CHANGE OF AN OLD NAVY BASE TO A MODERN MARINA IN BOLTENHAGEN (BALTIC SEA)

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1 CHANGE OF AN OLD NAVY BASE TO A MODERN MARINA IN BOLTENHAGEN (BALTIC SEA) Siegmund Schlie*, Bernd Opfermann** and Torsten Retzlaff*** * Heinrich Hirdes GmbH, Rostock ** b & o - Ingenieure, Hamburg *** Inros Lackner AG, Rostock Abstract: Near the village Boltenhagen on the German Baltic Sea Coast in 1934 an artificial peninsula was created to make tests with seaplanes. After the Second World War this place became a navy base of the East German Navy until In October 2006 the change of this area started. A modern hotel and holiday apartment combination for at most 930 people has been built, which includes a marina for about 290 boats. This marina will complete the net of boat harbours at the German Baltic Coast. At first an enormous quantity of old concrete structures had to be eliminated and recycled to material for road building. At a length of more than 1000 m the shore line had to be protected by different structures. About 650 m shoreline was secured by a combination of sheet pile wall, revetment and concrete walls for high water protection. The other part of shore line was protected by rock armour in combination with a concrete wall against high water. The space between the revetments and the concrete walls will be used as a promenade. The new marina consists of a water area of 100,000 m² with water depth up to 6.00 m. The sea side has been protected by 2 breakwaters with a total length of about 1000 m. These breakwaters were founded partly on soft subsoil and constructed with different types of geotextile, gravel sand as well as rock armour. Following the requirements for German coastal structures the top of the breakwaters has been carried out at 3.50 m above medium sea level. The different kinds of shore line protection were designed by two consultants. The paper will show the requirements and technical solutions in design and execution of work. The design of breakwaters had been changed by the contractor to find a more economic solution for the client. Design and execution of this project phase will be explained. I. INTRODUCTION Boltenhagen is a village at the Baltic Sea, situated between Wismar and Luebeck. More than 70 years ago an artificial peninsula was built by reclamation works and named Tarnewitz. It had been used for military as a testing field for special airplanes. The shore line protection of this peninsula consisted of sheet pile walls combined with revetments of boulders collected in the Baltic. After the Second World War this area was partly used by the East German Army as a navy base for small fast vessels to observe the border between the eastern and western block during the Cold War Period until This navy base was protected by a concrete wall and small breakwaters made of wooden piles and boulders as well as rock armour. The majority of the military area has not been reconstructed. After the German unification, from 1990 till 2005, the complete peninsula with its bunkers, subsoil tanks, houses and shore protection had been allowed to fall into ruin. A security service had to watch all the time to this dangerous area. One part of the peninsula became a natural reserve, but the former navy base was bought by an investor in This area, about 400,000 m² at the land side and about 100,000 m² at the sea side were taken over completely with all old structures, ammunition and environmental problems. A long permit procedure was needed to reach the goal, installing of a special recreation centre with marina. Such a centre had to be founded at a place where is no danger of high water caused by storm floods in the Baltic Sea. From October 2006 till April 2008 the new recreation complex called "Weisse Wieck" was built. Behind a more then 1000 m new shoreline protection the investors created a hotel with

2 rooms and an apartment hotel especially for families with about 400 beds. Two new breakwaters give the right protection to a marina for about 290 boats, a fishery port for 15 fishing boats and berth pockets for 2 small passenger ships. The location of "Weisse Wieck" recommended special measures of high water protection, because it is situated in front of the dike line. A new structure for coastal protection had to be built and the responsibility of maintenance the owner of that area had to take over. The wish of the owner was it to combine high water protection with a promenade and a marina. Different designers prepared all the necessary documents and drawings to get the building permits and to make a public tender process. There was a requirement for public tendering because the shoreline protection and the breakwaters for the marina were co-financed by the county government of Mecklenburg Vorpommern. In this paper we want to explain something about design and execution of works for the shoreline protection and the breakwaters. Figure 1. Sheet pile wall with anchor, flood protection, and promenade II. SHORELINE PROTECTION A. Design and Execution of Inside Marina Protection The main parts that had to be designed and executed were the follows: High water protection wall made from precast concrete parts Sheet pile wall with anchors and concrete arbor in the area of berth pockets for boats partly in front of an old sheet pile wall Revetment with sheet pile wall Rebuilding of several old concrete structures Retaining wall made by precast concrete elements Two-stage stairways by precast concrete elements Several movements of big soil quantities and preparing the placed soil for other works had to be designed and done as well. The old harbour basin had been existed on water depths of about 6.00 m below medium sea level. This fact was useful for the new marina, but it caused stronger sheet piles and anchors. The special requirements had been the combination of the sheet pile wall with the other elements of the new structure for flood protection and functionally with promenade (see Fig.1). In the other part of the marina and fishery harbour soil has to be dredged up to 3.60 m below medium sea level. Here the sheet pile wall led to 30 cm below the water level and a revetment protects the lower areas under the promenade. Details are shown in Figure 2. Figure 2. Sheet pile wall with revetment, flood protection and promenade The sheet pile wall was driven by vibration and partly by an IHC S-35 hydraulic hammer. The anchoring of this wall was made by drilling anchors pressed with concrete. The revetment was placed at a geotextile filter and consisted of a filter layer of natural rock (density 2.70 kg/dm³, thickness 0.30 m, size 80 mm to 150 mm) and an armour layer of natural rock (same density, thickness 0.65 m, weight 10 kg to 40 kg). The rock had been supplied by self discharging vessels from Norwegian quarries, handled to the land side and placed by hydraulic grabs. The precast concrete elements were placed at a concrete bed. There were particular requirements for the quality of concrete. The joints between the elements were sealed by special synthetic material. All together the following materials had to be built in to the shoreline protection inside the marina: 950 t sheet piles 91 drilled anchors 360 m³ monolithic concrete 2500 t rock 680 pieces precast concrete elements with a medium weight of 5 t. The design was made by b & o Ingenieure Hamburg / Rostock. The works had been executed 97

