Sheet Pile Steel (U-Type)

A U-Type sheet pile, historically known as a Larssen pile after inventor Tryggve Larssen, is a hot-rolled interlocking steel section with a symmetric U-shaped cross-section comprising a central web, two equal flanges, and thumb-and-finger interlocking clutches at the flange tips. The defining structural characteristic is that these clutches are positioned at the neutral axis of the combined wall cross-section when piles are paired and interlocked — not at the extreme fiber as in Z-type piles. This neutral axis clutch position has two important design consequences: first, shear forces transferred through the interlock between adjacent piles at the neutral axis generate relative longitudinal slip between the two piles of a pair, and most design codes apply an interlock efficiency factor (commonly 0.7–0.9 depending on interlock type and design standard) to reduce the effective combined section modulus below the theoretical value; second, the interlock at the neutral axis experiences minimal bending stress, which means it can accept angular deviation between adjacent piles in plan without generating high contact stresses — this makes U-piles uniquely suitable for curved plan alignments and circular cellular cofferdam construction that Z-piles cannot achieve. Z-type piles have their clutches at the extreme fiber (outermost point of the combined wall), which eliminates interlock slip effects and delivers higher effective section modulus per unit weight for straight retaining wall applications, but prevents use in tight-radius curved alignments. In practice, U-piles dominate temporary works and cellular cofferdam applications while Z-piles are preferred for permanent high-efficiency straight retaining walls and quay structures.

The GU, AU, and PU designations represent different product families within the U-type sheet pile category, primarily differentiated by pile width and section geometry optimisation. GU series (ArcelorMittal designation, 600–750mm wide single piles) is the most widely used global series, with GU 6N through GU 32N covering section moduli from 400 to 2,720 cm³/m for the combined double-pile wall. The GU series features a relatively wide pile width (600mm standard) that reduces the number of piles required per metre of wall, improving installation productivity. GU N-series (e.g. GU 16N, GU 18N, GU 20N) are the current production standard with optimised geometry for improved section modulus efficiency compared to older non-N suffix sections. AU series (ArcelorMittal, 500mm wide single piles) provides deeper section heights (up to 450mm) for a given wall width, delivering higher section moduli suitable for deeper excavations and heavier permanent walls with fewer metres of interlock per metre of wall — AU 14 through AU 25 cover section moduli from 1,390 to 2,640 cm³/m. AU series piles are well suited for anchor wall systems with large free height. PU series (metric designation, 400–600mm wide) are produced primarily by European mills including Maanshan Steel equivalents under EN 10248, covering PU 12 through PU 32 with section moduli 600–3,200 cm³/m — directly comparable to GU series in structural performance but with slightly different dimensional profiles that require confirmation of interlock compatibility when mixing sections from different manufacturers. Japanese FSP series (JIS A5528, FSP-III through FSP-VIL) are the equivalent U-pile standard for Japanese construction practice. Section selection should be based on required wall section modulus from structural analysis, available pile lengths for the required wall depth, site pile driving plant capabilities, and interlock compatibility with adjacent sections.

U-type sheet pile walls provide inherent groundwater retardation through the close-fitting thumb-and-finger interlock geometry, but achieving true watertightness for contaminated land cut-off, dewatered excavations, and environmental containment requires additional measures to seal the interlock gap. Four principal methods are used: (1) Interlock Crimping — after pile installation, hydraulic crimping tools compress the interlock clutch from the exposed pile head downward, mechanically deforming the clutch to close the gap along the full pile length. Effective for temporary dewatering applications but requires accessible pile head and may not achieve full depth sealing in hard or obstructed ground. (2) Continuous Interlock Welding — fillet welding of the interlock contact zone from pile toe to head before driving (pre-welding pairs) or after driving (post-weld of exposed head section) permanently closes the interlock. Pre-welded pairs are driven as a unit, ensuring watertightness of the welded joint along the full driven length. Suitable for permanent groundwater cut-off walls and contaminated land barriers designed for 25–50 year service life. (3) Sealant Injection — injection of two-component polyurethane foam sealant or cement-bentonite grout into the interlock gap from the pile head using pressure injection equipment after pile installation. The sealant flows downward through the clutch under injection pressure, filling voids and setting to form a flexible or rigid watertight seal. Effective for retrofitting sealing to existing walls and for long pile walls where pre-welding is impractical. (4) Water-Activated Sealant Strips — neoprene, bentonite-impregnated rubber, or hydrophilic polyurethane sealant strips factory-inserted into the interlock clutch before pile driving; the strip expands on contact with groundwater to seal the interlock gap without post-installation treatment. Suitable for moderate hydraulic gradients and temporary to semi-permanent cut-off applications. Watertightness specification and testing should follow EN 12063 or project-specific permeability requirements, typically expressed as allowable seepage rate in litres per metre of wall per day.

