Sheet piling is a construction technique where sheets made of steel are driven into the ground to create retaining walls, barriers, or provide structural support. Often these are for temporary use, being used to create a barrier between a structure and soil or water, but can also be used as permanent structures as retaining walls or excavation support.
Traditionally, sheet piling can be performed through vibratory or non-vibratory methods, however, this depends on the environment where the sheets are erected.
Ways to install sheet piling
Sheet piling can be installed in multiple ways that are dependent on the variables of the construction site:
- Noise and vibration pollution
- The size of the construction site
- The soil type
- Intended purpose of the piles
Pitch and Drive
Pitch and drive is a sheet piling technique where individual piles are interlocked and then installed consecutively. This method is best used for piles that are typically short in length and generally for temporary walls, short piles, and loose soil, where a strict vertical structure isn’t of concern. This is due to the piles being installed individually, which can lead to leaning, unless strictly monitored.
The pitch and drive method is the only method possible with the silent pressing drive method. This method minimises noise and vibration from traditional methods, making it ideal for working in urban areas or areas greatly affected by vibration and noise. Furthermore, this method is compact, making it a better option for sites with limited or restricted access.
Panel Driving
Panel driving is a technique where the sheet panels are pre-assembled using a guide frame before being driven into the ground in subsequent stages. This technique helps achieve verticality in very soft soils and is considered well-suited for other difficult types of soil due to it being threaded before installation. Panel driving is better suited for longer piles and commonly used for permanent walls in structures like underground car parks and basements.
Impact Driving
Impact driving is an installation technique that requires using a hammer to strike the top of the sheet pile repeatedly, forcing it into the ground. There are different hammers for impact driving:
- Drop Hammers- These hammers use gravity to drive the sheet piles into the ground. The falling weight of the hammer drives the piles in, using a driving cap to distribute the impact.
- Diesel Hammers- A type of drop hammer where a two-stroke diesel engine is used to generate the impact force that drives the sheet piles into the ground. However, this method is less common now due to noise and efficiency.
- Hydraulic Hammers- A modernised version of the diesel hammer, due to its efficiency and quieter nature, it uses hydraulic pressure to drive the sheet piles into the ground.
- Air-steam Hammers- Either single or double acting, these hammers use air/steam to power the ram that piles the sheets into the ground.
- Vibratory Hammers- By using a vibration hammer to create vibrations in the pile that reduce friction along its side, it can be driven into the ground with less force, making it faster and more efficient.
This technique is best for driving piles into hard soil and difficult ground that can include rocks. Although this is a loud method of installation, and shouldn’t be considered for places where noise and vibration restrictions are in place.
Hydraulic Pressing
Hydraulic pressing is a method of installing sheet piles by applying pressure from a hydraulic press to push them into the ground. One pile is installed into the ground, and by using the reactionary force, the next pile is pushed into the ground, creating a chain reaction during installation.
This technique is ideal for urban areas and sensitive areas due to its low noise and vibration output. Additionally, this technique minimally disturbs the surrounding soil making it good for temporary or permanent retaining walls, seawalls, and other underground structures.
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Water Jetting
Water jetting can be used to assist with sheet pile installation by attaching a high-powered jet to the inner face of the sheet pile. The pressure and volume of water can be adjusted while inserting the pile to ensure the water is only applied where necessary to stop waste. This process helps to reduce the resistance of the sheet pile penetrating the ground.
Pre-auguring
Pre-auguring a sheet pile before installation is advisable when dense soil predominates. A flight auger penetrates the ground at the piling line, loosening the ground without removing it, to make the piling easier to accomplish. Only a small amount of waste is produced in this process, where the pile is inserted. The loosened ground makes the installation of the pile much easier.
For both water getting and pre-auguring installation, there can be restrictions for these techniques, and it needs to be checked that the site is suitable first.
What are sheet piles used for?
