Breakwater on Soft Subsoil Problems of Design and Execution

Transcription

1 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 Heinrich Hirdes GmbH, Rostock Abstract The paper reports about problems in design of the breakwater system of the town Barth in Mecklenburg Vorpommern which is planned as a combination of harbour protection and reducing the risks of storm flood for the town of Barth. The projected breakwater system should consist of rock structures. Several boundary conditions can be identified as most important design factors. The paper shows how small mistakes in decisions of the client and designer will cause problems in execution of the construction works. These can cause problems in the realisation of works. Additional works had to be done, leading to the extension of execution time and the increase of construction cost. The paper presents the used technology and the problems with difficult soft subsoil in this project. Engineers learn mostly a lot of things by mistakes. So for the Barth project there is a chance to find better technical solutions for the second phase of the project. Possible solutions will be exemplified. 1 Introduction The town of Barth (Fig. 1) and the environmental authorities of Mecklenburg Vorpommern made a concept for a combination of harbour protection and reducing the risks of storm flood for the town of Barth. The implementation of the project was divided into two phases, the western and the eastern breakwater system. The western breakwater was partly carried out in The detailed design and the execution of the eastern breakwater are expected in 2008/2009.

2 Barth Rostock Figure 1 Map of the area round Barth ( Google) The continuation of the western breakwater will be combined with the design and execution of the eastern. The harbour of Barth is situated in the inner coastal waters, the so called Bodden. The characteristics of this area are shallow water and difficult hydrological and geological conditions. The planned breakwater system consists of rock structures. 2 General Design Aspects 2-1 Stages in the Life of Rock Breakwaters All projects start with a need: something that is desired, required or lacking the lack of a shelter of a harbour, for example. The project definition stage defines this need by setting project objectives. These objectives will present requirements and restrictions. Clear objectives will assist in establishing the appropriate engineering solution to meet the identified need. This is the starting point for the designer. At the concept design stage broad solutions are developed, such as structure type and feasibility. One of the main activities at this stage tends to be the identification of the functions, constraints and information requirements that will enable the design to go forward. Preliminary design is when many of the investigations and study activities must be carried out, including determination of wave climate or current regime, environmental assessments and economical analysis. This has to be an interactive process, involving many parties, to gain agreement and select a preferred solution. Once the various criteria have been satisfied, detailed design must involve the development of all structural elements, using further in situ investigation and physical and technical data, to produce drawings, specifications and bill of quantities.

3 During the working design phase further design modification may be necessary as a result of on-site difficulties such as unforeseen ground conditions or changes in working approaches. When this happens, the designer should ensure that the original design concepts are fully understood and the design changes do not compromise any other aspect of structure performance. In the operational period the continued performance of the structure is ensured by implementing a monitoring and maintenance programme. 2-2 Technical Considerations for Planning and Design Planning and design of rock breakwaters as well as other constructions demand the attention of different requirements. Economical, environmental and social considerations have to be checked with the same accuracy then technical considerations which are described in Table 1. If the engineer follows strictly these considerations in planning and design, the probability of big mistakes can be reduced. Sometimes the clients try to reduce the research of technical requirements because of financial problems. Mostly these reductions will cause much more financial and time schedule problems during the execution phase. Table 1 Technical considerations for planning and design of rock breakwaters Planning and design considerations Aspect Functional requirements (performance) Considerations - Achievement of functional requirements (e.g. limited overtopping, reducing wave activity in a harbour riverbank erosion) - Acceptable structural stability and residual risk of failure (factors of safety and choice of design conditions) - Changes in acceptable probability of failure over time - Adaptability (e.g. change of use of the structure over time) - In service health and safety requirements - Ground conditions (geotechnical) - Topographic and bathymetric conditions Physical conditions - Hydraulic forces - waves, currents, water levels - Morphological changes - Sediment load and movement - Uncertainties of physical conditions (confidence limits) - Material properties (e.g. rock density, quality, durability and availability) Technical data - Accuracy of design information, including analytical methods used - Structure-specific design methods - Nature of failure (progressive or instantaneous, complete or partial)

