Steel piles are a common choice for foundation systems, especially for large structures that require substantial load-bearing capacities. These piles, which can be rolled sections, fabricated shapes, or sheet piles, are designed to provide stable support for buildings, bridges, and other heavy infrastructure. With their exceptional strength and durability, steel piles are capable of supporting significant loads—up to 100 tons per pile. However, like all construction materials, they come with both advantages and challenges. This article explores the various aspects of steel piles, including their types, benefits, drawbacks, and protection against common issues such as corrosion.
Introduction to Steel Piles
Steel piles are structural elements made from high-strength steel used to transfer loads from a structure to the underlying soil or rock. They are often used when the surface soil is not capable of supporting the weight of a structure or when deep foundations are needed to reach stable soil or bedrock.
Steel piles come in several forms, including rolled sections, fabricated shapes, and sheet piles. The latter can be connected in a box shape and driven as a single unit. This makes steel piles especially useful for large construction projects like skyscrapers, bridges, and piers, where immense load-bearing capacity is necessary.
Advantages of Steel Piles
Steel piles offer several key advantages that make them a preferred choice in certain foundation applications:
- Large Bearing Capacity: Steel piles are highly efficient at bearing heavy loads. Unlike some other materials, steel can be designed to support up to 100 tons per pile, making it ideal for large structures with significant weight demands.
- Penetration Ability: Steel piles can penetrate through tough layers of soil, including stiff clays and even boulders. This ability to reach deeper, more stable layers makes steel piles ideal for projects where the surface soil conditions are challenging.
- Low Soil Displacement: During the driving process, steel piles cause minimal soil displacement, which helps preserve the surrounding environment and avoids disturbing nearby structures.
- Durability: Steel piles are robust and can withstand rough handling during transportation and installation. Their strength and flexibility make them resilient to damage during the construction process.
Disadvantages of Steel Piles
Despite their many advantages, steel piles also have some limitations and potential drawbacks:
- Corrosion and Electrolysis: One of the most significant challenges with steel piles is their susceptibility to corrosion. In certain environments, particularly in marine or industrial areas, the metal can degrade over time due to exposure to moisture, oxygen, and salts. This can compromise the pile’s structural integrity.
- Cost: Steel piles tend to be more expensive than other types of piles, such as concrete or timber. The high cost is often justified by the piles’ superior load-bearing capacity, but this can be a factor to consider, especially when the bearing stratum (the layer of soil that supports the pile) does not develop a large pile capacity.
- Effectiveness in Friction Pile Applications: While steel piles are highly effective for bearing capacity, they are generally less efficient than friction piles in certain soil conditions. In friction piles, the load is transferred to the soil around the pile through skin friction, while steel piles rely more on end bearing. In soils that do not provide significant friction, friction piles may offer better performance.
Common Types of Steel Piles
There are several common types of steel piles, each designed to meet specific project requirements. The two most widely used types are H-piles and pipe piles.
H-piles
H-piles are structural elements shaped like the letter “H” and are typically used for projects that require deep penetration and high load-bearing capacity. The design of an H-pile is carefully engineered to withstand large impact stresses during driving. Both the flanges (the horizontal parts) and the web (the vertical section) of the H-pile are rolled with equal thickness to avoid damaging the thinner sections. Additionally, the width of the flange is usually at least 85% of the pile’s depth, ensuring rigidity and strength, especially in the weak axis.
Pipe Piles
Pipe piles are made from seamless or welded pipes, which may be filled with concrete for added strength. Pipe piles come in two varieties:
- Open-ended Pipe Piles: These piles are driven with open ends, allowing the soil inside the pipe to be cleaned out during driving. This makes open-ended pipe piles suitable for deeper driving in certain soil types, as the internal soil can be removed as the pile is driven further.
- Closed-ended Pipe Piles: These piles have a solid driving point at the tip, which makes them more suitable for driving through hard soils, boulders, or other obstructions. In some cases, these piles are filled with concrete for additional stability.
The choice between open-ended and closed-ended pipe piles typically depends on the soil conditions at the construction site. Open-ended piles are more adaptable to deeper penetration, while closed-ended piles are better suited to overcoming dense or obstructed layers of soil.
Splices of Steel Piles
In some cases, steel piles need to be spliced together to achieve the required length. For H-piles, splicing is typically done using welded or bolted joints, with high-tension bolts providing secure connections. Splicing is usually required when the pile length exceeds the available size of a single pile section.
For pipe piles, splicing is often done using butt welding or by using a sleeve. It’s essential that splices are designed to handle the stresses that may arise from bending moments, particularly when the splice is located above ground. Piles are rarely perfectly straight, and the driving process may cause them to bend, creating additional stress at the splice. Engineers typically design the splice to resist bending moments that can range from one-third to one-half of the pile’s moment capacity.
Corrosion in Steel Piles
Corrosion is one of the most significant threats to steel piles, especially in environments where moisture, oxygen, and salts are prevalent. Over time, corrosion can weaken the pile, compromising its structural integrity. Several methods can be employed to protect steel piles from corrosion:
- Additional Thickness: To compensate for corrosion, an extra thickness of steel (usually around 1/16 inch) may be added to the pile’s section. This thickness helps ensure the pile remains structurally sound even as corrosion gradually erodes its surface.
- Removal of Corrosive Soils: In some cases, it is possible to remove organic or contaminated soils near the ground surface and replace them with non-corrosive materials. This method is most effective when the corrosive soils are located at shallow depths.
- Concrete Encasement: For piles near the ground surface, concrete encasement provides a durable barrier against corrosion. While this is an effective method, it can be costly, especially when the encasement extends to significant depths.
- Cathodic Protection: This technique involves introducing a slow electrical current into the steel pile to prevent the removal of electrons, thus stopping the corrosion process. Cathodic protection is particularly effective in marine or industrial environments.
While coatings, such as paint, are commonly used to protect steel from corrosion, they are not always effective for piles that will be buried underground. For exposed sections of steel piles, periodic repainting may be necessary. In polluted water environments, coal tar paint or concrete encasement is often the preferred method of protection.
Conclusion
Steel piles are a powerful solution for foundation support, offering excellent load-bearing capacity and the ability to penetrate challenging soil layers. However, they do come with challenges, primarily related to cost and corrosion. Understanding the different types of steel piles, their advantages and disadvantages, and the necessary methods of protection can help engineers and construction professionals make informed decisions when selecting a foundation system. By properly addressing potential issues such as corrosion and choosing the right type of pile for the soil conditions, steel piles can provide long-lasting, reliable support for large, heavy structures.