Holdfast Training Services Trial Blog

Cofferdams, Concrete and Coming to an End….

The last few weeks on site saw the start of the cofferdam preparations, which from a Monitoring Manager perspective I had a key negotiating and decision making role between third parties, the Client and Vinci.  The main issue was the level and type of  monitoring that would have to be installed prior, during and after the cofferdam works (sheet piling, dewatering, construction of cofferdam, de-construction and construction of tunnel) in order to satisfy all parties.

The Contractor’s consultant, convinced of the adequacy of his design did not deem any necessary, the Client was hinting at costly underwater scanning technology, Thames Water wanted the moon on a stick and London City Airport (originating from BA demands) wanted to charge 10 passenger seats for the additional risk of construction works in case of a plane crash!  Cut a long story short, the Thames Water issue was one of vibration, resolved by rearranging the sequencing and location of sheet piling works and proving that the service tunnel was actually steel and not iron!  A metallurgical test was carried out after we showed Thames Water a photograph of a drawing taking during a trip to the London Docklands Museum which showed the pipe as steel!  This doubled the allowable vibration limits solving at least one the the problems.

A monitoring proposal was eventually agreed which included prop , sheet pile, tie and dock wall monitoring.  The designer was most concerned about deflection of the southern dock wall (which is sheet piled and tied following expansion works in the 50s) during dewatering.  The siting and operation of the increased monitoring formed an integral part of the cofferdam planning process, since Crossrail will not allow any works to proceed without monitoring in place.  This clause also lost a week of production days after a £25k Total Station was stolen from the West Tunnel Portal!  Ironically this, and the Client EWN regarding monitoring requirements for the cofferdam, went a long way in getting the QS’s to acknowledge EWNs and CEs from the monitoring subcontractor (who were already owed over £150k)!

Monitoring issues increased exponential as works progressed elsewhere on the site, of note was movement of the DLR Prince Regent Street Station as a result of major excavation works adjacent to the retaining wall.  Fortunately because the excavation was only 400m lower that the final structural slab level (in order to carry out pile remedial works before laying the Load Transfer Platform (LTP)), the designer was probably more worried that the Contractor!  Hence, we had a number of very collaborative Engineer Review Meetings to conclude that the station was a piled box section, movement was conversant (albeit not entirely predicted) with the works and ground conditions and stabilisation was eventually established!

In addition to normal works, the Limo site a few kms west of the site (where a TBM shaft had been constructed) turned off their dewatering in November which had provided us with 3m of draw down.  There was then a 2 week period before we were had planned to turn on our Thannet Wells in preparation for the Central Sump shaft deepening.  It was interesting to monitor the effects, heave in the central section and increased water flow rates from our existing Chalk wells.  When we started to get our feet wet in the tunnel however, I managed to negotiate the Thannet dewatering date forward (at a cost of £800) – there had to be some perks to the Monitoring Manager job!

After severe delays with piling, and a curious bout of concrete chicken pocks (eventually put down to admixture quantities, resulting in an entire bay strip out), the reinforced concrete slab  programme started to progress well on both the east and west surface rail.

Drilling and Grouting works in the tunnel eventually finished at the revised extent (70m less that the planned invert replacement length (although the same as the OCI agreed length).  The delay, largely due to ground conditions initiated a design review of the invert replacement extents.  Vinci claimed it would cost £10m extra to do the extra 70m predominantly in the River Terrace Deposits (of which £4m were preliminary costs).  Crossrail has their Designers go back to the railtrack designers and make every tweak possible to achieve the requisite rail alignment within the OCI extent.  The Client won and as I left, were considering (albeit impossible to contractually implement) a pro-rata deduction based on the fact that Vinci were now not doing ‘£10m’ of work! An illustration on the ‘spirit of mutual cooperation’ by a Client on Cost Plus contract, and a Contractor experiencing the Pain aspect of the Pain/Gain share.  After their 12 week (turned 6 month stint at Connaught), Bachy will be returning after Christmas to grout the base of the Dock Walls to form a seal with the Lambeth Group as the 3^rd and 4^th side of the cofferdam.

In the Tunnel, the negotiation of Invert Replacement Bay and Prop size continues, but Target Cost, progress bonus driven Gallagers are continuing at pace with the Central Box Section replacing the Twin Tunnels.  The main issues have been timely issue of levels and reinforcement detailing. However, as Crossrail approach their Dock Passage Closure window (ironically not a Contractual date for Vinci), approvals and acceptances have been much more forthcoming!  A major logistical problem was concrete pumping over long distances which was solved by driving the concrete into the tunnel to a concrete pump adjacent to the works.  Production time was increased by bringing in additional formwork, mounting it on wheels and moving it from Bay to Bay without the requirement to re-errect it.  A commercial issue has been questioning Gallagers on their ‘bonus’ scheme.  They believe it drives production and ultimately saves the job money (which having seen the rate of work I would tend to agree), however our QSs do not believe it is conversant with the terms of the contract… the saga continues….

Aside from that, my last job outside the tunnel was planning the Parapet Wall removal which has Traffic Management (liaison with the local council), scaffold, crash desk, temporary propping and achieving requisite levels around existing arches aspects.  However, surprisingly the works have been delayed and will not start until after Christmas… Easter… next Christmas… have I become cynical at the end of my Contractor placement??!!

And finally, meet the Team….. my trusty Assistant Site Engineer Alex, the Tunnel Team (with left to right 2 new Assistant Site Engineers, fellow Section Engineer and ‘senior’ Assistant Site Engineer (vying for promotion) Alex…!) the Tunnel Construction Manager (left), mad Irish Foreman (centre) and my line manager the Tunnel and Central Section Production Manager (right), and the Works Manager recently turned Surface Construction Manager.