Sheet piles offer an economical solution

Dhairya Narayan Jha.

Steel sheet piles are long structural sections with interlocks which form a continuous retaining wall. To retain soil or water in most cases, sheet piles have been used for all kinds of temporary and permanent retaining structures being extensively used for quay walls, cofferdams, tunnels, underground parkings, shoring for foundation pits, sewerage and drainage works, micro tunnelling, to name a few.

Why steel sheet piles?

Ease and speed of construction and repeat uses are two of the most important reasons why steel sheet piles are preferred over other alternatives. A systematic and design-based approach to select sheet piles leads to an optimised solution, thus making it better than conventionally used materials.

Execution of sheet piles can be faster, with less resources and manpower. In the majority of the cases, just a vibrohammer would be sufficient for installation. Many of the contractors have their own vibrohammers.

For temporary works, a contractor can be assured of repeat uses and high resale value of the materials.

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Importantly, sheet piles are available with buyback and rental options too. Delivery with exact specification is possible from mill rolling and local stocks.

Sections with low to high elastic section modulus are available in the forms of trench sheets, sheet piles and combined walls such as sheet piles with tubes, box sheet piles and HZ system.

For large retaining heights where cantilever wall is not possible, sheet pile walls can be supported with struts, or an anchorage system can be easily designed and implemented.

Unlike other solutions, sheet piles don't occupy much space. It is easy to extract for reuse. For strict plot limits in construction projects such as buildings, roads and utilities, sheet piles have been extremely useful.

Sheet piles can be designed for to prevent corrosion. With the development of new grades, steel sheet piles have better corrosion resistance. Solutions like protective coatings and cathodic protection are available to prevent corrosion.

Sheet pile walls can also be made fully water tight. The increasing trend of having more underground car parking structures is one example of the benefits from water tightness of sheet pile wall. Steel sheet piles are completely impervious.

The only possibility of water infiltrating through a sheet pile wall is by flowing through the interlocks. For sheet piles with good interlocks, sealing systems are not necessary for temporary retaining structures, where moderate rates of seepage are acceptable.

With application of proper sealing system or welding of joints, the wall can be made completely water tight.

Which steel sheet piles?

When a project decides to use sheet piles, selection of the right sheet pile profile needs a careful examination of all the alternative sections available in order to reach an optimised solution to the design requirement.

It is unique, as unlike other steel sections, sheet piles do not have standard sections produced by all manufacturers. They produce different sections with different sectional properties. Cross sections are not common.

Selection of sheet piles starts with the design calculations after reaching a particular bending moment requirement. A sheet pile section would be designated with certain elastic section modulus and grade or quality of the steel material.

The bending moment carrying capacity of the section is directly proportional to the multiplication of its elastic section modulus and permissible stress depending on the grade of the steel material. For instance, a contractor or a project owner can choose a section with lower elastic section modulus and higher grade of steel in order to achieve a required bending moment carrying capacity.

Selection of lower elastic section modulus in combination with higher steel grade would lead to a lighter section and thus saving in material and total cost.

Of course, a higher grade of steel would cost a little extra. However, the saving in terms of material consumption and total cost is much higher than the incremental cost attributed to the higher grade.

These days, higher grades like S 430 GP are available without a big cost difference over normal grade of the steel. They are also advantageous in terms of greater resistance to the hard strata and driving obstacles, leading fewer deformations to the section and hence ensuring a high number of repeat uses.

Two more aspects are worth mentioning. They are higher width of piles and quality of interlocks. Higher pile width results in less number of piles to drive, would lead to saving in installation efforts and cost. It also reduces the number of joints, thus contributing to making the wall more water tight.

Quality of interlocks plays an important role when selecting the right sheet pile profile. By ignoring this, the wall becomes vulnerable to weakness.

The wall would be less water tight and migration of soil particles is possible from one side to the other during variation of water levels, in the case of tidal areas. Interlocks should not be loose and there should be proper gripping at the joints. Otherwise, declutching would appear.

Selection of the right steel sheet pile profiles has proven to be the best solution for retaining structures. With proper selection, steel sheet piles are the most economical solution.

If you would like to write for Construction Week in this column, please email rob.wagner@itp.com

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