Your Guide To Housing Foundations
When it comes to building foundations, they are one of the most critical aspects of any construction project. They may not be visible once the project is finished, yet correctly implementing and installing foundations ensures the longevity of a structure, avoiding errors that could have serious implications, even jeopardising a completed project.
Read on below as we cover the various types of foundations commonly used in construction, and the key steps you need to know to lay a solid foundation for your project.
What Does A Building Foundation Do?
A foundation is the lowest part of a building. Designed to distribute the weight of a new building equally, the foundations will keep the building level and anchor it in place.
Most commonly, foundations are referred to as concrete footings. Footings are a type of foundation that is constantly in contact with the ground, offering reinforcement and support with individual columns, rather than transferring the load to the ground like other types of foundations.
What Building Regulations Surround Foundations?
The right foundations are vital for a long-lasting structure and a safe construction site. Therefore, as per building regulations, all buildings must have adequate foundations, which will vary depending on the nature of the project and the circumstances.
As foundations are a structural element of a building project, house builders must satisfy Building Regulations, warranty providers and their own duty to the client. Approved Document A sets the context for loading, ground movement and disproportionate collapse, so that foundations can resist imposed forces and ground changes.
These foundations can be cast as either deep-fill, which fills most of the trench, or shallow-fill, where the minimum thickness is used to transfer the soil to the soil.
Building regulations require other factors to be considered when designing a foundation. This includes the type of soil the foundation will sit on, adjacent structures, surrounding trees, drains and sewers, the size and construction of the new building, the ground condition and local landfill sites.
Considering all these factors will ensure the structural integrity of the site, as well as the load-bearing capacity, and health and safety.
It is advisable to contact a structural engineer, a groundworks specialist, or building control for further advice when selecting the right foundation for the project.
Types Of Shallow-Fill Foundations
Individual Footing or Isolated Footing
Commonly referred to as spread footing or pad foundation, this foundation type is square, rectangular or circular in shape, supporting a single column. Uniform in thickness, this foundation can carry and spread concentrated loads, with the overall size calculated based on the load and ground conditions.
Combined Footing
These concrete footings are typically rectangular in shape, supporting two or more columns close together, so their individual footings overlap.
Strip Foundation
Strip foundations are typically used for load-bearing walls. This includes footings for extensions, conservatories, and house foundations. These foundations can accommodate a row of closely spaced columns, making a wider base of foundation well-suited to spreading weight over a wider area, and providing better stability.
Raft or Mat Foundation
A raft or mat foundation is a large reinforced concrete slab that supports several columns and walls. This foundation is spread across the entire building footprint, useful in situations where the soil pressure is low, weak, or compressible. These foundations are commonly used for large city tower construction or detached domestic garages, acting as a single, rigid ‘floating’ mat.
Types Of Deep-Fill Foundations
Pile Foundations
Pile foundations are commonly used where ground conditions near the surface are unsuitable for heavy structures. These foundations are driven or bored deep into the ground to transfer the structural load through the soil and create a more stable subsurface.
Drilled Shafts or Caissons
Drilled shafts or Caissons are high-capacity foundations constructed by drilling holes and filling them with reinforced concrete. These foundations are ideal for heavy structures, as they offer vertical and lateral load support, minimising noise and vibration. This makes them ideal for urban environments.
Identifying Your Plot’s Soil Conditions
Understanding the ground conditions on your plot is vital when deciding which foundation type to use. Across the UK, soil types can vary significantly from one area to another; therefore, it’s crucial to assess your soil and its properties, such as load-bearing capacity and susceptibility to movement.
A geotechnical soil survey must be completed before starting any work. This survey will report back on soil structure, makeup and the bearing pressure. From this, you can select the right foundation type.
There is no one-size-fits-all solution to building foundations. Therefore, the soil conditions on your plot will provide the information you need on the ground’s condition and its stability, as well as the water table. These are all important elements to consider when starting a construction project.
The most common soil types include:
- Clay soils
- Sand and gravel,
- Peat,
- Rocky ground.
How Are Foundations Built?
There are a few steps that need to be taken to build foundations.
Firstly, you will need a level datum point. This is important as all your height measurements will be relative to this point. Therefore, any errors at this point will be translated into problems later on, especially if you are working with a height restriction. When building an extension, this datum point is usually the finished floor level or damp-proof course of the existing building. New builds will use a fixed point, marked on the plans, such as a manhole cover or marker in the road or pavement.
Once this is marked, you must calculate how much material needs to be removed from the site to end up back at the finished floor level, after all flooring materials have been installed.
The next step is to mark out the string lines to map out your building foundations. You’ll also use timber hurdles at the end of each wall, out of the way of the digger. If your site is too small for hurdles, corner pins can be used to mark out the foundations. Then, hurdles can be set up once the concrete has been poured.
With the string lines and the datum point, you can now line up your footings and mark out the trenches for building foundations.
Once you’re ready to start digging, set the depth you need to dig to and check this often. A Building Control inspector will decide how far you dig.
You cannot pour any concrete until a Building Control inspector has inspected the trenches. They will sign off on the soil at the bottom of the trench if it is firm and consistent, the depth is sufficient, and any required steel reinforcement is correctly positioned. Choosing the pour without their visit, you risk having to dig it all out at your own expense.
After Building Control has visited, it will be time to pour your fittings. Depth pegs can be used to show how deep to fill, or you can mark the sides of the trenches. The more accurate you are at this stage, the easier the next phase will be. Any pipes or cables that cross the trench must be below the concrete.
Building foundations yourself may seem like an easy task at face value; however, once you consider the labour, equipment and various building regulations you must follow, calling in an expert may be the best course of action. An expert will have experience in working on a variety of sites, with different types of machinery, and also on time and to budget when it comes to projects of various sizes.
How To Get Your Building Foundations Right
Getting your building foundations right requires a careful, engineered approach. With a soil test, the right foundations that extend below the frost line
How Deep Should House Foundations Be?
The depth of your foundations will be site-specific. The NHBC notes that granular and frost-specific soils require footings that are at least 450 millimetres below the finished ground level. This will avoid frost action. In a desiccated zone, the footings must extend into bearing strata. Approved Document A states that foundations must be deep enough to avoid ground movement and frost damage when resisting loads. Planning guidance notes that different soils will have different load-bearing capacities and react differently to moisture.
Near trees in shrinkable clay, the safe depth will increase. This is to go beyond the zone of seasonal influence. In this situation, it’s best to refer to the NHBC Chapter 4.2, where worked examples, charts and tables are provided to help you in assessing the foundation depth requirements for specific conditions.
Using ‘rule of thumb’ measurements and blanket numbers is not helpful, and the depth you will need will depend on the site. An experienced groundwork specialist will properly inform you on the correct depth.
Planning Your Drainage System
Correctly planning your drainage system is vital when laying foundations to avoid costly complications later on. Effective planning involves mapping out pipe locations, ensuring adequate and consistent gradients for flow and providing access for maintenance.
Choosing To Work With A Contractor
If you want foundations that are right the first time, choosing to work with a specialist such as Wilsons Groundworks is the best course of action. With leading expertise and professionalism, we can ensure that your project meets Building Regulations and industry best practices. Speak to our team today for a free site survey.