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1. New metro line D and Pankrac station
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New metro line D and Pankrac station
The capital of the Czech Republic, the City of Prague, currently operates 3 metro lines - lines A, B, C. (Figure 1) The newly planned line D is designed to connect the city centre, namely Náměstí Míru station, with the southern district of Prague. The new line D will terminate in Depo Písnice station. It will consist of 10 new stations and will be 10,6 km in length. Pankrac D station is one of the stations that will become a part of the new line D.
Figure 1. Prague Metro Network
Pankrac station is designed as a single-vault station with side platforms (Figure 2). The station will be entered through southern and northern escalator tunnels. Understandably, the new station will be directly connected to the existing metro line C. The new station will be situated below the crossroads of Na Strži, Na Pankráci and Budějovická streets (Figure 3). The excavation of the station will be done in the depth of 33 metres below the ground level and in the southern part of the station just 4 metres below the existing tunnels of metro line C. The excavation area of mined Pankrac station is about 350 m2 in cross-section.
Figure 2. 3D visualization of Pankrac station located on line D
Figure 3. The situation of Pankrac station
Local geological conditions, in the area of the planned Pankrac station, are an extremely complicated 3-dimensional issue. In the northern part of the station, there are located older and younger Silurian formations, namely Kopanina and Liteň formations. The southern part was formed by the youngest Ordovician formation – Kosov formation. These two quasi-homogeneous units are divided by inclined layers of diabase. The area is very rich in massive failure zones.
See the bedrock quality in Figures 4.1 and 4.2. The photos show a geological borehole J179. In the picture, there is a borehole core of very low quality at the depths of 20 - 39 metres, which is the direct effect of failure zones. The groundwater level was measured at depths of 5 - 8 metres.
Figure 4.1 and 4.2 – Borehole core J179.
The complicated geotechnical conditions require the improvement of the bedrock quality to ensure safe and successful excavation. In this particular case, it has been decided to improve the bedrock by chemical grouting, which enables to fill up fine cracks in the rock mass. Grouting significantly improves the stability of the whole bedrock. To verify the efficacy of the chosen method, two test boreholes were drilled. Both are located in the vicinity of the new station. The main goal of the test was to determine the efficacy and range of rock grouting. At the same time, a series of pressuremeter tests were done to determine the deformational parameters of the rock mass before and after the rock grouting.
Although the effectivity of rock grouting was confirmed, the range of deformational parameters of the rock mass was very wide. To be able to responsibly design the station by FEM, the LHS (Latin Hypecube Sampling) method was applied to the inputs data of the grouted rock mass.
FE models of Pankrac station
For the preliminary static evaluation and to demonstrate the effect of rock grouting, the 3D model of the southern part of the station was created. The model also includes simplified geological conditions and existing metro line C. In order to simplify and shorten the computation time, the model was subdivided into three separate models A, B, C.
Model A – The access shaft with the pilot tunnel. From the inside of the pilot tunnel rock grouting in the vicinity of existing metro line C was done. The pilot tunnel was subsequently widened in the top heading part to the whole profile of the new transfer hall (Figure 5.1).
Figure 5.1. Model A
Model B – The rock grouting and excavation of the whole Pankrac station (Figure 5.2).
Figure 5.2. Model B
Model C – Excavation of bench and invert of the new transfer hall (Figure 5.3).
Figure 5.3. Model C
For each model A, B, C the reference calculation was done to analyse the state before rock grouting. Afterward, 20 calculations for each model with various inputs of grouted rock were performed. In total, 63 models were done. Based on the performed grouting tests, the inputs of grouted rock were determined. To compare the results, the superposition method was applied to a vertical displacement. The method was applied in the sequence, which corresponds to the actual excavation workflow. Final vertical displacement was compared in the five points placed on the lining of the new station and the existing metro tunnels.
Figure 6.1, resp. 6.2 -
Displacement of existing line C in natural, resp. improved geological conditions.
The current state of construction work
In June 2019 the construction on the Metro line D started. The project began with a geological investigation of the route. Within the survey, several shafts and pilot tunnels were made. These sections will in the future become a part of the metro tunnels.
Figure 7.1, 7.2, 7.3 – Photos from the ongoing geological investigation.
The newly planned Pankrac station (metro line D) poses a challenge for structural and geotechnical engineers. The complicated geology along with the existing metro line C in the near vicinity require a special design approach.
For the preliminary design, a series of FE models were created for verification purposes. Rock mass behaviour and reactions of the existing metro line C to the excavation works were examined. Based on these models the sensitivity analysis was performed. Final statistical evaluation has shown the necessity of rock improvement by chemical grouting to make the excavation possible and safe and minimize the impact on the existing line C.
Midas GTS NX software has proven to be an indispensable and powerful tool for 3D numerical modelling for complex design solutions. It is not only an advantage but given the complexity of the project, it is a must.