Section 8: Field instrumentation
COMMENTARY ON CLAUSE 50
Numerous techniques are available to be used in ground investigations to monitor movements and strains, total stresses and pore water pressures associated with known or expected ground behaviour (see Section 7). Such behaviours can result from construction processes, potential stability failures, tunnelling, subsidence and ground response in large-scale field trials. The types, advantages, limitations and appropriateness of the various techniques are discussed in detail elsewhere (see Dunnicliff, 1988 , ICE, 1990  and ICE, 2012 ). The principal types of instrumentation used in ground investigation are piezometers for measuring ground water pressures and inclinometers and extensometers for measuring ground movements. Other types of instrumentation are outlined in BS EN ISO 18674.
Utilities should be identified before any intrusive work commences (see 19.2.4). The design of an instrumentation programme, including the selection of instrument types, their location and their monitoring is a complex issue that requires experience and judgement; the geotechnical adviser should ensure that appropriate and skilled advice is obtained to guide this design. An instrumentation specialist should be used to install and read the instruments and possibly help interpret the results, where appropriate.
It should be taken into account that instruments might malfunction and give no measurements or, worse, give erroneous results, or that the installation of instrumentation could affect behaviour of the ground, particularly with respect to earth pressure measurements. The limitations of any instrumentation should be taken into account, particularly for long-term measurements.
NOTE 1 Electrical instruments are more likely to fail than mechanical ones, which are simple and robust. All measurements, however, are usually discontinuous in space and time. Data from instrumentation is not comprehensive and might not record the most adverse circumstances.
All instrumentation should be protected at the surface from the adverse effects of weather (including temporary flooding), construction activities and other potential damage, both accidental and deliberate. Electrical instruments and data loggers should be protected from power surges and lightning strikes using replaceable fuses.
When displacements are being measured, they should be referenced to a fixed or known point; in the case of downhole measurements, the fixed or known point can be at the bottom or the top of the borehole. If the bottom of the borehole is used, fixity should be confirmed by at least five sequential points showing no relative displacement. Measuring the position of the top of the borehole by surveying is also acceptable.
Following installation, an instrument should be allowed to stabilize and measurements should be taken at regular intervals to determine when the installation of the instrument has stabilized satisfactorily. The geotechnical adviser should advise on the required stability. When stability has been confirmed, a single "zero" measurement is taken; thereafter a series of "baseline" measurements are taken, the frequency of which should be provided by the geotechnical advisor and the purpose of which is to identify changes that occur due to causes other than those being investigated and to assign a reference measurement for subsequent monitoring.
NOTE 2 BS EN ISO 18674-1 provides comprehensive guidance on this process.
51 Planning a field monitoring programme
When planning a field monitoring programme it is as important to define the objectives as it is to choose the appropriate instrumentation. The following procedure for planning monitoring programmes using instrumentation should be adopted, and checked by the geotechnical adviser responsible for the instrumentation working with the designer (see Dunnicliff, 1988 ).
- Step 1: Define the project conditions;
- Step 2: Predict the mechanisms that control behaviour;
- Step 3: Define the geotechnical questions that need to be answered;
- Step 4: Define the purpose of the instrumentation;
- Step 5: Select the parameters to be monitored;
- Step 6: Predict the magnitudes of change;
- Step 7: Devise remedial action;
- Step 8: Assign tasks for design, construction and operation phases;
- Step 9: Select the instruments;
- Step 10: Select the instrumentation locations;
- Step 11: Plan recording of factors that might influence the measured data;
- Step 12: Establish procedures for ensuring correctness of the readings;
- Step 13: List the specific purpose of each instrument;
- Step 14: Prepare budget;
- Step 15: Prepare an instrumentation system design report;
- Step 16: Write instruments procurements specifications;
- Step 17: Plan the installation;
- Step 18: Plan regular calibration and maintenance;
- Step 19: Plan data collection, processing, presentation, interpretation, reporting and implementation;
- Step 20: Write specifications for field instrumentation services; and
- Step 21: Update budget.