Victor Chirilas, director of ground engineering experts, Mainmark, explores how building renovations can help meet housing targets, whilst addressing the risks involved and how to solve them
In 2015, the UK government announced the ambitious goal of delivering one million ‘net additions’ to the housing stock by 2020. This has led to an exponential growth in new builds and property conversions. Currently, the new housing supply is lower than the government’s ambition of 300,000 new homes per year; only 216,000 new homes were supplied in 2020/21. Whilst this is in part due to disruption to housebuilding caused by Covid-19 in early 2020, it’s clear from this data that more housing needs to be created quickly.
With extreme weather in the UK becoming more common, new buildings must be able to withstand the elements to prevent foundation damage and subsidence. Building as quickly and as cheaply as possible, however, can increase the risk of structural issues and cause longer-term problems. It’s common for new buildings to settle in their first few years and this shouldn’t be a cause for alarm but if the movement is severe or ongoing, it may be caused by subsidence.
Conversions and building renovations
Alongside the need for high-density new developments across the UK, if done correctly, building renovations can be a much quicker, more environmentally sensitive way to provide new homes. Starting with the foundations and ensuring they are sound is the first step to preserving a building’s life and will allow for safe and reliable refurbishments to take place. If this is not addressed first, upgrading expensive interior fixtures like doors and windows can become an issue, should the property subside.
Cracks in walls, close to doorways and windows, and sloping or sinking floors are often early indicators that a building has foundation problems that could be the result of subsidence. This can happen for many reasons, including neighbouring construction affecting a building’s integrity or when soil becomes too wet or too dry, which will affect the foundation structure.
The process of underpinning is the most common method to fix subsidence as it strengthens a building’s foundations but doesn’t always address the soil conditions. The two most popular methods of resolving foundation issues are traditional concrete underpinning and modern geopolymer resin injection. Both are proven methods, yet they involve very different procedures.
How to resolve foundation issues during building renovations
When assessing the foundations in an existing structure, understanding the soil type is critical. In the UK the most common soil types are clay, sand and silt, fill and organic:
Clays
Reactive clay soil swells when wet and shrinks when dry causing subsidence in drier conditions, or heave in wetter areas.
Sand and silt
Structures on sand and silt soils often experience settlement caused by flowing groundwater, a natural underground watercourse, a leaking sewer or stormwater pipe, causing washout.
Fill
Fill is earthen material placed and compacted in a hole or depression. Structures built on fill are susceptible to the same issues as those on sand and silt, especially when water washes away finer material. Settlement is worse if the fill is poorly compacted.
Organic
Organic soils, especially peat, are weak and highly compressible making them prone to settlement, as the weight of any structure above causes compaction. Primary settlement decreases over time but can total 100s of millimetres, resulting in significant structural issues.
If there is an issue with the foundations of a building that needs to be remediated, the modern method of resin injection allows both ground strengthening and re-levelling of the structure, increasing the likelihood of it staying as intended and serving its purpose for the future.
Concrete underpinning
This traditional method has been used for over 100 years and has been the go-to solution for strengthening foundations affected by soil subsidence. The basic premise of concrete underpinning is to increase the depth of the structure’s foundation to extend it to stronger soil. For this, large holes are dug beneath the structure and concrete is poured into the spaces in accordance with the project-specific plan. This process essentially creates new foundations underneath the original, to further stabilise the structure.
However, this method is slow, labour-intensive causes disruption and is therefore very expensive. Access is required to the underside of the original foundation, making it an invasive solution that takes time and causes disruption for the building occupier as they are generally required to vacate their home while the work is being completed.
A further consideration required for concrete underpinning is the strength of the soil below. Deeper soil is not necessarily stronger, combined with this the area of the concrete underpinning is often less than the area of the original strip-footing which might support a longer wall, for example, this means the underpinning is likely not as strong as the original foundations.
Furthermore, the extra concrete creates additional weight to the structure and in certain clay soils, the use of traditional underpinning can cause differential movement between the underpinned sections of the structure and the original foundations of the structure.
Resin injection
Although resin injection is a relatively modern method, it is a proven one, having been used globally for over 25 years. This process involves injecting an expanding geopolymer resin into the soil beneath the existing foundation through small holes drilled into the ground, typically only 16mm in diameter from outside or inside a home. As the geopolymer enters the soil the chemical reaction results in the material expanding, compressing the surrounding soil filling any voids and cracks, providing ground improvement and lift to the structure.
The injection process is controlled by technicians who monitor the injection as well as the movement using laser levelling instruments; the resin components can be tailored to suit unique soil conditions and the type of performance sought. By eradicating the need to excavate, this process is cleaner and less disruptive than concrete underpinning. As large excavations underneath the original foundation are not required, the project is usually completed within a day or two and occupants can often stay in the building.
