The Brombach Storage Reservoir

 

In extensive areas of northern Bavaria (Franconia and the Upper Pfalz) there is a shortage of water due to low precipitation, whereas southern Bavaria is very rich in water due to high precipitation and the proximity to the Alps. To balance out these differences, the "Water Channel System from the Altmühl and the Danube to the Regnitz-Main Region" is being constructed. Starting in the mid-1990s, the runoff of the river Regnitz below Nürnberg will thus be raised from 12 m³/s to 27 m³/s. An annual average of approximately 150 million m³ of water will be channeled into the river Main region. The major part of the channel system will proceed via the Main-Danube canal (125 million m³), and a smaller part via the Brombach storage system (25 million m³).

The Brombach storage reservoir is the most important structure of the project. The main Brombach dam. which is made of earth and has a height of 40 m and a length of 1,8 km, closes off a lake with a surface area of 12,3 km², which is larger than the Lake Tegern. Two upstream dams, the Brombach and the lgelsbach dam, create reservoir areas separate from the main reservoir that have only small fluctuations in water level.

 

Owner: The State of Bavaria, Department of New Dam Construction, Nuernberg

Execution: BAUER Spezialtiefbau GmbH

 

 

Scope of work

Bauer Spezialtiefbau GmbH was awarded the contract to construct a cut-off wall using the diaphragm wall method. The main scope of work included the following:

12,500 m² Cut-off Wall by means of diaphragm wall. Thickness 60 cm, depth up to 40 m, length 405 m

 

Geology

The high permeability and poor bonding properties of the middle and lower "castle" sandstone underlying the dam necessitated the sealing measures, which were unusual at the time.. The permeability of the variegated sandstone is between 10-4 m/s and 10-7 m/s and fissures between a few millimeters to several centimeters. Thus, the originally planned grout curtain could not provide the necessary sealing at the lower depths.

 

 

Miscellaneous

The 60 cm thick cut-off wall excavated with a BAUER BC 30 Trench Cutter provided controlled safety for the required sealing down to the final depth. Directional accuracy is ensured by cutter-inbuilt electronic inclinometers. After excavating a trench, a mixture of soil and concrete (Plastic Earth Concrete) was placed using tremie pipes. The mixture consists of sand, gravel, cement, finely grinded clay, water, a filler - for example rock flour - and, if necessary, some bentonite. This mixture gives the wall high impermeability and erosion resistance, and the plasticity of the material also allows for minor settlement and movement without the risk of cracking. According to the measurements, the finished wall had a permeability of k = 10-9 m/s and a specific gravity of = 2.0 t/m3.

 

 

Water Channel System from the River Altmühl and the Danube to the Regnitz-Main River Region (Site Plan)