Metal sheet piling was a natural advancement in the evolution of this product as we entered the “Iron Age” in the mid-1800’s. Cast iron was used to make some crude sections, but these were not successful due to lack of ductility. Toward the end of the century, Bessemer steel was developed and mills began hot-rolling I-beams, channels and angles, among other structural shapes. Freistadt-type piling appeared about 1890, fabricated from a rolled channel section as shown in Figure 1. Z-bars riveted to the web provided a groove into which the flange of a channel could slide, thus forming a crude but innovative interlock. A “Universal” type sheet piling introduced in Great Britain about 1895 utilized hot-rolled I-beams and special clips to join the flanges of the I-beams together. The efficiency of this wall was low because the I-beams were aligned in the weak structural direction.
Figure 1: Freistadt Sheet Piling
Inventors were striving to develop a sheet piling that would contain interlocks rolled into the beam during the manufacturing process, rather than attached afterwards by riveting. Gregson (USA) patented a bulb and jaw interlock in 1899, however this still resulted in production of a flat section with relatively small section modulus. Trygve Larssen obtained a German patent in 1904 for a deep, hot rolled section that greatly increased the strength and efficiency of steel walls and represented a major advancement. Larssen’s piling wall assumed a “wave shape” when assembled and all subsequent developments for efficient sheet pile walls are based on this concept. Larssen’s section still contained a partially fabricated interlock and it was not until 1914, that a rivetless Larssen interlock appeared in Germany.
Figure 2: Historical Sheet Pile Sections
In the United States, Lackawanna Steel Co. (later acquired by Bethlehem Steel Corp.) was a flat sheet piling shape and several arched types with rolled, integral interlocks as early as 1910.Carnegie Steel Co. (U.S. Steel Corp.) offered three flat sections with rolled-on interlocks and one fabricated section. By 1929, Carnegie’s catalogue illustrated four deep-arch, two shallow-arch and two straight sections. Some of these and other historical sections of sheet piling are showing Figure 2.
Z-shaped piles followed the Larssen concept for a wave-shaped profile but with the added advantage that the interlocks are formed on the outer elements of the section. The extra metalis put to best use, since it is well out from the neutral axis of the wall. Larssen interlocks are located on the neutral axis. Surprisingly, Z-shaped piles were produced in Europe as early as 1911. The Ransome profile looked very much like some of today’s lightweight Z-shapes. The deeper Lamp Z-pile introduced about 1913, resembles a modern ball and socket Z-type pile.
In Europe, Z-type shapes fell from favour when the Larssen U-types were developed. Two Z-shapes were introduced in the United States in the 1930’s and became quite popular. PZ-38 and PZ-32 offered wider and deeper sections than any of the arch shaped shapes then available. Z-shaped piles obtained some impetus in the U.S. from the long-standing controversy regarding the actual moment-resisting properties of U and Arch shaped sections.
Figure 3: Typical Hot-Rolled Steel Sheet Piling
Z-shaped piles interlock on the wall extremities and provide a solid web connecting the two flanges. When the PZ-27 section was introduced in the 1940’s, its section modulus of 30.2 in3/ft was almost three times that published for the arch section with the identical weight per square foot of wall. This section subsequently became the all-time most popular sheet piling section in history. Z-type shapes are now produced with section modulii ranging from 8.6 to about 85 in3/foot of wall.
The Z-type piling is predominantly used in retaining and floodwall applications where bending strength governs the design and no deflection (swing) between sheets is required. Most producers do not guarantee any swing although some can generally be attained or area can be built by providing some bent pieces in the run. Turns in the wall alignment can be made with standard bent or fabricated corners. Typical configurations are shown in Figure 3.
Z-piles are not used in applications when interlock strength is required such as filled cells. These sheets would tend to stretch and flatten in these cases. No minimum interlock strength is offered for this reason. When interlock tension is the primary consideration for design, an arched or straight web piling should be used.
STRAIGHT WEB SECTIONS
Flat profile sections were originally produced only because of mill rolling limitations. Competition and customer demand prompted the expansion into structurally efficient sheet piling. It was discovered that these flat profiles had strength in tension that was advantageous for building circular, filled structures from sheet piling. About 1908 a large cellular cofferdam was built on the Black Rock River in Buffalo N.Y. in order to de-water the site for a new lock. This conceptwas progressively expanded to include circular and diaphragm-shaped cells for piers and breakwaters that might have formerly been built of timber cribs or masonry.
The use of large diameter, cellular cofferdams was given a special impetus in the 1930’s when the Tennessee Valley Authority began a series of hydro dams and navigation locks on that river system in the south-eastern United States. Not only did TVA engineers develop new design methods for designing these large structures, they developed better ways of installing and maintaining them.
