Výskumný ústav vodného hospodárstva

1. Section new railway to the Airport of M.R. Štefánik in the meeting point with Ivánska cesta
2. Section Danube meeting point with existing railway (Bratislava Petržalka)
3. Section Bratislava Predmestie - Bratislava Filiálka, II. stage

Study of the impact of tunnel route on the ground water flow regime for „Project TEN-T the connection of railway corridor TEN-T with the airport and railway network in Bratislava“ was elaborated on the basis of the order of the main constructor of Dopravaprojekt a.s. and the Water Research Institute Bratislava. The objective of the study is to summon the materials for the project TEN-T and to assess the impact of the tunnel route on ground water flow regime.
The scope of the study was the first stage of the assessment of the impact in the section of new railway to the Airport of M.R. Štefánik as far as the meeting point with Ivánska cesta and the section in Bratislava Petržalka from the Danube to the meeting point with the existing railway as well as the second assessment of the section Bratislava Predmestie – Bratislava Filiálka.
Three partial studies were elaborated in cooperation with the Slovak University of Technology. These studies deal with the regime and changes of ground water levels in three sections. The task was performed using the method of final elements by mathematical model TRIWACO. Since the route TEN-T is placed mostly in complicated hydrogeological conditions (Danube quaternary alluvial deposit), a mathematical model of ground water flow was built and it precisely reflected existing condition and specified the impact of construction on development and change of ground water level regime in the area concerned. The model contained all available information on geological, hydrogeological, pedological, hydrological, meteorological and technical data provided by Dopravaprojekt a.s., SHMÚ or directly by the WRI. Basic input data on ground water level are the data from ground water monitoring.

Section new railway to the Airport of M. R. Štefánik in the meeting point with Ivánska cesta
Current ground water regime in concerned area is typical of decreasing trend of ground water levels. The highest ground water level in the section of related route was recorded in the area BA-Vajnory, level here is 1-3,8 m p.t. and southwards it decreases to 5,8 – 8,7 m p.t.
On the basis of modelling results the ground water regime changes were documented: in front of the sealing wall there was ground water level increase by 1,2 – 1,4 metres maximum in case of coherent sealing wall, or 0,8 metres maximum with the sealing wall having 6 holes. Behind sealing wall ground water level decrease was documented by 1,45 – 1,65 metres maximum v case of coherent sealing wall, or 0,8 – 1m maximum with the sealing wall having 6 holes.
Assessing the outputs of foreseen changes in case of the construction of the railway corridor TEN-T in Bratislava airport did not identify a significant critical spot in relation with the prognosis of increasing the ground water levels above 2 m p.t. in the assessed area (i.e. negative effect on engineering network, cellars and basements, underflooding...).

On the basis of abovementioned facts the following recommendations were proposed to limit negative impact on ground water regime in the territory:
SHORT-TERM MEASURES TAKEN DURING CONSTRUCTION
1.  To implement the proposal for dividing construction works into the stages
2.  Construction of the railway during dry periods (summer, winter).
LONG-TERM MEASURES TAKEN DURING OPERATION - alternative
A. Construction of designed openings according to developed model solution
B. Alternative scenario = construction of permanent interconnection of pipes in a bedrock as well as to deal with the parameters and location of interconnection according the 2nd stage of assessment.
TEN-T connection railway is located at boundary of this structure rich in significant groundwater resources of high quality and, therefore, it is important to prevent potential pollution of groundwater during construction works. There is no expectation of more significant impact on groundwater quantity.

The Danube river stretch  -parallel line with  the existing railway (Bratislava Petržalka)
Monitoring data proved stable or slightly upward trend of groundwater levels in the respective area. The most unfavourable conditions are in the vicinity of the Danube River where extreme water levels reach a value of 0.35 m.p.t  (100-year water levels).  
Model solution proved that the groundwater level will increase after the construction of tunnel only in a few centimetres short section.
Since the railway in the area of Petržalka will pass mainly through the tunnels and only short sections will be excavated under the protection of supporting and cut-off walls we have come to the conclusion that in case of the Danube River – Bratislava Petržalka railway section no measures are required to be taken for long-term decrease of groundwater level. However, it is important to deal with drainage of building pits during their deepening.   
Following the above-mentioned facts we recommend to implement short-term measures during construction process:
1. Construction of the railway during dry periods (summer, winter)
2. Building pits drainage
3. Consider the use of draining devices in rain water drainage systems (during operation).  

The railway section Bratislava Predmestie - Bratislava Filiálka,  2nd stage
The objective of the 2nd stage of construction Works in the section “Bratislava Predmestie - Bratislava Filiálka“ was to develop a model for affected groundwater regime providing that the construction will be divided into the proposed stages  and regulation measures will be implemented during operation (openings in cut-off wall). The project also deals with the issue related to potential superposition of the effect of TEN-T and tall building (between “Filiálka railway” station and Račianska street, which forming a barrier) on a change of groundwater regime.   
Model solution including the implementation of the above-mentioned measures and phased construction of five openings in designed cut-off wall proved that the groundwater will not increase above 2,5 m level below the surface in any stage of construction Works. The effect of tall building on present groundwater regime is negligible. 
A proposal of model solution is the basis for the team of project engineers to develop optimum solution and alternative scenarios. The result is a selection of the most suitable scenario in relation to technical and financial requirements as well as protection of groundwater in respective railway section “TEN-T Airport Bratislava”