Work that would be employed and managing all of the site activities.

The innovative design solution allowed cost savings of over £20M against the client’s business plan solution. Construction works were successfully completed in May 2019, within both the contracted programme period and the target cost.

Legislative Compliance

As the Principal Contractor and Principal Designer, SMB was responsible for producing the Construction Phase Plan. The McAllister Group fully complied with their Health & Safety and Environmental policies and the legislative regulations, standards, procedures and documentation requirements of the asset owner, Thames Water.

The McAllister Group supported SMB in their duties by providing the method statements and safe systems of work which were approved by Thames Water and, following completion, with as-built information and preparation of the Health and Safety File.

To ensure safe and robust working practices were in place we successfully staged several emergency drills while the works were being carried out. These included the evacuation of an injured casualty from the sewer, and the complete loss of communication between the top man and the entry team.