The advantages of modern surveying methods and systems are particularly evident in underground construction where the environment can be particularly troublesome for the surveyor, sometimes with a lot of work to do in a small space for which other functions are competing.

Most will be familiar with laser-based instruments and guidance systems used as references for accurate tunnel alignment and levels in guiding TBMs, roadheaders and blasthole drilling patterns, but it is how these can be set up accurately that is vital to the correct location of the required excavation. In addition, surveying can perform the following functions, to name a few:

Checking that surfaces are within design tolerances including over or under excavation, correct lining thickness, and non-interference with the operational ‘envelope’;

  • Support setting out tasks including rock bolts/anchors, arches, pipe roof, and formwork positioning;
  • Monitoring of any movement of excavation surfaces and the geotechnical instruments;
  • Accurate installation of any installed services, both temporary and permanent types;
  • Checking positions after any major incident, such as in an accident investigation procedures;
  • Generation of data records and reports for construction progress and final build for commissioning and handover.

In addition to increased accuracies, modern surveying methods can, if correctly managed, also achieve economies through ‘one-man’ surveying instead of two- or three-person teams, and the transfer of routine, usually small-scale, set-out and similar tasks to machine operators and skilled miners.

Design Set-Out
Over the years Amberg Technologies has developed a range of software packages for both survey office and in-tunnel use. The TMS suite covers functions such as setting out, comparison of design and actual profiles, using total stations within TMS Office. TMS Office also includes various levels of scanning functions using various proprietary scanning instruments. Finally ‘Amberg Tunnel’ covers geotechnical functions using total stations or special geotechnical scanning instruments.

Amberg Technologies recently entered into co-operation with Poltinger Precision System (PPS) for TBM guidance set up. The project can be defined within the TMS Office package, followed by direct export of the axis and theoretical sections to the PPS system for subsequent TBM reference and guidance. More features from this collaboration are promised for later.

Initial set-up for major alignment and profile measurements from control stations still requires professional surveyors, although procedures are now usually much less laborious than in traditional surveying methods, and may require less manpower. Subsequently, setting out tasks may be transferred to other skilled personnel if the procedures can be easily understood and/or are repetitive.

The complex excavations for Lot A2 of the Linthal development of the Linth-Limmern pumped storage power plant in Switzerland includes three rock caverns. Contractor Arge Kraftwerk Limmern, has been using the Amberg survey software packages TMS Profile, TMS Setout and TMS Tunnelscan. In this work TMS Setout was also employed for the rapid and efficient setting out of rockbolt positions as well as verifying the cavern profiles with TMS Profile.

The contractor’s surveyor, Bertrand Jeanguenat says, "Using TMS Solution we can evaluate the data direct at the tachymeter. This puts us in a position where we are quickly able to react to profile discrepancies, thereby avoiding construction delays and saving costs. The TMS Setout surveying application also simplifies the setting out of rockbolts."

Amberg’s enhanced Setout Plus software is designed to provide a ‘virtual’ surveyor to the tunnelling crew to carry out setout duties once the tunnel surveyor has set up the system in design mode. Subsequent setting out by the tunnelling crew uses the total station as a motorised tunnel laser, without calling on the project surveyor. In fact on-site surveyor requirements can be reduced by as much as two thirds. The total station in the production mode the total station offers predefined automated functions. Meanwhile the instrument is under continuous surveillance of its position, and can be operated by remote control. It also has log files for subsequent quality control checks.

Scanning
One the major advances allowed by the use of laser distance measurement combined with powerful software has been the possibility for mapping a wide range for surfaces for location. Multiple short-interval sightings produce a ‘picture’ of the object being scanned, such as the surface of a tunnel perimeter. Using any of the major scanning instruments Amberg’s TMS Tunnelscan software data can produce a 3D view, which can be used for comparison of measured scan data and the theoretical tunnel model. In practical terms this can translate to the planned and actual excavation, and also the inner surface of lining support and any reinforcement.

The latest instrument partner of Amberg TMS Tunnelscan is the Faro Laser Scanner Focus 3D. Under the agreement Faro is providing TMS Tunnelscan packages with the Laser Scanner Focus 3D for tunnel construction. Amberg can also offer TMS Tunnelscan, (together with the APM instrument position method as below) with the Leica HDS7000 and Z+F Imager 5010 instruments as well as its own Amberg Profiler 5033. These three instruments have very similar specifications whereas the Faro Focus 3D is lighter, with a longer lasting power supply, although smaller operational ranges.

Using Amberg’s own Profiler 5003, scanning can target up to 500,000 points per second; its speed giving a major advantage in not obstructing other tunnel works. Each setup typically needs less than two minutes for a full scan of up to 12.5 million points. In conducting an ‘as built’ survey TMS Tunnelscan can be used with the Amberg Positioning Method (APM) referencing back with a total station or TMS motorised laser. This can achieve a performance of 130m per hour in one-man operation.

