The term sheet piling refers to any retaining wall type that is a) installed into the ground by driving or pushing, rather than pouring or injection, and b) is of relatively thin cross-section and low weight so that the weight of the wall does not assist in the wall’s stability.
The modern sheet piling industry is a little more than 100 years old with perhaps the most important changes in type and selection of products occurring since the early 1970’s. Sheet piling have been used in a wide variety of applications, especially marine bulkheads and retaining walls where space is limited. In addition to these, a special type of retaining wall is the cellular cofferdam, which are used extensively for both temporary and permanent structures.
Sheet piling are made in a number of materials. The material chosen depends upon a number of factors including both strength and environmental requirements. The designer must consider the possibility of material deterioration and its effect on the structural integrity of the system.
U Type Sheet Pile
Most permanent structures are constructed of steel or concrete. Concrete is capable of providing a long service life under normal circumstances but has relatively high initial costs when compared to steel sheet piling. They are more difficult to install than steel piling. Long-term field observations indicate that steel sheet piling provides a long service life when properly designed. Permanent installations should allow for subsequent installation of cathodic protection before excessive corrosion occurs.
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.
Z Type Sheet Piles
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.
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.
U Type Sheet Pile
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.
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.