4 Construction Maintenance - Ability to build the structure - Contractor experience and resources - Health and safety issues - Conditions during construction (e.g. storm or flood frequency and magnitude) - Access of construction plant - Material quality - Alternative material availability (sources) - Site area for storage of materials and operations - Characteristics of structure response - Frequency and type of intervention - Availability of suitable resources for repair (materials, plant, expertise) - Funding - Accessibility for construction plant 3 Requirements on Subsoil Investigation Structures like rock breakwaters require especially accurate investigations of subsoil. The structure and its foundation must be designed in such a way, during the design life, foundation displacements and movements are kept within the limits that the structure can tolerate without effects on its structural integrity and functional capability. One important aspect of the design of foundation is the stability of the seabed and the possibility of scour and undermining around the structure under wave and current actions. Difficult ground conditions refer generally to the existence of unfavourable subsoil strata at the site. The presence of such conditions, if not properly handled, may lead to both problems during the construction stage and the future use of breakwaters. Guidelines for site investigation are given in different publications, e.g. the "Manual on the use of rock in coastal and shoreline engineering" and "Port Works Design Manual Part 4". The basic geological and geotechnical data requirements are summarised in terms of the physical and mechanical characteristics which need to be investigated. These data are obtained by means of a wide variety of field and laboratory tests, supplemented by detailed descriptions made by a geotechnical engineer or geologist. The values of geotechnical parameters for design should be determined from careful assessment of the range of values of each parameter. Particular attention should be given to the quality of ground investigation and the adequacy of test data with respect to the inherent variability of the materials encountered. For structures founded on silt or clay (low permeability) consolidation needs a long time. The most critical period for stability is during construction and just after completion. The undrained shear strength of the founding strata is the controlling critical factor for overall stability. The designer should determine the appropriate

5 undrained shear strength as well as the long term (drained) parameters, and assess the foundation stability under all conditions. Particular attention must to be paid to place the rock material of the structure when clay or silt deposits remain under the foundation. Rapid fill placement may induce instability on the foundation as the excess pore water pressure due to the fill loading will take some time to dissipate completely. The effect of the filling rate or the stages of loading on stability should be investigated with respect to the shear strength of the underlying soil at the time of construction. The settlement expected during the design life of breakwaters should be assessed to ensure that it is acceptable for the proposed use of the structures. In general, the settlement after completion of construction should be limited, depending on the type, importance, stability and usage of the structure and the site condition. 4 The Conditions at the Barth Area The following boundary conditions were identified in the area of Barth: - Hydraulic boundary conditions Barth is situated on the southern coast of the Western Pomeranian Boddenzone (see Fig. 1) with a very flat coastline. Due to the position of Barth to the Bodden waters the area is endangered by flooding during stormy high waters. The extreme water level was measured in the year 1872 with m about HN gauge. Such remarkable water levels will endanger as well the harbour as the city of Barth. Significant parameters for Barth are: Calculated high water level: Wind direction: Wind speed: Fetch Length: Wave length L: Wave period T: Significant wave height H s : + 1,95 m HN NNE 25 m/s 5,000 m m 2.84 s 0.78 m There are various water depths inside the harbour of Barth with a maximum of 3.00 m below medium water level. The waterways to the harbour are also dredged up to 3.00 m. Outside of the harbour and the fairways water depths between 0.50 m and 2.00 m exist. The last hydrographical survey was executed in These survey results had been used as basis for the design. The calculation of amour unit weights according to the formulas of HUDSON and VAN DER MEER requires smaller rock units compared with the influence of

6 ice pressure. - Ice conditions Ice pressure plays a decisive role in dimensioning of amour unit weight for the breakwater in this area. Ice thickness of 20 cm up to 53 cm can be expected. - Geological and geotechnical boundary conditions Geotechnical experts made some studies on the geological and geotechnical conditions of the planned site area. The geological situation formed following the last ice age. Generally three different layers are typical for this area: a) Holocene silty mud and peat with a low undrained cohesion and layer thickness of 0.3m up to 4.10 m b) Holocene silty sand c)pleistocene sand and bolder clay The detailed results of description of subsoil are presented in Table 2. Table 2 Soil characteristics of the site area Soil characteristic Friction Unit Layer Cohesion Elastic Coefficient of Layer angle weight horizon modulus Permeability Thickness effective undrained of soil in meter φ' c u γ E k [ ] [kn/m²] [kn/m³] [MN/m²] [m/s] -0,8 to -5,6 mud ,5 1*10-11 to ,5 peat ,5-1,0 1*10-8 to ,0 to -9,0 sand *10-5 to ,0 clay 28 5/ *10-5 to 10-4 The geotechnical expert recommended the exchange of the soft subsoil because a displacement of the muddy layers can be expected. - Sedimentation, shore evolution and environmental impacts The "Barther Bodden" can only be considered in connection with the complete system Bodden waters. This system is an ecological high sensitive one. The water exchange with the Baltic Sea influences the quality of water in the Bodden zone. The design of rubble mound breakwaters in this area has to take water exchange aspects into consideration. For this reason water exchange openings have to be arranged in the breakwater system. In several parts of the harbour the removal of organic sedimentation was recommended. - Construction and maintenance considerations The line of the breakwater was determined by functional requirements. The construction should be carried out in combination of floating and land based equipment. There are different experienced contractors for this kind of works available. Rock material can be supplied as well from Scandinavia as from

