Holdfast Training Services Trial Blog

So drilling through a basalt extrusion proves rather problematic when the drilling arm to your rig snaps off and you are unable to recover it.  You are essentially left with a snapped arm 20m deep in what should be a 1500mm pile which is now unusable.

The piling subcontractors are on a design and build contract; they caused the problem, they will fix the problem at their cost.  The solution, endorsed by the Multiplex team overseeing this part of the programme, is to concrete the original pile borehole (BP38) and construct two further piles (BP38-1 & BP38-2) either side of BP38, designed to carry the same loads.  Simples!!

What risk did they fail to see?

The position of BP38 is close to a secant pile retaining wall which is approximately 3.5m from the site boundary. There is a heritage façade which sits on strip foundations on the site boundary which is already in a pretty ropey condition and only supported laterally.  The northern retaining wall enables excavation down to Basement Level 1.  The construction of BP38-1 & BP38-2 was designed by the subcontractor’s engineer; construction went without a hitch and the two 1200mm bored piles have sufficient resistance equal to the original pile.

Position of BP38 in relation to site boundary and façade, and the northern retaining wall

Problem:  It turns out that when you adjust the position and number of piles beneath a pile cap it alters the pile cap design.  Funny that!  The structural engineer (a consultant) produced a new design for a 1.6m x 5.4m x 3m deep pile cap to ensure the load (37MN ULS axial load) could be safely transferred to the piles.  This means that an excavation of >3m is now required to construct the new pile cap.  This is a >3m excavation in soft Coode Island Silt within 1.5m of the heritage façade.

Cross section detailing the parameters of the problem

Unidentified risk: This is an obvious example of unidentified risk, which could easily have been identified.  Two subcontractors responsible for two areas of design and construct have not communicated before adjusting construction plans due to a foreseeable problem – everyone knew the basalt was in that location.  The geotechnical subcontractors have simply fixed the problem they caused, thereby fulfilling the terms of their contractual obligations, without due consideration for the next phase of the programme, i.e. the construction of the pile caps. Multiplex have ratified the change because, I would argue, the people overseeing the construction have not understood and/ or identified the second order effects of the change.

Discussions: The first time I was enlightened to the problem was when the co-ordinator (construction manager) overseeing this phase of the programme sat getting rather stressed at how he was going to solve it.  After two hours of ‘it will be fine’ and ringing round for advice he sent me the specifics when requested to have a look at.  Two heads are better than one.

Options on the table:

1. It is obvious that the excavation cannot be conducted to that depth without retaining the ground which supports the façade. But the question was how? My recommendations:
   • a) Stiff retaining wall functioning as a cantilever capable of withstanding lateral earth pressures and deflections (combi wall perhaps?). This would give clear space in which to excavate at the front of the wall but would require greater efforts to construct such a substantial retaining wall.  Also very risky due to the absolute necessity that the façade cannot move.
   • b) Prop a sheet pile wall using ground anchors so the load is transferred to the ground behind the wall. Due to the proximity of the boundary (<1.5m) I deemed this infeasible; there would not be sufficient space within the boundary to fix the anchors behind potential slip surfaces.
   • c) Prop in front of a sheet pile wall bracing against the northern retaining wall. This is very feasible ensuring that sufficient space is retained to permit excavation.

2. Review the pile cap design with the intent of reducing the depth of the pile cap and subsequent depth of the excavation. This may have been an option prior to the construction of the two piles, but unlikely to be feasible now the location of the piles is fixed.  A retaining wall would still be required but to a potentially shallower depth.

The solution:  I recommended Option 1c as the way to go, pushing the piles where possible rather than vibro.  The piling subcontractors are responsible for the design and construction of any temporary works to enable the excavation but were holding off to receive direction from Multiplex.  Multiplex in turn has sent the problem direct to the geotechnical engineer consultants for their advice.  Their recommendation: Option 1c.

Timing:  The problem was first identified by the groundwork subcontractor on receipt of the new design drawings of the pile caps.  This was on Friday, nearly three weeks after the failure to construct the original piles.  Construction of the pile cap was due to commence on Tuesday giving 1 working day to come up with a solution and have the designs authorised.  Needless to say, it is now Thursday and the team are still waiting on the design from the piling subcontractors and work is yet to commence on the retaining wall.

Lessons learnt? 

1. Communication is vital when the work being conducted by one subcontractor affects another.
2. Second + order effects must be considered prior to making any amendments.
3. Identifying risk early and consider this risk in re-mediation plans.

Additional remarks:  In this case, the solution would likely have still been the same had the problem been identified when the original pile was deemed unusable. The upshot of early identification was that a retaining wall could be have been designed early and constructed immediately after the piles, thereby causing no delay to the construction of the pile caps.