An embedded retaining wall generally comprises interlocking pile elements installed into competent ground, which can either retain material in an unsupported cantilever state or with additional support typically provided by tie rods and associated anchorage systems. Civil engineers have a range of construction techniques and materials available to them when planning new structures within a port. When an embedded retaining wall solution is sought, steel piling is often considered the preferred choice, for qualities including strength, speed of construction and suitability for over-water installation when compared to other options, most commonly in European port construction.
This article provides a brief overview of the principal structural forms available for embedded retaining walls including steel and concrete piling elements, and lesser utilised materials such as plastic. The key benefits and limitations of each relating to strength, durability and construction techniques for port applications are also discussed.
Steel retaining walls
Interlocking sheet piles
There are numerous types of interlocking steel sheet pile sections available from either hot rolled or cold formed steel. Typically they are of ‘Z’ or ‘U’ profile, the shape of which are being continually developed by manufacturers to provide greater efficiencies in stiffness to weight ratio and speed of construction. However, even when installed in combination with an anchorage system, walls constructed using steel sheet pile sections alone are likely to have limited retained height and are not suitable for quays required to accommodate deep draft vessels without some form of relieving mechanism. Steel sheet piles alone also have a limited vertical load carrying capacity and are therefore less suited to providing support to quayside gantry cranes.
Combi or high-modulus walls
When larger and deeper port structures are required, combi or high-modulus walls comprising deep embedded heavy tubes, box or ‘I section’ primary elements in combination with partially embedded light secondary sheet pile members are typically used due to their high strength characteristics. For deepwater berths accommodating deep draft vessels or where heavy lift cargos are being handled, secondary infill elements are often eliminated altogether and the high strength primary elements are joined to form a high-modulus contiguous quay wall. Figure 1 illustrates a tubular steel combi-wall system designed by Mott MacDonald for a heavy lift general cargo quay in the UK where offshore wind farm components were required to be loaded onto jack-up barges alongside.
Benefits and limitations
The key benefits of steel pile retaining walls include their potentially high strength and load bearing capacity as well as their suitability for installation over-water. Steel piling is also a widely recognised and understood method of construction for retaining walls with relative advantages in the availability of construction plant, speed of construction, reliability and robustness compared to other options.
Durability of steel pile structures is often highlighted as a primary concern for port owners and operators who increasingly demand infrastructure to require low maintenance resulting in minimal disruption to port operations. Even with corrosion protection systems in place, such as sacrificial steel allowance, coatings and cathodic protection, some form of maintenance is still likely to be required due to their limited longevity and susceptibility to physical damage. This durability issue has been reinforced in recent times with a greater awareness of bacterially induced concentrated corrosion problems associated with steel structures such as accelerated low water corrosion (ALWC). Solutions to protect steel structures against ALWC have been developed, such as the corrosion treatment system LATreat™ developed and patented by Mott MacDonald and BAC Corrosion Control. This industry award-winning system is proving to be effective in the protection of marine structures against ALWC in a number of ports using only the components of seawater to sterilise and then deposit a protective coating on to the steel using a process of passing phased electrical current through the seawater, with no requirement for port closure and no ongoing maintenance requirements.
Concrete retaining walls
Although pre-cast concrete sections have been used for embedded retaining wall construction, bored cast in-situ methods including secant, contiguous and diaphragm bored piles are generally employed when a concrete embedded retaining wall is required in a port environment.
The diaphragm wall is an increasingly common form of concrete piled retaining wall used in ports. A diaphragm wall is typically constructed by excavating a rectangular hole in the ground, supporting the excavation walls using a suspension fluid such as bentonite, installing a pre-fabricated reinforcement cage and stop-ends to produce a defined profiled joint and pouring of concrete which progressively displaces the suspension fluid. Diaphragm walls are often used for deeper retaining structures within ports, as they can be designed to take very high structural loads, comparable to those of a steel combi or high-modulus wall. Concrete diaphragm walls can also be designed to have low permeability, and are therefore also used for the construction of slurry or cut-off walls to contain ground water or contaminants.