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Going to new depths

Construction of a 70m deep cut off barrier in Bavaria’s slyenstein Dam has helped Bauer to set a new German record for diaphragm walls. GE reports.

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Part of the solution involved building concrete mixing plants at the foot of the dam

Bavaria’s Sylvenstein Dam was originally built over 50 years ago to help regulate the minimum water level in the Isar River to allow hydro-electric plants on the river to operate all year round. But with a capacity to hold 125M cubic metres of water, the dam has developed another role that has led to the need for the structure to be upgraded. In recent years the structure has also proved its worth by holding back flood waters that threatened the nearby town of Bad Tölz as well as the city of Munich.

Although the dam successfully held back flood water in 1999 and 2005, the Bavarian Ministry of Environment was concerned over the structure’s design to cope with this new role and also how climate change could add to the need for flood defences. To ensure the dam could cope with the changing demands, it was decided to improve the dam’s stability by constructing a 70m deep, 1m thick cut off wall.

The Bavarian Ministry of Environment called on Bauer to undertake the €11.5M (£10.3M) work on the 42m high, 180m long dam, which is part of a larger €25M (£22.3M) scheme that includes equipping the structure with a stateof- the-art water collection system and a monitoring system for water seepage.

The scale of the project was challenging and, at 70m below ground level, Bauer believes that the work set a new record for the depth of diaphragm wall construction in Germany.

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According to Bauer, the dam overlies a 100m deep gully cut into the underlying dolomite bedrock that is infilled by sediments. The watertight thin core of the dam is composed of plastic concrete - formed from gravel, fine sand, silt and bentonite - and of a filter layer of moraine gravel from both the downstream and upstream sides.

The ground conditions called for multi-layered grout injections to create a sealing curtain of claycement grout to be constructed when the dam was built between 1954 and 1959.

The dam has already undergone two previous upgrades - in the 1980s the structure was strengthened and then the dam height was raised in the 1990s.

Bauer believes that the work to 70m below ground level set a new record for the depth of diaphragm wall construction in Germany

The upgrade work called for a deep sealing structure to be built by carrying out two phases of cut off walls using diaphragm walling techniques. The structure was built offset from the core of the dam, slightly towards the downstream side. The depth of the cut off wall has been determined based on boreholes drilled during a site investigation stage to 140m below the dam.

According to Bauer, the interbedded layers of gravel and rock sediments combined with the fluctuating permeability in the bottom layers of the existing seal, meant that the cut off wall had to extend down to 70m below the crest of the dam.

The 10,000m2 cut off wall was constructed from the dam crest with the upper parts excavated using a grab and continued to the final depth by means of a BC40 cutter mounted on MC128 rig where the extremities of the wall had to be embedded in extremely hard rock.

Reaching the 70m depth was not the only challenge on this scheme - the topography of the site and the restricted working space at the dam crest meant that logistics were also a complex task. Part of the solution involved building concrete mixing plants at the foot of the dam and pumping concrete 50m up to the construction site on the crest. Bauer also undertook work to widen the road across the dam crest in summer 2011 in order to simplify the main construction phase.

The cut off wall construction started on site in June 2012 and has just been successfully completed in time to protect Bad Tölz and Munich from the risk of flooding this winter.

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