Quay Walls

Chinese-German Joint Symposium on Hydraulic and Ocean Engineering, August 24-30, 2008, Darmstadt

DESIGN CONCEPTS FOR QUAY WALLS FOR LARGE VESSELS Werner Richwien Institute of Soil Mechanics and Foundation Engineering, University Duisburg-Essen, Essen Abstract: The worldwide growth in container handling and other ship traffic leads to enormous challenges for harbour construction. Concerning container ships actually design studies are made for ship sizes of more than 15 000 TEU with a length of more than 400 m, a width of 70 m and a draft of 21 m. Along with these ship sizes the traffic loads and the operation loads of container cranes increase dramatically. This development has to be anticipated by new quay concepts with extended dimensions especially in the height between harbour bottom and top of the structure. Thus a number of innovations in quay wall design have established during the last 10 years. New calculation techniques permit better modelling, and thus the possibility for extension existing quay walls for higher water depth and entirely different types of structural forms than the conventional ones. In the construction phase the quality of the structure in situ could be improved and at the same time high strength pre fabricated elements with hoisting loads up to 4000 tons can be accurately placed by new technologies. The paper discusses design concepts for large sheet pile quay walls, from relatively simple one dimensional structures to integrated structures with highly complex interactions between the soil behind the retaining wall and the wall itself. Structures of these types are actually discussed controversially in Germany, the JadeWeserPort at Wilhelmshaven is such an example. The paper discusses advantages and risks of these new design concepts and thus might help to objectify the actual discussions on this question.

I. INTRODUCTION Quay walls are earth retaining structures at which ships can berth. They are equipped with bollards and fenders, and they are used for the handling of goods by cranes and other equipment moving alongside the ship. In ancient times ship landing had been restricted to natural bays in which ships were drawn to the dry. At places where ships could moor villages and towns grew up. Mooring places grew into quays and developed into ports and trading places. Early types of quays were gravity walls, their retaining function is obtained by the self weight of the structure. Gravity walls have been constructed from stone blocks, since the first century BC already from concrete. Especially at coast lines with weak soils sheet pile walls have been developed. They get their soil retaining function and stability from the fixation capacity of the soil. The sheet pile wall then is a cantilever beam elastically fixed in the ground. With increasing height the deflections at the top become to large so that the top needs to be anchored. Once more the fixation capacity of the soil is the stabilization element for the anchors. Early sheet pile structures had been made from wooden elements, connected by groove and tongue. The anchors were chains leading to anchor plates back behind of the structure. In case of weak soils forming the upper soil strata the walls have been anchored by inclined piles or by pile racks.

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Later the wall elements were U-shaped steel profiles, connected together with bolted on Z-profiles. These elements combined a higher stiffness with a length depending to the construction height. The invention of the Larssen interlock in 1904 was the starting impulse for an accelerated development of the sheet pile construction method which is not yet finished. Nowadays sheet pile structures are most widely used for quay walls. Due to the development in shipping during the last decades design depth of quay walls increased dramatically, resulting in combined sheet pile walls with heavy profiles. This development is a challenge for the designer to find structural solutions for an optimal capacity utilization of standard sheet pile structures. But it is recognized, that the conventional anchored sheet pile wall, fixed in the ground, has reached dimensions which can not be enlarged. II. SOIL STRUCTURE INTERACTION Already the simple structures of the early period used the same soil mechanic principles as known from the nowadays large sheet pile constructions. Soil is at the one hand the loading element, it takes the structure loads on the other hand. Thus design of quay walls is firstly an evaluation of active and passive earth pressure. Early in 1776 Coulomb developed a theoretical approach for the calculation of earth pressure, in which he postulated that active earth pressure on a retaining wall withdrawn from the soil behind becomes a minimum when the soil shear strength is activated to its maximum.

