Lay of the land: why homeowners need to know what lies beneath before making more space underground

With the cost of moving continuing to rise, Britain has seen a 183% increase in planning applications for basement extensions, as homeowners seek to create more space for less. For those in highly populated urban areas, where space and new home options are even more limited, turning their attention underground is now often considered the norm. In London alone, a study found that over 4,650 basement planning applications were given the green light between 2008-2017 (Newcastle University, 2018).

While an easier, and often cheaper option for expanding families, when compared to the costs associated with moving home, building, converting or extending basements without careful planning can be a risky business. For those considering building below ground, knowing what lies beneath the land is crucial when it comes to planning a safe and successful extension.

Brian Poole, Senior Geologist from Mining Searches UK, outlines the key challenges posed when excavating beneath an existing building, as well as why mining searches – often considered a minor inconvenience – are a crucial step in any planning process.

Buried below

Mining may be dismissed as a chapter in the nation’s history – but this is far from the truth, with historic mines still having an impact on today’s housing and construction market. Over 290,000 dormant and closed mines exist in the UK, criss-crossing the entire country. Whether under busy cities and remote countryside, mines are frequently found where they would be least expected, including in the heart of Kensington and grounds of Wentworth House.

Mine workings commonly run long distances beneath the ground from mine shafts and extend beneath existing residential properties. These shafts and tunnels could potentially cave in if not properly dealt with, causing subsidence and damage to buildings above them – so it’s important that these are addressed before a problem arises to not only save further financial investment in the future, but to also safeguard the lives of those living above.

Identifying and remediating mining risks can not only help in the construction of a subterranean development but can also provide valuable information as to the stability and composition of the land. For those in known mining areas where there is a risk that a basement extension could intersect a mine working, mining searches are a Building Control requirement, and a crucial step in the initial planning process. A mining risk assessment should always begin with a search or desk study, carried out by a professional, which will determine the level of risk and if further investigation is required.

This is important even for areas where there are no obvious signs of mining as closed mines leave very little, if any, evidence on the surface, but still have the potential to cause major problems if not investigated. Urban areas can also be at risk when development has been carried out without due diligence, with ground investigations helping to clarify the presence or absence of potentially underlying adverse ground conditions – such as shafts, workings quarries and pits.

Digging deeper

There are several engineering challenges facing a basement construction. Excavation under an existing building will require temporary works, such as piling, to ensure the structure remains stable, while the impact on neighbouring properties – particularly in urban environments – requires careful handling.

With the potential to significantly affect construction costs, understanding ground conditions is vital, with the excavation of hard or soft ground presenting different problems. While soft ground is easier to excavate, it normally requires more extensive temporary works, as well as specialised foundation design. In comparison, hard ground is normally considered a positive, requiring less temporary works, such as shoring, but can prove more problematic when it comes to excavation.

A thorough geotechnical assessment will provide an understanding of the ground conditions, and is a staged process, usually starting with a desk study. This involves gathering all recorded information about the land and surrounding areas, which includes analysing records of mining, geology and railway tunnels – all helping to dictate and outline the scope of an investigation. An investigation will improve understanding of the ground conditions even further, by analysing its potential impact on a proposed development, investigating mining risks, and informing the need for temporary works and foundation requirements – as well as other issues which could impact construction, such as drainage, and soil chemistry.

Safe space

With space only set to become more limited and house prices on the rise, more homeowners will likely continue to look below ground when faced with the issue of creating more space. However, understanding what lies beneath a current home, can provide assurance that a development is not only cost effective, but also safe to both occupants and the existing building.

Ignoring or underestimating the impact of the ground, can prove to be a costly mistake to today’s homeowners. When considering a basement extension, it is always wise to consult experts, who will be able to provide a detailed and accurate outline of not just the feasibility of the project, but also the associated risks – helping to save homeowners time and money in the long term.