Constant face surveillance
Constant face surveillance Nov 2009
Boris Schukoff and Helge Grafinger, Surveying Engineers, COMET Consulting Metrology
Failures of tunnels are initiated often at the face. Yet the face is the most difficult to monitor. For open-face methods, particularly NATM, SEM or SCL excavations in softer ground, detecting movement of the face is especially vital, but has been an illusive capability until recently. Introduction of a system that continually monitors the face without interrupting progress or needing reflectors has proved its mettle on a highway tunnel projects near Vienna in Austria.
Monitoring ground movement at the tunnel crown and side-walls of conventional or open-face excavation methods is a standard technique in modern tunnelling, however the behaviour of the tunnel face when using these methods has been measured only in special cases so far. A new system called OBM (the German acronym for tunnel face monitoring) uses a fully automated, continuous and reflectorless measuring process to monitor the face. It does not involve the need for additional staff and there is no interference with the construction works. The system provides important additional information for geotechnical engineers.
The common method of measuring rock deformation underground is to install pins in the tunnel lining, equipped with optical targets. The position of those targets is measured with surveying instruments at regular intervals, normally on a daily basis. Modern total station systems, have an accuracy of the point positions of as good as +/-1mm, both in relation to neighbouring points and in relation to the precedent measurement.
In conventional tunnelling, while the newly excavated part of the tunnel is sprayed with shotcrete and supported with steel arches and rock anchors, the face is frequently left untouched and not affected by construction works.
Pic 1

An OBM station in the tunnel crown with an ACI dust blower alongside

The tunnel face however is also exposed to ground pressure which is created by the loosened rock lying ahead of, and above, the tunnel. Whereas the tunnel itself normally has the advantage of a good load distribution thanks to the vault effect, the vertical tunnel face is a relatively weak and sensitive zone. In the worst case, with a so-called face failure, the face wall will collapse, creating extremely unstable conditions in front of and above the tunnel.
Normally, one round of excavation and support takes some hours before the tunnel face is advanced again. Due to this relatively short time-span, the face has not been monitored, the installation of pins and reflectors, plus the manual surveying process being too expensive.
In 2006, the Austrian company COMET, now part of Rhomberg Bhantechnik GmbH, and working in cooperation with iC group of consultants and on behalf of the Austrian Federal Railway Company, developed a new monitoring system to measure tunnel face deformation. The main requirements were for an automatic and continuous monitoring system that would need no stabilised points on the face.
The OBM system consists mainly of an electro-optical motorised total station that is connected to a computer. Data transmission between the instrument and the computer is established via a cable that serves also as the power supply for the total station. The data transfer from the tunnel to the surface is by GSM radio transmission.
Pic 2

Displacement diagram (vertical axis: millimetre, horizontal axis: hours)

The most important feature of the OBM is that the system does not rely on reflectors or any other installed points or targets, but measures tunnel face in reflectorless EDM mode. As a matter of course, the only component of the displacement vector that can be detected by this kind of measurement is the one parallel to the tunnel axis - i.e. the movement against the heading direction.
The total station is usually mounted on a bracket in the tunnel crown in order to minimise the risk of being damaged by machines. At the same time, the system is set in a position from where most of the tunnel face is covered despite being generally obstructed by excavation equipment. To keep the instrument's lens dust-free, a fan is used which blows constantly towards the total station.
The computer is housed inside a waterproof and dustproof steel cabinet and is operated via a touch screen. The screen serves mainly for displaying the monitoring results in real-time, as the system runs operator-free except for maintenance works. The OBM software is part of the Axis3D program, an Austrian surveying software product. Prior to installation, the system parameters are defined in the office. They comprise of the general data (the tunnel axis, cross sections, communication parameters etc) as well as information specific for the OBM; the number and position of points to be monitored on the tunnel face, the typical round length and various values required for the filtering of the measurement data.
A setup file is then fed into the tunnel computer that guides the monitoring process. As the monitored points do not exist physically on the tunnel face, the laser beam's position of incidence on the face has to be checked at every measurement. This is done by verifying the point's position in relation to the tunnel axis (vertical and horizontal offset). If necessary the position is corrected in an iterative process. (As a point of interest this specific function has been accepted for patent registration.)
After each single measurement, a filter program checks if the result is either a valid consecutive reading, a new zero-reading or if the measured point lies on an obstacle in front of the tunnel face. Various user-defined parameters make sure that the results are smoothed out which allows the system to detect automatically if the difference between two measurements is due to natural reasons (rock displacement) or to construction activities (excavation in progress, spraying of shotcrete etc).
In addition, by defining deformation thresholds, the responsible geotechnical engineer on-site can be alerted via a text message in case the displacement or the rate of displacement exceeds a certain trigger value.
Pic 3

