One component of the plan emerged early as particularly challenging and troublesome. Work on one section of the TBI would require installation of 162 feet of 12’x9′ box culvert, which in turn would require removal and replacement of 120 feet of rail operated by BNSF, and in the area of track affected, the top of the sewer tunnel was just five feet beneath the top of rail. Understandably, BNSF was concerned about the effect of any work in the area. This is a very busy section of commercial railway–30 to 40 trains daily–and shutdowns are almost unheard of, and stoutly resisted by BNSF. Still, BNSF agreed to an unprecedented 30-hour suspension of rail activity.

To further complicate work in this section, high groundwater and difficult soils made subsidence an issue, and railway settling was a real concern even without construction work. To allay BNSF’s concerns, Barr devised a technically progressive subsidence monitoring system based on 250 track-mounted prisms and two Leica TM30 optical monitoring sensors. With this worked out, the 30-hour “Big Dig” was set to proceed.

Why should you read this? Monitoring is getting easier, cheaper… and more important. (full disclosure—I wrote this awesome article) The American Surveyor, January 2015