From the magazine, Microtunnelling, Tunnelling

Around the curve

Tunnel Guidance is currently supplying 13 Roadheader Guidance Systems and five Bolter Guidance Systems for works on the Airport Link tunnel in Brisbane. Further to this another two large diameter tunnel boring machine (TBM) VMT Guidance Systems will be utilised for the TBM-driven part of the Airport Link Project.

In the smaller diameter tunnelling market in Australia, VMT currently has five Guidance Systems on small diameter TBMs, constructing the Melbourne Main Sewer Replacement, the Northern Sewerage Project in Melbourne and the Adelaide Desalination Intake and Outfall tunnels.

In the curved pipe jacking section in Australia, its extended distance and curved pipe jacking guidance systems have been used on the Sugarloaf Pipeline in Victoria and the Nerang River Crossing on the Gold Coast in Queensland.

As part of the pumps and pipes project, the 140 m long crossing of the Nerang River was carried out with an AVN1500 TBM. The local conditions made it necessary to cross the river with an 800 m vertical curve. After finishing the drive, a 960 mm diameter steel pipe had to be pulled in. Due to the restricted flexibility of the steel pipes and the changing and challenging ground conditions the variations had to be minor. In close collaboration with the TBM operator and the site engineers, another curved drive was successfully finished.

The Sugarloaf Pipeline Project gives a good indication of how the field of curved pipe jacking has significantly progressed.

The TBM used to construct the 830 metres of tunnel at the project was an AVN1800TB with an extension kit to suit 2,000 mm inside diameter and 2,400 mm outer diameter pipes and a horizontal radius of 825 metres. The tunnel was necessary to avoid a sensitive and difficult to access section of the pipeline route to transfer water savings to Melbourne from irrigation modernisations in the Goulburn-Murray Irrigation District. The contractor, John Holland Group, began tunnelling on 1 April 2009, completing the tunnel in ten weeks.

Choosing the most appropriate navigation system for the project involved detailed knowledge of the alignment, geometry, diameter, technical specifications (machine type), geology, specifications in the tender documents and economic considerations. For long distance and curved drives the choice of navigation system is either a Laser Total Station-based System or a Gyro Navigation System.

The Gyro System is best for small diameter drives (ID 800-1,200 mm) or for drives with muck skips and bounded views inside the TBM. A Gyro-based Guidance System uses a self-levelling, north-seeking gyro compass for the determination of the horizontal position and azimuth of the TBM.

The calculations of this system are based on the principle of the dead reckoning, which means that it is assumed that the TBM will move exactly along the direction which is defined by its axis.

The difference between the direction along which the TBM is moving and the axis of the TBM is called the TBM drift. This drift will influence the calculations of the system. Check measurements for the calibration of the system must therefore be carried out every 20-40 metres in order to determine the TBM drift and reduce its influence to a minimum by pre-setting this value for further calculations.

The Laser Total Station System is a guidance system designed for the navigation of long distance and curved pipe jacking applications for pipe diameters above 1,200 mm. The main component of the system is a servo motorised Laser Total Station which is mounted inside the tunnel on a special bracket and moves along together with the pipeline. The actual position of the laser total station is continuously calculated with help of the known as-built position of the already installed pipes.

The calculation principle of this system is based on the theory of the invariance of the pipeline which means that all pipes must follow the same course, which is given through the drive of the TBM.

Even the TBM drift will be recognised and will not influence the accuracy of the system calculations. As the complete hardware of the SLS-Microtunnelling LT System is installed in the front part of the tunnel (first 100 m) refraction will not influence the measurements for the vertical TBM position.

The advantage in the use of the Laser Total Station Guidance System is the ability to determine continually the TBM’s current position independent of drift or refraction. The system will determine the position of the TBM plus the position of one pipe which is 30 m behind the TBM. This additional information is useful to analyse the pipe behaviour regarding vertical up floating effects.

Control measurements for system calibration are only required every 80-120 m with this system, which is a big advantage in comparison to the Gyro System. Reduced machine down time and high accuracy makes this guidance system extremely cost-effective.

The choice of which is the most appropriate system depends on the specific circumstances of the project.

For the Sugarloaf Project the decision to use the Laser Total Station system was made, as the number of frequent control survey measurements was estimated between eight to ten rather than 20 measurements by utilising a Gyro Navigation System. Each manual control survey requires, depending on the drive length, between three and six hours of machine down-time.

Considering the required machine down-time for each control measurement, the right choice of guidance system can have a big impact on the project duration and costs. The use of the SLS-Microtunnelling LT System enabled the site to perform the control measurements only once a week.

The success of the TBM breakthrough depends on the accuracy of the direction and co-ordinate transfer from the start shaft into the tunnel. The required tolerance at the end of the tunnel is usually defined in horizontal and vertical between 50 and 100 mm. To reach that accuracy, especially in horizontal for a drive length of 830 mm, the local survey company Surex recommended performing a special survey Gyro Measurement campaign, to control whether the surface survey network and the tunnel survey network had the same orientation. These kinds of measurements are usually only known for larger tunnel projects with tunnel lengths of several kilometres. However, pipe jacking projects can require this type of measurements because of the special ratio of small jacking shafts with a short survey reference and long tunnel sections.

The combination of a precise and reliable automatic navigation system and proper survey measurements will guarantee that from the start to the end the tolerances will be reached.

A new option of the Guidance System to provide remote visualisation of navigation and machine data from the internet was utilised. All data of the actual TBM position, the TBM history, the TBM position in Google Maps and available machine data from the TBM were displayed in the internet browser and frequently updated. The machine and navigation data can be displayed in web charts to analyse the jacking process. The use of that feature meant that the important information was readily available online for people on site and the management in the head office.

Similar to the Sugarloaf Project, the Nerang River Crossing Project decided to use the Laser Total Station System due to the less frequent control survey measurements required in comparison to the Gyro System. Only two control measurements were required, because of the relative short drive length.

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