The borehole extensometer measures the extension (displacement) which takes place with time in a bored hole or in several bored holes in a rock mass. It essentially consists of one or more anchors and a reference plate. The anchor or anchors are set in the same borehole or in different boreholes drilled adjacent to the first borehole. They help to accurately measure the distance between the various anchors with respect to the reference plate and monitor their relative displacement with passage of time. It is usually assumed that the deepest anchor is in stable ground and so any change in the anchor spacing gives information about the settlement of foundation taking place.