Self Build Advice: 8 – Building: Groundworks

With plot, plans, permission and finance firmly in place, it’s now time for construction. Building a home can be roughly divided into five stages; groundworks (substructure/foundations), superstructure (timber frame), first fix, second fix and decoration. For those new to construction, most of the terms used will be unfamiliar. It can be helpful to understand the terminology and be aware of the different phases, even if you are instructing a main contractor to carry out the works for you. Here we breakdown and explain the processes required to build your new home. 

Self Build Advice Groundworks

Groundworks / Substructure / Foundations

The first requirement is to create your working space. Access and facilities for tradespeople need to be considered. A site office or cabin is not only useful to keep drawings and safety equipment but is also a health and safety requirement, as are onsite toilet facilities and storage containers. Fencing to surround the site is required to ensure it is secure. Water must be available so a temporary or permanent connection needs to be made. A power supply is also essential. 

Groundworks is the first stage of your project, this term refers to the work carried out to prepare your land for the building. 

The area will need to be cleared. If any structure currently sits on the site, demolition will take place. Vegetation, rubble and rubbish will need to be removed from site. Anything that can be reused (topsoil for example) should be put to one side. 

The substructure is all the work below floor level, including foundations. The purpose of foundations is to distribute the weight of the building safely to ensure structural integrity and provides the support for your home. It is therefore fundamental that foundations are suitable. The building’s structural load, soil conditions, trees, adjacent properties, and drainage all need to be considered when calculating the foundation design.

Standard shallow foundations are used where the structure’s load is low relative to the bearing capacity of the soil, examples include strip, trench fill and raft foundations. Strip foundations provide support to walls in strips and consist of a thin layer of solid concrete poured into trenches and blockwork built up to ground level. Trench fill foundations are similar but a larger quantity of concrete is used and fill the trenches to just below ground level. Trench foundations are the preferred option for many as it allows a project to get out of the ground quickly, but it is only suitable for stable ground conditions such as clay or chalk soils. Raft foundations are steel-reinforced concrete slabs that cover the footprint area of the proposed building. If the soil is weak then raft foundations are ideal as it spreads the weight of the building over a large area, reducing the pressure on the soil. Raft foundations can only be used on level surface areas.

If you have a plot with difficult conditions (presence of trees and or poor soil), then these types of foundations are unlikely to be good enough and a deeper solution may be necessary to ensure loads are sufficiently supported. One of the most common deep foundation solutions is piling. Piles are strong cylinders that are driven into the ground, they are capable of taking higher loads than spread footings. End-bearing piles are pushed deep into the ground, bypassing weak soil to reach the strong layer below. The load of the building is then safely transferred through the pile onto the high bearing earth. Friction piles on the other hand transfer the load to the soil along the sides of the pile for the full height via frictional resistance. This is useful when hard layers of soil are too deep to reach. Piling has to be installed using specialist machines and is significantly more expensive than standard ‘trench’ foundations.

The drainage system also needs to be planned at this stage.  The trenches need to be carefully dug to the right depth and gradient.  Gravel is used to support and protect the pipes.  Utility connections also need to be considered – the cables can be laid in position now – ready for the supply companies to connect when the house is complete.

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