Borehole rod extensometers serve to measure lengths between one or more anchor points in a borehole and a reference head at the borehole collar. They ensure high precision, even at great installation depths.

Extensometer measuring rods are made of either fibreglass or stainless steel. Extensometers using fibreglass rods can be pre-assembled at the factory coiled and shipped to the site, ready for installation. In general fibreglass rods are easier to install in confined areas such as tunnels and are preferred over stainless steel in alkaline environments.

Extensometers using stainless steel rods (supplied in 3 m lengths) must be assembled on site. Stainless steel rods in general are stiffer than fibreglass rods and are therefore preferred when compression rather than extension is likely or when larger lengths of rods are required. Furthermore stainless steel rods are easily adapted in length when the exact installation depths are not known at time of order.

Typical areas of application include the monitoring of:

• Movements of rock and soil, caused by fractures, landslides and weathering
• Underground settlements and deformations of foundations and abutments, as well as man-made caverns
• Relaxation and deformation of rock around tunnel walls, shafts, pillars, roofs and caverns