Holdfast Training Services Trial Blog

I’m now back! After a reasonable period of time offline in the blogging world here is blog no 4.

My work has continued at breakneck speed along C620, hence the lack of blogs, and for those who can recollect blog 3 I had just finished the piling and had started the excavation of the foundations.

So here is a shopping list of what I have facilitated or directly completed over the last couple of months:

17 piling platforms excavated, backfilled and CBR tested.
170 PC RC piles, positioned, driven, tested and cut.
8 Steel H-Section piles, positioned, driven, tested and cut.
28 permits to dig completed.
Excavations set out.
1000m3+ of material excavated.
Heights checked.
22 service ducts and water mains positioned, excavated and placed.
136m3 of blinding poured.
Heights checked.
170 PC RC piles marked and cropped to expose rebar.
8 Steel H-Sections cut and modified for a deep foundation in tension.
Over 200m3 of groundwater pumped out of excavations.
Over 200 nails positioned for setting out.
Reinforcement steel fixed, positioned and checked against RC drawings.
Heights checked.
56 earthing cables set out and welded.
Over 1kM of shuttering placed.
Height, vertical and horizontal cover checked.
Foundations blown out.
69 individual bolt pads positioned.
Heights checked.
500m+ of chamfer filet set.
Heights checked.
11 notification to pour forms completed.
581m3 of C32/40 concrete poured.
60 sets of cubes taken.
Heights checked.
Shuttering struck.
Excavations backfilled.
1 large QA folder still to be completed!

The last base was poured today and I now have over a week’s worth of QA paperwork to complete for the whole section. 3 of the steel trestles have been constructed onto the early bases and they all fitted perfectly onto the bolt pads – so far, so good – fingers crossed for the rest!

So what have I learnt? Loads of stuff! But for now ill conclude the piling episode with a few remarks as I know those who followed my previous episodes cannot wait for the conclusion?!

As you will remember many PC RC piles broke, steel H-Sections were used as replacements. This was the focus of TMR1 and as some of you won’t be lucky enough to read that particular gem so I will expand on my observations.

Risk.

The risk within the piling sub contract was shared reasonably effectively between Graham Construction and Balfour Beatty Ground Engineering (BBGE) and reads ‘‘The subcontractor has not allowed for overcoming man-made or naturally occurring obstructions (whether they are below ground, surface or overhead) which impede, or deflect the Works, or result in them failing to achieve the design and/or specification requirements. Any additional time and costs associated with overcoming or attempting to overcome such conditions (including but not limited to set ups, displaced and/or damaged works, replacement and/or additional works, lost consumables, and damaged plant) shall be in variation. The subcontractor however has allowed for the cost of 30 replacement piles on this occasion. Any further replacements will be a variation in accordance with this clause. Should less than 30 replacements be used the cost saving shall be shared by both parties’. This figure of 30 piles was agreed by negotiation as initially BBGE would only accept 10 breakages as part of their standard contract. It still proved an underestimation as these replacements had been consumed during the first stages of the operation whilst piling for the RC storage silos. Subsequent replacements were charge at the agreed original pile cost calculated by the length required: 18m and below at £450 per pile, 18m and above £620 and driven via a variation order. All the piles for C620 were 18m and above and therefore the replacements were charged at £620 per pile with the additional cost of pile testing at £120 per pile (1 in 10 tested). This risk of breakages had been identified by the Graham design team from studying the borehole data and hence the 30 pile float had been negotiated. This risk and associated cost was covered by Graham Construction.

However BBGE also took a risk within the contract. The calculations conducted by BBGE detailed the design depth for the piles to achieve the specified working loads across the various areas of the site; the price was quoted from these calculations. However the piles were installed to a dynamic resistance, or set, calculated using the Hylie formula (less than 25mm per 10 blows) and therefore the actual length varied from those suggested by the static calculation. The additional sections required to achieve the extra depth and set, beyond the calculated and quoted for depth, were at a cost to BBGE. In addition, BBGE priced the contract on the number of piles driven at the calculated length, not on site time. The re-organisation of the piling during and after the breakages resulted in an amount of standing time charged at a set rate, higher for the first hour and then reducing for subsequent hours. This time whilst at a cost to Graham, was at a greater cost to BBGE as the greater profit for them is in the driving of piles succinctly and quickly. Whilst the rigs are standing the BBGE profit is reducing and it also delays the rigs from moving on to other sites where the more profitable piling can continue. When looking at the number of breakages across the whole site coupled with the additional lengths required on a significant number of occasions, it is quite possible to envisage a reduced profit margin for BBGE.

I feel that the risk was actually well shared by both parties. On the surface it appears that the majority of the risk was held by Graham with the low number of potential breakages (30) written into the contract but on closer inspection of the ground investigation data it was clear that deeper piles would likely be required and hence a risk to BBGE.

Ground Investigation.

A large amount of ground investigation data was available to both the designers (HBPW) and BBGE as the pile designers from a series of surveys conducted at various times across the site from when it was reclaimed from the sea in the 1990’s. From the vast amount of data available BBGE took the unusal step of not using a design borehole to design the piles but used the actual borehole data. Whilst not particularly risky the boreholes used where not, in my opinion, the ‘worst case’ boreholes that in my logical mind would choose. In addition not all of the data was referenced by the main designers for BBGE to use to design the piles and this missing data contained the worst case boreholes, including the one closest to the area where I broke all the piles that had a 2m deep layer of chalk made ground. Has all the information been thoroughly considered it is likely that the piles may have been redesigned at a deeper depth, costing BBGE less and invariably saving on the number of breakages. This in turn would have led to a reduced cost to Graham and the steel section solution being reached earlier, if not from the outset.

The ground is a risk. The importance of GI cannot be underestimated and whilst in this case the problems were not from the lack of information, as is often the case, but from a lack of exploitation of the available data.

Apologies for the long blog and the lack of pictures. My iphone slipped out of my pocket on Thurs afternoon, and as is always the case with sods law, got immediately run over by a dumper with all my exciting photos. Happy with that.