Akershus Fortress

Since 2007, several walls have been demolished and rebuilt to obtain new stability. Each summer, a new part has been restored by using historically correct materials and procedures. A specialist team has worked on these labour-intensive processes, including the removal of huge blocks and using NHL-based mortars.

Norway
Oslo
Contractor
Forsvarsbygg National Fortification Heritage
Project Date
2007-2014

Issues and stakes

Following the National Fortification Heritage procedures, this huge fortress has undergone an intensive upgrade. The fortress includes 21 buildings, most of them masonry buildings originally built and still used as military offices. The most prominent is the fortress and castle itself dating back to 1299 AD. The restoration work includes both buildings and the massive fortress walls surrounding the oldest parts. Regular wear and tear, wrongly used repair material and a lack of regular maintenance have resulted in worn buildings with the danger of debris falling down. Wrongly used paints have resulted in cracks and scaling. The massive walls have been restored previously but using cementitious materials that made joints watertight, trapping moisture inside the walls. This has lead to the lime bedding being washed out, slow decomposition of the original lime joints and unstable walls. Opening the joints reveals decomposed lime with no strength, frequently as deep as 20 cm into the wall. This has resulted in falling rocks, bulging and the collapse of walls.

Achievements

A thorough investigation by prescribers identified voids in the walls. Some walls have been injected with lime-based mortars, while others have been demolished and rebuilt. During this process, to secure the historic appearance of the wall, each stone are numbered and photographed. When rebuilding the wall, the visible stones are put together like a puzzle in their right place. Due to weather conditions, work is only possible in the summer months, starting with the lime works in April and ending in August. This provides a curing period so that the mortars can withstand freezing in the winter. The restoration follows guidelines in the National Conservation Plan developed by the National Fortification Heritage. The buildings are upgraded inside and outside using NHL-based and weak, lime-cement mortars where appropriate. Colours and paints have been chosen that are in keeping with the year of construction and original materials. This means an extensive use of lime-based materials.