Definition of monitoring points on the tunnel face

The monitoring is performed within an endless loop where a number of predefined points are measured one after the other. Once started, the monitoring process only stops when aborted by an authorised user. The OBM takes approximately five minutes for a set of 10 points, with four repetitive measurements per point. Thus, there will be, theoretically speaking, 12 measurements for each point ever one hour. In practice, many measurements turn out to be invalid because the four results per point do not fit together or because the view to the tunnel face is blocked by a machine or a worker. All actions are logged in a file, and the monitoring data are stored in a database.
The distance to each single point is measured several times in a row to increase the accuracy. During field tests on a tunnelling job site and over a longer Christmas shutdown period, the inner accuracy of the distance measurements could be shown to be noticeably better than +/-1mm. Under normal tunnel conditions (high temperature gradients, dust, humidity, staff and machine passing through the laser beam), the accuracy can be estimated at 2-4mm depending on the distance. Another important influence is the quality of the monitored surface. While reflectorless distance measurement is limited to a range of approximately 50m when the surface is dark, moist and rough-textured, such as fresh shotcrete, it can work properly up to a distance of 90m and more on a grey and dry surface, like dried shotcrete. Modern total stations feature reflectorless distance measurement under optimal conditions of up to 500m and more.
Optionally, the station's position and orientation can be checked by means of control points in the rear part of the tunnel after each monitoring cycle. If the instrument itself is set up in an unstable area, it can be necessary to correct those two values from time to time.
Pic 4

OBM view towards the tunnel face

If the maximum range of the system is exceeded, the OBM has to be set up on a new position closer to the tunnel face. This is actually the only time when the interference of a surveyor is required. At every set up of the instrument, it is positioned within the local site network in order to establish the connection between the measurements and the project axis. Usually this is done with a free station, using the existing monitoring points on the tunnel lining as reference points. The absolute accuracy of the station's coordinates is not too important, since all results always refer to the initial reading and thus are only relative.
From November 2006, the OBM system was used at the Wienerwald Tunnel, close to Vienna, Austria, where it was refined and tested. Three units were then deployed at the Lainzer Tunnel in Vienna, part of the new high-speed rail link between Vienna and St Poelten in Lower Austria. The Lainzer Tunnel is constructed predominantly in an urban area, beneath buildings and existing railway lines, with an overburden of less than 20m in places, and through soils comprising silt, clay and sand. OBM has been part of an extensive monitoring program in the Lainzer Tunnel since 2007.This includes optical monitoring, precise levelling, inclinometer and extensometer measurements and geophoning. The thresholds for the face deformation were 5mm and 8mm respectively.
Apart from the system's real-time functions of supervision and alarming, all measurement data were transferred at regular intervals via GSM to a PC in the site's computer network. Here, the data was available for all persons involved with monitoring and was available for further analysis. As a relatively new system, OBM is constantly being refined and adapted to the conditions and requirements on site. Future options include a high-capacity data link between the tunnel PC and the office PC, supplying the geotechnical staff with monitoring data in real-time.
Technical specification
Total station Leica Geosystems TCRA 1205 R300
Absolute accuracy +/-2mm at 40m, +/-4mm at 90m
Inner accuracy +/-0.5mm, tested under clear conditions
Connection PC-TPS RS232
Monitoring rate 10-12pts/5min
Power supply 220-230V DC
Data volume Approximately 1MB/day
Data transfer to surface GSM
Furthermore, the total station could be used to monitor not only the tunnel face but also the conventional deformation targets in the tunnel lining in its vicinity, as long as those are equipped with prism targets that can be measured automatically. This would provide a constant recording of data for points in critical areas which are nowadays measured manually one to three times daily. Excavation of the Wienerwald and Lainzer Tunnels was completed in Spring 2008 and summer 2009 respectively.

The data collected, and experience of working with the OBM system is now being reviewed and analysed by a team of researchers at the University of Gratz.
As part of the normal geotechnical monitoring of the excavation process, the OBM provides a valuable additional tool for the process of risk management. Movement in the face will always precede movement in the crown and sidewalls and being aware of face movement is vital in managing successfully open face and sequential excavation methods.
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