Fixed Extensometers

Sisgeo’s Fixed Extensometer is a single‑rod settlement/heave measurement device, installed either in embankment fill or boreholes. It provides direct deformation tracking between two fixed points without requiring a removable probe.

Two primary versions include:

1. Tell‑Tale Extensometer

·        A galvanised‑steel riser rod, anchored at the bottom (plate in soft ground or grouted in borehole), extends upward through a corrugated sheath.

·        Ground movement shifts the rod; measurements are taken by optical levelling to a survey pin on the top head. Sliding caps minimize friction.

1. Automatic (Electrical) Extensometer

·        Same mechanical setup as above but integrates a DTM displacement transducer within the top head.

·        Offers continuous remote monitoring via a 4–20 mA output, ±0.15 % FS accuracy, IP68 protection, and measurement ranges of 250, 500, or 1000 mm

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

1.      Measuring Rod (Riser Rod): 25 mm outer diameter, 2000 mm long galvanized steel rod with M25 threaded ends. Transmits movement from anchor to top head.

2.      Anchor / Settlement Plate: Bottom anchoring element. “Spider” type for boreholes or 500×500 mm flat steel plate for embankments/soft soils. Ensures deep ground reference.

3.      Measuring Head (Manual) Top-end housing with stainless steel survey pin for optical levelling reference. Used in Tell-Tale version.

4.      Measuring Head (Electrical): Protective head housing the displacement transducer. Weatherproof and supports permanent installations.

5.      DTM Displacement Transducer: High-accuracy linear transducer with 4–20 mA output. Measurement ranges: 250-, 500-, or 1000-mm. Accuracy ±0.15% FS, repeatability <0.08 mm. IP68-rated. Used in automatic systems.

6.      Corrugated Sheath (PVC): 55 mm OD corrugated PVC tube enclosing the riser rod. Allows free vertical movement, minimizes drag/friction. Typically supplied in rolls.

7.      Sliding Caps: Low-friction plastic elements placed inside the sheath to isolate the riser rod and prevent soil drag (“downdrag”).

Applications of Fixed Extensometers:

·        Embankment and Earth Fill Monitoring: Detects settlement of soil layers after placement of new fill or during construction of embankments for highways, railways, or levees.

·        Dams and Levees: Measures long-term consolidation of core or embankment material in dams and levees.

·        Transportation Infrastructure: Monitors subsidence or heave under highways, railways, and airport runways.

·        Landslide and Slope Stability Monitoring: Detects vertical movement related to deeper slip surfaces or subsidence zones in slopes.

·        Excavation and Tunnelling Impact Studies: Assesses how underground excavation (e.g., tunnelling or shaft sinking) affects nearby soil or structure settlement.

·        Structural Foundation Monitoring: Tracks building or bridge foundation settlement or heave, especially in variable or reactive soils.

·        Ground Improvement and Soil Treatment Validation: Verifies the effectiveness of ground improvement techniques such as preloading, vertical drains, stone columns, or deep mixing.