Whether you are building a side-return kitchen extension or a full wrap-around extension, before you can start building, you first need to get your groundworks in order. More than just the foundations, this refers to the work done to prepare any sub-surfaces for the start of a construction project, and can include trenching, substructure construction, laying pipes and cables, drainage, landscaping, and tree removal. Groundwork is critical to prepare the area for foundations, which determine the structural integrity of the entire building.
Who carries out groundwork?
Groundwork is a specialised form of construction, and it is common for builders to subcontract groundworkers to complete this primary stage of a project due to their expertise with ground investigations, site clearances, drainage, and building the foundations.
What are the four stages of groundwork?
Ground investigation
Before clearing a site, it is important to carry out a thorough investigation of the area to identify past land uses, potential stability issues, and uncover any subterranean sewer systems or pipework that could be affected by the construction above. These investigations are required to comply with national and local regulations, but more importantly, they will let you know what is beneath the site you plan to build upon and what works need to be carried out so that you can lay down strong foundations for the build.
Site clearance
Once the ground investigation is complete, the site needs to be cleared to lay the foundations and begin construction. This would include clearing any trees, roots, or bushes that may be growing on the site, as well as any paving stones, masonry, soil and any other debris. If the site is on a slope, this would be the stage where it would be levelled to create a flat development platform onto which foundations can be built.
Drainage
It is critical for drains, pipework, and cables to be taken into account during the groundwork phase of the build, as fixing these after construction has started and the new foundations have been laid can be difficult, time-consuming, and expensive.
As part of the ground investigation, the groundworker will determine where your existing drains run and whether they will need to be moved for the extension to be built. Moving drains is a complex and costly process that requires permissions from your local water authority and adherence to various building regulations, with the new drainage system is properly designed, at the correct depth, and accessible for future maintenance.
After any changes to the drainage system have been completed, the pipework should be tested for leaks, generally via an air or seal test, to make sure that all issues are discovered and fixed before the foundations are laid.
Foundations
Once the site has been prepared and the ground levelled, it is time to build the foundations for the extension. Most modern extensions utilise either trench fill or strip foundations.
Trench fill foundations
Trench fill foundations are generally the preferred method for most extensions (and house builds). They are typically 600mm wide trenches, which are then filled with concrete to within 150mm of the surface ground level, which means that the construction can get started quickly, saving time and money. Trench fill foundations are ideal for clay soils like that beneath London as well as chalk soils, but are less suited if the project is on more unstable ground, such as sand.
Strip foundations
Strip foundations tend to be shallower and wider than trenches and use less concrete on the base layer, which spreads the load and therefore makes them better suited to softer soils and sand. However, as these foundations are further below surface level, the build will require bricklayers and blocks below ground level, which can increase the time and cost of the build.
Whichever foundation option is decided upon, pouring concrete is a time-sensitive process, and should be poured within two hours after mixing to avoid it going off, which can affect its strength and durability. Once poured, it is critical that the concrete does not dry out too quickly, so it is generally left to “cure” for 48-72 hours by covering it with plastic sheeting, making sure the edges are sealed, and carefully monitoring the temperature.
After the groundwork has been completed and the concrete foundations have cured, you can finally start to build the extension of your dreams.
If you are looking to build an extension to your London property, call in the experts at Good London Builders to help you through the entire process, from producing a tender and finding the right team to overseeing the project from groundwork to completion. Call us on 0207 978 5097 or get in touch online.
