Monitoring road settlement over a TBM: TotaLite and an ATS, side by side

At a glance

A tunnel boring machine passed under a reinforced haul road built for ITER’s oversized convoys. Geo-Instruments mounted TotaLite on the same column as its automated total station (ATS). Both instruments recorded the same 1.5 mm of settlement through winter fog, rain and black ice.

The challenge: millimetre settlement in the worst light of the year

The ITER fusion project in southern France depends on a dedicated road to move oversized, heavy components to site. That road is built from dense, heavily reinforced bitumen. When a tunnel boring machine was scheduled to pass beneath it, the road had to be watched for settlement throughout the crossing.

Geo-Instruments, the geotechnical and structural monitoring specialist on the works, was responsible for measuring that movement. Any settlement would be small, so the monitoring had to resolve changes at millimetre level.

The site made that difficult. Beside a river and deep into winter, the team worked through dense fog, humidity close to 100%, black ice, and only two to three hours of sun on a good day. Water pooled on the road prisms and fog left droplets on every optical surface.

“We rarely have conditions this bad for such an endeavour.”

Julien Lextrait, Lead Engineer, Geo-Instruments

Those are the conditions where optical monitoring is most often questioned. Geo-Instruments used the project as a direct test: run TotaLite alongside the ATS it already trusted, then compare the records.

The setup: one column, two instruments, shared targets

To make the comparison fair, both instruments had to see the same thing from the same place. TotaLite was mounted on the ATS tower bracket, sharing its rigid steel column. The column stood on a 150 kg concrete base anchored into the road.

Both instruments read the same reference targets spread across the site. Eight targets were positioned across the site, several fixed to rock faces more than 80 metres away, providing stable reference points well outside the zone of movement.

No filtering was applied to the TotaLite data.

The results: the same 1.5 mm, the same gaps

The road settled 1.5 mm as the TBM passed beneath it. TotaLite and the ATS both recorded that movement.

The data gaps tell the same story. Fog and water on the optics interrupted readings on both instruments, at the same moments and for the same length of time. That points to the weather, not either instrument, as the cause.

TotaLite’s wiper kept its optical window clear between those weather events, so readings resumed as soon as visibility returned.

What the engineers said

“For levelling with such requirements, TotaLite works just as well as an ATS in wet conditions.”

Why it matters for TBM and settlement monitoring

A fair test needs a shared viewpoint. Mounting both instruments on one column and reading the same targets removes setup differences from the comparison. Matching gaps then show the weather caused them, not the instrument.

Bad weather affects every optical instrument. The question is whether a system recovers quickly and records honestly. Here, both did, on the same timeline.

TotaLite works alongside a total station. It can add continuous coverage to an existing ATS network or cover sections where installing another ATS is not practical. Engineers keep the instruments they trust and gain more data to judge with.

Frequently asked questions

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