Steel piling in marine applications: Types, durability and installation
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Demand for larger and deeper berthing facilities at harbours and ports is constantly increasing to satisfy the expansion of world trade. The Port Engineer has a range of construction techniques available when planning new structures, and steel piling is often the material of preferred choice for qualities of robustness, reliability, and speed of installation compared to other options. This paper presents an overview of steel pile types, their durability, and the methods of installation and driving equipment most commonly used in maritime situations.
Steel pile types
Universal H Piles are low displacement piles particularly beneficial for penetrating dense stratums and hard rock. They are generally used in buried structures where the pile shaft is fully supported to prevent buckling failure about the weak axis of the profile. Soil heave and transmission of ground vibrations during installation are low compared to other pile types.
Tubular Section Piles are the most widely used steel bearing pile type and are suitable for supporting deep water structures such as jetties and dolphins. They are available in a large range of diameters, wall thickness and steel quality. A symmetrical radius of gyration allows long unrestrained lengths in open water to resist buckling failure and provide flexural strength for horizontal berthing and environmental forces.
Fabricated Box Piles are formed by connecting together individual sheet piles to form close sided sections. They can be transported from the rolling mill in single bars to be welded together on the job site. By using special corner elements and driving sequentially, it is possible to create significant load bearing foundations without the need for heavy pile driving equipment. In respect of design and installation, they have similar attributes to tube piles.
Interlocking piles for earth retaining structures
Interlocking steel sheet piles supported by tie rods and anchoring systems are prolific in harbour works. However, the demand to build very deep structures often requires profiles which have much greater strength and robustness than standard proprietary pile sections. CombiWalls and High Modulus Walls consist of tubes, box or I section ‘primary’ members interlocked with ‘secondary’ panels of Z or U profile sheet piles, and are used for deep water berths because of their high strength characteristics.
Straight Web Piles are commonly used in circular cofferdams to form self supporting gravity structures for dock closures, quay walls and breakwaters. Maximum depths and diameters of cells are limited only by strength of the bulb interlocks and also practicalities associated with installation.
Durability of steel structures
Up until the mid 1980’s it was accepted that corrosion of steel piles in marine applications was most severe in the splash and intertidal zone just above low water. However, engineers have become increasingly aware of a sinister form of bacterially induced concentrated rust termed ‘accelerated low water corrosion’ (ALWC) which manifests itself as a bright orange bloom giving off a pungent odour when disturbed. The rate of corrosion is extreme, and, unlike other corrosion mechanisms, ALWC is unpredictable as to where it will occur and its severity.
Selecting the right type of pile and other design criteria can create sustainable solutions to the problem of concentrated corrosion. The design life of the steel depends on the combined effect of imposed stresses and corrosion. For earth retaining walls, avoiding maximum stresses at or near low water by astute positioning of tie rods is desirable. Steel grades can be improved so that corrosion does not impair the strength to the same degree and the same effect can be achieved by selecting heavier section piles to provide a sacrificial thickness of steel. Protective coatings can also be used to delay the onset of rusting. High quality durable paint systems increase effective life by up to 20 years.
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