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Talking Point with Andrew Ridley

In response to the feature on instrumentation in GE’s April issue, traditional monitoring approaches can still meet the needs of challenging projects

I read with interest the recent GE article on monitoring “reaching a new level” in which the author suggests that “developments in monitoring capabilities in recent years means the problem that led to that event (the Heathrow Express tunnel collapse) could be consigned to history.” Contributions to the article advocate a new dawn for remote monitoring solutions. This may be so, and as a purveyor of such systems I wholeheartedly agree with many of the sentiments, but I also feel strongly that the age of high quality manually read instrumentation is not necessarily dead.

For example, with the right approach it is still possible to obtain accurate and precise displacement measurements from traditional probe inclinometers. To do so however requires, in my opinion, the organisations carrying out the measurements to be directly responsible for all aspects of the work, from drilling the boreholes to assisting in the interpretation of the data. This further requires those organisations to be diverse. They must employ geologists to understand the ground, technicians to read the instruments but also to maintain and calibrate the instruments and engineers who understand how the ground and structures behave.

“Engineers from the monitoring subcontractor checked the data and processed it on site.”

This approach was put to effective use during the monitoring on Heathrow’s Terminal 2B. The client, designer, contractor team was supplemented by a single monitoring subcontractor who was made responsible for all aspects of the instrumentation.

Over 100 inclinometers were installed into and behind about 2km of diaphragm wall. All of them were installed using stage grouting, whereby grout is added to the void formers in stages to avoid buoyancy issues. In addition each void former was painstakingly topped up and no readings were taken until this was completed and the grout had set.

Each inclinometer casing was read at the beginning using three independent inclinometer probes. One of which was only ever used for this purpose and for checking the results where displacements were not as expected.

Two readings were made with each probe and the results were compared with one another to check that they did not lie outside the random error for the instrument/casing combination.

Engineers from the monitoring sub-contractor checked the data and processed it on site, producing daily reports, including profile and trend plots with the ground profile and construction progress clearly shown to help identify the causes of any displacements. These engineers would also meet regularly with the designer and the contractor to discuss the results.

The approach adopted at Heathrow instilled trust between the monitoring contractor, the main contractor, the designer and the client. Remote monitoring solutions, such as in-place inclinometers, were used at Heathrow T2B in sensitive locations, but the quality of the data recovered from the manually read
inclinometers and the approach taken by the team to deliver this was equal to, if not better than, that from the remote systems.


A longer version of this article is published in GE’s Linked In group with a link to the original feature.

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