Cut-off Wall Main Dam at Brombach, Germany

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 has a rich water supply because of high precipitation. 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-1990´s, the runoff of the river Regnitz below Nuremberg will thus be raised from 12 m3/s (190,000 US-gpm) currently to 27 m3/s (430,000 US-gpm). An annual average of approximately 150 million m3 (200 million cu.yds.) of water will be channelled into the Main region. The major part of the channel system will proceed via the Main-Danube canal (125 million m3/164 million cu.yds.), and a smaller part via the Brombach storage system (25 million m3/33 million cu.yds.).


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 (131 ft.) and a length of 1,8 km (1,1 st miles), closes off a lake with a surface area of 12,3 km2 (4 3/4 squ.miles), which is larger than Tegern Lake. Two upstream dams, the Brombach and the Igelsbach dam, create reservoir areas separate from the main reservoir that have only small fluctuations in water level.

 


The Subterranean Sealing

The high permeability and poor binding qualities of the mid-level and lower "castle" sandstone that lies beneath the dam make unusual sealing measures necessary. With the permeability of the variegated-sandstone at between 10-4 m/s and 10-7 m/s and gaps of between a few millimeters and several centimeters, the originally planned grout curtain could not ensure the necessary sealing in the lower depths.
The 60 cm (24 in) thick cut-off wall excavated with a Bauer trench cutter, yielded a controlled safety for the required sealing down to the final depth. The precision of direction is controlled by electronic inclinometers ensured down to the final depth.


After a panel has been excavated, a mixture of earth and concrete is installed using tremie pipes. The mixture consists of sand, gravel, cement, finely ground clay, water, a filler - for example rock dust - and if necessary, some bentonite. This mixture gives the wall a high impermeability and resistance to erosion, and the plasticity of the material also tolerates minor settlements and movements without danger of cracking. According to measurements, the finished wall had a permeability of k = 10-9 m/s and a specific gravity of = 2,0 t/m3 (1 3/4 ts/cu.yd.).

The Bauer Trench Cutter


Two contra-rotating cutting wheels with teeth cut or break the earth or the rock and transport it to the middle, where it is suctioned off with the supporting slurry. The slurry and the earth are separated in a desanding plant, the supporting slurry is pumped back and is once more available in the trench.


The rate of revolution of the cutting wheels, as well as the suctioning of the cuttings, the power input and torque output of the cutting wheels, the contact pressure on the cutting teeth and the sinking speed, are all constantly monitored. All moving parts are infinitely adjustable from zero to maximum. The trench cutter is driven by the high-performance hydraulic system of the crane, control and monitoring instruments are installed in the operator´s cabin of the crane. The hoses and measurement cables are automatically kept constantly taut and are held in an exactly vertical upwards direction with preselected forces, which helps substantially in maintaining verticality. The production per shift after the start-up period was up to 250 m2 (2'700 squ.ft.) of wall with a thickness of 60 cm (24 in) at depths of up to 40 m (130 ft.).

Scope of work:

12,500 m2 (135,000 squ.ft.) cut-off wall
Thickness 60 cm (23 1/2 in), depth up to 40 m (13 ft.),
length 405 m (450 yds.)