Cellular cofferdams are gravity retaining structures formed by connecting sheet piles in closed circular or diaphragm cell plan configurations, filling the enclosed cells with granular material (sand, gravel, or crushed stone), and relying on the weight and internal friction of the fill — rather than the bending resistance of the piles — to resist lateral earth and water pressures. Common cellular cofferdam types include: (1) Circular Cells — independent circular plan cells of U-piles connected by arc-shaped connecting piles, suitable for straight or gently curved alignments such as river diversion cofferdams, navigation lock approaches, and offshore island construction; (2) Diaphragm Cells — main circular arcs connected by straight diaphragm walls of U-piles, providing continuous aligned structures suitable for port bulkheads, quay walls, and breakwater structures; (3) Cloverleaf Cells — larger diameter cells with internal cross walls for very wide cellular structures requiring high lateral load resistance. U-type sheet piles are the only standard hot-rolled section used for cellular cofferdam construction for a fundamental geometric reason: the clutch at the neutral axis can accommodate the angular deviation (plan curvature) between adjacent piles as the wall follows its circular plan shape — in a typical circular cell of 12–20m diameter, each interlock must accept 5–15 degrees of angular deviation between adjacent piles. Z-type piles have their clutches at the extreme fiber under high contact stress, and the Z-profile geometry physically prevents angular deviation between adjacent piles beyond approximately 1–2 degrees, making them unsuitable for circular cellular construction. The tensile hoop forces generated in the cell perimeter piles by the internal fill pressure are resisted by the interlock in direct tension — interlock tensile capacity and watertight sealing of the interlock (by crimping or welding) are therefore the critical design parameters for cellular cofferdam U-pile selection.

U-type sheet piles are available in standard mill lengths from 6m to 24m, with most production in 0.5m or 1m increments. Custom lengths up to 30m are available as special mill orders with minimum quantity requirements. For temporary excavation works requiring pile embedment plus retained height exceeding 24m, splicing by full penetration butt welding per EN 1011-1 / AWS D1.1 is the standard practice, with splice joints located at low-moment positions and staggered between adjacent piles. Extraction and reuse is a major economic advantage of steel sheet piles over competing systems such as secant pile walls, contiguous bored pile walls, and diaphragm walls, which are non-recoverable. Successful U-pile extraction depends on soil type and pile surface condition: extraction is most straightforward in cohesionless soils (sands and gravels) and soft to firm clays where piles have not been driven to refusal on hard stratum. Extraction becomes progressively more difficult in stiff to hard clays, dense sands, and cohesive fills where soil adhesion and negative skin friction resist upward extraction force. Pile extraction is performed using vibratory pile extractors (reversing the driving process) or hydraulic pile pullers applying static upward force combined with water jetting to break soil adhesion on the pile surface. Extracted U-piles are typically cleaned, straightened if bent, and inspected for interlock damage before reuse — interlock damage during driving or extraction reduces reuse value and may require local welded repairs. Typical reuse cycles for U-piles in temporary works applications range from 3 to 8 uses depending on soil conditions, pile handling care, and extraction method. Mill surface coating with anti-rust oil prior to first installation reduces surface corrosion during storage between uses and slightly reduces driving resistance and extraction force in cohesive soils.