Sheet piling can be used for many construction, commercial, and civil engineering projects, including:
- Retaining walls
- Excavation Support
- Underground structures (car parks, pump houses, basements)
- Marine structures (cofferdams, seawalls, canals)
- Land reclamation
- Riverbank Protection
- Flood defense schemes
- Bridge abutments
- Boundary walls
- Quay walls
- Retail yards
Benefits of sheet piling
Sheet piling can be used for all of the projects outlined above, and many advantages come with doing so:
- Versatile- Due to its ability to be used for permanent or temporary structures, its durability, and its adaptability for different soil types, sheet piles are adaptable for many construction projects. Moreover, as sheet piles can be welded or bolted together, any desired length or depth is achievable.
- Durability- Sheet piles can be made from many materials, but are typically made from steel. This can give them a lifespan of up to 125 years. They can withstand harsh environments and have the strength to provide safe, high-pitched walls.
- Ground conditions- Any type of soil, from soft to hard, can be made penetrable by sheet piling.
- Cost and time efficiency- Sheet piles are relatively easy and flexible when it comes to installation, maintenance, and extraction. This can save time and cost on both labour and more expensive alternatives like secant piled walls.
- Watertight- For water-facing projects, sheet piles can be secured or welded to ensure a watertight barrier.
- Anti-corrosive- Sheet piles can be protected with anti-corrosive sprays to further their lifespan.
- Minimal disruption- Compared to other piling techniques, sheet piling causes less damage to the surrounding ground and area.
- Sustainable- The materials used to make sheet piles are mostly recyclable and reusable, making them a sustainable alternative.
Things to consider when sheet piling
Shape of sheet
Sheet piles come in three main shapes: Z-shaped, U-shaped, and straight web sections. Each type of shape has different attributes and applications in construction.
Z-Shaped Sheet Piles:
- Continuous web that interlocks on the outside flange of the section, optimising the section modulus-to-mass ratio.
- The continuous web design enhances water and soil tightness.
- High resistance to bending offers greater stability. This makes them ideal for deep excavations, sea-facing structures like cofferdams.
- Z-piles allow for more uniformity, making them ideal for retaining walls, deep excavations, and other heavy-duty tasks.
U-Shaped Sheet Piles:
- Symmetrical shape that has interlocks located at the web’s centre. This gives them a weaker joint performance when dealing with water-facing projects.
- U-shaped piles are simpler to manufacture and install than Z-shaped piles.
- Better for lighter duty applications due to their ability to be stacked, however, this makes their resistance to bending less.
- Due to being a lighter-duty application, U-shaped sheet piles are better for soft retaining constructions that are either permanent or temporary.
Straight Web Sections:
- Designed to form closed cylindrical structures retaining a soil fill.
- The stability of the straight web sections comes from the material they are made from, as well as the internal body of soil.
- Mostly used on projects where rock layers are close to ground level or where anchoring would be difficult, such as dry docks, deep-sea quay walls, and breakwaters.
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Soil type and properties
Different soil conditions have different properties that can cause challenges when installing sheet piles. Due to this variable, the different techniques of installation are designed to handle all the different types of soil workers may come across.
- Soft Soil- Made of loose sand, gravel, and slit, impact driving can be preferable for this soil
- Dense Soil- Made of medium-sized coarse sand and gravel, vibratory driving is more suitable here, but impact driving may be possible.
- Hard Clay and Rock- For harder ground, techniques like jetting and pre-auguring may be more suitable.
- Firm Soil- Panel driving would be better suited for ground that is more resistant to piling.
- Moist Soil- These have a lower resistance to piling.
Before work begins, engineers must perform a soil investigation to determine the properties of the ground and how the sheet piles installation will react to its behaviour.
Vibration and noise
Sheet piling can be a loud process, which can cause structural damage to buildings and utilities through vibrations. These vibrations can further damage the structural integrity of nearby buildings or pipes with cracks or deformities. The process can also cause noise pollution through sound, which can cause disturbance to residents and businesses alike.
Things to remember when it comes to sheet piling
When sheet piling, it is important to remember that every project is unique, and none of them can be replicated one for one. There are many considerations, from the shape of the pile to the method of installation, as well as the area. But further issues such as the sustainability of the material, as well as the effect it can have on residents in the area. It is essential to make sure all proposed and planned work has been approved before the process can be carried out.
