Instrumentation and monitoring during construction

How bridges get built · Site investigation and testing · Instrumentation and monitoring during construction

Also called: settlement monitoring, piezometers, inclinometers, structural health monitoring

How it is done

Define what must be controlled and why (fill settlement, pore pressure for stability, wall deflection, deck geometry, stay forces), and state the predicted values and ranges.
Select instruments with redundancy at critical points: settlement plates and profile gauges, vibrating-wire piezometers, inclinometers, extensometers, survey prisms, strain gauges, load cells and temperature sensors.
Install and baseline before loading or excavation starts, protect instruments from plant, and record as-built positions and several initial readings.
Set green, amber and red trigger levels, each tied to a predefined action, a named owner and a response time.
Read at a frequency matched to the rate of change, automating with loggers or robotic total stations where the work is fast or access is unsafe.
Compare readings with predictions (Asaoka or hyperbolic fits for consolidation, calculated deflections for staged construction), back-analyse, and adjust the work: hold a stage, add support, or reset camber.
Close out with final readings, decommission or hand over to the permanent monitoring system, and record lessons in the as-built file.

Key numbers

Settlement plates every 25–50 m along soft-ground approaches, closer near abutments, with piezometers in each soft layer.
Precise levelling repeatability about ±1 mm; automated total stations about ±1–2 mm on prisms at typical ranges.
Reading frequencydaily during loading, excavation or segment erection; weekly to monthly once movements slow.
Trigger levelsamber commonly at about 70–80 % of the allowable value, red at 100 %.
Costabout 0.5–2 % of the works protected.

Where it fits

Soft-ground approaches, deep excavations, balanced cantilever and stayed construction, launching.

Where it does not

Installing instruments without a plan for what to do when a trigger is reached.

Choosing it

Instrument when the design relies on a predicted response that the construction can still change (staged fills, drain release, excavation support, cantilever geometry); skip it when no action would follow.
Choose automated monitoring over manual readings where movements develop quickly or access is unsafe (live roads, railways, water).
Prefer the observational method with monitoring over a conservative design when the ground is uncertain but the response can be controlled in time.
Keep construction monitoring separate from permanent structural health monitoring unless someone will use the long-term data.

Plant, pace and money

PlantInstruments, data loggers, survey total stations, sometimes automated systems.

ProductivityContinuous.

Cost0.5-2 % of the works it protects.

Risks and controls

What goes wrong

Damaged instruments during earthworks; data that nobody reads.

Quality assurance

Named person responsible for readings and for the trigger response; weekly plots in the progress meeting.

Origins

Peck's 1969 Rankine Lecture set out the observational method, building on Terzaghi's practice of designing from measured field performance; Dunnicliff's 1988 handbook standardised instrument selection and installation.

Examples

PVD-treated approaches are released for pavement only when settlement plates show the predicted consolidation; cantilever decks are cambered from survey data segment by segment.

Case studies

Muar Flats trial embankmentsMalaysia · 1989The Malaysian Highway Authority built instrumented trial embankments on soft marine clay to compare ground treatments; one was taken to failure, and published predictions were compared with the measurements.

Leaning Tower of Pisa stabilisationItaly · 1990–2001Controlled soil extraction beneath the north side, steered by continuous monitoring, reduced the lean by about half a degree.

Tsing Ma BridgeHong Kong, China · 1997A permanent wind and structural health monitoring system (WASHMS) was installed when the bridge opened, an early example of construction instrumentation carried into service.