Chinese-German Joint Symposium on Hydraulic and Ocean Engineering, August 24-30, 2008, Darmstadt At the other hand passive earth pressure on a wall which is pushed against soil becomes maximum when soil shear strength is fully activated. Up to today this soil-structure interaction is the basic concept of quay wall design. It has been adopted by Blum for his sheet piling calculation method and it forms the background for the design concept according EAU 2004. According to the soil structure interaction earth pressure is redistributed to the more stiff parts of the structure due to activated shear strength in the soil. III. STRUCTURAL DESIGN ASPECTS At first sight the designer of a quay wall seems to have a high degree of freedom to chose between many different design concepts. In practice however the local construction conditions predominate the design. Nevertheless some general design aspects have to regarded. A. Type and position of achor The most simple type of a quay wall is an anchored sheet pile wall without any superstructure (Fig. 1). All surface load is directly transferred to the wall by the earth pressure. The wall is supported by soil resistance in the bottom and by the anchor. Bending moments of the wall depend on the position of the anchor and the stiffness of the support in the soil. They can be reduced, when the anchor is placed in a deeper position, this is however often restricted with respect of the water level when the wall is constructed in the open sea.

slope beneath of the superstructure, which again reduces earth pressure. In tidal zones the linking of ships beneath the superstructure can be prevented by fenders. When the superstructure is positioned deeper the length of the sheet piles can be reduced to any desired length and additionally the active earth pressure is further reduced. This option is however restricted when the wall is not constructed in a dry pit. C. Position of sheet pile wall In addition to soil retaining function the quay wall has to bear vertical loads from cranes. So the position of bearing elements directly under the crane track is optimal for the load transfer into the soil. The wall itself than can be placed under the rear side of the superstructure, the bottom in front of the wall is sloped, so the system length of the rear side wall can be slightly shorter (Fig. 3). However the full, not relieved earth pressure acts on the back-positioned wall and the passive earth pressure in front of the wall is due to the sloped bottom small, so that a the benefit of this design is restricted.

Figure 2. Quay wall with superstructure, Hansaport, Hamburg [1] Figure 1. Structure of single anchored sheet pile wall [1]

Improvement of the support in the soil is possible by densification or replacement of the soil in front of the wall. Horizontal anchors with anchor walls or anchor plates are easy to install, but their bearing capacity is often restricted when soils in the upper regions are weak. Inclined anchor piles reach down to soils with higher strength. B. Superstructure A compact superstructure placed on the wall and on raked piles allows the transfer of crane beam load and all traffic loads directly into the subsoil (Fig. 2). Thus the retaining wall itself is not affected by the operational loads and has to keep soil pressure only. Additionally to this load relieving the superstructure screens the earth pressure. Further on it allows a

Figure 3. Back-positioned retaining sheet pile wall [1]

D. Inclined sheet piles Because the crane track is some distance from the front of the structure, the available space can be

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Chinese-German Joint Symposium on Hydraulic and Ocean Engineering, August 24-30, 2008, Darmstadt used to drive the sheet pile wall inclined (Fig. 4). In combination with anchor piles incline by 1:1 the axial load in the sheet pile wall is reduced, and additionally the sheet pile wall acts as a stabilizing foundation member. On the other hand active earth pressure, but mainly passive earth pressure are reduced due to the inclination of the wall, so that the effect of inclination is restricted. An important advantage however is that the inclination of the wall gives space for the structural design of the bearing of the superstructure on the wall. E. Static system The sheet pile wall is a beam. loaded by soil- and water pressure, anchored at the top and fixed in the soil by passive soil resistance. Within certain limits the depth of the wall can be varied, depending on the choice of the static system. Free earth supported walls just ensure the stability, with increasing depth fixed end moments can be activated. Between free earth support and fully fixed support, partly fixed support offers a variety of depths. Fixed sheet piles minimize the risk of loss of stability caused by insufficient passive earth pressure and offer an extra capacity for extreme load situations. The choice of fully free support might be considered in case of subsoil which could cause driving risks.

IV.

TWO COMPETING SOLUTIONS

A. Tender design For a new container harbor at Wilhelmshaven (Jade) the tender design was made as a vertical anchored combined sheet pile wall with a concrete superstructure, placed on inclined steel piles. Top of the superstructure is NN +7.5 m, the sheet pile wall and the steel piles under the superstructure reach down to NN –45 m in the very dense pleistocene sand (Fig. 5). Harbor bottom is NN –19.8 m, thus the height of the quay wall is 27.3 m. The length of the sheet pile wall is 46.5 m, the length of the anchor pile is 47 m. The sheet pile wall is placed 4.5 m behind of the front of the superstructure, in front of the sheet pile wall circular piles of 1.2 m diameter, spaced 4.32 m, form the support for the fender system. The front sheet pile wall is backfilled only up to NN –3.0 m to reduce earth pressure. Even with this design trick the largest available profiles (PSp 1035 S) are necessary as primary piles. At the rear end of the superstructure a sheet pile wall Larssen 605k retains soil behind of the superstructure. The tender design makes use of all design options to optimize the structure, but more or less it is something like a final evolution for this type of quay wall construction.

