Settlement Monitoring And Survey Marks

When a building cracks, the first question is not how bad it is. It is whether it is still happening.

A crack that stopped moving two years ago is a repair. The same crack still opening is a problem that has to be addressed before any repair is worth doing. Monitoring is the only way to tell them apart, and it needs a starting point.

Set the baseline before the work, not after

The single most useful monitoring decision is taking the first reading before anything happens.

Which means survey marks installed and read before an excavation starts, before dewatering begins, before piling, before a load is added. Without that reading, any movement later is unattributable, and every argument about who caused it is a matter of opinion.

That is the same logic as the photographic record in [the importance of dilapidation reports](/blog/importance-of-dilapidation-reports) and its counterpart in [post construction dilapidation reports and why they matter](/blog/post-construction-dilapidation-reports-why-they-matter). Monitoring adds numbers to what those record visually.

What gets monitored, and with what

Precise levelling to survey marks, which is the standard method for vertical settlement. Marks are fixed to the structure, usually at column or wall positions, and read to a reference benchmark well outside the zone of influence. Accurate to a fraction of a millimetre when done properly.

Crack monitors, either a simple graduated gauge fixed across the crack and read by eye, or a demountable gauge read with an instrument for far better precision. The second kind is what you want on anything consequential.

Tilt measurement, on a wall or a retaining structure, where rotation matters more than settlement.

Inclinometers, installed in a borehole or a shoring wall, measuring lateral movement with depth. The direct indicator of whether a retaining wall is behaving as designed, which is part of [excavation near existing structures, best practices](/blog/excavation-near-existing-structures-best-practices).

Piezometers, measuring groundwater level, which is the cause rather than the effect. Monitoring settlement without monitoring the water table on a dewatering job tells you half the story, as set out in [dewatering and groundwater control in excavations](/blog/dewatering-and-groundwater-control-in-excavations).

Vibration monitors, which answer a different question and are covered in [vibration monitoring on construction sites](/blog/vibration-monitoring-on-construction-sites).

The reference point has to be stable

A monitoring system is only as good as what it is measured against.

Which means the benchmark sits outside the zone of influence of whatever is being monitored, founded in stable material, and itself checked against a second benchmark. A reference point that moves produces a complete set of readings that are all wrong in the same direction, and the data looks perfectly consistent.

The same applies to the marks on the structure. A mark fixed to render rather than to the structural element behind it measures the render.

Frequency follows the activity

Before work, at least two readings separated in time, so the baseline is a confirmed value rather than a single number.

During the activity that causes movement, frequently. Daily or every few days during bulk excavation, piling or dewatering, because the point is to detect movement while there is still time to react.

After the activity, tapering off as the readings stabilise.

On a long term diagnosis, monthly for at least a full year, because seasonal movement on a clay site is the thing you are trying to separate from progressive settlement. A six month record through one season proves nothing about a site that moves with moisture, which is the behaviour in [reactive clay sites and footing design in NSW](/blog/reactive-clay-sites-footing-design-nsw).

Set the trigger levels in advance

Monitoring without trigger levels is data collection.

So the system has three thresholds agreed before work starts: a level at which the readings are reviewed, a level at which the method is changed, and a level at which work stops. Each with a named action and a named person.

Which turns a number into a decision, and it removes the argument that happens when a reading arrives and nobody agreed in advance what it meant. The levels come from what the structure can tolerate rather than from what the contractor would prefer, and that tolerance is a structural assessment of the building being protected.

Reading the results

Three things to look for in a settlement record.

Is it still moving, which is the slope of the line rather than the total. A flat line at twenty millimetres is a settled building. A rising line at five millimetres is an active one.

Is it seasonal, which shows as a line that goes up and down across the year rather than steadily in one direction. Seasonal movement is clay responding to moisture and the remedy is moisture management, not underpinning.

Is it differential, which is the comparison between marks rather than any one of them. A building settling evenly is in far better shape than one settling unevenly by half as much, for the reasons in [bearing capacity versus settlement](/blog/bearing-capacity-versus-settlement).

Where it changes the decision

The decision monitoring most often changes is whether to underpin.

Underpinning an active settlement problem is appropriate. Underpinning a building that stopped moving years ago is expensive and unnecessary, and it introduces a new stiff support beneath a structure that has already found its position. Which can create the differential movement it was meant to prevent. The methods and their consequences are in [underpinning adjoining structures explained](/blog/underpinning-adjoining-structures-explained).

Monitoring also decides when a crack repair is worth doing, because a repair to a moving crack fails. That is the first question in [crack injection and structural concrete repair](/blog/crack-injection-and-structural-concrete-repair).

ACSES provides geotechnical engineering and structural monitoring across Sydney. Talk to us about a project.