Microtunneling has become an essential part of the underground construction industry globally, not least for the installation of new and or the replacement of water and wastewater...
Whilst the correct choice of microtunneling equipment for the prevailing ground conditions is of high importance, without the right guidance system to ensure that the machine hits the target shaft/position at the end of the drive the installation could be a waste of time and money. With wastewater systems it is also usually vitally important that the correct line and level is maintained throughout the drive, another vital requirement for accuracy from the guidance system.
Two recent projects in Zabbar, Malta and Shenzhen, China highlight how the microtunneling equipment and guidance system have to marry together well to achieve the desired outcome. Malta
Tunnel guidance equipment specialist VMT of Bruchsal, Germany recently supplied the guidance systems for a microtunneling project on Malta which was required as part of water treatment network scheme. The project was designated TP2816 Ghaxaq, Malta. The microtunneling works involved the installation of two pipelines each of 1,600 mm internal diameter.
One pipeline was 1031.76 m long whilst the other was a little longer at 1138.57 m. Both pipelines were designed as straight bores which meant that no specialist guidance was required to handle in-built curves. The drives were completed in the district of Zabbar West, between October 2015 and November 2016 by local contractor Rockcut Limited which specializes in tunnel, shaft and rock excavation as well as ground support and geological surveys. Zabbar is located in the South East of Malta and it is Malta's fourth largest town, with a population of around 17,100.
One of the major challenges for the project was that the ground comprised relatively soft rock and chalk which was quite porous at times and in places included huge cavities. This was at least in part why the project took some 13 months to complete. The difficult ground conditions meant that the Herrenknecht AVN 1600 microtunneler, fitted with a hardrock cutterhead, encountered some of these cavities which needed to be filled with concrete before tunneling could continue. This happened once on the first drive and at least twice on the second.
For its part VMT provided the guidance system and a support team to the project. The guidance system used was the Universal Navigation System (U.N.S.) modular navigation system developed by VMT's sister company Herrenknecht AG. VMT markets these systems for Herrenknecht in the international market. The U.N.S. system was specially designed for use on straight bore routes of up to 200 meters in length and utilizes a laser that is installed in the drive launch shaft. The laser line maintains a direct 'visual contact' with the guidance target unit at the rear end of the TBM that allows the machine operator to monitor horizontal and vertical deviations from the predetermined target values and make the necessary steering corrections to maintain the required drive course.
For longer drives such as those at Zabbar West, the navigation system can be extended using a reference module. To protect against refraction interference, an additional Hydraulic Water Levelling system (HWL) can be used to determine vertical deviation for longer, straight drives of up to 400 meters in length. For even longer drives a north-seeking gyrocompass can be installed in the TBM, which does not require direct visual eye contact to the reference module in the launch shaft and makes it possible to master even longer distances, gradients and curves.
The VMT support team visited site to ensure that particularly for the first drive the machine crew were fully up to speed on the functionality of the guidance system, especially the HWL. They were also useful to the project given that whilst no particular problems occurred with the guidance system, the ground circumstances meant that drift calculations for the machine were challenging as it went through porous chalk and then just a few meters later through the harder rock. Ultimately both drives were successfully completed to within the necessary target tolerances required by the client.
Shenzhen
The Futian sewage treatment plant project in Shenzhen is an important emission reduction, sewage cleaning project for Guangdong Provence, China. As part of the construction works a pipe jacked sewer pipeline some 713 meters long with an internal diameter of 3 meters was required to protect the National Mangrove Nature Reserve. Once constructed the whole pipeline will create an underground pipeline that will connect the Shenzhen Bay coastal recreation belt, Shenzhen Expo Park and Tanglang Mountain area enabling the sewage flows to be kept away from the Reserve. The underground pipeline will transport 600,000 cubic meters of sewage water per day, and provide effective protection for the wetland ecosystem in Shenzhen.
Whilst the concept of the plan is simple in reality the situation is somewhat different. The pipe jacked tunnel runs alongside Binhai Avenue, as close as 5 meters in places. To the south is the National Mangrove Nature Reserve which has around 1,000 visitors per day. The pipe jacked tunnel also runs under the South pedestrian walkway and the green areas along Binhai Avenue. Ground conditions along the route also varied considerably from silty conglomerate and medium weathered granite, through totally weathered granite and slightly weathered granite to plain fill and rock fill and conditions in between. The tunneling machine also passed close to a gulch, which only added to a very complex geological environment. So accuracy at the cutterhead was vital. As well as the complex geology the tunnel itself was design as two straight sections with one curve 388 meters in length with a radius of 888.88 meters.
Given that this was the first project in the region to attempt such a complex pipeline installation it was vitally important that the right guidance system for the tunneling machine was selected. Having investigated the options an SLS-LT system from VMT was selected based on its very successful history on complex pipe jacking projects. VMT's SLS-LT system is designed for the guidance of long distance and curved pipe jacking applications for pipe diameters above 1 m using a servo-motorized laser total station which is mounted inside the tunnel which moves 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 previously installed pipes. The SLS-LT System also recognizes TBM drift and with the system installed in the front part of the tunnel (inside the first 90 m) laser refraction does not influence the measurements.
Engineers from VMT SH (the local VMT distributor) were on site throughout the project to initially set up the system and monitor progress throughout the work. The local team was also trained to use the system effectively as the project took place. Despite access difficulties at the start of the drive in February 2016 which were overcome with team work and experience in such set-ups, the project holed successfully on target on 24 April 2016.