Example of a numerical model at Bratislava, SNP square

This model was part of a large project to complete SNP Square at Bratislava Center. The aim was to assess the influence of underground objects (e.g., garages, tunnels) on the groundwater level. The main goal of this model was to document how the groundwater flow would change after four construction stages that were defined by the project. The situation required a 3D model due to the complexity of the construction and the hydrogeologic situation. This numerical model of the steady groundwater flow used the BEM.

Layers of the model

The model was divided into three horizontal layers. The top layer simulates the quaternary sediments consisting of gravel, sandy gravel, and concrete constructions. The middle layer was created in order to model the underflow of the concrete constructions in the water-bearing gravel and in the thick Neogene series of pervious strata below. The bottom layer consists of Neogene sediments which are, according to geophysical measurements, hydraulically active up to 48 m below the terrain. Each layer is composed of triangular elements that match the shape of building constructions, horizontal geological boundaries, and hydrogeologic inhomogeneities. All three layers are divided into numbered homogeneous zones with respect to the coefficient of hydraulic conductivity. There are 29 homogeneous zones in the top layer. Highly permeable quaternary gravel is found in zones 14, 15, 27, 28, and 29 with the coefficient k=2.1×10-3m s-1. Less permeable terrace gravel (k=5×10-4 m s-1) occupies zones 9-13 and 22-25. Zone no. 1 is the remainder of the water-bearing, higher terrace gravel that has a different coefficient value, k=4.1×10-4m s-1. Zones 2-5 and 16-21 represent the sandy and loamy water-bearing diluvium of the Neogene sediments. Their coefficient of hydraulic conductivity is k=6.4×10-6 m s-1. The last three zones represent parts of buildings in which some parts of their construction are below the groundwater level. Their permeability was set to k=1×10-15 m s-1. In the four construction stages, other zones have been added with the same permeability value (all data above and below are for the current state). The number of zones in the middle layer reaches 23 (Fig. 8.14). Very permeable quaternary gravel composes zones 38, 39, 51, and 52 (k=5×10-4 m s-1). The Neogene sediments with the coefficient k=6.4×10-6 m s-1 occupy zones 30-32 and 40-44. Zones 33 and 34 (k=1×10-15 m s-1) correspond to the only two buildings that reach this middle layer. In the bottom layer there are only eight zones. In zone 53 the Neogene sediments have a permeability value of k=1.3×10-5 m s-1; in the remaining zones 54-60, the Neogene sediments have a value of k=6.4×10-6 m s-1. There are no building parts that penetrate this layer in the current state. All coefficients of hydraulic conductivity in all layers and zones were determined using a hydrogeologic survey. The vertical coefficients of hydraulic conductivity were assumed as kz=0.1kx, analogous to other similar problems.

The model of current state

The described 3D model was prepared for the current state of the free surface of the groundwater and the model was verified by comparison of the predicted and measured data. The determined water levels (see Fig. 8.16) served as a prognosis of the change caused by the construction. The piezometric level in the bottom layer is larger than the level of the free surface of the groundwater, implying a trend of vertical water flow from the Neogene to the Quaternary. The differences in water level between the top and the bottom layers are 0 to 2 m.

Model of the first stage of construction

There are only two new buildings below the groundwater level of the upper part of the square. These objects are gray in the picture. The elevation of the groundwater level after the first stage can be clearly seen on the contours of change in the groundwater level.

Model of the second stage of construction

There are four buildings below the groundwater level. These objects are gray in the picture. The elevation of groundwater level after the second stage can be clearly seen on the contours of change in the groundwater level.

Model of the third stage

Here, we consider the third stage of the construction. This stage represents the whole construction of three parking silos that reach the last Neogene layer and a large underground parking garage with entrances that are under almost the entire area of the square. All of these buildings are gray in the figure. The elevation of the groundwater level after the third stage can be clearly seen on the contours of change in the groundwater level.

Model of the fourth stage

Here, we consider the last stage of the construction. This stage represents the whole construction of three parking silos that reach the last Neogene layer and a large underground parking garage with entrances that are under almost the entire area of the square. Moreover, further underground objects were added to the lowermost side of the square. These objects are gray in the picture. The newly designed protection wall, along with an existing barrier of the West Theatre, cause an elevation of the groundwater level. This is also caused by a recharge of groundwater from the Neogene subsoil of the side barriers. The elevation of the groundwater level after the fourth stage can be clearly seen on the contours of change in the groundwater level's. The maximal elevation of the groundwater level is 4 m, even reaching the sides. On the NE side of the model there is an elevation of 0.5 m. In the wake of any building in the fourth stage, there is a decrease in the groundwater level of 1 m. The elevation in the fourth stage is significant enough to require remediation.