7 southern Germany. Execution time is only limited by frost conditions because of safety requirements. The size of amour layers of the breakwater was designed for zero maintenance with normal weather conditions. - Cost considerations. During the planning phase a budget for the construction was estimated in the right level. When the designer made the bill of quantity he didn't use any position for unpredictable circumstances. The budget for execution had been settled on basis of the winning offer of tender process without any reserves because of limited aid from the county government and the European Union. 5 The Breakwater System in Barth 5-1 Design of the Western Breakwater The breakwater system for Barth consist of a western part with a length of 830 m, which should be constructed in the first construction period, and an eastern part, that will to be implemented in a second stage respectively. The planned structures of the breakwater system are shown at Figure 2. Figure 2 Top view of the planned breakwater system in Barth The Barth breakwater system consisted of rubble mound breakwaters made of natural rock. For the western part a breakwater with the following main parameters was designed: Top level after settlement: m HN Slope inclination sea side: 1:2.0

8 Slope inclination harbour side: 1:1.5 Slope Inclination of breakwater heads: 1:2.0 Amour layer sea side: Amour layer harbour side: Core material in different layers: > 1.0 t per unit > 0.5 t per unit 0 to 60 kg Blockade against sediment passage to sea side:by non woven geotextile Content of hollow space inside the rock structure:42 % Figure 3 demonstrates a general cross section of the western breakwater. The designer held the opinion that the exchange of the soft subsoil which the geotechnical expert had recommended is not necessary. For foundation on the soft subsoil a woven geotextile with high tensile strength was designed to reduce settlements on a level of 0.45 m on maximum. 5-2 Execution Figure 3 General cross section breakwater We started the execution with hydrographical survey and detected water depth that were 0.3m less then the survey from The placement of the woven geotextile with high tensile strength was the first step of construction of the western Barth breakwater. The single tracks were 5.00 m wide and had to overlap 1.0 m. The start was at the western shore line. On this geotextile a gravel layer of 4 60 mm had to be installed.

9 Figure 4 Displacement of mud in the direction of construction With the placement of gravel a displacement and strong settlement of the soft subsoil started. This process continued and became stronger during the placing of core material. Following the results of our measurements on the settlement gauges the core of the breakwater sunk into the soft subsoil up to the level of the sustainable soil layer. The muddy subsoil was displaced to sea and harbour side as well as to front side, the construction direction (Fig.4). Some additional dredging works had to be done to continue the construction. The general technology was disturbed by some additional works. The new process is demonstrated at Figure 5. Figure 5 Technology influenced by replacement of mud Thus, the displacement of the soft subsoil happened by expensive rock material. The supposition of the designer referring the content of hollow space inside the rock structure was completely wrong. Due to the wide spread size of rock material this content averaged only 28%. These both facts caused a massive increasing of the quantities of rock and the construction costs. The budget of the client was limited because of the missing of reserves for unpredictable circumstances. Then the client had to come to a decision, the construction of the breakwater had to be interrupted about 200 m before the end. The next stages were profiling of the core and placing of the armour layers by hydraulic grab (Fig. 6 and 7).

10 Figure 6 Profiling by hydraulic grab Figure 7 Completely profiled breakwater In spite of all problems we built this part of the western breakwater in a good quality. For continuation there will be a big disadvantage. A water based technology has to be used which is more expensive then the land based in the first stage. 6 Conclusions and Recommendations a) The client and the designer have to find out a budget that is flexible enough for unpredictable circumstances. b) The designer should never use a hydrographical survey which is some years old. That is saving money at the wrong place. c) If the designer hasn't got experience in the field of rock armour works, he must lock to some experts and/or experienced contractors for advice. Then it is possible to choose the right details for the rock structures, e.g. the hollow space inside. d) The designer should never ignore recommendations of geotechnical experts referring to the properties of soil and necessary measures. e) There are various types of improvement of the ground for a foundation of a breakwater in the case of bad soft subsoil, such as - Vertical drainage - Vacuum consolidation - Embankments on different kinds of piles - Geotextile encased columns - Vibrated stone columns - Soil exchange. f) For the relatively thin layers of soft material in the Bodden area soil exchange is the cheapest solution, but there are actual requirements on storage area for the organic soft soil.

11 7 References Ciria, Cur. Manual on the Use of Rock in Coastal and Shoreline Engineering. CIRIA London, CUR Gouda, Ciria, Cur and Cetmef. The Rock Manual. The Use of Rock in Hydraulic Engineering (2 nd edition). CIRIA, London (published in advance in the web), The Government of Hong Kong Special Administrative Region, Civil Engineer Department, Civil Engineer Office,.Port Works Design Manual Part 4. Guide to Design of Seawalls and Breakwater, Pilarczyk, K.W. and R.B. Zeidler. Offshore Breakwaters and Shore Evolution Control. Balkema Rotterdam / Brookfield pp , , Arbeitsausschuss "Ufereinfassungen" der HTG, Recommendations of the Committee for Waterfront Structures Wasserstraßen EAU Ernst & Sohn, Berlin, 2004.

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