BRS Magnet Extensometers

BRS Magnet Extensometers

Sisgeo’s BRS Magnet Extensometer is a manual, multi-point settlement monitoring instrument designed to measure vertical and horizontal displacements in soil and rock. It uses a reed switch probe to detect magnet rings that move with the ground, enabling accurate tracking of settlement, heave, or subsidence over time at various depths.

BRS Magnet Extensometer is ideal for geotechnical projects that require cost-effective, reliable, and easy-to-install systems for long-term monitoring, especially in Embankments, earth fill dams, tunnels, landfills, foundations, mining and excavation zones.

Working Principle

The system consists of:

• An access tube (PVC) inserted vertically into a borehole or soil layer.
• Magnet rings (targets) installed at predetermined depths along the tube.
• A portable reed switch probe connected to a graduated cable reel.

During a survey, the probe is lowered manually through the tube. Each time it passes a magnet ring, the built-in reed switch is activated, triggering a beep or signal. The depth is read from the graduated tape on the reel.

By recording the depths of these rings over time and comparing to the initial readings, users can determine:

• Amount of settlement or heave
• Depth at which the movement is occurring
• Rates and trends over days, months, or years

Typically, the user should select bill of materials based on deployment scenario:

1. Access Tube: A PVC guide tube (typically 33 mm OD, 3 m length segments), installed inside a corrugated sheath to reduce friction and allow tube movement with the surrounding soil.

2. Corrugated Sheath: A flexible outer sleeve that protects the access tube and helps reduce drag during soil movement.
3. Magnet Rings (Targets): Installed at specific locations, either Spider-type magnets for borehole installations (expand inside the hole) or Plate-type magnets for fill or layered ground. They are fixed to move with the ground while the access tube remains relatively free.

4. Datum Magnet Section: A fixed reference point, typically at the bottom of the system, which serves as the base measurement marker.
5. Suspension Head: Located at the top of the tube; allows for easy entry of the probe and serves as a stable reference.

Applications of BRS Magnet Extensometer:

• Construction Monitoring: Detect differential settlement in buildings, bridges, or highways.
• Embankment Stability: Monitor settlement profiles during and after fill placement.
• Dam Safety: Evaluate fill consolidation or foundation settlement in embankment dams.
• Tunnel Impact Studies: Assess surface and subsurface settlement above tunnelling zones.
• Ground Improvement Verification: Confirm performance of preloading or surcharge methods.
• Mining and Sinkhole Monitoring: Detect subsidence or void migration in soft ground zones.
• Landfill Monitoring: Track waste mass settlement and cap performance over time.