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Figure 4. Inclined sheet pile wall [1]

A typical cross section of the specific proposal can be seen in Fig. 5. The retaining wall is anchored by 4 layers of anchors and anchor plates reaching up to 45 m in the backfill. The construction of the quay wall was planned as follows: After the retaining wall, existing of primary elements HZ 775 A and secondary elements AZ 26 is positioned and driven to its design depth (NN – 34.5 m) the soft soil behind of the wall was intended to be dredged under water to a level of NN –17 m. On the cleaned up dredging base the anchor rods, diameter 6.25”, of the bottom anchor layer should be placed, ready assembled together with the anchor plates, one anchor for each primary element (a = 2.27 m). The placing of the anchor is patentregistered by the Josef Möbius Bau Aktiengesellschaft Hamburg. In the next step the bottom anchor layer should be embedded in under water dumped sand up to NN – 12.50m, starting in front of the anchor plate and moving towards the retaining wall. Thus the anchor capacity is guarantied before the wall is loaded by earth pressure. In the same way the anchor layers at NN – 12.5 m and NN – 5 m should be placed and embedded in sand. The topmost anchor layer at NN * 2.5 m was finally planned to be placed in dry. The superstructure was planned as a relatively light concrete beam, placed on the retaining wall and on the fender piles in front of the wall. It is at the same time the bearing beam for the front crane rail. The rear crane rail was planned to be founded separately on concrete piles. V. COMPARISION OF TENDER DESIGN AND SPECIFIC PROPOSAL At first sight the specific proposal is recognized as a multi anchored retaining wall, a construction which is known since long from deep excavation pits. It forms a statically multi-indeterminated system with the advantage that the span is cut in small sections, so that the design bending moment is small compared to the tender design. Consequently the needed elastic section modulus of the primary elements decreases from W = 16 656 cm³ in the case of tender design (PSp 1035 S) to W = 7230 cm³ for the specific proposal (Hz 775 A). Thus this specific design is a typical example for an advanced engineering solution, it is an evolution of quay wall

Chinese-German Joint Symposium on Hydraulic and Ocean Engineering, August 24-30, 2008, Darmstadt design which offers a large potential for increasing cuts. Apparently the structural safety of the quay wall is relatively insensitive to differences in the density of the backfill in front of the anchor plates, since due to overlapping load distribution zones differences in the deformations to activate passive earth pressure are smeared. Thus the separately placed anchor plates interact with the soil like a throughout back wall of a coffer dam. The process of activating passive earth pressure is however accompanied by horizontal deformations of the wall, and it is not clearly to identify in which

phase these deformations occur. In case that they occur during construction phase they can easily considered and compensated when the superstructure is placed. In case that the deformations go along with the operation loads their magnitude may however be of paramount importance for the serviceability of the structure. Since a structure of this type never has been build before, it was not possible to identify clearly the structure deformations to be expected during operation and over lifetime. This was the main reason why finally the specific proposal has been rejected by the awarding authority.

Figure 5. Typical cross section of the quay wall according to the tender design [2]

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Chinese-German Joint Symposium on Hydraulic and Ocean Engineering, August 24-30, 2008, Darmstadt

Figure 6. Cross section of the quay wall according to the specific proposal [3Specific proposal

VI. CONCLUSIONS The classical design concepts of quay walls for large vessels with only one anchor placed close to the top and a free span down to the harbor bottom has reached dimensions which hardly allows for the ship dimensions needed for today traffic. If it is realistic that the development of ships draught will increase in future as in the past, new concepts for the quay walls are needed urgently. A very promising one is multi-anchored combined sheet pile wall. The statical concept is well known from deep excavation pits, the static stability of the ready build structure is relatively robust. Open question remain however with respect to structural deformations during lifetime. REFERENCES [1]

[2] [3]

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Handbook Quay Walls, Center for Civil Engineering research and Codes (CUR), CUR-Publication 211E, Gouda, The Netherlands, 2005 JadeWeserPort Realisierungsgesellschaft Wilhelmshaven, Tender design 2005 Bietergemeinschaft Bunte u.a.; Specific proposal JadeWeserPort, 2005