Introduction

When the surface soil is not strong enough to sustain the massive loads of heavy structures such as skyscrapers, bridges, or marine terminals, engineers must go downward to the world of deep foundations. Steel pipe piling is the leading solution in that field, in which the high-strength steel geometry penetrate through soft, compressible soil layers and transfers the structural load to strong bedrock or bearing layers found meters underground. This engineer’s guide analyzes steel pipe piling, examines the geotechnical mechanics of load transfer (friction versus end-bearing), and breaks down the most critical ASTM A252 specification to enable engineers to rightfully specify materials for safe, secure foundations.

How it Works

What is steel pipe piling To the layman, steel pipe piling seems to be just a hollow tube that is driven into the soil; however, that basic tube is part of a complex force transfer mechanism. The steel pipe acts as a geotechnical structural column. The capacity of the pile is governed by the interaction of the steel surface with the soil around it.

There are two primary mechanisms by which steel pipe piles transfer loads from the superstructure to the ground:

1. End-Bearing Piles (The “Column” Mechanism)

In this set-up the steel pipe pile functions as a true column.The pile is pushed through layers of soft, loose, or unstable soil until the tip (the “toe”) contacts a layer of solid bedrock or is embedded in extremely dense soil.

· Load Transfer: The entire load of the structure is transferred vertically through the walls of the steel pipe directly to this hard stratum.

· Engineering Focus: The determining factor is the ability of the steel to carry the load and withstand the compressive strength of the bedrock. The pipe has to be sufficiently rigid so that it doesn’t buckle under the load of the building on top of it.

steel pipe piling load transfer end bearing vs friction.

 

2. Friction Piles (The “Grip” Mechanism)

Frequently, the bedrock is too far down to be reached economically. In these situations, engineers rely on Friction Piles. These piles do not seat on a firm base to transfer load, rather they develop skin friction on the pile surface and surrounding soil.

· Load Transfer: When the pile is driven into the ground, the soil applies pressure to the pipe walls. This causes friction (sometimes referred to as ”skin friction”) for the full length of the pile. The friction sum is that which supports the structure.

· The SSAW Advantage: This is where manufacturing style matters. SSAW (Spiral Submerged Arc Welded) pipe is frequently the best choice for friction piles. Using an SSAW pipe’s helical weld seam creates a small external ridge or protrusion. This roughening of pipe surface increases the soil-to-steel “grip” and the friction coefficient accounting a perfectly smooth pipe.

ASTM A252 Specification

While API 5L is fluid transport’s bible, ASTM A252 (“Standard Specification for Welded and Seamless Steel Pipe Piles”) is the deep foundation industry’s law. Understanding of difference between ASTM A252andAPI5L is essential for any purchasing engineer to save him from liability and loss of money.

The Core Philosophy: Structure vs. Pressure

The fundamental difference lies in the intended use.

· API 5L (Line Pipe): Designed to hold internal pressure. It requires strict testing for leaks (hydrostatic tests) and toughness to prevent bursting.

· ASTM A252 (Piling Pipe): Designed to hold external structural loads. It treats the pipe as a structural member (like a beam or column). Therefore, ASTM A252 does NOT require hydrostatic testing. Specifying a hydro-test for a piling pipe is a common “copy-paste” error in procurement that adds unnecessary cost without adding structural value.

The Steel Grades: Why Grade 3 is the Standard

ASTM A252 defines three grades based on yield strength.

· Grade 1: 30,000 psi (Rarely used today).

· Grade 2: 35,000 psi (Common for smaller, general construction).

· Grade 3: 45,000 psi (The Industry Standard).

Why ASTM A252 Grade 3 Dominates: For today’s heavy duty infrastructure, the default is ASTM A252Grade3. The reason is certain strength to weight efficiency. With Grade 3 steel (which has a minimum yield strength of 45,000 psi), engineers can achieve the needed axial load capacity with a thinner wall thickness.

· Example: To support a 500-ton load, a Grade 2 pile might require a 20mm wall. A Grade 3 pile might only require a 16mm wall.

· The Result: Significant savings in total steel tonnage, transportation costs, and welding consumables, without compromising structural integrity.

Open-Ended vs. Closed-Ended

The shape of the pile tip alters how the pile engages with the soil. When ordering steel pipe piles for deep foundations, engineers have two basic options: open-ended and closed-ended designs.

1. Open-Ended Pipe Piles

As the name suggests, the bottom of the pipe is left open.

· Installation Mechanics: The soil enters the pipe as it is driven. The pile goes deeper and the soil within creates a “Soil Plug.” This plug is eventually compressed to the extent that it behaves like a solid bottom, but at initial driving, it enables the pile to penetrate through more difficult soil strata more readily than a closed pile.

