Crossrail, London – Diaphragm Removal Case Study
Removal of Top 1 Metre of Diaphragm Wall
• Sector: Civil Engineering – Transport
• Task: Removal of the top 1 metre section of diaphragm wall to enable joining of capping beam
• Location: Durward Street, London
• When: September 2012 Client: Crossrail
• Main contractor: BAM Nuttall Kier JV
• Precision Hydrodemolition sub-contractor: RGL
• RGL plant: Ultra High Pressure portable diesel driven water jetting unit (24 litres per min. /2500 bar) with hand held low recoil safety lance. Single high velocity sapphire nozzle. Portable noise reducing acoustic working enclosure
RGL were approached by the joint venture of BAM Nuttall and Kier (BNK) in August 2012 to assist with a tricky concrete Hydrodemolition task on one of their Crossrail sites situated in London.
This site is one of many currently in operation within the Crossrail project that will provide a new rail connection from Maidenhead in the west with Canary Wharf, Abbey Wood and Shenfield in the east. The site work was well under way when RGL were contacted. The diaphragm wall that had been constructed needed to have the top 1 metre removed in order to allow the steel reinforcement to be integrated effectively with the capping beam which was to be added.
On previous projects, where similar height reduction was required the use of percussive tools to break down the concrete resulted in a damaged edge at the cut off line. This edge required extensive and costly concrete repair remedial work before the capping beam could be added.
All the Health and Safety requirements for BNK were completed and RGL prepared a detailed Risk Assessment and Method Statement that were approved for the job.
The work was undertaken within a 2 metre square work enclosure, covered with a double layer of debris netting to eliminate the dangers of flying concrete on site. Acoustic panels were also used to minimise noise and disruption as the site was in the vicinity of residential properties. The enclosure provided a safe area for the Hydrodemolition work to be undertaken, while ensuring the safety of other site workers in the immediate vicinity.
RGL provided one trained jetting crew (2 qualified men) to undertake the work. It was agreed that the top portion of the diaphragm (approx. 600mm) was to be removed via the traditional hydraulic breaker method, as this would not affect the integrity of the wall, and the steel reinforcement to this level was not required for affixing the capping beam. This reduced the amount of concrete to be removed by Hydrodemolition and optimum progress was made on this time critical operation.
RGL were then able to concentrate on removing concrete from within the critical steel reinforcement, leaving it intact. The hydraulic breaker work was undertaken directly by BNK in a staged process, ensuring the next section of the diaphragm wall had been broken down, ready for jetting, typically to within 300mm of the proposed line cut.
During the project BNK requested RGL to provide a second jetting team to satisfy an identical requirement to be undertaken on another BNK Crossrail site.
Iggy Hoey was the contact from BNK that RGL communicated and discussed the proposed changes to the works package with; “I found RGL to be very helpful and accommodating during this project. Once the scope of work was changed they were able to provide an additional jetting team when requested to ensure the project was completed on time and within the constraints imposed on BNK by our client”. The edge of the cut line was preserved, which avoided the need for costly concrete repairs. This enabled BNK to cap both walls as planned and move onto the next stage of the project in a timely manner.