Holdfast Training Services Trial Blog

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:

1.  Below 62mm  – continue to monitor twice a day
2. 62mm < Deflection < 68mm – 24hr continual monitoring.  CERT decides which mitigation method to use (Surcharge, ground anchors, jet grouting) Implement Mitigation Method.
3. > 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!!