Structural and Construction Systems of Reinforced Earth Marine Wall
|دسته||رسوب شناسی و زمین شناسی دریایی|
|گروه||سازمان زمین شناسی و اکتشافات معدنی کشور|
|مکان برگزاری||بیست و پنجمین گردهمائی علوم زمین|
|نویسنده||Dr. Younes Daghigh|
|تاريخ برگزاری||۰۱ اسفند ۱۳۸۵|
One of the most contemporary applications for reinforced earth is in marine works. Although the total number of reinforced earth marine structures is not very high in comparison with the other applications, this is very much a rapidly growing area. The great number of reinforced earth structures including marine structures that have been built by Reinforced Earth Pty Ltd confirms this fact (RE Co., ۱۹۸۶, ۱۹۸۸a, ۱۹۸۸d and ۱۹۸۸e). Figure ۱ shows the total number of reinforced earth structures that were built by Reinforced Earth companies between ۱۹۷۵-۱۹۸۶. The geographic distribution of these structures and their distribution by type has been shown in Figures ۲ and ۳. As shown in Fig.۳, about ۷% of reinforced earth projects were seawalls and other types of marine structures.
In this paper, the structural systems, construction systems and design considerations of reinforced earth marine walls will be presented based on the available information about several structures built in marine environment.Case Studies۲>
Available information about nine reinforced earth structures built in marine environment is presented briefly in Table ۱. This information is included the project sites, dimension of the structures, special features and the problems which the structures were facing with.
Regarding the case studies reviewed in the last section, two different structural systems are advisable for reinforced earth marine walls. Depending on the type of project site, these system are: exposed site system and sheltered site system.
An exposed site system is suitable for exposed sites where breaking waves are expected and can be used as a seawall or as a breakwater to protect the coastal area against severe storms and waves. In this system a concave concrete panel is used for wave deflection and armour or riprap is used for toe protection (See Table ۱). It is clear that in this system, due to standing wave action, thicker facing panels are required.
A sheltered site system is suitable for sheltered sites such as harbours and can be used as a quay wall or wharf structure, or as a bulkhead for protection of land sliding. Sheet-piling in this system provides toe protection and also allows for construction to proceed in dry conditions (like a coferdam). Table ۱ shows the general features of the structural systems.
Each of the REMW structural systems need special design considerations. For an exposed site system, hydrodynamic efficiency is an important factor, so porous panels and wave reflectors are recommended to provide hydrodynamic efficiency. Hydrodynamically efficient coastal structures would :
· posses a steep or near-vertical slops
· be highly porous to absorb wave energy rather than reflect it (Hall, ۱۹۸۵).
Considering a suitable wave reflector and especially using a porous shape for facing panels can decrease the following deleterious effects (Hall, ۱۹۸۵):
· wave uprush (runup and overtopping)
· locally doubled wave height
· quadrupled power in the wave
· accelerated bed erosion
· creation of deeper water and so
· greater wave height admitted to the wall
For the sheltered site system, hydrodynamic characteristics are not important. In this system special loading due to wharf traffic, bollards and fenders must be considered. The special design considerations for both these structural systems are summarised in Table ۱.Construction Systems۲>
The main problem for designing both the exposed or sheltered site structural systems is the problems due to construction. Based on the case studies presented already, three different construction systems for reinforced earth marine structures can be recognised. These are: construction systems for dry conditions; tidal conditions; and underwater conditions.Each of these construction systems, have some special problems and require special design considerations. For example, underwater construction presents some important problems such as: foundation excavation; base levelling; panel aligning; connecting strips to panels; backfill placing; and compacting must be considered in structural design, while in construction in dry condition conventional methods like those used for construction of other non-marine structures could be used but a little additional consideration is needed. In Table ۳, the different construction systems for REMW, their problems and special design considerations have been summarised.
An evaluation of available existing reinforced earth marine structures consisting of design considerations and construction problems was presented in this paper. Based on this evaluation, suitable structural and construction systems of reinforced earth marine wall were presented.
According this study, two different structural systems are advisable for REMW: an exposed site system and a sheltered site system. The first is suitable for sites where breaking waves are expected. It can be used as a seawall or as a breakwater to protect the coastal area against severe storms and waves. For an exposed site system, hydrodynamic efficiency is an important factor, so porous panels, wave reflectors (concave concrete panel), and a steep (or near-vertical) slope are recommended for providing hydrodynamic efficiency. In this system, armour or riprap is used for toe protection. The second system is suitable for sheltered sites such as harbours, and can be used as a quay wall or wharf structure, or as a bulkhead for protection from land sliding. In this system, toe protection is provided by sheet-piling which also allows construction to proceed in dry conditions (like a coferdam). Use of sheet-piling instead of armour and riprap for toe protection, provides enough depth at the front of the wall for ship berthing. For sheltered site systems, hydrodynamic factors and wave force are not so important and hence a thicker facing panel is not required.
Three different construction systems were proposed for reinforced earth marine walls which are: dry condition system; tidal condition system; and underwater condition system. Each of these construction systems have special features and therefor require special design considerations. In dry condition construction systems, the conventional method used for construction of other reinforced earth walls can be employed. Dry conditions can be provided by using a coferdam (eg sheet-pile coferdam). In tidal condition construction system, the construction process is carried out during low tide and the structure is left semi-finished to become submerged during high tide. This method takes advantage of the tide, therefore, the construction operations must be synchronised with daily tide cycles. Underwater construction system presents another alternative for construction of a reinforced earth marine wall. For this system some special considerations must be paid to foundation excavation; base levelling; panel installation and aligning; strip-panel connection; backfill placement, compacting and testing in both design and construction stages.
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