Chapter 9 — Measurement of Load, Strain & Temperature¶

9.1 Monitoring forces in structures¶

Where geotechnical works interact with structures — struts, anchors, piles, linings — monitoring the load and strain in those members confirms they behave as designed and reveals distress early.

9.2 Load cells¶

Load cells measure force. Common types:

Vibrating-wire — automated, widely used in struts, anchors, and piles.
Hydraulic — robust, good for high forces.
Electrical (strain-gage) — versatile, sensitive to lead effects.

Applications include braced-excavation strut loads, anchor prestress, and pile load tests.

9.3 Strain gages¶

Strain gages measure deformation per unit length, from which stress can be inferred:

Bonded wire / foil — surface-mounted on steel or concrete.
Vibrating-wire — embeddable in concrete or attached to steel.
Telltales — simple devices indicating relative movement across a section.

9.4 Temperature measurement¶

Temperature affects nearly every other instrument: it changes sensor output, concrete strain, and pore pressure. Thermistors, RTDs, thermocouples, and mercury thermometers are used to:

Compensate other readings for thermal effects.
Detect freezing, hydration heat, or thermal-driven movement.

9.5 Interpretation cautions¶

Strain in concrete includes shrinkage and creep; separate these from load effects using temperature and companion specimens. Load-cell readings must be referenced to a verified zero.

Temperature is not an afterthought

Ignoring temperature compensation is a leading cause of misinterpreted strain and pore-pressure data. Measure temperature wherever thermal effects are plausible.

Figure: dunnicliff-load-cell-strain-gage

Figure. Load cells and strain gages. A center-hole load cell measures anchor or strut force; a sister-bar gage measures strain in a rebar (Dunnicliff, Ch. 13).

9.6 Key takeaways¶

Load cells and strain gages confirm structural members behave as designed.
VW, hydraulic, and electrical types serve different needs.
Temperature affects almost every reading — measure and compensate it.
Separate shrinkage/creep from load effects in concrete.