Preparation for Repair of the Charles Bridge in Prague

REHABILITATION OF THE CHARLES BRIDGE

INNOVATIVE DATA BASE SYSTEM FOR PRAGUE CHARLES BRIDGE STONEWORKS

On the 650th anniversary of the laying of the bridge foundation stone by Charles IV, the King of Bohemia and Caesar of Germany in July 1357, the Charles Bridge is getting finally a thorough rehabilitation. The last extensive reconstruction of the bridge superstructure was performed during years 1965 – 1977 with only moderate success. Problems caused by water soaking through the bridge superstructure appeared already in 1980 and gradually became more severe. After many setbacks, a new rehabilitation project was launched on August 20th 2007. The bridge rehabilitation is divided into two main phases. First phase is the reconstruction of the bridge superstructure. This includes reconstruction of bridge drainage, installation of new water-proofing system and construction of the pavement structures, new gas street lighting and other services. Stone parapets from below level of waterproofing membrane will be also reconstructed during this phase. First phase of the rehabilitation works seamed to be straight forward reconstruction process. But the fact, that we are working on one of the most precious heritage structures, with strict requirement on highest quality possible of every detail of rehabilitation works - for example, new waterproofing system must be supplied with at least 30 years guarantee, makes the project extremely demanding. The rehabilitation works should be done per partes, enabling the visitors to walk always through the bridge and enjoy without restriction most of the bridge area. Strict requirement on minimal adverse environmental impact of works makes project even more difficult.

Not less demanding will be the second phase – rehabilitation of over 54 000 bridge ashlars (6 000 ashlars are dealt with in 1st phase of the bridge ‘parapets rehabilitation). To develop most appropriate rehabilitation strategy, the Institute of Geology, Mineralogy and Mineral sources Faculty of Science, Charles University Prague was invited and there was developed innovative methodology of stone elements diagnostic survey which includes: 1) Coloured photography of each of 60 000 individually numbered bridge ashlars, including the date and position in the 3D wire model of the Charles Bridge. 2) Visual investigation of all built in ashlars, which determines petrographical type, origin of material and extent of deterioration. 3) Determination of the type of deterioration – blistering, flaking, granular disintegration, honeycombs, alveols, cracks, efflorences, subflorences, black deposits, flow deposits, dissolutions, sprays, algae, lichens 4) Conclusion whether the deterioration process is stabilised or continues 5) Taking samples of the material to determine types and sources of deterioration. Sample analysis will include the petrographic survey, survey of extent of material deterioration, analysis of surface layers composition – RTG diffraction, DTA/TG analysis, FTIR spectrometry, IEC water soluble salts, SEM/EDS analyses and mass spectrometry of stable isotopes. Above analyses will be instrumental for the decision on the type of treatment prescribed for every ashlar. The treatment of ashlars will than range from “doing nothing” to “stone replacement”. According to the above mentioned, each of the bridge ashlars has its own set of data in digital form as a part of the whole bridge data base information system. Then it will be easy possible in future to regularly (in 5 or 10 years or as required) assess and evaluate the degree of deterioration of individual ashlar. To make it possible, the laser-scanning technology was applied for creation of 3D wire model to form the bridge lines and edges with precision of about 5 cm. After that, the detailed 3D model with all stones was created by application of digital photogrammetry. All of about 60 000 face stones were processed in this model in Auto CAD. As an example, we can see the north side breast wall of the XVth vault of the bridge. Stones are numbered – each stone has own information list with data delivered from detailed survey based on the methodology mentioned above. Brown coloured stones were replaced, the green ones surveyed or eventually treated and returned back in to their original position.

The first phase of rehabilitation is scheduled to take 1029 days. The Client is Prague City Hall, the Contractor SMP CZ a.s. and Mott MacDonald Praha supervises the works.

The second part of the rehabilitation works will be very much time demanding and will present mainly careful stone-cutting work. The bridge’s vaults, breast-walls and piers will be gently repaired. We expect that two vaults will be rehabilitated each year. Crucial condition for this phase is that transport of materials and works will be done entirely from below of the bridge without any restrictions to pedestrian traffic on the bridge. Works will commence after completion of superstructure rehabilitation and will take approximately eight years.

We want to perform a very thorough, but careful and gentle restoration of the Charles Bridge, which should allow at least 50 years o regular maintenance only. Every large restoration works can negatively affect the structure. Stress distribution changes and cracks may appear while the structure struggle for new balance. From an engineering point of view, the Charles Bridge is a tremendous technical achievement. Despite all the wear and tear, it has survived for six and a half centuries in its original form. Bridge architects, Otto and Peter Parler, were pioneer bridge builders who designed the structure which, with the help of modern technology, will last for many generations to come.

Jiri Petrak Mott MacDonald Praha