3 by a Joint Venture of Heinrich Hirdes GmbH Rostock and Colcrete -von Essen GmbH & co. KG Rastede. B. Design and Execution of the Outside Marina Protection The North Eastern part of the shore line protection required a stronger structure, because North East is the most dangerous wind direction combined with the highest water levels and waves in this part of the Baltic Sea. This piece of the area would not be protected by any breakwater. The following parts of the structure and works had to be designed and executed (see Fig. 3): Removal of the old revetment and other constructions New heavy revetment Retaining walls for storm flood protection and stabilization of the revetment made of monolithic concrete Movements of soil for slope profiling and filling. Figure 3. North Eastern shore line protection Revetment with flood protection wall and promenade The old shore protection construction consisted of a sheet pile wall, which was at the top corroded and a revetment of boulders and rock armour. Old destroyed concrete constructions were situated close to the revetment on the land side. The corroded part of the old sheet pile wall had to be cut, boulders and rock armour had to be stored for the new revetment and old concrete had to be recycled for site way improvement and road building. The retaining wall at the land side reduces finally splash water and wave overtopping for all the buildings of the apartment hotel. The top level of this wall had to be built constant at HN m. The design water level had been determined according the general arrangement plan for coastal protection of Mecklenburg Vorpommern at HN m and a medium overtopping quantity of about 5.0 l/s per running meter had been accepted. Significant wave hight was determined at 0.9 m for the Northern protection structure. The surface of the retaining wall had to be carried out as fair faced concrete. There were several requirements for the quality of the concrete and the design of the surface of the concrete. Another retaining wall bordered the promenade to the revetment on the sea side. The top level of this wall was defined at HN m to make dewatering of the promenade possible. Both walls had to be made from monolithic concrete C 30/37. The revetment was placed at a profiled slope and consists of the following elements: Filter layer of natural rock density 2.70 kg/dm³, medium weight 175 kg, layer thickness 0.40 m Armour layer of natural rock same density, medium weight 1750 kg, layer thickness 1.75 m, partly use of the old stored rock material. The geotextile was placed by hydraulic excavator with a long reach stick and special handling equipment. The rock had been supplied by self discharging vessels for small rock and big barges for the armour rock from Norwegian quarries, handled to the land side and placed by hydraulic grabs. For the shoreline protection outside the marina the following materials were built in to the structure: 26,000 t rock 9,000 m² geotextile filter 700 m³ monolithic concrete The design for this part of shoreline protection was made by Inros Lackner AG Rostock. The works were executed by the Joint Venture Hirdes / Colcrete as well. III. BREAKWATER SYSTEM A. Design The main part of marine construction the breakwater system had been. It is a specialty to combine a recreation resort with a marina and a small fishery port. Two breakwaters have to protect the marina, the fishery port as well as the hotel building. They were designed for overtopping and overflowing of the crest and have to reduce wave movements in general in this area. The following waves could be accepted inside the harbour, if there would be a high water level of HN m (according PIANC recommendation 1995): Eastern harbour area for small passenger ships about 0.4 m Western harbour area for sailing boats about 0.2 m. On the crest of the breakwaters no way was planned. Layout and length of the breakwaters had been checked by analysis of diffraction. Design high water level was determined at HN m. The significant wave hights had been estimated at 2.20 m for the Eastern breakwater and 1.60 m for the 98

