Holdfast Training Services Trial Blog

Appreciate there’s been a bit of a deluge of blogging lately but something came up in my office that I thought was interesting. Also apologies for no pretty pictures – I haven’t been able to get hold of anything relevant yet. You may or may not have read in the news recently about a magnitude 7.7 earthquake that hit the Caribbean yesterday. This was quite a large earthquake in a region that has a risk of seismic events due to the North American and Caribbean plate boundary close to Cuba.

Although Sir Robert McAlpine concentrate their construction operations in the UK they also ‘randomly’ have projects in the Cayman Islands for which MDG do some design work. The Cayman Islands are in the Caribbean region south of Cuba close to the epicentre of this recent earthquake. In the office this morning our team had a call to say that a self-supporting tower crane (on pile foundations) on a Cayman site was ‘dancing about a bit’ yesterday. Secondly a sink hole appeared just over a metre away from the foundations of a mobile tower crane (with ballast foundation). At this time we don’t have a lot more additional information other than there were no injuries and the cranes appear to be still standing. The sink hole appeared in an area of limestone and so far no instrument has been able to determine its depth. Whether there are other voids in the limestone is unknown as I don’t think this was picked up in the GI.

It seems that the crane foundations produced by MDG were designed for tropical weather conditions including hurricanes but not for the seismic conditions. The original brief from site did not mention a seismic requirement but I think it would be reasonable for designers to take this into account. I think due to a lack of experience in this area in the UK it wasn’t considered. The problem facing us now as a result is to determine if the cranes can still be used and a possible redesign of the foundations. Furthermore there is a large risk of foundation failure near the sink hole. Whist the crane is still standing, additional loads either from wind or during deconstruction could undermine the weakened foundations. Clearly there is a cost and time impact to the project as a result. Particularly with addressing the sink hole problem which may require significant amounts of concrete to infill.

Does anyone have any experience with seismic and/or sink hole problems? How do you account for large voids in the rock – I imagine desktop study with GPR might be able to pick this up? I can see if we’re deployed anywhere with seismic activity as PQEs this may be something we have to consider and design for. There is guidance in EC8 for seismic design but potentially other more local codes based on experience might be more applicable (in this example US design codes might be more suitable).

TLDR:

1. If you end up doing design work on sites outside of the UK don’t forget to at least consider seismic risks.
2. How do you mitigate against large voids in rock during design and how do you resolve a sink hole issue?
3. Any suggestions on foundation design for cranes against seismic loads?
4. Sorry no pictures yet.
5. Yes there is a whole load of other things that come out of this like who holds the risk etc but this blog is already long enough.