Holdfast Training Services Trial Blog

Situation.
The northern wall of the Blomfield Box Superstructure runs parallel to the London Underground District line. A heavy duty fence had been erected to provide the required barrier between the two sites and to protect both personnel working on site and trains from construction activity. Concerns over safety and constructability of the current design of an insitu RC wall and aluminium rain screen cladding.
Build-ability: With the heavy duty hording in place there is no means to access the other side of any formwork to strike the formwork once the concrete has been poured. Left in place form work was looked at to overcome this issue but the issue of affixing the rain screen cladding would still be a problem.
H&S: In order to construct the rc wall and cladding access from the track side would be required to erect formwork and to fix the rain screen. Due to the live track this would only be achievable during engineering hours (0100-0500 daily) during which time the track is in places still live other than those sections that work is taking place.
Commercial: The closure of the line and works completed during engineering hours is very expensive with cost payable to both London Underground for line closure and in overtime for the Laing O’Rourke operatives. It would also have a knock on effect of additional labour cost as those working over night would not be available to work the next day shift, therefore additional operatives required to ensure the day work is not affected.
The proposed solution was to use a precast concrete cladding section that could be lowered into place and fixed from the inside to remove the problem of live track side working and to reduce the need to work in engineering hours. The requirement for the precast is as per the Project Managers Instruction below:
1 Change the cladding arrangement at Blomfield Box North Elevation from the current “aluminium rain
screen on RC/ Blockwork substrate”, to single skin Pre Cast concrete plank spanning vertically as per

a. Liaise with C138 – Peter Churton, and agree: 1) Loading; 2) Fixing restraint requirements; 3) Any
changes required to the concrete profiles; all to be subsequently recorded and agreed in an ICD.

b. Design, detail, manufacture and install cladding in such a manner that the works can be achieved
without the need for temporary access/disruptions to the railway face of the building.

c. The joints in the PC planks to be detailed with solid mortar/ grout infill with adjacent planks locked
together by interlocking rebar detail, generally as provisionally agreed with C138/ CRL during visit to
Explore this winter.
The issue that has now arisen is that the PMI was issued having not consulted all departments and has been to constraining in its direction and has resulted in a period of abortive works that have subsequently lead to compensation proceedings.
The new issues:
Non Structural: the precast wall is now to be a cladding/fascade and not a structural wall. This means all connections must not become load pathways and load the wall. As the superstructure will now be acting as canterlever and we have been told to assume a 20mm deflection the connections must be flexible enough to account for this.
Fixed grout connections: The connections have been stipulated as grouted. This has given us a number of issues: Firstly the grouting of joints means that there will be no flexibility in the joints, therefore the panels will need to be stacked on top of each other and that the 20mm deflection will need to be accounted for in the connection between the superstructure and the precast panel and not between the panels themselves. Secondly the purpose of using precast panels was to remove the requirement to access from the LU live track side. The use of grout to seal the connections still requires some form work to prevent the grout flowing out of the connection during pouring and then the form work would have to be struck from the track side.
Architecture and planning: the accepted precast drawings by the architects and the planners show the precast panels fixed externally to the superstructure. This however requires that the precast panels are fixed to the original rc wall location as there is no structural plinth directly at the base of the precast panels new location.
Heavy duty fence line: Having set off down a proposed method of affixing the precast panels to the superstructure and having though we had resolved the structural and architectural issues I thought I would just check the fence alignment. One survey and painful lesson in using AutoCad I discovered that the space between the heavy duty fence and the precast panels would only leave 40mm of space in which to manoeuvre the panels into place. Time spent in reconnaissance and all that….In during my investigation of the heavy duty fence line I raised concerns that should we proceed with installing any wall be it RC or Precast once fixed in place there would not be room between the electrical cable rails and the new wall to access the fence to allow for its removal. As a result the fence would become a permanent feature. This I knew to be unacceptable to LU.
M&E: The show stopper however has come in the form of permanent services. The original design of the services was hard against the RC wall. The new precast panels once fixed external to the superstructure would only leave 200mm gap. This presents a maintenance issue as the connections would have to be exposed in order to account for any deflection and would therefore require inspection and maintenance. Having raised this with Crossrail and that we would need alterations to the M&E it soon transpired that this was not possible without incurring huge redesign costs. IN addition the possibility to use access hatches through the M&E ducts to conduct future inspections and maintenance would also cause large commercial issue in the future. The ducting is to be used to provide ventilation to the shaft, station platform and running tunnels. Any interruption to the ventilation would result in the closure of the station. As a commercial business this requirement for periodic station closure to maintain the precast panel connections is not acceptable by Crossrail.
Conclusion:This fire and forget approach is becoming a bit of a theme on site with departments and individuals reacting to issues and concerns raised without fully working through the consequences. While in this case it was for us to resolve the issue the constraints placed on the changes allowed little room for manoeuvre. After all the design changes and arguments over the structural design of the panels and waterproof connections the elephant it the room turned out to be the heavy duty fencing. Originally proposed as a means to avoid live track side works the precast panels still require the fence to be removed and this can only be done live track side.
We are back to square one, the PMI has been retracted and we have been instructed to now propose a new solution or accept the original designs and the risk that are associated with this. The abortive works that have been completed have now been passed to the commercial team to proceed with compensation proceedings against Crossrail and all for a getting out of the office and fully understanding the situation on the ground. This episode harks back to John and his lessons on retaining walls in that the designs only show the wall in place but if temporary situation won’t allow you the path to the permanent state then you may need to rethink the permanent state. And a lesson from myself in that sometimes the temporary and permanent conditions must consider not only the engineering risk but also the commercial and other department risks.