Holdfast Training Services Trial Blog

SURFACE WORKS

Piling

CFA and CMC piling continues, as does rectification works for non-conformance reports (NCRs) (raised by Crossrail and subsequently Vinci and the sub-contractor upon realisation that it is commercially advantageous to flag up your own defects) on 1200 out of 1800 CMC piles.  The principal cause of these defects was the use of a crack inducer that had not previously been trialled on this scale.  The plastic yellow cone pictured contains a wooden ball which, when filled with grout at high pressure from above acts as a non-return valve forcing the grout out through the sides and hence inducing a crack in the pile.  The induced crack, combined with the use of an excavator to crop the piles proved highly ineffective and was eventually replaced by a pile cropper.   A 67% defect rate has proved extremely costly, and unfortunately for the sub-contractor and Vinci, unlike the first few piles which disappeared into the river terrace deposits, these NCRs cannot be attributed to unforeseeable ground conditions.

Those rectified CMC piles that have now been accepted by the Client have subsequently been overlain with the Loading Transfer Platform (LTP) and Load Tested in preparation for the reinforced track slab works.  There have generally not been any problems with the CFA piles, which have been cut off exposing the starter bars in preparation for the track slab walls and overhead cables.

Reinforced Concrete Delivery

The Vinci delivered reinforced concrete track slab and walls are awaiting final ‘acceptance’ from Crossrail, and a trial has been carried out to refine methodology prior to works commencing.  Making good trials will follow to establish best practice for the main works.   With a large amount of concreting starting on site (and within the tunnel), Vinci engineers have been receiving in-house training to in order to be able to carry out re-bar and pre and post-pour concrete inspections.

TUNNEL

Temporary Services

Temporary Services have now been 90% installed, partially handed over and the sub-contractor is now in contract in a maintenance contract.   Having seen a sub-contract ‘almost’ full circle my main observations and lessons learnt are as follows:

Sufficient  time and resources (the right people) should be spent drawing up the scope and specifications – what will be needed during construction (what will be the maximum capacity + contingency) and for final handover (ability to reduce capacity for the Client)?  E.g.  Having to issue 6 x PMIs for additional power requirements.

Planning should be continuous and communicated to the right people  – how will other works interact with the installation, during and after construction.  E.g. The Invert Replacement Trial required 4 x M&E services to be moved and the props and temporary works for the main works conflict with the fire main, compressed air line and drainage pipe.

And finally…. Final handover is extremely difficult!  The longer it takes to sign and accept the works, the longer any defects have to appear and the longer the risk remains assigned somebody else!  Not forgetting the difficulty in achieving the requisite level of quality.

However, I also accept that hindsight is a great thing and issues and snags often only become apparent during or after the work.  And time = money at the start of the project,  in the same way that  time = money during and after the project.  I would still argue however, that time (and money) spent in planning is seldom wasted, provided of course there is sufficient information to plan with…..which in this case may by a significant contributing factor!

On the issue of money, at an average of £100k a month to discharge water in the sewers either side of the tunnel, a re-design (including a filtration system) is now underway to construct a drainage system that discharges water from a central sump into the docks…… hence negating the requirement for 200m of pipe line and £500k in Thames Water bills….maybe that’s why they chose to do it like that in the 1890?!

Drilling & Grouting Works

Drilling suffered a further setback after high water pressures hampered the insertion of plastic TaMs, despite the additional contact grouting directly behind the tunnel lining.  The design was reviewed and sacrificial steel TaMs with a conical end will be used, which can be drilled into the gravels allowing the micro-fine grout to permeate directly into the gravels.

In-Tunnel Well Points

Localised dewatering was to be provided by well points drilled at 10m intervals, 4m below the invert into the Lambeth Group.  All were found to be dry, however, in the event of any water entering the wells, a passive pumping system will be installed.

Invert Replacement 

Prior to the replacement of the concrete invert, two 1.8m trail bays were required in order test the methodology and monitor the loads induced in the props as a result of the excavation.  The trial got off to a bad start when a fully charged cast iron pipe (part of the original construction temporary drainage, but fortunately for Vinci and the subbies not marked on any drawings – good ol’ Clause 61!) was hit causing water inflow at a rate of approx 50l/s.  A packer was eventually connected and 1000l of grout solved the problem 3 days later (the delay was a result of a drawn out Client decision to grant permission to break open the pipe for fear of inundation  – despite not being able to stop the flow without opening up the pipe)!

Pipe fixed, reinforcement installed and concrete poured.  All that remains now is a prolonged battle between the Client and the Contractor regarding permissible bay widths and size of props for the main works.  The Client arguing that whilst the loading in the props was monitored to be less than 2% of the actual capacity (less than the differential caused by thermal expansion and contraction), and that the tunnel, whilst extremely strong will act as a brittle structure and if failure does occur it will be catastrophic.  The Contractor on the other hand is using the prop load data, combined with an independent consultants report arguing that the Ko used by the Client is far too conservative to allow for a feasible and cost effective methodology……

….in the mean time….. Central Box Construction through the Twin Tunnel, Cofferdam preparations and newly appointed Instrumentation, Monitoring and Dewatering Manager…. (I would recommend avoiding flying from London City Airport during my tenure!)