With major tunnelling projects it is as important to monitor rock faces for signs of possible geological deformation as it is to continuously verify the tunnel’s alignment, travel and dimensions. This is because tunnel excavation can cause geological disturbances.
Manifestations of this phenomenon might be settling of the tunnel crown, a convergence of the tunnel walls or distortion at the tunnel face. By early detection of such deformations, it is possible to initiate appropriate remedial actions and/or modify work patterns to effect a degree of stabilisation.
On Austrian Railway’s 12.3km Vienna Lainzer rail tunnel project, for example, a company called Consulting Messtechnik (Comet) is employing a motorised high precision surveying instrument to monitor excavation work and check for any deformation of the tunnel face.
This system, dubbed OBM, was developed and engineered by Comet and, unlike most other alternative systems, which use prism targets to obtain readings, it employs electromagnetic reflector-less distance measuring technology. The unit is designed to operate around the clock without maintenance, taking readings from some 20 different points in 12-minute cycles, to an accuracy of +/-1mm.
To ensure maximum efficiency and accuracy at all times, Comet has introduced a novel way of keeping the lenses of this equipment clean – an Air Control Industries (ACI) blower, supplied by the latter’s German distributor Carl von Gehlen.
Whilst other systems require surveyors to clean lenses manually at frequent intervals, the Comet system’s lenses are kept dust free at all times by the blower. According to Comet technical manager, Boris Schukoff, this has been achieved successfully thanks to the performance and reliability of the ACI blowers in a very difficult, dusty environment.
In addition to helping maintain accurate reading, the blower has a safety function, as it avoids the need to send technicians into potentially hazardous locations, such as tunnel ceilings, for cleaning duties.
The blower used for this application is ACI’s VLB5/3, a unit based upon an ‘inside-out motor’ with the rotor on the outside driving a forward bladed impeller. This blower, with its forward curved blades, is a compact, efficient unit, delivering high air volumes at high pressure - the precise characteristics required for this application.
Currently, Comet has two deformation measuring devices with ACI blowers in service on the Vienna Lainzer rail tunnel project, the excavation phase of which is expected to be completed by March 2009.