City Tunnel Leipzig, Germany

Leipzig Station front view The City Tunnel of Leipzig is the last and central component of the railway system restructuring process in Greater Leipzig. Not only can the suburban train system be restructured after the tunnel has been put into operation but also the traffic connections between the region and the City will be improved and give major impetus to the development of the City’s traffic system.

One section of the tunnel is scheduled to go under the existing Railway Station.

Leipzig Station view inside The main Leipzig Railway Station is the biggest terminus and most significant passenger station in Europe. There are 26 platforms and 5 external platforms. The station has a shopping area on three floors with a total of 30,000 m2 for strolling, shopping and eating.

Under the effected area of the shopping centre there are several piles which may be undermined by the tunnel construction and a method of improving their capacity and the effect of the potential processes were to be evaluated on some similar dummy test piles.

Summary
The most challenging task was to evaluate the change in pile performance – while under load – as injection grouting was used to improve the general capacity of the piles. For this evaluation to take place a series of expendable test piles were installed adjacent to the affected piles, so that several injection processes and techniques could be evaluated. This would have presented a specifically difficult problem for top-down loading as the loading arrangement would be in the way of the process equipment and would impede process execution.

Bi-directional load tests
Tests were performed on one control pile, two process evaluation piles and one dedicated test pile to determine the in situ end bearing performance of these 1500 mm diameter piles constructed under limited headroom to approximately 22 m deep and fitted with Geokon model 4855 pile-tip pressure cells.
Three 405 mm diameter O-cells were installed in each pile giving a maximum loading capacity of 30MN. These allowed upward evaluation of level reduction, application of constant load during boring for low pressure and high pressure injection grouting and evaluation of the friction and end bearing of the lower section of the pile (below the level of influence of the tunnel) without interfering with the numerous other site activities.
In order to achieve sufficient reaction on the upper section of one of the test piles, shaft grouting and jet grouting was employed allowing the test load to exceed 17MN in each direction.

Conclusions
Skin friction was less than expected, making the enhancement of skin friction challenging without causing additional unwanted settlement. Ultimate end bearing capacity was excellent and larger than expected.

Main Contractor: Wayss & Fritagg

Foundation Contractor: Bilfinger Berger AG

Local LOADTEST office in Germany: Scanrock GmbH: Carlos Fischer

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City Tunnel Leipzig, Germany