Hydro-demolition integral to interchange repairs
The Golden Valley Interchange – Junction 11 on the M5 – is a three level roundabout between Cheltenham and Gloucester. It forms the junction between the M5 and the A40 and the main commuter route between the two large towns.
It was opened in 1971 as part of the construction of the M5 Junction 10 to 13 extension towards Bristol. Fifty years on, it was ready for scheduled major refurbishment. Not least due to the thundering effect of ever growing volumes of traffic, rising well above the number it was expected the structure would have to take.
Work began on the project, on behalf of Highways England, in November 2020. The principal contractor was MJ Church. Also involved in the project has been Freyssinet UK. It had previously worked with Kier Highways on the design of key elements of the project.
Freyssinet has been delivering a range of specialist services, including concrete repairs, bridge bearing replacement, grouting of post-tensioned ducts, and cathodic protection installation.
Another key contractor has been Aquaforce Concrete Services, a high pressure and ultra-high pressure water jetting specialist, and member of the Water Jetting Association, whose teams were to carry out the extensive hydro-demolition needed during the planned 55 weeks of work.
Aquaforce Concrete Services is a subsidiary of Freyssinet and had previously worked with MJ Church on the A338 Tidworth Junction 22 Widening scheme so was well placed to offer coordinated support.
The refurbishment project has involved the refurbishment of abutment walls, ballast walls, wing walls, string courses and parapets on bridges that take the interchange’s roundabout over the M5.
MJ Church divided the work into four zones, based on the four bridge abutments, allowing work to be sequenced efficiently between each zone.
The main contractor excavated to the rear of the abutments and installed scaffold access/encapsulation systems to allow Freyssinet to access the ballast walls and deck ends.
This allowed Aquaforce to get to work carrying out the required hydro-demolition. The project has been characterised from the beginning by very good partnership working.
For example, scaffolding plans were shared with the Aquaforce team before the required scaffold platforms were erected, giving the water jetting teams time to see if they were optimum for the work they needed to do.
All hydro-demolition has been carried out with hand-held lances. The Aquaforce teams have used Calder Multijet 120 rigs with water pressures commonly of 1150 bar (16,700psi) and water volumes of 48 litres per minute. This was delivered onto the concrete surfaces through nozzles with diameters of between 1.45 and 1.55 millimetres.
Up to three two-person jetting crews have been working on the project at any one time. Each crew has needed up to 20,000 litres of water a day to work optimally.
At the start of the project, this was brought in by a 30,000-litre Aquaforce tanker. However, as the works progressed, MJ Church were able to install an on-site mains supply of water. Water for high and UHP pressure jetting must be of a potable quality.
This was directed via hoses into twenty 10,000-litre water tanks, located at key points around the site’s four zones to support the jetting teams.
Hydro-demolition wastewater then has to be processed. Aquaforce set up three Siltbuster filtration units on the site to rectify the pH of the process water and to remove a large percentage of the suspended solids within the water.
The Siltbuster units bubble carbon dioxide through the water to restore its pH neutral score of 7. Concrete wash water can have an alkaline pH score of 11 to 13, equivalent to water used in oven cleaning. This pH neutral and clean water could then be disposed of in a foul sewer running adjacent to the site.
At the start of the project, it was calculated that around 300 cubic metres of concrete would be removed through hydro-demolition. By July 2021, this prediction was found to be largely on track.
Each hydro-demolition team removed about one cubic metre of concrete per shift. Structures that gave up the most concrete were wing walls and bridge parapets, with around 10 cubic metres of concrete being removed from each set of wing walls.
Following the hydro-demolition works, the exposed rebar could be inspected by Freyssinet engineers. Where necessary, it has been replaced, repaired and strengthened, reflecting the need to build in additional resilience to cope with rising traffic levels.
Additional cathodic protection has also been installed to improve rust resistance before new concrete was poured to rebuild the bridge structures.
Concrete released by hydro-demolition has been removed from site using an escalator – and has been reused to help create hardstand parking areas for site vehicles, reducing the amount of hard core that has had to be brought to site for this purpose.
A total of 32 bridge bearings have also been released by hydro-demolition. Freyssinet colleagues have first jacked up the bridge. Then up to 0.4 cubic metres of concrete has been removed with high pressure water jetting to allow old bearings to be replaced before the deck is lowered back into place.
Hydro-demolition has also been essential to gain access to the deck end and post-tensioning anchors to allow specialist surveyors Corrosion Control Services to undertake a post-tensioning investigation before concrete repairs were carried out.
The Golden Valley interchange repairs remained on course for successful conclusion. Hydro-demolition has been at the heart of the process to ensure this strategically vital junction remains resilient and is able to withstand transport demands placed on it for decades to come.