Holdfast Training Services Trial Blog

I was in the process on drafting a general update on the progress on site but late on Friday the piling sub-contractor’s world started caving in and it dragged me in. On Friday at 1900 (perfect timing like all great catastrophes) I, with the Project managers for FLO and Cementation Skanska were scratching our helmets, one of the diaphragm wall grabs was stuck at 60m. The situation worked itself out in the end but at the time, all options seemed bleak.

The background

The grab first got stuck at a depth of 60.5m depth around 1300. This meant it had 0.5m left to dig before the sub contractor long weekend. Either before or in the process of trying to release the grab, the rig had a hydraulic failure and lost 50 litres of hydraulic fluid. At this point it was hoped that the hydraulic failure was the reason it was stuck.

Unfortunately after the endless calls to Germany, diagnostic and repair the grab was still stuck. Shock loading and constant load of 42 tonnes for 3 hours didn’t work. The rig is about 160 tonnes on crawler tracks. The grab itself weighs 20 tonnes and is about 10m long. The lifting cables are rated to 100 tonnes.

The diaphragm wall rig and grab unstuck

Options

As we stood around the rig in the quiet site we discussed three options.

1/ Use a 100t crawler crane and get it as close as possible. Then wrap a lifting chain around the cable and use both to lift together. This should add about 15t in addition to the rig.

Risks: very high risk activity using a crane to do something it isn’t designed for. Requires lift plans, method statements. The connection of chains to cables isn’t normal practice (shackles were not an option). Any slip in the cable could cause a potential collapse onto Battersea Park Road.

Impact to project: Least impact on rig if recoverable.

2/ Pay out as much cable as possible and cut all the cables and hydraulic lines  from the grab. Then fix the cables through an anchor block to two100t cranes. Use both cranes to lift in tandem to get the grab unstuck, and in turns bring the grab to the surface.

Risks: very similar high risk activity using a crane to do something it isn’t designed for. Requires lift plans, method statements. Additionally

Impact to project: It would take a week or two to re-condition the grab and reconnect with new cables and lines.  Lost time to project.

3/ If all else fails, cut the cables and hydraulic lines and bury the grab.

Risks: Safest in terms of H+S.

Impact to project: Despite the significant cost (£350,000) more critically it would take a long time (months) to find another grab. This would have massive effects on the project.

As we left it on Friday evening, the plan was to spend Saturday completing the necessary paper work and risk assessments to prep for option 1 and if not attempt option 2. The site was open on Sunday which suited as it would be quiet. In the end on Saturday morning the sub-contractor re-programmed or ‘re-baselined’ the software on the rig essentially telling it that it had a shorter jib allowing it to pull harder. It worked…

Why?

Now that the storm has passed there doesn’t seem to be any effort into working out why this happened. I calculated that at the depth that the grab was at, that the bottom 5m was in Thanet Sands. These are fine granular sands that are very hard (SPT tests gave an N value above 50, during some borehole testing we did across the site). I assumed that if stuck in the sands that the ground would tighten with time. or that continued pulls would only make the situation worse.

Has anyone seen anything like this before? We were lucky in this case as all the options presented significant risks. I never got to review the method statement or see the lift plan but if something had gone wrong I don’t know if the measures would have justified the risks.

Apparently Skanska got a grab stuck whilst in the Netherlands. Apparently the Belgian PC was preparing to send divers into the bentonite!!!

On a different note. The first bit of TBM has started arriving onsite.

Cutter head number 1 and the 250t crawler crane to unload the TBM bits as they arrive

In my 7.5 months on site I have seen plenty of suspect activities been undertaken by one of the many sub-contractors. In almost every occasion so far the perceived risk to life has been reasonably minimal. In these instances we have had opportunity to discuss the problem with the senior construction team and multiple engineers of varying experience employed by the project.

This morning however, whilst conducting a check of some works on another part of site, I spotted a section of works that concerned me to such a degree that it caused me to cancel everything I was doing in order to have an informed debate (It turned into an argument) with the sub-contractors on-site engineer.

Now I am happy to be wrong, particularly with Geo-technic stuff, but the image below triggered some long archived memories of a JM lecture back in PEW. We effectively have two primary issues that concerned me:

Issue A: Workers operating in a deep unsupported excavation in stiff blue clay. In the worst position the walls were over 2m tall and near vertical. This was compounded by numerous enormous diggers moving around the perimeter providing a particularly unhelpful surcharge.

Issue B: In the background you can see a very, very  large stockpile of disturbed excavation material piled up directly adjacent to this work area. This pile must be at least 7m tall and was worryingly close to where the guys were working in the base of the excavation. If that slipped I have no doubt it would end up in the base of the excavation. Given the amount of spoil involved it would take us days to find people under that heap.

The sub-contractors on site gave me a load of excuses about the stiffness of the clay. I asked them who knew how long it would stand up for, he couldn’t answer. He then told me he was allowed a 1.35m vertical wall (Not sure where that has come from) before it became a problem. I quite quickly pointed out that unless all of his men are unusually short the wall was still well over that limit. I am 185 CMs tall on a good day in my boots (As generously stated on my MOD 90) and it was definitely taller than me.

In the end the answer was simple. It didn’t look right and I knew enough from PEW to have the confidence to stop work until we had some answers. I told them to prove I was wrong and that the excavation was safe in that condition and I would allow them to recommence work. They obviously couldn’t so are now working to make the area safe before anyone else enters the excavation. I also got the temporary works engineer to confirm he agreed with me and funnily enough he did. In summary, when programme comes under pressure expect sub-contractors to cut corners. Alarming but true.