ISO 18674-7:2025

Overview

ISO 18674-7:2025 - Geotechnical investigation and testing - Geotechnical monitoring by field instrumentation - Part 7: Measurement of strains: Strain gauges specifies how to measure strain in geotechnical monitoring using strain gauges and strainmeters. It complements the general monitoring rules in ISO 18674-1 and applies to performance monitoring of 1‑D, 2‑D and 3‑D structural members (e.g., piles, diaphragm walls, tunnel linings, gravity dams, embankments). The standard covers instrument selection, installation, measurement procedures, data processing, evaluation and reporting for strain measurement in geotechnical contexts.

Key technical topics and requirements

• Scope of measurement: Measurement of strain using surface‑mounted and embedded strain gauges, and strainmeters, with guidance on sensor types (vibrating wire - VW, electrical resistance, Fiber Bragg Grating - FBG).
• Instrument classification: Distinguishes instrument type (strain gauge vs strainmeter), mounting location (surface vs embedment), measurement principle and host material (steel, concrete, fill).
• Gauge lengths and typical configurations: Common gauge length ranges indicated (e.g., steel surface gauges 5–150 mm; concrete surface gauges 50–350 mm; embedment gauges 50–500 mm; strainmeters 1 000–3 000 mm).
• Installation and measurement procedure: Practical guidance on installation of strain gauges and strainmeters, instrumentation checks, calibration and measurement routines.
• Data processing and evaluation: Procedures for converting strain to stress/force using material stress–strain relationships (see related parts for 1‑D and 2‑D/3‑D conversions).
• Reporting requirements: Content and format for installation and monitoring reports to ensure traceability and consistent interpretation.
• Annexes of practical relevance: Annex A (data processing), Annex B (informative overview of Distributed Fibre Optic Sensing - DFOS - noted as excluded from normative scope), Annex C (temperature effects), Annex D (geotechnical applications) and Annex E (measuring examples).
• Sign convention: Compressive strain is taken as positive for many geotechnical applications (clarified for consistency within the series).

Practical applications

• Performance monitoring of structural members (piles, sheet piles, tunnel linings), dams and embankments.
• Design verification and observational design: checking geotechnical designs and adjusting construction based on measured strain.
• Stability assessment during or after construction by converting measured strains to stresses/forces where appropriate.

Who should use this standard

• Geotechnical engineers and structural monitoring specialists
• Instrumentation and field monitoring contractors
• Dam, tunnel and infrastructure owners and asset managers
• Construction QA/QC teams and consulting engineers
• Research laboratories developing strain‑based monitoring programs

Related standards and cross‑references

• ISO 18674-1 (general rules for geotechnical monitoring)
• ISO 18674-2, -5, -8 (related measurement types and conversions)
• Annex B provides guidance on DFOS (informative only)

ISO 18674-7:2025 is an essential reference for setting up reliable strain‑based geotechnical monitoring programs, ensuring consistent instrumentation, data handling and reporting.