What a 2am movement alert actually looks like

Our monitoring systems run 24 hours a day, 365 days a year. The longest single installation we have had running went four years without a crew standing next to it. So what actually happens at 2am, when one of them decides something has moved?

What is physically on the site

A Leica robotic total station in a solar-powered enclosure, mounted on something that is not going anywhere. Prisms fixed to the structure being watched. And a second set of prisms on stable ground outside the zone of influence — the reference network, which is the whole reason the system can tell real movement from a bad day.

The instrument sits on a self-levelling tribrach. That sounds like a detail and is not. Over a deployment measured in years an instrument gets nudged, thermally cycled and settles on its mount. A self-levelling tribrach absorbs that instead of quietly poisoning every reading after it.

The cycle

Leica GeoMoS runs the instrument. On a set interval — minutes to hours, depending on what the structure is doing — it turns through the full round of prisms, observes on both faces, reduces the round against the reference network, and compares every target to its baseline. Then it does it again. Nobody presses anything.

Who sets the thresholds, and it is not the surveyor

Amber and red thresholds come from the designer or the geotechnical engineer. They are a function of what the structure can tolerate, not of what the instrument can measure. Our job is to build a system that measures comfortably inside those numbers and reports honestly against them — not to decide how much a retaining wall is allowed to move.

Worth checking when you appoint someone. If a monitoring quote arrives with trigger levels already filled in and no geotechnical input behind them, ask where they came from.

2am

A target crosses amber. GeoMoS sends an SMS. Depending on the response plan that goes to the site engineer, the geotechnical engineer and us at the same time. The client logs in from a phone and sees exactly the data we see. Round-the-clock remote access is not a nice-to-have on a monitoring contract — it is the thing that stops a 2am alert turning into a 2am phone tree.

Then the actual work starts, which is deciding whether the structure moved.

Most alerts are not the structure moving

Atmospheric refraction. A long sight across a big temperature gradient bends the line. The tell is that the reference prisms move too — and real structural movement does not shift a prism sitting on stable ground well outside the works.

Something in the line of sight. A truck, a crane boom, scaffold, a stack of pallets. The tell is that the target drops out entirely or returns one wild epoch, then comes back clean.

A knocked prism. The tell is a step — instantaneous, then flat. Structures do not move like that. They move as trends.

Thermal movement of the structure itself. Real, measurable and reversible on a daily cycle. The tell is that it comes back at dawn.

And then actual movement. The tell is a trend across several adjacent targets, in a consistent direction, that does not recover. That is the one worth a phone call, and reading the difference at 2am is the part that takes experience rather than equipment.

What else feeds the same system

A total station only sees what it can see. For the rest: inclinometers for lateral ground movement down a borehole, extensometers for extension across a joint or a face, settlement points for level, and thermal imagery where heat is the signal. All of it reports into the same platform, so the 2am picture is one picture instead of four.

The test of a monitoring installation

It is not whether the system alerts. Any system will alert. It is whether the alerts it sends are worth waking up for. Get the reference network right, put the instrument on a self-levelling tribrach, take the thresholds from the engineer who owns the structure, and they will be.

How we scope and run continuous monitoring: Monitoring Solutions.

Jay Rowley, Director, Elite Survey