TBM reaches Brandenburg Gate3 November 2015
GERMANY – The TBM mining the U5 extension in Berlin has reached its destination. The 6.67m Herrenknecht Mixshield TBM was launched from the Max Engels Forum in June 2013 and reached the Brandenburg Gate in October. The 1,620m-long parallel drives were constructed with overburden ranging from 5 to 17m, and passed underneath the River Spree, Spree Canal, and Schlossplatz.
Ground was heterogeneous soil, comprising fine sands, stones and blocks of granite, boulder clay and high-pressure groundwater. The contractor was Implenia.
To safely pass under the waterways, the contractor originally planned elaborate ballasting work along the banks of the river and canal, using 400mm-thick steel plates. A Herrenknecht spokesman explained the changes in the final works: "[A change in the original plan] has led to a significant improvement in the ability to use Mixshields for tunnel sections with small overburdens: in some stages the machine was driven using a higher density bentonite suspension (HDSM = High Density Support Medium). This meant that in comparison to conventional suspensions, with considerably higher transferable support pressure the penetration depth into the soil ahead of the machine was significantly reduced.
"Thanks to the HDSM used (up to 1.4t/m3), [the machine] was able to safely cross under the Spree... large sandbags lowered into the water near the banks provided sufficient weight. Shipping remained unimpaired. The modification of the machine for the HDSM drive was already taken into consideration during design, factory assembly and jobsite assembly. During the advance no further alterations were necessary."