Diaphragm wall work for one of the deepest shafts on the Crossrail project is calling for the highest level of logistical planning. Claire Symes visited the Moorgate site to find out more.
A real postage stamp of a job, is how Bauer Technologies’ senior project manager Carl Dunsire describes the work he has been overseeing on Crossrail’s Moorgate shaft for nearly 18 months. Dunsire is not exaggerating either – the site measures just 40m by 35m and is tightly constrained by sensitive buildings and infrastructure – but his team is on the verge of completing the deepest shaft on Crossrail.
At 60m deep, the shaft is a key part of the new Crossrail Liverpool Street station which will connect the existing Moorgate Underground station with Liverpool Street’s main line and Underground stations. Nonetheless, the diaphragm walls of the shaft are far from straightforward, with some complex connections and design elements to contend with, as well as the logistics of getting materials onto the site.
“It has been a real logistical challenge,” says Dunsire. “We have used a 3D model of the site to correctly sequence the movements of equipment and construction materials.”
The reinforcement cages for the diaphragm walls have all had to be brought into the site by night with road closures in place from midnight to 5am two to three times a week to allow this to happen. “The larger ones have had to be ‘walked’ from the Old Street roundabout with someone on foot operating the rear steer on the low loader from behind,” says Dunsire.
The average size of the cages is 18.98m long and 2.6m wide, which may not sound large but four sections must be connected to create the 53m long cages used for much of the panels on the project.
The cages weigh up to 60t each. “There are two basic designs of reinforcing cages, but with over 6,000 couplings needed for this project, in reality no two will be the same,” says Dunsire. “We are using the highest level of reinforcement allowed under European standards here and getting the concrete in was a concern, so we are using a high flow C35/45 mix that will stay ‘live’ for 10 hours.”
“We have to fit the cages together vertically as we insert them,” says Dunsire. “We can lay three cages down on top of the covered walkway above the entrance to Moorgate station but we have also created a dummy panel in the south -east part of the site where we can store two connected cages vertically in the ground.”
The dummy panel is an 18m long steel structure that has been inserted into a diaphragm wall panel excavation to allow up to two cages to be installed vertically. The outer face of the steel structure was covered in Visqueen with grease between the layers before it was grouted into position. “The Visqueen and grease, along with the collapsible mechanism within the panel, means that we will be able to extract the structure at the end of the work,” says Dunsire.
According to Dunsire, the biggest challenge was “panel five”, located on the north east corner of the diaphragm wall. Panel five is the largest panel on the wall and called for three bites to excavate it and three tremmies fed by three different concrete pumps to deliver the 500m3 of concrete over a period of 16 hours to form the finished panel.
The panel was not only large in profile but very deep at 60m, which took it close to the Hammersmith & City Line.
The proximity of other infrastructure has also impacted on the approach to the work. The Northern Line Underground tunnels follow the line of Moorgate road immediately to the east of the site and the Hammersmith & City runs along the northern boundary. Moorgate road also contains a major brick-built sewer and to the south are some listed buildings that were underpinned ahead of the shaft work. To the west is Moorgate Tube station.
“We closed Moorgate road for several days at the end of April in order to complete the work on panel five,” says Dunsire.
“We used the road to store the cages and located a spare concrete boom pump on the road in case one of the others on site failed.”
One of the biggest risks to the work is that there was only room for one diaphragm walling rig, so a breakdown could have potential to put the work severely behind programme, especially with 24-hour working. “We have a brand new Bauer MC64 rig on the site and engineers on standby to ensure we can keep working,” says Dunsire.
The work so far has focused on the more complex elements of the job, but there is one major challenge to go before the work can concentrate on completing the diaphragm walling work. Through the centre of the shaft are two barrettes that are formed in eight sections to depths of 60m below ground level.
The bottom 20m of the construction is a conventional barrette but the upper 40m is formed by king posts that are encased in a lean mix concrete that can be broken out during the excavation. The king posts will extend past the base slab and be used to support a platform for the crane that main contractor BNK will use to carry out the bulk dig of the shaft.
“The normal method of constructing a barrette with a king post would be to plunge the column through the concrete but with a 40m long king post, this approach would not give us enough control,” says Dunsire. “We decided to put the column in as part of the cage, but this was not a simple solution.”
The king post columns are split into three sections so Bauer and its steel supplier Express had to develop a new system to trap off the column while the next section of the king post was bolted in place.
“Although the column sections were lighter than the barrette cages at 15t, compared to 21t, there are less points to connect to for lifting, so it made the columns difficult to handle,” explains Dunsire. “We also had to use 128 bolts to connect each section of the column before lowering the next section.”
The king post columns are 356mm by 406mm by 340mm UC sections and weigh in at 340kg/m.
To ensure concrete flowed evenly around the column during construction, Bauer used twin tremmie pipes on either side which were fitted into temporary steel guides bolted onto the king posts.
“The geology has also made the work complicated as once we get 51m below ground level, we are digging into the Thanet Sands which are very dense and difficult to excavate,” says Dunsire. “We had to adapt the grab to pulverise the sand and also deal with issues related to desanding the bentonite after excavating these panels.”
So far Dunsire’s team has built four of the eight barrettes and work on the next four will be undertaken in July. Work on the rest of the diaphragm walling is now at the halfway stage and, with most of the complicated elements completed, the team expects to finish the work by mid-August.
“BNK will start work on the bulk dig straight after,” says Dunsire. The shaft is critical to construction of the east-bound sprayed concrete lined tunnel into Moorgate and there are a number of adits from the shaft that also need excavating