The BOF-EX is best described as a multiple-point single tube extensometer. It consists of four main components: the mechanical anchor, the measurement module, the extension tubing and the centralizers. An installation consists of a number of measurement modules, in series in a borehole, each one being mounted on the length of extension tubing required to span a lower and upper mechanical anchor. Displacement measurement is therefore made in the hole, in sections distributed along the borehole length.
Mechanical Anchor: The special design of the BOF-EX mechanical anchor allows complete system retrievability. The anchor consists of a cylindrical body and three contacting shoes spaced at 120° angle. Using the installation tool and rods, the anchor is screw actuated from the collar of the hole until the shoes make contact with the borehole walls. The anchoring capacity is very high and the contacting shoes are designed to adjust to small borehole deformations while still exerting the anchoring force.
Measurement Module: The measurement module of the BOF-EX is a watertight capsule in which a spring-loaded linear displacement transducer is mounted. The moving spindle of the measurement module comes in contact with the lower anchor of each monitoring section.
Extension Tubing: The extension tubing is made of individual lengths of flush-coupled tubes bridging the two mechanical anchors of each monitoring section. Centralizers are mounted at regular spacing over the extension tubing to prevent sagging.
The BOF-EX is designed to be installed in an “N” size 76 mm (3 in.) borehole using an installation rod. The installation procedure is simple and straight forward due to the modular design of the instrument. The BOF-EX can also be fully grouted in a borehole. In unstable, poor quality rock, a BOF-EX can be installed using the telescoping plastic casing previously grouted in the hole.