GEO-Instruments have collaborated with WSP consultants to design and install a comprehensive bridge monitoring scheme using an array of Optical Displacement Sensors (ODS).
The monitoring scheme required the measurement of the vertical displacement of the bridge at various points as well as the horizontal displacement across the jack arches that comprise the structure of the underside of the bridge. ODS sensors were installed in pairs at multiple positions across three distinct spans of the bridge with the key focus of the monitoring on the outermost jack arches and edge girders.
Manufactured by Senceive, the ODS sensors accurately measure distance to their target and tilt values across three axes. Evaluating the changes in these values over time, it’s possible to build a detailed understanding of the movements and deformation of the structure the ODS is mounted on.
Where necessary, targets for the ODSs were installed to aid the installation process and provide more accurate distance measurements. Several of the vertical sensors could not be aimed directly down at the tracks where it was not possible to install targets. In these cases, the sensors were aimed diagonally down and vertical displacement is calculated using the tilt angle measured by the ODS.
As the sensors measure distance optically, they are ideal in many situations where other techniques are not. However, for this reason they are vulnerable to obstructions. On this project the sensors installed vertically were particularly susceptible to obstruction by vegetation growing adjacent to the bridge and tracks. Careful clearance of this vegetation was a necessary precaution to maintain line-of-sight.
As the monitored structure is a road bridge over active railway lines, installation works were undertaken on nightshifts during Network Rail possessions. This time constraint significantly complicated the installation as access to the underside of the bridge also required the use of a MEWP, meaning an extended mobilisation every shift.
To eliminate potential risk of equipment falling onto the tracks, the sensors were additionally secured to the structure using lanyards as a backup to the primary fixings.
Tilt and displacement data are transmitted from the ODS nodes to a local gateway then sent to Senceive’s cloud platform where any necessary calculations are applied. This is viewed by the client in GEO-Instruments’ online portal QuickView. Project displays, triggers, alarm systems and reporting features are set up to client specifications.
Read more about our road monitoring projects.