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CPD technical bulletin Monitoring of deformation and structural movement Author - Stefan Allen (office@kempengineering.co.uk)
When something starts moving and it's not supposed to then the alarm bells start ringing and prompt action is usually required to arrest the problem. Before effective action can be taken to create a viable long term solution a number of questions need answering;
How much movement is there? What is moving? Which way is it moving?
Cracking is a familiar problem that is traditionally solved by the use of studs with callipers or crack monitors. But what if there is limited or difficult access to the crack? What if the cracking needs close monitoring, maybe safety is an issue?
In these circumstances it's now possible to install a "black box" The sensors enclosed are able to measure to sub millimetre accuracy at predetermined intervals, ten times a day, once a day, once a week. Data is then transmitted by GSM to a central server for reporting. For extreme situations it is possible with this type of equipment to have instant alert - if you want to know of any excessive movement then you can have an email or text alert sent direct to you instantly!
Tilt/Plumb can be monitored using total stations to measure simple offsets or by taking reflectorless observations over the structures height. Longer term monitoring projects may require targets to be fixed to the structure.
When subsidence is suspected level datums can be established and precise digital level observations reduced and compared. Again black box technology similar to that used for crack monitoring can be employed.
Case Study 1
Structural Load Testing - Eden Project
During construction of The Eden Project microscopic flaws in some welds to the main arches of the biomes were detected. To ensure that they posed no structural risk the main design & build contractor decided to carry out a load test by hanging water filled weights from the arches and monitor the corresponding movement.
Prisms were fixed to the arches at suitable locations where the structural engineer calculated the movement that should occur, as the arches were loaded. Observation was undertaken and downloaded to a laptop with custom made spreadsheets enabling instant analysis of the results on site and thus enabling the structural engineer to manage the loading sequence.
Case Study 2
Historic Wall Movement - Robinsons Shaft
This historic mining site in Cornwall is being regenerated after years of neglect. Whilst work is carried out the Architect is keen to ensure that one of the sites significant features doesn't move excessively.
The Architect requested that a 1m rectangular grid on the wall be monitored for subsidence and plumb. Reflective targets have been epoxy glued to the wall to form the grid. Prisms have been fixed to surrounding structures to enable a Leica Total Station to resect it's position prior to observing each target.
The data is downloaded to digital terrain modelling software with height transformed to produce a contoured elevation drawing to demonstrate any variations.
Case Study 3
Replacement of Piers - on Viaduct
The main contractor was employed to demolish the piers on a busy trunk road viaduct in Devon without disrupting the traffic flows above. Each pier would be individually removed whilst temporary props would support the deck whilst the new piers were constructed.
The client required reassurance that any movement in the deck whilst the work was carried out could be detected and that there would be no risk to the public or workforce.
Survey pillars were constructed and two high end Leica TCRA 1800 robotic total stations were positioned with weather protection. Generators were positioned at a suitable location to maintain power to the instruments. A number of prisms were fixed to the deck of the viaduct.
The 1800s were set into a continuous monitoring mode 24 hours a day 7 days a week for the next 6 months whilst all of the piers were replaced.
The data collected was downloaded by GSM link to on site computers to enable our surveyor to analyse the results and produce daily reports. The system was also set to trigger an alarm to text and email staff should any excessive or sudden movement be detected.
Case Study 4
Unstable Rock Face - Lanearth Quarry, Cornwall
Following a period of heavy rain a rock face in a redundant quarry earmarked for commercial development was showing signs of instability and a stabilisation solution was required.
A number of control points were identified on the rock face and reflectorless measurements were observed using a Total Station. A series of stereo digital photographs were then taken using a calibrated camera. The photographs were then processed and using pixel recognition software a three dimensional ground model created to assist the consultants in designing a soil nail solution.
Once work was completed and with a steel mesh covering the rock face photogrammetry would no longer be suitable to monitor the rock face. A leica HD Scanner was then employed to take observations monthly and produce point clouds that could be compared with previous models and variations reported with sections and isopachytes.
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