Rod extensometer is a common instrument used to accurately monitor small displacements in soil, rock and concrete along axis of a borehole. It can be used to monitor the vertical or horizontal displacement of soil and rock - depending on the borehole orientation - in different geotechnical applications such as tunnels, shafts, retaining wall, natural and cut slopes, dam abutments, mining and other open excavations.
In general, there are two types of rod extensometers: single point or multipoint rod extensometers. The first type measures the displacement between only one fixed point and a reference point; while, the second type measures the displacement between several fixed points and a reference. Thus, a multipoint rod extensometer can provide an accurate distribution of displacement along a borehole axis.
The main applications of rod extensometers in geotechnical monitoring are, among others:
The rod extensometer consists of a reference head, anchors, rods and protective pipe. A rod extensometer may have only one rod and anchor (single point) or more than one - usually up to eight - anchors and rods (multipoint). The operation principal for both types are the same and the difference is in the quantity and precision of measured displacement data.
The rod extensometers are installed in boreholes usually drilled for this purpose. The anchors with the rods attached are installed length installed at different distances inside the borehole. The rods of different lengths are spanning between the anchors and the reference head which is located on the surface at the borehole collar. The rods are insides a pipe and protected from the backfill grout and other possible damages.
The movement of the rod between the anchored point and the reference head indicates the occurrence of movements in that zone. The displacement of the rods are measured at the reference head either with a micrometer gauge or a displacement transducer. The displacements are measured with reference to a fixed point which is often assumed to be the down-hole anchor. Reading of multiple points at the reference head provides a distribution of displacement along the borehole axis.
Different types of anchors exist and can be used depending on the ground condition.
The extensometer rods can be either in stainless steel or fiberglass. Steel rods are used for greater depth range and can be assembled on site. Fiberglass rods are easier and quicker to install at shorter depths mainly due to their flexibility and are assembled in the factory prior to delivery to site.
The reference head can often accommodate up to 6 or 8 rods. They are equipped either with micrometer gauge or displacement transducer to measure the displacements. The gauge is often used in cases where the reference head is easily accessible and less regular measurements are required; whereas, the transducer option is often used when real-time or continuous measurements are required or if the reference head cannot be easily accessed.
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