Are Sheet Piles Watertight?
Individual pieces of sheet piles especially steel, PVC, and FRP are impervious materials which means they are 100% watertight. But as a sheet pile wall, it needs welding or application of sealant at interlocks to achieve desired water-tightness. Sheet piles are interconnected side by side using clutches that employs either the ‘Larssen’, ‘tongue and groove’, or ‘ball and socket’ system. These connection joints create voids or gaps that make the wall susceptible to seepage or leak.
While moderate seepage is tolerable for applications such as temporary retaining walls, high seepage resistance is required for most applications such as contamination cut-off walls, bridge abutments, tunnels, basement walls, and more. It is necessary to determine the required degree of seepage resistance in order to come up with a cost-effective solution to employ for a certain type of project.
Factors Affecting Water-Tightness of Sheet Piles
Sheet pile walls are not inherently watertight, but they can be designed to be one. This imperfection is simply because pile driving requires certain amount of ease leaving spaces or clearances between interlocks. The water-tightness of sheet pile wall is determined by major factors, including the material used, the interlocking mechanism, and the installation method.
Material: The choice of material can affect the water-tightness of sheet piles. Besides being corrosion-resistant, steel and plastic sheet piles are non-porous materials which make them impervious by liquids. With their interlocks properly sealed, these materials are expected to deliver high level of water-tightness. Concrete and wooden sheet piles at the other hand, degrades with time and has extent of porosity. Cast concrete sheet piles if not vibrated properly, results to honeycombs which are potential causes of leaking. Concrete sheet piles as they age, also develops cracks especially when exposed to movements especially seismic activities. Steel in contrary does not. The choice of material should be based on the site conditions and the required level of water-tightness.
Interlocking Mechanism: The interlocking mechanism is a critical factor in ensuring the water-tightness of sheet pile walls. The interlocking system must be designed to provide a tight seal that prevents water from seeping through the joints. There are different types of interlocking systems, including tongue & groove, Larssen interlock, and ball & socket, each with varying degrees of water-tightness.
Installation Method: The installation method can also affect the water-tightness of sheet pile walls. The sheet piles must be installed with the correct driving parameters to ensure that they are properly interlocked and that there are no gaps between the sections. The quality of the installed sheet pile wall depends on the selection of driving equipment, method, and level of workmanship. With high quality, the wall is expected to exhibit better performance against water intrusion. If the installation is not done correctly, it can compromise the water-tightness of the sheet pile wall.
Over time, voids at the interlocks are also believed to be “self-sealing”. Fine particles from the environment gets inside the voids, accumulate, and completely fill the gaps becoming a natural filler. The build-up of rust in between the interlocks also helps the sealing. However, “self-sealing” is not a design solution that should be relied upon as it is uncertain and takes effect in a considerable length of time
Additional Measures for Water-Tightness
In addition to the prior-mentioned factors, there are other measures that can be taken to enhance the water-tightness of sheet pile walls. These include:
1. Sealing: Joints or interlocks can be sealed using various methods, such as welding, grouting, or injecting a sealant.
Welding – Like any other sealing methods, welding applies to both vertical and horizontal sealing. Vertical sealing prevents the migration of water or any liquid from one side of the sheet pile wall to the other side. Horizontal sealing stops capillary rise of water from any sub-surface to the ground. Depending on the sheet pile section, ‘Z’ or ‘U’, either steel plates or round bars are welded to conceal the interlocks from the pile toe up to the top of the sheet pile, which is usually embedded into a reinforced concrete capping beam. In horizontal sealing, a steel ‘puddle flange’ water bar is welded perpendicularly to the full profile of the sheet pile wall
Sealant application - Interlocks can be sealed with bituminous materials to achieve a watertight joint. Such materials can be applied in the workshop or on site. The watertightness is achieved according to the displacement principle - excess sealant is forced out of the interlock when threading the next pile.
Grouting – Seepage resistance can also be done by injecting bentonite or any swelling and hardening chemical fluids at the free interlocks during sheet pile installation. Another way is by drilling holes besides the interlocks and filling it with cement bentonite slurry, this process is called grouting.
2. Waterstops: Swellable or hydrophilic waterstops are special seals, polyurethane or elastomeric compounds, that are placed in the joints between the sheet piles. They are designed to provide an additional layer of water-tightness and prevent water from seeping through the joints. Swellable waterstops are preformed plastic or rubber cords which are caulked into the interlocks and swells in water to create a tight seal between spaces.
An example of these materials that demonstrates high and proven performance in any condition and environment is The Hyper Seal DPS-500, a product of ESC Steel. Hyper Seal DPS-500 is a highly effective hydrophilic polyurethane sealant with a high expansion coefficient of 500%- or 5-times volume increase. Reach us about the product at: www.escpile.com/interlock-sealant
Wooden wedges forced into the interlock gaps can also be used as remedial or emergency material as it also expands in time when immersed in water.
3. Other means: As an augmentation or an ultimate remedy, water-tightness for special cases of high seepage resistance requirements can be achieved by installing a separate impervious sheet behind the sheet pile wall such as high-density polyethylene membranes.
1. The Larssen interlock is found to provide substantial water-tightness for sheet pile wall applications where minor seepage is allowed or manageable. Applications includes retaining walls, cofferdams, diaphragm walls, and other temporary walling structures.
2. In projects where moderate to high seepage resistance is needed, the used of double piles with a seal-welded intermediate interlock is recommended. The free clutch of the double pile which will be threaded at site should come with a filler. Applications requiring this degree of water-tightness includes cut-off walls, bridge abutments, and walls below ground.
3. 100% water-tightness can be achieved by welding all interlocks present in the entire steel sheet pile wall. In the case of multiple piles, the interlocks are factory-welded, this means that only the remaining interlocks between groups of sheet piles have to be welded on site.
Achieving 100% watertightness is a real challenge especially when difficulties are encountered during pile driving and sealant application. However, combination of well-designed and manufactured sheet pile interlock and properly applied sealant can practically eliminate fluid seepage through sheet pile walls. This has been proven in so many projects successfully delivered by ESC Steel, including scenarios where sheet pile walls are subjected to huge hydrostatic pressures. View these projects at: www.escpile.com/projects
Sheet piling projects, whether steel, PVC, or FRP sheet piles are used, needs professional assistance from the design stage until the implementation phase and completion. This is to avoid painful losses in time and money due to undesirable results, especially intolerable water seepage or leak.