Magnetic Spider Extensometer
The CEP Magnetic Extensometer is designed to measure subsurface settlement or heave in earthfill embankments, excavation, ground improvement and land reclamation projects.
Inclinometer casings or pvc access pipe with telescoping couplings are used where the soil SPT value is less than 20 blows/30cm or where total vertical or lateral movements are expected to exceed 50mm. In each installation a magnetic datum is installed at the bottom of the inclinometer or pvc access pipe. The BH is usually drilled to stable ground where the soil SPT value is at least 100 blows/30cm. The magnetic spider anchors are attached along the inclinometer casing at the specified depths. A simple triggering device is used to release the compressed spider legs to anchor at the specified depths in the soil.
The ground settlement or heave is measured by means of measuring the position of the spider magnets with respect to the distance from the bottom datum. The readout consists of a reed switch probe attached to an electrical tape with 1 millimeter graduations. A buzzer in the tape reel sounds when the reed switch probe passes through the magnetic fields of the spider magnets and datum. The depth of the spider anchors are read on the electrical tape.