3D imaging for rock TBM headings 29 Sep 2015

A new 3D imaging and assessment system for hard rock tunnelling with TBMs is introduced by 3GSM GmbH of Austria. Known as ShapeMatriX^TBM, this innovation allows for a previously unavailable comprehensive, reproducible, and objective acquisition of geological conditions of the tunnel face.

Fig 1. 3D image recorded by a single camera installation through a TBM rotation

3D imaging systems for geological mapping at conventional open-face tunnelling are state of the art and 3GSM has provided such systems since 2005. For open face tunnelling projects, digital photographs are taken, processed to a 3D image, and used subsequently for geological mapping and the measurement of geometrical features, as well as for assessing the amount of overbreak.

But what can be done on a TBM project site where there is no direct view of the tunnel face?

The solution is to use small, rugged video cameras and install them in working or inspection openings of the cutterhead of a TBM during maintenance shifts. As the cutterhead turns the video camera records a full 360° sweep of the face. The imaging unit must work autonomously and is equipped with its own light units, laser pointers for providing scale information, and a smartphone to control the camera and log the orientation of the imaging unit using the camera’s built-in sensors.

After the rotation, the imaging unit is removed to at another opening on the cutterhead to record a different 360° sweep of the face. Alternatively, more than on camera can be installed at different locations to record several 360° rotations of the face simultaneously.

The video is then processed. Single images or frames are taken from the video resulting in a series of overlapping frames. These are comparable to images captured with continuous shooting.

Using the latest 3D processing strategies, combining elements of the Photogrammmetry, Computer Vision, and Structure from Motion programmes, allows for an automatic computation of a 3D image (Fig 1). The information from the laser pointers and smartphones enable spatial orientation.

Once the 3D image is ready, measurements are taken with the focus on two goals:

• * geometric assessment of the face quality and
• * mapping of geological features.

Geometric assessment of the face quality includes the quantifying of particular outbreak of the tunnel face, in terms of depth or volume (Fig 2), and assessment of the evenness of the face compared to an ideal condition. The assessments are processed in an objective manner and following procedures that can be reproduced.

Mapping of the geologic features is performed using existing software that is used as standard on conventional open face tunnelling sites (Fig 3).

The resulting 3D image provides a straightforward representation of the actual tunnel face.

Extensions of the system are already underway at 3GSM. These include the combining of several installations of the imaging unit and automatic recording of several circular rings simultaneously, and fixed installation of cameras into the cutterhead.

The system has been trialled on the two TBMs that are currently working on the Koralm high-speed railway tunnel project in Austria and proposals are developing for application on other TBM projects. Contractors, machine manufacturers and clients have expressed keen interest in the system, as it promises - for the first time - an objective documentation and characterisation of the tunnel face during TBM excavation.