APM is a multi-point alignment method that can be related back to the high-precision survey methods developed for the installation accuracy requirements at the turn of the century for linear colliders. This geodetic survey solution to avoid problems with variable light-beam refraction was developed by the Deutsches Elektronen-Synchroton DESY at Hamburg with the Stanford Linear Accelerator Center (US), the Bauhaus-University Weimar (Germany) and the Technische University Dresden (Germany). The accuracy requirement for DESY installation was 0.5mm transverse and 0.2mm vertical over a range of 600m. With the development and availability of commercial scanning instruments, applications have now become widespread for surface applications such as coastal profiles as well as underground applications in tunnels and mining. Multi-disciplined surveyor company Six-West of Belfast, Northern Ireland, recently undertook a detailed topographic survey of a gold mine at Curraghinalt at the request of Canadian exploration company Dalradian Resources. Six-West suggested a full, high-definition laser scan of the mine to produce the highest possible level of detailed topographic information. This would give a 3D model allowing shareholders and potential investors the opportunity to ‘virtually’ walk through the mine from their desktops.

Six-West director Paul McCabe explained that the surveyor selected the Leica C10 HD scanner (now superceded by the Scan Station P20) over the Trimble Faro due to it being more suitable for wet sections of the mine. These two scanners are the most popular today. McCabe explains, "These instruments measure a full 360-degree array at a rate of up to one million points per second. The resulting point cloud is a complete 3D model of everything within range of up to 100m of the instrument to a user-defined resolution of anything from 2mm by 2mm upwards."

Describing the procedure McCabe says, "The C10 was set up at intervals of about 50m for the full length of the mine drives, totalling about 20 set-ups. Each scan took around ten minutes and was related to survey control stations that Six-West had previously installed, and related to Irish Grid and Ordnance Survey Datum using a Trimble S6 total station. Leica Geosystems HDS Cyclone software was used to reference and combine all the scan data."

The Cyclone software, which offers a wide of set of process options for 3D scanning from point cloud observations, in this case providing final output in AutoCAD dxf, ASCII xyz and as a ‘fly through’ video in avi format.

Another useful facility of scanning is available through the intensity of the laser reflection. Explains Paul McCabe, "The lighter the colour of the object scanned, the better the return. As a result, the scan data also picked up important painted up references on the sidewalls such as changes and markers indicating particular geological intersections." McCabe also points out that scanning using a remote-control, unmanned vehicle has safety advantages. In mining it can provide high-resolution surveying of open stopes or, perhaps in tunnelling, particular hazardous areas of unstable ground such as fall cavities.

Within a wide range of instruments for all types of surveying duties Leica offers two series of scanning instruments. The ScanStation P20, mentioned above, is the top of the range ultra-high-speed scanner that gives a high level of detail at long ranges (up to 120m). Other ScanStation models are the C10 and entry-level C5.

The other Leica series is the High-Definition-Surveying (HDS) instruments using phase-based technology to achieve high-speed, detailed results. Phase based instruments use multiple laser transmissions, the return signals of which are differentiated by methods such as wave-length/colour filters. The Leica HDS6200 and HDS7000 are particularly recommended for tunnel work.

The specialist Amberg Geotechnics software produces a 3D view of the tunnel model input as the basis for subsequent collection of geotechnical data by limited scanning as below.

Geotechnics
In addition to setting out and alignment checks, surveying instruments can now be used as a means of collecting data for geotechnical monitoring. Recently introduced software from Amberg Geotechnics, for example, allows efficient and cost-effective observation and evaluation of deformations of the excavation or support, and of settlements. Of course it can also be used for recording installation and maintenance of instrumentation such as extensometers. The Amberg system can accept data from a wide range of instrument outputs from different suppliers, with automatic point allocation without the need for separate inputs. The data can be used to produce tunnel images in 3D over several construction stages.

Geotechnical reports are produced with defined filters to give fast evaluation to identify hazardous areas. One advantage of this and other scanning procedures for project supervisors is the possible elimination of numerous spreadsheets from which useful data has to be extracted or highlighted.

Amberg recently announced a new capability for handling images produced by the Leica TPS within its office software. This integrates a facility for photographic-type image storage on the Amberg database for recovery when required. Oliver Schneider says, "Pictures in the tunnelling world are sometimes useful for geological mapping and for evidence purposes." The facility can therefore be used to accurately locate geological features or rock anchor installations (perhaps before being covered up by lining support) or to show the layout as well as location of items following a hazardous incident.

Use of motorised total stations on the surface is now common to check on any movements caused by settlement or geotechnical anomalies at pre-set stations. It would also be possible to scan critical structures for any displacement. Many organisations such as VMT in concert with ITC Engineering (IRIS system) are striving to perfect the integration of data so collected with that generated by tunnelling operations, chiefly of TBM operational feedback such as excavation rates, or grouting operations. Data can be compared on both time and location basis so that project engineers can have a better understanding of the ground and surface reactions to tunnelling operations, be warned of any hazardous situations by pre-set alarm limits, and be able to better suggest action.