With the advent of mechanised tunnelling at the beginning of the 19th century, faster rates of boring with increased safety for the workers became feasible. It soon became evident that an effective method of determining the position and orientation of the machine should be established, since the machine operator cannot see where he is going. He is always dependent upon receiving information about the position and orientation of the machine from external measurements.
The final accuracy of any finished tunnel is heavily dependent upon three critical elements:
This article covers the final item.
While it was, and still is, possible to survey the precise position of the machine with conventional surveying techniques, the operator would need to interpolate the position of the machine between surveys. The provision of a continuous intermediate reference between surveys enables the operator to avoid this interpolation. Ideally, a real-time indication of where the machine is located, in relation to where it should be, needs to be provided. This will enable immediate feedback about the consequences of control actions in keeping the machine as close as practical to the desired tunnel alignment.
String lines, light sources, mirrors and various other techniques have all been used in the past to give a intermediate reference from which the movement of the machine can be measured. However, the creation of the first laser on 16th May 1960, gave the tunnelling industry the simple device that was to become the standard intermediate reference for the next four decades.
Setting up the laser
The determination of the coordinates of the reference station as well as the azimuth and elevation of the reference beam is normally achieved by conventional surveying methods. As a check, and to ensure that the azimuth or elevation of the reference has not changed during the rigors of tunnel construction, one or more check gates are typically used.
Targets
Determination of position and orientation of the machine, with respect to the laser reference, was initially by the interpretation of the point of impact of the laser beam on each of two transparent target boards, mounted on the tunnelling machine. Significant spacing between the target boards was necessary to give the required accuracy and various methods to effectively reduce this longitudinal spacing by the use of mirrors have also been used.
However, as machines became more crowded in terms of the equipment installed on both the machine and in backup gear, space became a premium and the need for a compact active target became apparent. The first of these was created in 1975 by ZED in the UK. (Reported in Tunnels and Tunnelling, July 1975)
This active target enabled the precise centre of the laser beam to be measured using photo-electronic sensors as well as determining the angle of incidence of the point of impact of laser beam on the target. The addition of twin inclinometer transducers enables each of the X and Y positions and the Roll, Pitch and Yaw angles of the machine to be displayed to the machine operator.
Alignment
The advent of computers in the 1970’s enabled the Desired Tunnel Alignment (DTA) to be stored in the main processing unit of the guidance system. This stored alignment, usually in the form of a point coordinate table of the tunnel centreline, enabled a comparison to be made of where the machine was in relation to where it should be.
Nowadays the operator is presented with comprehensive graphical displays showing the position and orientation of the machine with respect to the desired alignment. The proposed future alignment of the tunnel ahead of the machine can also be depicted to the machine operator to enable him to anticipate the necessary steering adjustments.
Laser reference developments
Evolution of guidance systems over the past 25 years had typically been based around this original concept of laser and active target. As the establishment of the azimuth and elevation of the laser reference has proved a somewhat time consuming exercise, various ways have been developed to ease this task.
Initially, the laser was projected through the telescope of a manual theodolite. This enabled the azimuth and elevation of the laser reference to be read from the theodolite rather than calculated. With the advent of Electronic theodolites these values could be continuously sent to the guidance system. The introduction of Electronic Distance Measurement devises (EDM’s) permitted the final variable (the distance between the laser reference and a retro reflective prism located at the active target unit) to be automatically entered into the guidance system.
Recently, with the addition of servo motors to theodolites it is now possible to create interactive control between the projector of the laser beam and the point of impact on the active target unit, thus ensuring that the laser beam is kept on the target at all times.
The development of servo controlled theodolites and their ability to “seek” retro-reflective prisms, now enables the laser station to search for retro-reflective prisms and to coordinate its own position. The most recent range of such theodolites, manufactured by Leica, also incorporates a purpose designed tunnel laser whilst still permitting full mobility of the theodolite. Software control of this type of instrument by the guidance system also permits the laser beam to be switched off during its coordination routine as a safety measure to protect the work force from any unexpected exposure. As with all laser products (both visible and infrared), strict safety regulations are applicable. The reference laser and EDM lasers used in guidance systems all fall within, or are of a lower classification than, the latest regulations specified for use in tunnel type environments (Class 3R laser product in accordance with IEC825-1:1993 + Am1:1997 + Am2:2000 : “Radiation safety of laser products” or Class IIIa laser product in accordance with FDA 21CFR Ch.I §1040 : 1988 US Department of Health and Human Service, Code of Federal Regulations).
Active target developments
Since the initial development of the active target several manufacturers have developed a range of active target units based around photo electronic sensors and video camera techniques. All typically incorporate inclinometers for the measurement of roll and pitch.
