Client: National Highways
Designer: Kier Highways Design Services
Principal Contractor: MJ Church
Works Commenced: ECI February 2020 / On site November 2020
Works Completed: 2022
Bridge Strengthening & Post-tensioning Special Inspection (PTSI)
The M5 junction 11 Golden Valley Interchange comprises of two bridges supporting a gyratory system over the M5 and A40 in Gloucestershire.
In 2017, National Highways set out plans for a £200m road improvement scheme in the South West to include refurbishment of the Golden Valley structures.
Kier Highways engaged Freyssinet under an Early Contractor Involvement (ECI) scheme in 2020 to provide design assistance on methodology for grouting empty post-tensioned ducts, post-tensioning special investigation (PTSI), cathodic protection design, concrete repairs strategy and methods of jacking and bearing replacement. Freyssinet provided a programme of activities to ensure the complex works could be sequenced efficiently.
Following the ECI period, Freyssinet collaborated with National Highways project delivery team and Principal Contractor MJ Church. The combined approach with assured a good level of understanding in buildability and programme knowledge for the specialist refurbishment works required.
Freyssinet’s scope of works included temporary works design, concrete repairs, PTSI, site surveys and investigation, tendon grout trials, jacking, bridge bearing design/supply and installation, cathodic protection, hydro demolition and reinstatement of ballast walls and stringcourses.
Work on site started at the beginning of November 2020. MJ Church excavated to the rear of the abutments and installed scaffold access/encapsulation systems to allow Freyssinet to access the ballast walls and deck ends.
Freyssinet’s in-house company Aquaforce Concrete Services carried out hydrodemolition to the existing ballast walls to gain access to the deck end and post-tensioned anchors. This gave access to undertake the PTSI to check the anchorages were in a suitable condition before the concrete repairs could take place.
Concrete repairs to the bridge deck soffit and deck end took place in early 2021, followed by the jacking and bearing replacement and stringcourse parapet reconstruction.
A successful post-tensioning grouting trial was carried out at Freyssinet’s Telford factory in October 2020 witnessed by National Highways and Kier representatives. Freyssinet operatives prepared for the grouting of the post-tensioning duct that was found to be un-grouted during a PTSI undertaken by Freyssinet in 2017. The techniques developed and proved during the trial were subsequently used to grout the duct within the north bridge deck in order to restore the integrity to the structure. The un-grouted duct extended 71m across the whole length of the deck.
Freyssinet also trialled the use of Tomography and GPR techniques to ascertain the extent of voids in the PT system as well as conventional techniques. Freyssinet adopted a void measurement technology, developed internally to accurately quantify the volume of any voids encountered within PT ducts.
This assisted the designer in finalising any remedial works and re-grouting that may be required. This technology has since been used successfully on other PTSI schemes delivered by Freyssinet.
Due to the complexity of the structure the activities concerning investigation were challenging and required Freyssinet to find some innovative methods for dewatering the PT ducting system.
In order to minimise the risk of rainfall having detrimental effects on the PT system whilst the ducts were open and the tendons exposed to the elements, Freyssinet utilised ‘work tents’ in order to shelter the work areas from rain/water ingress. We also developed a technique to seal the work area (deck break out) against water ingress in the case of heavy rainfall, when water was likely to run down the deck and enter the ducts, which could cause the onset of corrosion.
This process involved using a ‘water-plug’ cementitious material that could be mixed and placed around the break-out in the bridge deck followed by the installation of a waterproof cover plate, thus sealing the breakout against water ingress.
In order to achieve the required outcome and restore integrity to the PT structure, Freyssinet worked closely with the Client, Permanent Works Designer and Principal Contractor to ensure that each area of post-tension ducts were investigated, the results reviewed/analysed and appropriate remedial works undertaken. With all parties adopting a collaborative approach, these specialist works were completed on time and in compliance with the specification.
During the scheme, it was necessary to undertake elements of the PTSI from the motorway below the structure. Through collaboration with the Principal Contractor and the Traffic Management specialist, the necessary Impact Protection vehicles were deployed in order to fully protect the workforce, whilst the motorway traffic was allowed to pass by the works in a series of lane closures.
The project was completed successfully in line with the programme and to the satisfaction of National Highways, Kier and MJ Church, and was received the ‘Over £8m re-engineered project’ award at the ICE South West Civil Engineering Awards in November 2022.