Precast Concrete Quay wall blocks in the UAE are the large gravity retaining structures that hold back backfill, resist wave and berthing forces and provide a stable edge for ships to dock at, and they are vital to the UAE’s port and coastal infrastructure.
They are not simply precast, like a manhole ring or a boundary wall panel, but are engineered, heavy-mass concrete units, frequently of tens of tons each, that are expected to serve for decades in one of the most demanding environments that any concrete can encounter: constant exposure to the sea, tidal fluctuation, and the mechanical stresses of vessels, cranes, and cargo handling.
With the UAE moving ahead with the development of ports, marinas and offshore structures, precast quay wall blocks are still the chosen way to build structures with reliability and on time.
What Quay Wall Do
A quay wall is a gravity retaining structure. The wall does not need to be anchored or piled to withstand lateral pressures from backfill and water, it relies on the mass of the wall and the friction between blocks to hold it up. Precast concrete blocks are made off site, hardened to full strength, and then delivered to the site and stacked to the desired height. The use of this blockwork system has been practiced in the Gulf region for many years and continues to be one of the major techniques used for port and marine works. Concrete blockwork quay walls are a proven type of retaining wall for use in marine and port facilities throughout the Gulf for their durability, suitability for harsh marine conditions, and more recently, in offshore artificial island infrastructure during construction and operation.
Each block is connected to the adjacent ones by friction, weight and in many cases shear keys formed by pressing the keys into the block shape, so that the completed wall is a mass, not a collection of individual blocks. This is why a quay wall constructed from precast blocks is able to resist berthing pressure from ships, hydrostatic pressure differences between ebb and flow tides and surcharge loads due to cranes and stacked cargo along the quay.
UAE Marine Conditions and Design Standards
A mix of environmental challenges unique to marine structures in the UAE are not commonly found in precast products found in inland applications. The chlorides concentration in seawater is much higher compared to the usual concentration found in the UAE groundwater, and the tidal area is subjected to repeated wet and dry cycles which strengthen the process of chloride attack on the embedded steel. Mix design, cover, etc. requirements can be different elevations within the same wall due to the different risks associated with each part of the wall below the permanent waterline, in the splash zone, and in the tidal zone.
The designs of such structures have traditionally been done in line with BS6349, the main reference document for marine and coastal structures, which also was originally based on an allowable strength design approach before a revision in 2010, in which ultimate limit state design methods were introduced based on Eurocode. While this update is not necessarily followed by all marine projects in the region, some engineers reviewing or specifying quay wall works must ensure that they understand which design basis a project or consultant is applying to the work, as the two methods may result in different block sizes and stability margins for a wall of the same height.
The blocks are not the most important thing, the foundation is. The walls are usually not constructed directly on seabed material but on a prepared bed of rock or crushed rock that is capable of spreading the extremely high bearing pressures imposed by the stacked block wall. Anti-scour protection is added on the seaward side to stop the foundation becoming undermined over time by the action of the waves, and the landward side is filled with graded fill with sometimes geotextile fabric on top to help control drainage and the loss of fines behind the protection.
Technical Range and Block Sizing
As there is no standard size for quay wall blocks, they are constructed in different sizes depending on the wave loading, tidal range and berthing forces to be expected at a particular port. Precast suppliers have turned out thousands of single quay wall blocks weighing in the order of 50 to 80 tons each and meeting critical tolerance requirements to ensure each course fits over the previous one. Where the required height of the retained material is comparatively small, and where the cost of a gravity wall is more expensive than the cost of a reinforced earth structure, lighter stacked blocks, often nearer to a metre, can be used instead of relying only on the mass of the blocks, tying the blocks together with geosynthetic reinforcement.
Block geometry is not based on a catalogue, but is created by engineers. Shear keys, rebate joints and cast-in lifting points are all specified to the specific project drawings and formwork is usually custom-made for each block size and profile that is required per project. This is one of the reasons why the manufacture of quay wall blocks is not regarded as part of the more commoditised classes of precast blocks – each project is a different set of tools and casting plans.
Manufacturing Process
The initial step in the production of quay wall blocks involves creating a mix that has a long marine service life and not only high early strength. Low-permeability concrete, sometimes with supplementary cementitious materials like fly ash or slag in the mix, will delay the migration of chlorides through the block to the embedded reinforcement; and a controlled water-cement ratio and care of the heat of hydration will minimize the risk of thermal cracking in the curing process, a true concern given the mass of concrete in a single block and ambient temperatures in the UAE. Reinforcement is specified and detailed with greater concrete cover than normal structural members to cover the aggressive chloride environment to which these blocks will be subjected throughout their service life.
When producing large mass-concrete blocks, attention needs to be paid to the temperature of the concrete in the inside curing process to prevent the formation of excessive thermal gradients between the core and the surface of the block, or else the block will crack due to the uneven cooling process. To address this risk on large blocks, some manufacturers use thermal maturity monitoring during the casting and stripping process to control the risk. Cured blocks are then removed from formwork, checked for dimensional tolerances and surface quality and stored in the yard awaiting transport and installation.
Quality Assurance and Compliance
The products of this step are considered a continuous process not a one-time quality assurance, due to the structural role that these blocks play and the challenges faced when working on remedial steps after a quay wall is in service. All blocks are tested for compressive strength on the concrete batch being used, dimensions checked to approved drawings and documented as each block is cast, cured and dispatched. On larger port projects it is part of the contract, as a block is much harder to replace if it has been placed and backfilled than it is to be caught at the yard.
To ensure compliance with the BS6349 design provisions, as well as any additional specification provided by the port authority or main contractor, the compliance needs to be determined early in the design process, as the design basis has a direct impact on the dimensions of the blocks, amount of reinforcement and number of courses of blocks for the desired wall height.
Applications of UAE Marine and Coastal Projects
Quay wall blocks are installed at any point where a load-bearing edge needs to be formed between land and water. They are used by commercial ports for the construction of berths to accommodate big ships and big cargo handling machines. Developments around Marina’s apply the same principle but in a less intense manner, to hold back backfill around boat berths and walkways. Precast block structures are also used for breakwaters and revetments, which are not strictly quay walls, to prevent erosion of harbours and coastlines by waves. The blockwork method has been also successfully employed in both the construction and permanent works phases of offshore and artificial island projects where a relatively certain environment is provided by precast production.
Delivered and Installed on Site
The blocks are transported in most cases by marine transport or by barge to the installation site, as very large blocks may not be convenient on road transport. Each block is installed by crane (rated for block size) usually on a barge or on the completed portion of the quay, and is set precisely against the course below, as per the setting-out drawings. Courses are added behind the wall in stages as the backfilling process is carried out to ensure the wall is loaded in stages rather than all at once.
Talk with Your Marine Project Requirements
Quay wall block projects require a correct design basis, a correct block size and the specification of the marine grade mix, from the initial stages, as these structures are designed to last for decades in the continuous presence of seawater. As one of the UAE’s most dependable providers of precast concrete solutions optimized for marine applications, Solid Rock Concrete works , provides precast concrete components and services to contractors and developers on all UAE port, marina, and coastal infrastructure projects. Talk to our technical team about your project drawings, height of your walls and where your project is located, and we’ll start planning your block size and delivery.
