Sensor Technologies



Measure Tunnel Wall Deflections:

The Challenge

Concrete, steel, or rock lined tunnels are subject to deflections from the pressure exerted by the overlying strata. In areas prone to seismic events, geological instability, or significant pore pressure, small movements in the tunnel wall may be a precursor to more significant structural changes – in the worst case – possibly tunnel collapse. A conventional approach used to measure and track these changes involves the installation of a series of linear displacement transducers to determine the shape of the tunnel. This technique is cumbersome to implement, partly due to the physical size of the transducers.

Is there a sensor system that can measure structural integrity in tunnels? Can problems such as concrete swelling or wall strain be tracked? Can the system be applied to existing structures? Can the sensors survive a wide range of environmental conditions? Is the system safe to use?

The Solution

FOX-TEK ’s FT fiber optic sensors and monitors are part of an engineered system with these capabilities. The sensors are light weight, flexible, are easily bonded to concrete, steel, rock, and may other materials. For very large diameter tunnels, sensors are available with gage lengths up to 100 m. The sensors are passive, non-electrical devices and are immune to electromagnetic interference (EMI). Although the sensors are quite rugged by themselves, additional protection from environmental is easily achieved with hermetic coatings.

FT sensors measure structural displacement and are readily compensated for thermal effects. With one or more FT sensors bonded around the tunnel circumference, any movement of the wall is detected and summed along the gage length. When a tunnel wall is under stress, even small strains are measurable. If the concrete in a tunnel swells due to corrosion of the steel reinforcement, the array of sensors identifies a change from the baseline response. Periodic site visits or continuous monitoring of the FT sensor strain signals are convenient ways to assess the structural health of the structure in comparison to the initial state.

Summary of Benefits

  • Early detection of fracture growth or internal corrosion effects
  • Timely repairs to minimize further/structural damage
  • Track changes from seismic events
  Cost Savings
  • Maintenance/repair schedules based on actual integrity
  • Reduced risk of premature structural failure
  • Automated identification of problem areas
  • Reduced risk of structural failure
  • Lower risk to maintenance personnel
  • Increased assurance of public safety