John Dunnicliff reports on initiatives to improve geotechnical instrumentation practice.
When instrumentation data is disappointing, we can either blame the instrument or we can blame the people who planned and executed the instrumentation programme.
Personal experience has shown that most of the blame belongs with the people, through a lack of knowledge or a lack of commitment. In response to this, the instrumentation community is carrying out a number of initiatives to improve practice.
The internet offers an exciting opportunity for practitioners to share their experience. The Field Measurements in GeoMechanics (FMGM) website www. fmgm. no (see box) aims to be a neutral, noncommercial site, where anyone interested in field instrumentation can meet, exchange ideas and find useful information.
Product data sheets
Many manufacturers' product data sheets (sometimes called 'specifications') contain information that can readily be misunderstood, leading the user to expect unrealistically high performance.
The terminology used is often confusing. One common example is the quoting of a figure for resolution (defined as the smallest division on the readout scale) but nothing for accuracy (closeness to truth), so that the user believes the accuracy is higher than it is.
Figures quoted for accuracy are often based on calibrations in the laboratory and accuracy in the field may be much less. An obvious example is embedment earth pressure cells. Product data sheets usually indicate an accuracy based on calibration tests in air or water, whereas in the field accuracy is greatly reduced by inclusion effects.
DiBiagio et al (1999) identify the problem of misleading product data sheets and recommended establishing appropriate contents and uniform use of terminology.
These ideas are being pursued on the discussions page of the FMGM website, under the Specifications heading. Please visit the site and contribute to the discussions.
here are four on-going efforts to gather and disseminate knowledge relating to sealing piezometers in boreholes, and for selecting a grout mix as backfill for other instruments.
There is very little in the literature to help in the selection of a grout mix for use while installing instruments in boreholes. A US group has decided to mix various proportions of cement and bentonite, and fly ash and bentonite, and test for strength, permeability, compressibility and volume stability.
Erik Mikkelsen, a consulting engineer in Bellevue, WA, USA is preparing an article to help engineers understand the why and how of using cementbentonite grouts as backfill for borehole instruments. The article is due to be published in either the December 2002 or March 2003 issue of Geotechnical News .
There is a growing body of opinion that vibrating wire and pneumatic piezometers should no longer be installed in boreholes by surrounding the tip with sand and then sealing either with bentonite chips and/or grout. Instead, the entire borehole should be filled with an appropriate grout. This has become known as the fullygrouted method.
More than 30 years ago Peter Vaughan of Imperial College, London, raised this suggestion (Vaughan, 1969), but practice has lagged far behind. The method has been used by a few of practitioners in the western US and Canada over the past 20 years, and in Eastern Europe.
A definitive paper is in preparation by Erik Mikkelsen and Gordon Green and will be submitted to FMGM 2003.
Many engineers have experienced extreme frustration when bentonite chips or pellets become bridged part way down a borehole during an attempt to seal piezometers. RST Instruments is planning a test programme to evaluate the properties of various commercially available bentonite chips and pellets and will provide guidelines for their use during installation of piezometers in boreholes.
Fibre optics for monitoring deformation of tunnels An extensive research programme was recently begun to develop fibre optic sensors for monitoring deformation in tunnels.
The two basic types of fibre optic sensors of interest to geotechnical and structural engineers are Fabry-Perot and Bragg Grating.
Fabry-Perot sensors are available commercially for monitoring strain, temperature and pressure, and each incorporates an individual fibre optic cable.
Bragg Grating systems incorporate a series of sensors on the same cable, and have the capability of monitoring deformation and temperature at each sensor point.
Significant efforts are under way to develop Bragg Grating systems for monitoring deformation of embankment dams (Sweden) and highway bridges (USA).
The new research project is directed at tunnels, under the name Ofstunn (optical fibre sensors for remote tunnel displacement monitoring). The objectives are to design and manufacture an array of fibre optic sensors that can be fixed to tunnel linings and are able to measure accurately, reliably and economically tunnel strains and displacements associated with settlement, rotation and distortion.
The three-year research programme, funded by EPSRC/DTI and worth £0.9M, is being carried out by Birmingham University with the support of Smart Fibres, London Underground and SolData. For further information, contact Professor Chris Rogers at c. d. f. rogers@bham. ac. uk Courses Three courses on geotechnical instrumentation are planned for the near future.
The first will be in Delft, The Netherlands, on 26-29 November.
Lectures will take place on the first three days. On the fourth day there will be a discussion of topics chosen by delegates, followed by a visit to the North-South line in Amsterdam, focusing on the very extensive instrumentation programme (GE September 2001).
For details visit www. geodelft. nl
The second is a one-day seminar and discussion, on Santorini Island in Greece on 28 May 2003, after the 11th International Symposium on Deformation Measurements. Details are on the International Federation of Geodesy website: www. heliotopos. net/ conf/11fig/ The third is in Florida on 10-13 March 2003. There will be lectures on the first three days but the fourth day will be for discussion of topics chosen by delegates.
Ralph Peck is one of the lecturers. For details see the University of Florida website: www.doce-conferences. ufl. edu/geotech/ FMGM
The 6th FMGM international symposium, which is devoted to instrumentation, will be held on September 15-18, 2003 in Olso, Norway. There is an early deadline for submission of abstracts - the exact date is unknown at the time of writing, but it is expected to be during October. Visit the FMGM website for details. The 7th FMGM is planned for USA in 2007.
In-place inclinometers A comprehensive test programme to determine the performance of tilt sensors for in-place inclinometers (IPIs) has now been completed (GE July 2001).
Sensors were purchased from eight commercial manufacturers, and tested by an independent testing laboratory in France for the characteristics needed by users of IPIs. A copyrighted report by the testing laboratory is now available for purchase on Sol Data's website: www. soldatagroup. com
For the past eight years part of the North American-published magazine Geotechnical News has been devoted to articles on geotechnical instrumentation (www. bitech. ca).
Its purpose is to provide a forum for sharing information that requires much less effort than preparation of a paper for a geotechnical journal.
References DiBiagio E, Pezzetti G and Bruzzi B (1999).
Classification, certification and specification of instruments for field measurements. Proc Symp on field measurements in geomechanics, Singapore, pp125-134.
Vaughan PR (1969). A note on sealing piezometers in boreholes. Géotechnique, Vol 19, No 3, pp405-413.
John Dunnicliff is an independent geotechnical instrumentation consultant based in Devon, email: johndunnicliff@attglobal. net