In-place inclinometers (IPIs) are being used in large numbers in many countries to monitor subsurface horizontal deformation, for example around excavations, and to define landslide movements. Rasmussen (2001) indicates that in Europe, IPIs with electrolevel tilt sensors are widely accepted and used, and that many of the recent major UK tunnelling projects in and around London have specified comprehensive and extensive instrumentation systems which incorporate electrolevels as their sensors.
Rasmussen also reports on successful and unsuccessful use of two commercial versions during a project in London.
Sweetman and Carayol (2001) report on successful short-term use of IPIs with vibrating wire sensors in Hong Kong.
An IPI typically consists of a series of interconnected hinged rods, installed in a vertical borehole, with a tilt sensor mounted on each rod. In this way the rod lengths and the tilt sensors' output can be used to provide horizontal deformation data throughout the depth of the borehole.
The most commonly used tilt sensors are electrolytic levels (electrolevels), but vibrating wire, accelerometer (both the servo type and the capacitive type) and magneto-resistive tilt sensors are also used.Often the tilt sensors are connected to a datalogger, with pre-set warning levels and alarm features.
In addition to their use in vertical boreholes for monitoring subsurface horizontal deformation, the same interconnected rod and tilt sensor systems are also used for monitoring vertical deformation. In this configuration the system is installed horizontally at the ground surface or in horizontal or near-horizontal boreholes.
Applications include monitoring settlement of railway racks while tunnelling beneath, and monitoring the effects of compensation grouting.
Performance has been very mixed, with temperature sensitivity, zero drift and ground-borne electrical noise as significant problems.The above two publications are helpful when evaluating performance of different versions, but there have been so many competing claims from manufacturers that some users have found it difficult to decide which commercial version to use in a particular project situation.The authors are two of them!
We have therefore initiated a comprehensive test programme on eight different commercial versions of IPIs, which will be well under way by the time this article is published.We expect the tests will be completed in September this year.
Testing laboratory The tests are being performed at the French National Testing Laboratory (LNE, Laboratoire National d'Essais) in Paris.LNE is one of the major independent testing houses in Europe for testing and reporting on the quality and technical conformity of measuring equipment.The state owned company has 600 staff and more than 100 years of experience in this field.
Procedure before start of testing We have followed the customary procedure of LNE, and of other organisations that make comparative tests among commercial products (such as Which magazine in the UK), by obtaining the IPI hardware without informing the suppliers about the test programme.
A letter was sent to eight suppliers of IPIs, requesting a quotation and full specifications for an in-place inclinometer.The letter was sent from a separate company, with which the second author's company has working relationships, so that there was no evidence of a link to LNE or to the authors of this article.
Requirements included one string of three sensors with uniaxial sensors, for a vertical installation with 3m spacing between adjacent sensors. Sensors were required to have a range of +/-10infinity and have a built-in system of temperature correction.
However, the suppliers were invited to recommend a different product if they thought it would be more appropriate. It was not necessary to order dataloggers and related software, because the second author's company already owns these.
The separate company's address was given for shipping purposes.
All eight quotations were accepted. After receipt in France of all eight IPIs, a letter was sent to each supplier to inform them of the planned test programme, and enclosing a detailed step-by-step test procedure.
Each was encouraged to review and comment on the procedure 'so that we can do our best to make sure that our tests meet with your approval' We also stated: 'If we receive any questions/comments/ requests from you, we reserve the right to inform each of the other seven suppliers about these, including our answers' After receipt of the reviews, appropriate changes were made to the test programme, and the tests were started.
Outline of test programme Before testing, a general description of each sensor was made, and the suppliers' specifications were reviewed. Significant interaction was necessary with most of the suppliers to ensure that there was agreement on the methods for reading the sensors and on the methods for converting the measured data to tilt.
A brief outline of the test programme and a list of the eight IPI suppliers is given on the facing page.
After testing has finished, a team of four people, including a mechanical engineer and an electronics engineer, will make a critical visual inspection of each sensor. This will include all the rods and other ancillary equipment that have not been subjected to testing.One sensor from each supplier will be opened for visual inspection.
Reporting procedure A decision on the method for disseminating the test results within the instrumentation community will be made later.We expect to provide a brief report on the internet, and perhaps also in the geotechnical literature. In addition we expect to make a detailed hard copy report available.
Budget and funding The cost of the test programme, including buying the instruments, is approximately £13,000 for each set of three IPIs, for a total cost of a little over £100,000.
Part of the funding is provided by the French government, the remaining being supported by the second author's company.
References Rasmussen C (2001). Electrolevels - A European view.
Geotechnical News, vol 19 no 1, March, pp31-33.
Sweetman J and Carayol S (2001).Vibrating wire in-place inclinometers - A case history.Geotechnical News, vol 19 No 1, March, pp26-31.
This article is based on an article published in the March 2001 issue of Geotechnical News, and is reprinted here with the kind permission of BiTech Publications, Richmond, British Columbia, Canada.
John Dunnicliff is an independent geotechnical instrumentation consultant based in Devon, email: johndunnicliff@attglobal. net Jean-Ghislain La Fonta is managing director of Sol Data, email: jg. lafonta@soldata. fr