What is Sheet Piling?
Sheet piling is a piling technique. What is piling? Piling is a construction technique which has been in use since ancient times. The purpose of piling or creating piles is to build new foundations on construction sites which do not permit traditional foundations. For instance, the type of soil is one fact which has to be considered when building a foundation. The load-bearing value, or LBV, has to be estimated before you select and build any foundations. Soil behaviour, i.e., soil reaction to weather and climatic conditions or sudden changes, must be studied or predicted before selecting and building foundations. The presence of other structures which may be affected by construction activities also has to be considered. When the soil and construction site conditions are not favourable and do not allow the construction of shallow foundations, deeper foundation building methods such as piling are employed.
What are the Different Type of Piling Methods?
There are numerous types of piling methods and techniques used by engineers. They can be broadly classified as end-bearing piles and friction piles. They are further divided into techniques such as Driven piles, bored piles, aggregate piles, CFA piling, SFA piling, micro piling, and many more. Sheet piling is a type of driven piling.
Sheet Piling Explained
Sheet piles are commonly used for excavation support and earth retention. They comprise sheets made from steel or reinforced concrete with interlocking edges to ensure a perfect fit. Multiple sheets can be interlocked together to create a barrier. Although steel and concrete are preferred, sheets can also be made from timber and composite materials. Some sheet piles are treated before installation to protect against corrosion and other environmental influences.
How is Sheet Piling Selected?
Selecting the type, material, and size of sheet piles depends on numerous factors.
- Temporary or permanent projects will require different sheet piles. Some may only require simple timber sheet piles, while others may require the permanent installation of steel sheet piles.
- Site conditions such as soil quality, soil properties, load-bearing values, moisture content, reaction to climatic conditions, presence of adjacent structures, and building regulations will all influence the choice of sheet piling.
- The design depth.
- The nature and purpose of the structure.
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Timber Piles
Sheet piles were historically made from timber. As timber is not as resilient and strong as steel or reinforced concrete, timber piles are mostly used to provide temporary support to structures and for resisting less heavier lateral loads. Tongue and groove joints are carved into the timber piles to connect them. Timber piles need preservative coatings and cannot be used for soils with stones.
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Reinforced Concrete
Reinforced concrete sheet piles are interlinked with tongue and groove joints. They have high strength and are used for creating permanent marine structures, canals, and river embankments. The driving end of the piles usually has oblique faces so that they can be easily interlocked and driven into the ground. The heads of the piles have a capping beam.
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Steel Sheet Piling
Steel is the preferred material for constructing sheet piles. Steel sheet piles have very high resistance and water tightness. The length of the Steel sheets can be increased by bolting or welding. The sheets are interlocked with each other when installed.
Applications of Sheet Piling
- Sheet piling is often used for creating permanent foundations.
- They can also be used for constructing temporary or permanent retaining walls.
- Sheet piling can be used to create the foundations for underground structures, including car parking and basements.
- They find extensive application in marine construction, including providing support to harbour walls, riverbanks, and canals.
Sheet Pile Designs
Sheet piles usually have three designs.
- Z-shaped: These are the most efficient type of sheet piles and are used for cantilever retaining walls, deep walls, and intermediate wall construction.
- U-shaped: This sheet pile profile is similar to z-shaped piles. However, they have a lower resistance to bending.
- Straight Web Sections: These sections are used for interlocking and forming enclosed or circular structures. They are usually backfilled with gravel or soil.
How Are Sheet Piles Installed?
Impact or vibratory hammers are employed to drive sheet piles into the soil. Construction sites that restrict noisy construction work use hydraulic methods to drive the sheet piles. Before sheet piles are installed, they have to be checked thoroughly for cracks and straightness. Their interlocking ability should also be checked to ensure a perfect installation.
Sheet piles are driven in a controlled manner and carefully monitored throughout the process. The driving should be stopped immediately if the pile cannot penetrate further. At times, several piles might be unable to reach the design depth. The obstacle should be removed first instead of forcing the sheet piles in such a scenario. These obstacles are usually removed either through excavation or water jets. Sheet piles often deviate when they are being driven and would tend to lean sideways. This is because they encounter obstacles while driving, which deflect their direction. Guide controls or panel driving is often used to counter this problem.
Anchored Sheet Piles
When less penetration is required, and the height of the piles is more than 6 meters, anchoring the sheet piles becomes more economical. Anchor walls provide higher back slope subsidence as they face less lateral deflection. The usage of anchors gives rise to large gravity walls by retaining the soil mass between the wall in compression and the anchors.
Cantilever Sheet Piles
In geotechnical construction, cantilever sheet walls are used for building temporary retaining structures. They are also used as pile walls and diaphragms for constructing permanent retaining structures.
Cofferdams
In bridge construction, cofferdams are often used as temporary structures for keeping water and soil away from the excavation site. Cofferdams are often used for building and repairing bridge piers, oil platforms, and permanent dams constructed within a body of water. They have typically welded structures comprising cross braces, wales, and sheet piles.
Advantages of Sheet Piling
- The installation time of Sheet Piles is much less than constructing reinforced concrete walls.
- Sheet piles are incredibly narrow, enabling construction workers to install them in the tightest places.
- Sheet piles do not damage their surroundings, unlike contiguous or bored concrete piling.
- No earth excavation is required to install them.
- Treated Steel piles can be used in a variety of soils.
- Sheet piles provide an instant increase in LBV (load-bearing capabilities)
- Sheet piles are often used for curtain-walling, containing and securing the construction site from environmental and visual exposure.
- They are pre-cast in a factory, which enables excellent quality control.
Disadvantages of Sheet Piling
Sheet piling also has some disadvantages, although the benefits outweigh them.
- It can be challenging to install sheet piles if the construction site soil has boulders or rocks.
- Vibratory or impact hammers can cause a lot of noise, and they might not be permitted to be used in specific areas such as sensitive properties, archaeological sites, hospitals, and highly populated areas.
- Sheet piling is mostly a temporary structure which is removed once the project is complete resulting in the wastage of raw material.