Flat sheets have little strength to resist bending, but do have very strong interlocks to resist “hoop” stress. These piles are used almost exclusively for building large, filled cellular structures. Flat sheets must provide some ability to “swing” between sheets so that a circle can be closed. Most manufacturers will guarantee a minimum swing of 8 to 10 degrees between adjacent sheets for standard lengths of piling. For overly long pieces, these warranties must generally be negotiated.
Available interlock strengths must be known in advance in order to design a structure that will be safe against bursting. Most manufacturers will guarantee a “minimum” interlock strength based on tension tests conducted on a number of representative production samples. It has been determined from experience that interlock dimensional tolerances that fall within certain limitations will provide tension values characteristic of the entire production run. Flat sheet piling is available only as a hot rolled product, since the cold-finishing process does not provide an interlock with sufficient strength in tension. Interlock strengths have been gradually increased due to the demand to build larger cells for deeper cofferdams.
Most flat sheet piling has been used to construct temporary cellular cofferdams. After the initial use, the sheets are pulled and used in other portions of the project or perhaps sold for another project elsewhere. Other flat sheets are used in permanent structures such as breakwaters, earth containment sites, piers and other applications.
COLD FINISHED PILING
Since the early 1970’s another method of producing steel sheet piling has greatly expanded the availability and the selection of sections. This new method uses hot-rolled sheets in coil form, fed through a series of cold-rolling stands to form “Z” or “arch” shapes complete with a simple, hook-type interlock. This involves a relatively inexpensive capital expenditure compared to the hot-rolled product and has attracted a number of new producers.These steel pilings are shallow-depth sections, cold formed to a constant thick-ness of less than 0.25 inch and manufactured in accordance with ASTM A 857. Yield strength is dependent on the gauge thickness and varies between 25 and 36 kips per square inch (ksi). These sections have low-section moduli and very low moments of inertia in comparison to heavy-gauge Z-sections. Specialized coatings such as hot dip galvanized, zinc plated, and aluminized steel are available for improved corrosion resistance. Light-gauge piling should be considered for temporary or minor structures. Light-gauge piling can be considered for permanent construction when accompanied by a detailed corrosion investigation. Field tests should minimally include pH and resistivity measurements.
Figure 4: Typical Cold-Rolled Sheet Piling Sections
See Figure 4 for typical light-gauge sections.
HIGH MODULUS SECTIONS
There is a limited but regular demand for sheet piling with strength properties that exceed those available from standard products. These may be required for deep excavations, poor soil conditions, deeper dredge lines and other special conditions.
SHEET PILING NOMENCLATURE & IDENTIFICATION
U.S. producers of sheet piling standardized the identification of sheet piling sections so they could be specified without reference to a particular manufacturers product. The identification included a “P” (piling”) “Z” (type or shape) and “27” the weight, or PZ-27. Arch and flat shapes were similarly described. Non-U.S. and cold-finishing producers have their own “in house” identification systems. There is now no universal nomenclature system. It is common practice recently to specify the bending moment to be satisfied which then allows the contractor considerable flexibility in his selection of a section and a supplier. This bending moment specification should not be used blindly, however, as many sheet pile designs (especially those using vinyl or pultruded fibreglass sheeting) are principally governed by deflection.
ORDERING SHEET PILING
Like other steel products, steel sheet piling may be ordered by reference to a standard specification. In the United States this standard is published by the American Society of Testing Materials (ASTM) 1916 Race Street, Philadelphia, PA 19103-1187. The basic ASTM Specification A-328 and others listed may be obtained by writing to the Society or visiting their website//www.astm.org.
This specification covers the steel making process, the chemistry requirements, the minimum yield and ultimate strength. Delivery is referenced in ASTM Specification A-6. The ASTM Specification does not cover interlock tolerances, straightness, interlock strength, nor does it cover rental or second hand material. These are between buyer and seller.
Other Specifications include:
- Canadian Specification CSA 44 W, CAST 44W/70
- British Specification BS4360 – Various Grades
- European Specification: ST SP 37; ST SP 45; ST SP 5.
STEEL SHEET PILING TODAY
While the annual consumption of sheet piling in this country rarely exceeds 250,000 U.S. tons, the number of producers and the availability of sections has increased dramatically in the last ten years. In 1960 there were two U.S. producers, each offering nine sheet piling sections. Today there are at least 14 U.S. and non-U.S. producers offering over 200 sections in this country. Competitive factors have generated development of new, wider, more-efficient sections. Large Z-shapes are now available for deep construction with section modulus of almost twice that previously available. A wall system has been developed using large H-sections combined with light Z-shapes that greatly increases the section modulus. Light weight “gauge” material is produced on the cold forming mills for economical shallow bulkheading and trench work.
Higher strength steels up to 60 ksi yield point have also been effectively used in sheet piling design. These grades offer the opportunity to save weight or to extend bending or interlock strengths beyond those of conventional grades. For those applications that require it, corrosion resistant steel can also be specified as well.
(This article comes from pilebuckinternational.com editor released)