Pretty Steady: 16 June – 16 July
So it’s been a steady few weeks after all the excitement of the last month. Dewatering is working well and though I have picked up assisting in a few areas my work areas has been pretty consistent, in part as result of work on site slowing due to number of boundary/external issues with the client.
The good news is that work is starting to build up again. We have now completed the retention piling (less one small annex) so site is now boxed out. As a result our destiny is nearly in our hands rather than fighting against all the outside stakeholders!!
I say nearly as the problem of the moving chimney is back again…this led to a day when a 100 m exclusion zone was imposed by the client whilst mitigation strategies and emergency plans were revised.
This is a very interesting (if not worrying) part of the project. The chimney will be dismantled in October, when the new energy building is commissioned, but until then it is causing some concern.
A Plaxis (finite element) model predicted that the chimney would settle as the excavation was dug. However, the as built drawings of the chimney are not available and there is limited knowledge of the foundations.
All of this has made the model a little bit academic and as you would imagine finding an engineer that will categorically give a deflection limit has been impossible because of all the unknowns – the Australian code is not very helpful. As a result a 1/1000 rotation limit has been set which means a 68mm deflection of the top of the chimney is the acceptable limit. Deflection is currently 48mm but has slowed considerably since initial movement.
Currently the chimney is monitored twice a day by the site surveyors monitoring 3 points (bottom, middle, top) which tracks movement and they are responsible for raising the alarm. In addition to this the capping beam (at the top of the retention system) is monitored daily for any movement.
The project has a Chimney Emergency Response Team (CERT)…a naff name but the intent is correct. This is a group of nominated JHG personnel, consultants and contractors who are involved in the monitoring and corrective action if required. There are three levels of action:
- Below 62mm – continue to monitor twice a day
- 62mm < Deflection < 68mm – 24hr continual monitoring. CERT decides which mitigation method to use (Surcharge, ground anchors, jet grouting) Implement Mitigation Method.
- > 68mm 100m exclusion zone around the tower and consider dismantling chimney early.
As for the rest of the project the focus is now on firmly constructing the pile caps and cores, and I have picked up the responsibility for the deepest Core – Core A which will be dewatering nightmare!
This afternoon I briefed the team on the construction options for this, (un)fortunately the cofferdam option has been rejected and we are going to just dig a big hole (23 x 28 m, 4.5m deep). Though this has a number of advantages there is a very real possibility that the dewatering system will not cope and we will have one very large swimming pool!!
Also the 1 month tender freeze has been lifted by the client so my involvement in the Post Tensioning Tender will start to ramp up.
Finally, Fay and I popped across to Bali for a long weekend – a brilliant trip with plenty of activities, Bali is a fascinating country with amazing topology, I would recommend a visit if you ever get the opportunity!!
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Stephen
Where no calculations completed with regard to the need for or otherwise of a cofferdam, or was it a wet finger in the air, cost driven solution?
Jim
PLAXIS well known- I’ve never used it …but I think I detect the correct level of cynicism
The sort of things that go tits-up are:
1 mis modelling of actual structure ( you’ve picked this up)
2 parametric study but with orders of magnitude of difference in parameters ( since this is a settlement issue the parameter is stiffness) You excavated in the stuff what guess could you make?
3 THE BIG ONE. If you dewater there are two effects:
a) The soil volume changes ….derrrrrreerrrrr
b) (Vicky Pollard might not get this one) the pore pressure goes down, the effective stress goes up so whatever the stiffness ( reprise on this below) settlement occurs. Did the PLAXIS model envisage dewatering and (if you’ve decided not to drive a cut-off to pull down the level of dewatering required… ) when you turn up the pump to ’11’ ( c.f. Spinal Tap) ,to avoid the swimming pool , did the model anticpate the level of dewatering and the radius of drawdown you can associate with this dewatering
I noticed from something that Rachel sent from her job that the GEO consultants ( bunch of charlatans the lot of us) had used the self-same fag-packet method I will have mentioned in the classroom linking settlement to effective stress change thorugh an estimate of E’
OK reprise on E’……bastard thing to guess …if you’ve cone petetrometer data ( or SPT if you must) you have , from my notescorrelations ranges ….other corellations are, of course, also availble!
Thanks for your comments -I’ll do my best to answer them
1. I wrote the option study for the construction methodology for the core, this included some refined flow calculation and a sensitivity analysis (dewatering calculations/permeability changes/ coffrerdam flow nets etc). All very enlighting and show that as you would suspect a cofferdam would be an excellent option with a cut off of over 10m…..however the nature of the contract means that as this option ahs not already been priced, it was very quickly cast aside by the JHG hierachy…a little sole destroying but I await with my calculations if we are forced to go back to the cofferdam option!!
2, Is dewatering having an effect….interestingly the groundwater level by the chimeny has not moved since dewatering has commenced…again I am slightly suspicous as we have now removed 35,000M3 from the ground but I ahve installed a monitoring well that site of the retention system only 5m away so I have to trust the monitoring results…
However, a consultant (sadly not me as it is very enlightening) has mapped chimney movement to rain events….these correlate perfectly. My gut feeling is the rain was washing away the fines through the CFA before it was shotcreted (see previous comments about stupid boundary dispute and State giving priority to the other contractor and not our site). Maybe something more clever is happening but with the groundwater level not moving I can’t think of it!!
Core A piling starts tomorrow with excavation and dewatering in 10 days time…this will put more strain on the chimeny as the hole gets deeper and dewatering cranks up…