Although the mechanics and methods of determining the X & Y values for the point of impact of the laser and its angle of incidence to the target vary, the end results are similar. Typical accuracies being in the order of ±1mm for positional and ±1mm/m for angular measurements.
Any guidance system, whether manual or state of the art high technology, must continually detect and compensate for any movement of its primary reference. Movement beyond a pre-set allowable amount must generate a warning or error message to the user.
Optical effects such as refraction can also interfere with the results. Refraction will have an effect on all types of optical measurement. Careful selection of target and reference locations can minimise, but not completely eliminate, these influences. Some guidance system technologies, such as Gyro-based systems are immune to the effects of refraction, but have other more serious methodical errors or adverse cost.
Shuttered prism system
To reduce the cost of guidance systems, some manufacturers have replaced the active target with retro-reflective prisms. This type of system uses a standard motorised theodolite, incorporating the automatic target recognition (ATR) feature, but does not need the addition of a laser thereby further reducing costs. The ATR feature enables the theodolite to seek and measure the position of one or more retro-reflective prisms.
When two or more prisms are used, they can be measured on regular basis, and by determining their position the system is able to determine the position and orientation of the machine. If more prisms are used or are used in conjunction with inclinometers the precision of the system is increased due to cross checking capabilities.
This, however, also has the effect of increasing the time needed for each measurement cycle. The time taken to locate and measure to each prism means that the system is carrying out frequent, although not continuous, measurements and calculations of the machines’ position that are based on historical (albeit sufficiently recent) not real time measurements.
To achieve the necessary accuracy the spacing between the prisms should be, according to some manufacturers, at least 2m longitudinally and at least 300mm laterally, although the use of a top of the range (i.e. ±1.5″ ±0.5mgon) theodolite would be needed to achieve the necessary accuracy. Alternatively, increasing the spacing between prisms, if space on the machine and backup gear allows, would permit the use of a standard (±3.0″ ±1.0mgon) theodolite.
To avoid incorrect prism identification or sighting to erroneous reflective surfaces, shuttered prisms are used. These are linked to the computer timing control for the theodolites’ measurement cycle, so that only the selected prism has its shutter open for the ATR to seek and measure.
Irrespective of the type of guidance system to be used on a tunnelling machine, a suitable survey window must be available to enable periodic precise measurement of the machines’ position to take place. This sight path runs from the survey station, through part or all of the backup equipment to a reference target on the forward section of the tunnelling machine. Obviously, this path must be unobstructed for the measurements to take place. Since this path must run reasonably parallel with the machine axis and its backup gear the size of the survey window tends to be limited and its position is not always in the most useable location for measurement purposes.
Infra red active target
While the use of ATR and prisms has many merits the spacing needed severely limits its use on many machines. ZED Tunnel Guidance have taken the concept of using a theodolite with automatic target recognition a step further, by incorporating a special retro-reflective prism into its new Infra Red Active Target.
This special prism, in association with electro-optical devices within the target, enables the angle of incidence on the prism of the infra red beam from the EDM in the theodolite to be determined. This substantially reduces the space needed on the tunnelling machine for the target. The effective working range of this type of target is also substantially increased over that of a laser based one in similar atmospheric conditions
Ram differential measurement system
While primarily a machine control and optimisation system, the method utilised by the French company CAP for position determination involves the short-term dead reckoning of the machines position by precisely calculating the differential lengths of the ram extensions. Extremely accurate transducers are necessary in order to give the required accuracy, and the machine must be resurveyed on a regular basis to give an absolute position with which to update system, and eliminate cumulative, errors. Any of the previously mentioned guidance systems could be used in conjunction with this system.
There are now several hundred guidance systems in the field and both guidance system manufacturers and customers of existing systems would like to include features from competitive systems. It has therefore become more common place for collaboration between suppliers to take place, to enable previously used equipment to be upgraded to the latest specifications.
Gyro based systems
Several manufacturers, mainly in Japan but with one or two European manufacturers also now entering the market, have developed guidance systems based on a gyroscope mounted on the TBM as the primary sensor of machine position and orientation. These North seeking gyros intrinsically measure the direction of its fixed axis against NORTH. Position indications are based on a sequence of accurate dead reckoning values.
However there is no intrinsic tie-in to the survey in the tunnel and as one of the fundamental properties of all gyroscopes is drift, it is necessary to survey the machine and update the systems values at least each shift and more frequently at the advance rates that are frequently experienced.
The level of the machine is normally determined using a sophisticated water level system that automatically compensates for atmospheric pressure variations and density changes in the fluid in the system. The advantage of this type of system is the ability to negotiate very tight curves, typical of many Japanese projects.
The temporary reference is not subjected to the refraction effects of laser based systems and there is no necessity to have a specific laser window throughout the back-up and into the machine. However, there will still be a requirement for an adequate survey window for the frequent machine surveys that this type of system needs.
