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What's a long base extensometer?
A long base extensometer is an instrument for measuring variations of the distance between far points. Each of the two points are constructed by means of anchors and
connected through a sheathed rod: this assembly constitutes a "base". An extensometer can be formed by one or more bases. Typical installations are made inside boreholes, with bases extending from the surface to
various depths.
How does it works?
One end of each rod is fixed and the other is free, so that variations of the distance between the two anchors come about from the sliding of the free end.
How do you take the measurements?
For infrequent measurements in accessible locations a removable dial gage can be used. As an alternate assembly electrical transducers (potentiometers, LVDT or others) can
be fitted inside the extensometer head; they can be linked to a data logger or to an acquisition system. Ranges vary from 10 to 100 mm typically.
What are the main applications?
- Monitoring of the convergence in underground excavations
such as tunnels, mines.
- Monitoring of the subsidence induced by open or underground excavations.
- Monitoring of the foundations of large structures, e.g. dams, bridges.
- Monitoring of abutements and walls.
- Monitoring of slopes.
How long can the bases be?
Length is limited only by the resulting accuracy and by installation considerations. Lengths of some tens of meters are common, but bases up to over 200 m were successfully put in
practice.
Can the extensometers have any orientation?
Yes. Of course upward installations require appropriate instruments.
Which diameter is needed for extensometers installed inside boreholes?
Diameter vary typically from 50 to 100 mm. The smaller the diameter, the lower the installation costs.
How much accurate can be the measurements?
Care should be paid not to give full credit to the accuracy coincident with that of the readout unit or of the electrical transducers, as variations of rods'
length degrade the overall accuracy. Please note
that the shorter the base, the higher the (absolute) accuracy.
How are the rods made?
Two main families exist:
- Jointed metal rods (carbon or stainless steel, aluminium).
- Continuous pultruded rods (carbon, aramid, glass), usually delivered fully preassembled.
Note thate rods greatly influence the performances of the instrument.
What kind of base should you prefer?
If good accuracy is needed the rods should have high longitudinal stiffness, low thermal expansion coefficient, and should move with low friction inside their sheaths.
Various materials and design solutions are adopted, resulting in very diverse performances (say from some 0.01 mm to over 1 mm for a 30 m extensometer). Request detailed informations on instrument behaviour from the
manufacturer.
How are the anchors made?
Groutable anchors should be chosen if best performance is required. In some case mechanical or hydraulic anchors can be practical.
Long base extensometers model LBE-C
Applications include:
- Monitoring of the convergence in underground excavations such as tunnels, mines.
- Monitoring of the subsidence induced by open or underground excavations.
- Monitoring of the foundations of large structures, e.g. dams, bridges.
- Monitoring of abutements and walls.
- Monitoring of slopes.
- ... and all situations where you need to monitor changes in distance
between downhole anchors and a reference head.
Efforts where made developing LBE-C extensometer to obtain an highly accurate yet simple to install instrument.
Accuracy takes advantage from the exceptional
features of carbon rods compared with conventional glass, aluminium or steel rods.
Note that stiffness is of main importance if long bases are planned, and thermal expansion coefficient if large temperature
variations are expected.
Friction too is kept low by a proper choice of sliding materials.
Complicated installation procedures can rise significantly the cost per installation; for this reason LBE-C
extensometer is made so to need low work to assemble it on place. The bases are extremely low-weight and flexible, so that also restricted places are not a concern. Further, the borehole is easily plugged so to
allow pressure-iniection of the cement grout, also in upward holes.
Standard main features:
- 1 or 4 bases, other on request.
- Preassembled base with carbon rods.
- Groutable anchors.
- 25 to 100 mm measurement range.
- Manual (mechanical) or electrical readout.
- Weatherproof and rustproof.
Technical details:
- Borehole size: 80 mm
- Borehole lenght: such as longest base + 0.5 m.
- Base weight, per meter: 105 g
- Base stiffness: 115 GPa
- Base thermal expansion coefficient: 1.6(10-6/K)
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