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Restoration of the Venus Grotto in the years 2015 to 2025

Origins and structure of the construct

The Venus Grotto in the palace gardens is a unique construction designed as an artificial flowstone cave and the highlight of King Ludwig II’s illusionary architecture. This grotto not only depicts the 1st act of Richard Wagner’s opera "Tannhäuser", but also reproduces the motif of the Blue Grotto in Capri by means of artificial illumination in various colours. For this purpose, one of the world’s first electric power stations was created. The grotto was erected from 1875 to 1877 by court building director Georg Dollmann and landscape sculptor August Dirigl. In the 19th century, it presumably was the largest construction of its kind even though it had been designed only for the king’s own private use.

The basic construction consists of 1.7 m thick walls made of limestone rubble and solitary pillars made of rubble stone or cast iron. A rubble stone arch spans over the entry and exit area while the main grotto has a vast vault made of brickwork. On the outside, the vault was sealed with liquid tar and then covered with soil.

The shell visible from the inside is made of plaster applied to burlap or iron mesh. To achieve an authentic look of the surface, some of the plaster was poured or fabric soaked in plaster skilfully applied. The surfaces are painted in colour and partly covered with mica to achieve a sparkling shiny effect in combination with the changing coloured illumination.

 

Problems with moisture and damage to the construction

Shortly after completion of the grotto, problems with moisture became apparent. For this reason, a roof was erected above the main grotto as early as in 1889/90. This, however, could not fully solve the problem as water flowing underground still penetrates through the thick natural stone walls, in particular after heavy rains or when the snow melts.

After around 140 years, the Venus Grotto was severely damaged by the penetrating moisture. The filigree inner shell made of iron mesh and Roman cement corroded and disintegrated. The first protective nets had to be installed as early as the 1960s to ensure the safety of visitors during tours. In 1997, additional protective scaffolding and nets were added in the entrance and exit areas and along the entire tour route.

In addition, the original appearance had changed and there was no longer any sign of the former modern technology. The floor coverings and parts of the grotto shell surface were distorted, the lighting was outdated and did not correspond to the historical lighting concept. Only remnants of the historical furnishings were still present.

 

Restoration measures and objectives, description of the project

The primary aims of the restoration were to conserve and permanently secure the still extensive historical building fabric and furnishings and to restore the original, undisturbed spatial impression of the grotto. The protective nets and scaffolding were to be removed.

To achieve these objectives the following root causes had to be eliminated:

• Entry of artesian water and precipitation from the hill side into the areas not covered by the roof construction

• Defective roof construction above the brick stone vaults

• Unsuitable climate conditions inside the Venus Grotto (cave climate, entry of moisture through visitors, waterfall, slope and artesian water) leading to progressive corrosion in the mesh and plaster shell and damage to the monumental painting and other fixtures

 

Layout of Venus Grotto (State building authority Weilheim)

 

The following measures were necessary to address the existing problems:

• Erection of a dam, mostly underground north of the Venus Grotto, including drainage in order to minimise the entry of slope and artesian water into the Venus Grotto

• Improvement and extension of the existing historic drainage ducts inside the grotto

• Demolition of the existing roof and installation of a new seal above the vault including green roof construction extending to the northern side of the dam

• Reduction of air humidity in proximity to the mesh and plaster shell to a relative humidity of 80% by installing air conditioning in order to stop the corrosion process

• 

  Restoration work on the wire plaster shell, construction site photo
  Photo: Bayerische Schlösserverwaltung / Heiko Oehme

  Structural strengthening of the suspended construction of the wire plaster shell and of two historic cast iron columns with elementary significance for the supporting structure

• Restoration of the wire plaster shell including stalactites, stalagmites and special constructions, as well as installation of galvanic protection for the permanently damp areas

• Protection, restoration and in part reconstruction of the interior (monumental painting "Venus Scene from Tannhäuser", gilded shell boat, crystal throne and shell throne, flower garlands and artificial plants, tree branch railings, tiled stoves)

• Recreation of the historic illumination with lights in different colours; improvement of overall illumination

• Replacement of the existing toilet facilities with a largely underground extension to the south of the Venus Grotto, which also contains staff and technical rooms

 

Conservation of nature and bats

Due to the location of the construction site in the ‘Ammergebirge’ nature reserve, extensive nature conservation requirements had to be observed. The construction work was permanently monitored by nature conservation experts in order to avoid negative effects on the protected fauna and flora. In particular, the bats living in the Venus Grotto, including the lesser horseshoe bat, required special measures. Specially designed new entrance tubes were provided for them in collaboration with bat experts.

 

Realisation of the construction project over a period of approx. 10 years

• Stage 1: Construction site set-up / access routes: August 2015 – 2016

• Stage 2: Dam – diversion of water: 2015

• Stage 3: Technology and technical rooms: 2016 – 2024

• Stage 4: Demolition of existing roof: 2016 – 2018

• Stage 5: Restoration of inner shell: 2018 – 2024

• Stage 6: Sealing of vault, construction of green roof: 2021 – April 2025

Sectional view of Venus Grotto (State building authority Weilheim) with shelter during construction works

 

Authors: Susanne Hempe, Wolfgang Eichner, Martin Bosch