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THE MAGAZINE OF THE BRITISH GEOTECHNICAL SOCIETY

John Dunnicliff identifies some current major issues with geotechnical instrumentation.

When using geotechnical instrumentation it is essential that we maximise the likelihood of obtaining high quality data. Some current major issues and developments in our attempts to gather good data by using geotechnical instrumentation include the following:

Systematic planning

The primary shortcoming in practice is the inadequacy of sensible and systematic planning of monitoring programmes. People often think of the instruments first, rather than the questions that leads to the use of instrumentation. My golden rule is every instrument on a project should be selected and placed to assist with answering a specific question: if there is no question, there should be no instrumentation.

Contract practices

When I worked for Binnie & Partners in the UK during the 1950s and 1960s, instrumen- tation was regarded as a professional service, and not as an item of 'low-tender' construction work.

Purchase of instruments was under the control of the resident engineer, using the 'prime cost item' method. Instruments were installed by a nominated subcontractor. The resident engineer's staff read the instruments and made the interpretations. This arrangement led to high quality data.

With this background, I moved to US in 1967, and found that instrumentation was always part of the low-tender construction package. I made a lot of noise about the better English practice, and changed some minds. I have just moved back to England, to find that English practice now appears to be on a lowest cost basis. I question why, and wonder what has brought about this change. I believe that this is a major issue, that needs to be fully addressed.

Improvements to established technologies

The following two developments are of particular note. First in-place inclinometers with electrolevel sensors have often led to disappointments, either because of sensor drift or because of temperature sensitivity. However improvements appear to be here

or nearly here, using vibrating wire sensors or more stable and repeatable electrolevels.

Second, attempts to install more than one piezometer in a borehole have often failed, because of the difficulty in sealing around and between two or more cables, pipes or tubes.

A vibrating wire piezometer is now available, which has a filter set against the wall of a borehole by releasing a spring. Several can be installed in one borehole, and the borehole filled with a bentonite grout.

New technologies

There have been in recent years dramatic improvements in automatic data acquisition hardware, and in data management software. To say that we are at the end of the learning curve would be foolish, but we can now have a strong comfort level.

Time domain reflectometry, a technique developed by the telephone industry for locating cable breaks, has been enhanced for use in the mining industry to determine locations of roof falls. TDR technology is now available for monitoring deformation

in soil and rock by grouting a coaxial cable in a borehole (both axial and shear deformation), water levels in a borehole, contaminant transport, and volumetric water content.

Optical fibre technology is also on its way for monitoring strain and other parameters, although I think we must wait a few more years before these systems are proven.

Forum for exchange of experience

Our current methods of sharing experiences are either very formal or very limited - but I believe that the industry would benefit if we had an informal forum for sharing instrumentation experience. For the last four years I have attempted to achieve this in the North American magazine Geotechnical News, by writing a column in which I try to share things that I have learned and which might be useful to others, and also by soliciting articles on the experiences of others.

Now that I have returned to the UK, it is my intention to set up a similar network here, and I am pleased that Ground Engineering has agreed to the idea in principle. Looking ahead Paul Wheeler and I plan periodically to elaborate on current major issues associated with instrumentation - and we'll be looking for your help. Please help us make this potentially valuable forum a success and benefit for

us all.

John Dunnicliff is a self-employed geotechnical instrumentation consultant, now practising in Devon, after living in US for 30 years. He is the author of the book Geotechnical instrumentation for monitoring field performance. He can be contacted by phone at 01647 432209, fax 01647432379, e-mail johndunnicliff@ibm.net.

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