In this blog, I will be discussing what Piled Foundations are, when they are needed, whether the use of piled foundations is notifiable under the Party Wall etc Act 1996 and other considerations that may come into play with the use of Piled Foundations.
What are Piled Foundations?
Before understanding what, a Piled Foundation is; it is important to understand what a Foundation is. Foundations can be mainly categorised into 2 main sections and then several subsections within each of those main sections.
Foundations can be categorised as Shallow or Deep. Typically, Shallow Foundations are used where the loads are considered low in relation to the sufficient bearing capacity of the surface soils. Deep Foundations are required when the bearing capacity of the surface soil is insufficient to support the loads imposed by a structure, this means that the foundation needs to be filled deeper into the Hard Soil Layer.
Whilst it may seem obvious a trench-fill foundation is more suited to smaller-scale works such as residential ground floor extensions and would be deemed insufficient to support a skyscraper such as the Burj Khalifa.
When & Why are Piled Foundations needed?
The purpose of a pile foundation is to transfer and distribute the load throughout its long slender, typically columnar shape and made from steel or reinforced concrete and used usually when their inadequate support, risk of sliding or ground movement. The pile, due to its length and strength allows for penetration to deeper and firmer grounds that are capable of supporting loads without any significant movement. There are a wide variety of piles that can be used based on the relevant soil types.
As each pile carries a lot of loads, they must be designed and arranged with great care, it is important to consider that the soil is not overloaded beyond its bearing capacity and is separated far enough apart so that the loads are distributed evenly over the full area of soil as opposed to being concentrated to one particular area of soil.
Several situations when piled foundations are more common are listed below;
- When groundwater is high
- Heavier loads to be used for larger-scale structures
- Can be seen as a cheaper alternative relative to the scale of the structures it is typically used for
- When the soil at shallow depths is compressible
- When there is the possibility of Scouring, slippages, erosion typically due to being located near a river bed, seashore etc
- When the location is close to a deep drainage system or canal
- When soil excavation is not possible due to the required depth due to the poor soil condition
- When the foundation trenches are penetrated by water seepage
Whilst the above lists some situations when piled foundations are required, it is also important to understand the type of pile foundation suitable for a specific situation.
The different types of Pile foundations suitable for different soil types are described below;
End Bearing Piles:
In end-bearing piles, the bottom side of the pile rests upon a layer of strong soil/rock and pierces through the weak layer of the soil until it reaches the harder layer of the soil. The idea is that the load of the structure built on top of the foundation is transferred through the piles onto the strong layer of soil.
Friction piles function by using the stress/friction produced along the sides of the pile. The pile transmits the load to the surrounding soil. The deeper it goes the more stress is produced and in effect the more load it can support. It is typically used when the harder layers of soil is too deep and therefore it is unsuitable to use End-bearing piles.
Driven piles are driven, jacked, vibrated or screwed into the ground by displacing and compressing the soil around the pile outwards and downwards instead of removing it.
Bored piles remove soil from the ground to form a hole and then the pile is formed by pouring the material into the hole. Typically used in environments close to other buildings to avoid damages to the surrounding buildings.
Screw piles have a helix at the end of the pile which is then screwed into the ground in the same way a screw would be screwed into a plank of wood.
Are Piled Foundations Notifiable under the Party Wall etc Act 1996?
Piled foundations are notifiable under the Party Wall etc Act 1996. More specifically under Section 6 of the Act, Subsection 2.
Section 6 of the Party Wall Act covers adjacent excavations within 3-6 metres of an adjoining owner. Subsection 2 of Section 6 is more relevant to Pile foundations as it is only triggered when you are digging to a greater depth to the foundation of an adjoining owner and are within 6 metres of them. This differs to Subsection 1 as this is triggered when foundation depths are greater than the foundations of an adjoining owner and are within 3 metres of them.
The reason Subsection 2 is triggered with foundation types such as piles is that they root much deeper and is considered to be more of a risk to adjoining owners within wider parameters.
What other Precautions may be considered with the use of Piled Foundations
The use of piled foundations suggests that the works taking place are quite extensive and goes beyond the general precautions that are to be taken on-site when carrying out a basic extension with a trench-filled mass concrete foundation.
Some additional precautions that may be considered with the use of Piled Foundations can be found below;
- A Risk Assessment is usually produced alongside a separate Method Statement. The purpose a Risk Assessment serves is to make apparent all possible risks that may arise on a building site and the safety precautions considered to control those risks. Usually, you’ll find details on protective equipment on the building site, Health and Safety procedures and contacts and the general protocol to prevent issues but also the actions to take when an issue arises.
- A Method Statement put simply is a document that details a step-by-step process on how work is to be carried out safely on a building site. Typically, the Method Statement will break down the project from beginning to end and will indicate usually on a weekly basis what phase of the project will be taking place each week. Each of those weeks are further broken down into descriptive words on how that phase of work is to be carried out in detail. This helps reassure the client and any concerned parties as they can somewhat grasp and follow the project and what the expectations are. It also provides structure and a general timeline the project should follow.
- Movement monitoring which is also known as a ‘Deformation Survey’ is used to monitor the deformation and movement of a structure during the building phase or after the completion of a structure. The main purpose of it is that it allows you to track if any movement has taken place whilst also alerting you at the initial stage of movement so you can assess and apply further prevention measures in place if necessary. You might find this used on large scale projects where they use piled foundations and with basement excavations
- Checking Engineers are typically appointed on projects where the build is structurally complex and includes much higher risk. The original appointed structural engineer will assess all risks and produce structural plans for the build to follow. The role of a checking engineer is to review the plans, consider all the potential risks and raise questions for clarity until they are satisfied all risks have been considered, the structural plans are sufficient, and any requests are met and implemented.
It would not be uncommon to find all of the above aspects included in a project that involves Piled foundations.
Here at Icon Surveyors, we are happy to provide a free 30-minute consultation to any building or adjoining owners who may be affected by the Party Wall Act and the subject matter raised in this blog.