Ground movement tolerance limits for underpinning adjacent to active LRT tunnel

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francesca.struct•2mo

Ground movement tolerance limits for underpinning adjacent to active LRT tunnel

We’re underpinning a 5-storey commercial building adjacent to an operational LRT tunnel. The tunnel is shallow — crown sits at 6.2m below grade — and our excavation depth for strip footings goes to 3.8m. The geotech model predicts settlement < 12mm and minor horizontal displacement, but the transit authority flagged it as high risk. They’re asking for 3D monitoring, plus settlement < 5mm and lateral < 3mm, which feels overly conservative given the distance and soil type (stiff glacial till, 150 kPa ULS). Have you successfully negotiated monitoring-based thresholds instead of fixed ones?

Comments (10)

imani.amlo2 months ago

If this affects service reliability perceptions, you’re not just in a technical space — you're in political space. We hired a third-party reviewer trusted by the transit agency to validate our model. That helped unlock approvals faster than any analysis.

emma.builds2 months ago

We added a contractual clause that tied monitoring thresholds to pre-approved remediation steps. It bought us breathing room when movement briefly passed limits — no shutdowns, just trigger our response.

joe.bridges2 months ago

We've got an LRT viaduct project where differential vertical tolerance of 8mm between pile groups was deemed fine — and yet here you are with stricter limits for adjacent foundations. It’s inconsistent. Push for a review of their internal risk criteria.

mkale.codes2 months ago

I built a 3D visualization system tied to inclinometer + extensometer sensors on a sewer dig near light rail. Translating the data into displacement cones helped the city review board approve variance in 3 days — visual storytelling matters.

zoe.assetlife2 months ago

From a long-term O&M angle: if you breach their settlement protocol and a warranty event occurs later, they’ll blame you even if unrelated. Documenting your thresholds and real-time compliance gives you protection for years after handover.

neil.mateng2 months ago

Model limitations aside, your tunnel-soil interaction might benefit from 2D or 3D FEA, depending on structure spacing. We did a Plaxis run with stage-wise excavation modeling and it helped our case tremendously for LRT next to an excavation shaft.

chandra.water2 months ago

I'd focus less on thresholds and more on velocity — if movement trends are within stable rates, a few mm above limit won't matter. On our site, we set velocity-based alarm bands and avoided false triggers from minor surface crust shifts.

civ_pioneer922 months ago

We used optical fiber settlement gauges in lieu of prisms near a tunnel junction. Lower profile and more granular data. Combined with weekly InSAR snapshots, we proved negligible movement and got them to loosen thresholds.

terra.solid2 months ago

You might want to offer a compensation grouting backup plan. Even if it’s not triggered, authorities get more comfortable when mitigation tools are specified upfront. That helped us on a courthouse retrofit next to a twin-bore alignment.

deeptank.pm2 months ago

Transit agencies over-conservatism isn’t new. We negotiated 10mm vertical, 5mm horizontal for a subway cut in Toronto’s Annex. Best path forward is a staged instrumentation plan, real-time alerts, and a signed-off response threshold. They’re protecting against press coverage, not collapse.

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