4 Western. The fairway for the harbour entrance was required with a width of 33 m and medium water depth of 3.50 m. The following parts of the breakwater system had to be designed and executed: Partly demolition and integration of the old wave protection structure into the new Eastern breakwater New Eastern breakwater New Western breakwater Berth pockets for some fishery boats. Rebuilding of on old breakwater and using the rock for different layers of the new structure The breakwaters had been designed in general as conventional rubble mound breakwaters. Fig. 4 shows all the elements and layers of the Eastern breakwater such as: Sinking mattress to lead to evenly settlements of the soft sub soil in some parts of the Eastern breakwater Filling and part of the core made of gravel and sand Core material 0.5 kg to 50 kg natural rock, density 2.7 kg/dm³ Filter layer natural rock, same density, weight 60kg to 300 kg, layer thickness 0.80 m Armour layer natural rock, same density, weight 1 t to 3 t, layer thickness 1.75 m Width of the crest 2.4 m. The loads for the Western breakwater were determined much lower then for the Eastern. The Western Breakwater consists of the following element and layers: Partly core material of gravel and sand Core material natural rock 0.5kg to 50 kg, density 2.7 kg/dm³ Filter layer natural rock, same density, weight 40 kg to 200 kg. layer thickness 0.60 m Armour layer natural rock, same density, weight 0.5 t to 1.5 t, layer thickness 1.35 m Width of the crest 1.9 m. Figure 4. Cross section of Eastern breakwater The original design was made by Inros Lackner AG Rostock. But the contractor, again the Joint Venture Heinrich Hirdes GmbH Rostock / Colcrete von Essen GmbH & Co. KG Rastede, changed the design partly, because the client had some budget problems and had asked the contractor for some practical ideas to save costs. The proposal of the contractor included to substitute some core rock material by gravel and sand and to cover this completely by a geotextile filter (sand mattress). Additional sand slopes in deep water had to be protected by rock furthermore. The client accepted the contractors proposal and had saved more then 1.5 million costs. The adaptation of the execution design had been carried out by designers of the contractor. The main advantage of the gravel and sand had been in the logistics. Marine gravel and sand borrow pits were situated only 30 km from the building site. The sailing distance to the rock quarries in Norway had been more than 800 km. This advantage had led to a shorter construction period for the core as well. B. Execution The execution of all works had started with the partly demolition of the old protection constructions. The old structures had been rebuilt up to 0.3 m under the level of the new filter layer. Boulders and rock armour were stored for the use in the new filter and armour layer. A part of the Eastern breakwater had to be founded at mud layers between 0.6 m and 1.8 m. Special sinking mattresses should lead to evenly settlements of the breakwater. The mud had been replaced mostly by the cheaper gravel and sand material. The sinking mattress was made of a combination of woven and non woven geotextile fitted out with in square placed fasces of pasturage. The prefabricated mattresses had a size up 35 m by 60 m and had been placed by special equipment. The gravel and sand core and filling material had been dredged by a small Trailing Suction Hopper Dredger with about 1,500 m³ hopper volumes, transported to the site and sprayed via a spraying pontoon in to the right place. Later a backhoe dredger had profiled the gravel sand surface. After that the geotextile sand mattress had been placed by a floating cable grab and other special 99

5 equipment. Core rock material covered the geotextile to protect it against the more heavy rock of the filter layer. The core material was supplied by self discharching vessels from Norwegian quarries, which loaded barges for the site transport. Hydraulic grabs based on pontoons had placed this material into the right profiles supported by computer based monitoring systems. The filter and armour layers had been supplied by an 8,000 t barge from Larvik in Norway. Site transport was organised by smaller barges. The under water profile had been carried out by hydraulic grabs based on pontoons. The crest had been profiled by a hydraulic grab based on the breakwaters. All the works required a high accuracy especially because the four different layer profiles and several changing of the water depth, which caused some crossover of the profiles. Totally the following materials had been placed in to the both breakwaters: 45,000 m³ gravel and sand 160,000 t rock 30,000 m² geotextile filter 9,000 m² sinking mattresses 60 pieces wooden piles as a pier construction for the fishery boats 6 steel piles for small passenger ships The very hard time schedule for a winter time project had been implemented in time. Dedicated designers and site managers as well as experienced operators on board of all the used equipment had cooperated with the client very well and all together they made this project to a success. This project is a very good example to change old military constructions into peaceful utilisations. 100

Breakwater on Soft Subsoil Problems of Design and Execution

Third Chinese-German Joint Symposium on Coastal and Ocean Engineering National Cheng Kung University, Tainan November 8-16, 2006 Breakwater on Soft Subsoil Problems of Design and Execution Siegmund Schlie