Prague-based engineering consultant Pudis was recently selected through a tender process, and work will start this month (May) on preparation of the final design. The repair work must be carried out carefully to minimise intervention into the bridge structure and without disturbing tourism during the 2006 and 2007 seasons. The principle objective is to preserve this remarkable, 648-year old monument for further generations.

The breakdown of the waterproofing layer on the bridge has allowed water to penetrate the argillit infill material of the structure, which creates pressure on the breastwalls when it freezes and consequently has caused the walls to bow outwards. Water penetrating through the cracks in the masonry facing is also damaging the structure.

In October 2003, consultant Mott MacDonald Praha was commissioned by the Capital City of Prague to review the existing survey works and design documents. The following April, the Professional Coordination Commission for Charles Bridge Repair approved Mott MacDonald's proposal of careful intervention into the stone structure of the bridge to carry out repairs; a solution that was confirmed following additional survey work.

The 16-arch stone bridge is naturally permeated by cracks that have been caused by loading and wear throughout its 650 years. The bridge was built as a very rigid structure – its vault thickness of 900mm for spans of 16.7m to 23.4m at f/l = 1/3.6 ratio is unusual. The breastwalls and infill masonry also contribute to its rigidity. In a structure of this type, cracks would have originated during early volume changes – first in the mortar, and then also in the stone ashlars of the vaults, which is natural, with a view to the high bending rigidity. At present, the bridge condition is statically consolidated and engineers are keen not to modify the distribution of the inner forces in the structure because of the risk of further development of faults, particularly cracks. Thermal expansion of the stone masonry and stresses substantially affects the potential for cracking, and this needs to be taken into account when calculating stress and deformations of the stone structure. It is not possible to evaluate the structure as homogeneous or continuous; the approach needs to comply with non-linear mechanics theory, including both reduction of the masonry rigidity relevant to the origin of cracks, and reduction of thermal expansion relevant to the rate of deterioration by cracks.

The physical qualities of the monitored rock samples, taken during surveys in 2002 and 2004, correspond to petrographic types and do not show any marked deterioration when compared to fresh rocks from the source locations. In 60% of the new samples, water-soluble salt content is zero, or it is at the detection limit for the analytical equipment. An increase in the water-soluble salt content was recorded in one case only, namely in the sample of the original hydraulic mortar. The existing nitrate, interpreted as the tetrahydrate of calcium nitrate, is associated with the type of glue used in mortar, that is with hydraulic lime use. There was no evidence of extensive contamination of the inner fabric of the Charles Bridge; this was believed to be due to the long-term washing of the structure by rainwater in the period after the use of salt had been prohibited on the bridge.

The face ashlars of the bridge are made of quartzy, fine-grained sandstone dating from the Upper Cretaceous period, sourced from the north of Prague and Horice and of coarse-grained arkoses from the Carboniferous age and sourced from the Kladno-Rakovnik Basin, mainly from Kamenne Zehrovice. These stones were found to have sustained damage from the weather and from atmospheric pollution in the city. A board of experts from the Faculty of Natural Science of the Charles University and from the National Historical Monuments Institution was appointed to determine the degree of damage and to ascertain how best they could be repaired, including the appropriate composition of the joint mortar. The same board of experts was also appointed to act as a professional supervision body during the rehabilitation.

Repair of the face ashlars and vaults will proceed independently of the overall bridge reconstruction.

The main problem identified on the bridge was its non-functioning waterproofing system, which is responsible for most of the problems the structure is experiencing. Consequently, the main purpose of the repair contract will be to replace the deck waterproofing. This has to be done with extreme care, particularly around drains and gargoyles and at locations where the road pavement meets the parapet walls. The life of the waterproofing system will depend on the physical and mechanical qualities of the materials used and on good, professional work. Hence the waterproofing system subcontractor will be required to provide a guarantee of at least 30 years for the new system.

Another part of the works, which has already been completed, was to strengthen and protect the piers and foundations of the bridge.

Direct subsoil under the piers is formed of the recent sandy-gravel terrace of the river Vltava. In the lower part of the terrace, sandy gravel becomes rough gravel with occasional boulders up to 1-2m³. The bedrock is 6-8m below the river bed and consists of clayey shales of the Ordovician period.

For the foundation of those piers in shallow water, in the 14th century gravel in the river bed was levelled, and on top of this subsoil base, a layer of black (hydraulic) lime was placed. Presently, the compressive strength of the mortar varies between 6.3MPa to 11.2MPa – after 650 years in the water this is regarded as an outstanding result. The foundation area of the individual piers varies between 160m² to 170m².

In deep water, where precise levelling and placing of the mortar could not be achieved, the foundation was made on timber rafts and boxes built to the foundation shape, which were sunk to the levelled river bed. Timber piles only appeared in later repair work. In some places, where the beams of the raft cross, millstones of approximately 1m diameter and 250mm thickness were placed under the beams as a support of the raft. The remaining parts were bricked-up. It is obvious that a bridge with such foundation structure was liable to be frequently damaged due to floods, nevertheless, it was always repaired again, being the only stone bridge in Prague until the middle of the 19th century. At present, the foundations of all the piers are sufficiently protected, including the foundations of piers eight and nine, the rehabilitation of which started at the end of last year. Larsen steel sheetpile walls were erected around the pier foundations in parts of the upstream and downstream pier cutwaters.

Below the arches, concrete walls 0.5m thick were built, formed of jet-grouted columns and anchored, like the sheetpiles, approximately 1m into the bedrock. On the top, between the pier stem and the jet-grouted columns, a reinforced concrete slab was poured and anchored to jet-grouted columns or sheetpiles, to create a closed frame, which is designed to protect the foundation against scour.

Water alone would not be sufficient to severely damage the Charles Bridge; the main danger has always come from the accumulation of large floating objects such as rafts, round timbers, ice floes and so on. These apply dynamic pressure to the bridge and increase the water speed and turbulence by blocking or reducing its flow through some of the arches, which could lead to the undermining and collapse of some piers.

The bridge piers are massive and faced with sandstone ashlars. The infill material is formed mostly by arenaceous marl masonry with hydraulic mortar. Geo-radar and seismic measurements undertaken between September and October last year did not identify any caverns filled with water in the piers; only areas with differing humidity were detected. Hence it was concluded that the piers, save for replacement of some of the face ashlars, do not need to be repaired.

Jiri Petrak is project director and Vladimir Tvrznik is project manager for Mott MacDonald

Written in the stars

The foundation stone of the bridge was laid by the Czech king and Emperor of Germany and the Holy Roman Empire, Charles IV, on 9 July 1357 at 5:31 in the morning as part of the Royal Route. The date and time of this event was carefully planned to take place at what was believed by astrologers to be an auspicious moment. The bridge was originally called simply 'Stone Bridge', but was re-named Charles Bridge in 1848. Until 1850, when Negrelli´s railway viaduct was completed it was the only stone bridge over the River Vltava in Prague.

Construction of the bridge was led by Petr Parler, one of the most notable medieval architects, who, when he died in 1397, left the bridge operational but not quite complete. The final element to be constructed was the higher 'Lesser Town Tower', which was finished in 1407. The bridge is highly decorated in the European Baroque style and is lined by more than 30 sculptures.

Charles Bridge has 16 arches ranging from 16.6m to 23.4m span, 15 piers and two abutments. The total length of the bridge is just over 515m, its width is 9.5m and the stone balustrades on either side of the bridge are 0.4m wide. The bridge also has three gate towers.

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