· Best Application: Used when the pile needs to penetrate dense soil layers to reach deep bedrock, or for offshore platforms (like jacket legs) where the pile must go very deep.

2. Closed-Ended Pipe Piles

The bottom of the pipe is sealed, usually with a flat steel plate or a specialized conical point.

· Installation Mechanics: The pile is a so called “displacement pile.” As it is driven in, it displaces the soil to the sides, compacting the soil around it and enhancing friction resistance.It acts as a full end-bearing column.

· Best Application: Used in loose sands or soft clays where increasing the soil density is beneficial, or where the pile rests directly on rock.

Allland’s Capability: Standard pipe mills do not sell plain ends. But as specialized piling manufacturer, Allland provide full range of fabrication services. We can also pre-weld high-strength Conical Points (for rock penetration) or Flat Plates (for soil displacement) straight from our factory. We also weld Pile Shoes (stiffener rings) to the tip of the pipe to avoid buckling when it impacts hard rock, substantially saving time and energy at the construction site.

Why SSAW Dominates Piling

Although LSAW pipe is the choice for very high pressure applications, when it comes to piling, SSAW pipe for piling (Spiral Submerged Arc Welded) holds the largest market share. It’s not just  a question of cost; it’s a question of geometry and physics.

1. Diameter Capabilities (The Cofferdam Factor)

Deep foundations have to be very large in diameter. Bridge cofferdams, and dolphin piles for ports, as well as wind turbine foundations, can require pipes with diameters of 80 inches, 100 inches, or even 120 inches.

· LSAW Limitation: LSAW is limited by the width of the steel plate (usually maxing out around 56-60 inches).

· SSAW Advantage: SSAW is made of steel coil. By varying the angle of the spiral, a normal width coil can be formed into a pipe of quite large diameters.Allland can manufacture SSAW piling pipe to a diameter of 3000mm(118 inches), which they hold the unique feasibility for large ocean engineering.

2. Unlimited Length (Reducing Field Splices)

Piling jobs are not uncommon that need to be driven 50m+ Standard pipes are 12m long, and must be field welded (spliced) to join them, which is expensive and hazardous.

· SSAW Advantage: SSAW pipe can be made in any length that can be transported, as it is formed from a continuous coil. Allland provides piling pipe in lengths up to 30 meters (100 feet) or higher. This significantly decreases the amount of splices in the field needed, resulting in a faster project schedule and overall better integrity of the pile string.

3. Structural Integrity (The “Hoop” Effect)

During the driving process, a pile is subjected to immense impact energy from the pile hammer.

· SSAW Advantage: The spiral weld seam encircles the pipe like a bandage or a spring. When the pile is driven, this spiral configuration plays a role in stress equalization and crack propagation resistance.It acts as a “hoop reinforcement,” which renders the pipe extremely resistant to splitting even in hard driving conditions.

Conclusion

Steel pipe piles form the foundation of modern geotechnical engineering, and its performance is largely dependent on compliance with the ASTM A252 standard and knowledge of load transfer mechanisms. As a professional manufacturer of ASTM A252 Grade 3 piling pipe, Allland is able to manufacture super-large diameter SSAW and high-strength LSAW piles with customized end treatments such as splicing and pile shoes to be the ideal technical partner for your next major deep foundation project.

FAQ

Q1: Can ASTM A252 Pipe be utilized for Water Transmission?

A1: It is not recommended. ASTM A252 is a structural. However, it does not require hydrostatic testing for leak proofing of pipe like API 5L or ASTM A53. If you do not want an additional hydro-test during purchase, each A252 pipe is only warranted for structural support, not for fluid holding.

Q2: What is the difference between Grade 2 and 3 piling pipe?

A2: The only significant difference is Yield Strength. ASTM A252 Grade 2 has minimum yield strength of 35000 psi (240 Mpa), and Grade 3 has minimum yield strength of 45000psi (310 Mpa). Upgrade to Grade 3 for about 30% additional strength. This enables Engineers to either design piles which can carry substantially heavier loads or utilize thinner wall thicknesses for the same capacity and strength, thus making Grade 3 the most economical option for extensive size projects.

Q3: Why is SSAW better for piling than LSAW?

A3: SSAW is preferred for two main reasons: Dimensions and Cost. Due to higher productivity of SSAW technology much larger diameters (max. 100 inches+) and longer continuous lengths (up to 30 meters+) than those of LSAW can be offered, which minimizes the costly field welding. Moreover, the continuous spiral process is more efficient, and has a lower cost per ton and high radiai strength (“hoop strength”) to resist driving stresses.