Everyone knows that if a building doesn’t have a strong and stable foundation, there’s a risk of structural collapse. As the connecting element between a building and the ground it sits on, it’s crucial for the foundations to support the structure’s weight effectively. The ground must also be secure enough to keep the supporting components in place so the weight doesn’t shift around.
There are many factors that can contribute to foundation failure, but these should only occur as a result of negligence in the planning and/or construction of the building. Knowing what to avoid can help you to prevent the foundations from failing, causing damage that becomes expensive to fix.
From poor materials to plumbing problems, shifting soil to tree roots, let’s run through some of the most common causes of foundation failure so you know what to watch out for to make it safe.
Poor conditions and preparation
Choosing a suitable site to build on and preparing it properly is key to ensuring the stability of a structure. Low-density soil that’s too soft or wet isn’t safe to build on; nor is soil that isn’t properly compacted. Loose soil under a foundation will gradually compact itself, but can shift in doing so, especially if water builds up underneath. The risk of destabilisation is too great in these conditions.
In many cases of building site preparation, soil is removed from one area and stacked in another (known as ‘cut and fill’). If soil and debris isn’t managed appropriately, this will also affect the stability of the ground when the soil is compacted, increasing the likelihood of the building sinking into the ground. There must also be adequate gradation to divert water away from the foundations.
Carrying out a thorough ground investigation and regular inspections on the soil itself and the substructure should prevent negligence from leading to structural defects like foundation failure.
Design or construction errors
Structural planning tends to start with the design, which should be left to a qualified architect with knowledge of structural safety requirements as well as artistic skill. A structural engineer should also be involved to confirm the viability of the building plans. If the designers make a mistake, such as miscalculating load-bearing capacity or specifying unsuitable materials or placement, it can be costly.
Similarly, if the construction workers deviate from the building plans or standard procedures, either accidentally or deliberately, it can compromise the integrity of the whole structure. When it comes to laying foundations, errors with temporary shoring, concrete quality, and other such elements can lead to foundation failure. Corrective work can be expensive and difficult for these kinds of mistakes.
To prevent faults like these, multiple people should sign off on each step of the process. This makes it more likely for someone to notice a mistake that can be corrected before it becomes a problem.
Changing water levels
The water content of the soil around and beneath the foundation can fluctuate and affect the supporting structure. If the water table changes too much, or the soil is oversaturated due to a plumbing leak or poor drainage, this can make the foundation unstable. Forceful water can erode the soil, or excess moisture build-up can make the soil swell and push up against the foundation.
On the other hand, the soil being too dry can also cause problems. If it dries out too much from hot weather causing the moisture to evaporate, the soil can shrink and pull away from the foundation. This leaves space for the structure to shift, potentially causing cracks from the strain. This can also happen if nearby trees or vegetation are too close to the foundation, as their roots pull water away.
This is why it’s so important to make sure the year-round ground conditions are suitable for building on, and to design the structure with effective waterproofing and drainage to control displacement.
Heave and subsidence
Two particular ways that shifting soil can affect foundation stability are heave and subsidence (also known as drag-down). For the foundation to be stable, it must compress the soil evenly to settle safely. Disturbances in the soil below the footings, either due to expansion from excess moisture or compression from a new load nearby, can result in irregular upward movement known as heave.
When the soil around foundations constantly swells and shrinks from flooding or desiccation, it can either pull away or press up against the foundation structure. Subsidence can occur when the added weight drags parts of the foundation down, or when it leaves space for the foundation to shift and drop, leading to sloping floors. This shifting and sinking can also happen if there is a gradual landslip.
If the ground not only beneath the structure but around it isn’t sufficiently stable, with consistent soil conditions, building on it increases the risk of the soil moving and compromising the structure.
Lateral displacement happens as a result of improper soil stabilisation. If there is insufficient base support for the walls and columns of the structure above, such as bracing and shoring, even the smallest sideways movement of soil can cause increasing levels of damage. The over-saturation or washing out of soil can reduce resistance and intensify pressure against parts of the foundation.
When this happens, the lateral pressure can cause walls to collapse and structures to cave in if they haven’t been reinforced to withstand such events. The effects of lateral movement below the surface can be seen in the aftermath of earthquakes, for example. It can also happen when careless digging or demolition on nearby land causes the soil to shift away from or towards the foundation.
If provisions of underpinning or other support systems are inadequate or omitted completely, the structure is likely to crack from the unresisted pressure, and there’ll be an increased risk of collapse.
Load transfer failures
Foundations help to distribute the weight of the superstructure so the soil beneath settles evenly. If loads are distributed unequally, the compacting of the soil will also be irregular, increasing the stress on elements of the foundation that can eventually cause them to fail. The design and construction of the foundation and building frame should be rigid enough to tolerate some differential movement.
Without spreading the weight of concentrated loads across a wider area, in accordance with the load-bearing capacity of the subsoil, the points where walls transfer their loads to the foundation will be under too much pressure. The connected substructure should be reinforced in enough places to ensure that loads are never transferred through single supports that don’t have the capacity.
It’s impossible to design and build a structure with effective load transfer without first conducting geotechnical surveys of the ground, so it’s necessary to test the load-bearing capacity of the soil.
Earthquakes are one of the most common causes of ground vibrations that can disturb foundations and cause the structure above to fall down. However, there are many more sources of vibration that can also disrupt the settlement of the soil. For example, heavy machinery or constant traffic in the vicinity, or construction or demolition work nearby – especially blasting explosives and pile-driving.
Activities like this can affect the soil volume by either compacting or loosening it. Constant vibrations repeatedly displace particles in their path, eventually forming cracks and gaps that allow pieces of the structure and the loads they bear to move around too much. If the soil is pushed up against the foundation or dislodged away from it, this leaves the structure open to the issues discussed above.
For these reasons, the state of the ground should be monitored throughout construction work, and any controllable vibrations should be kept within pre-set limits to avoid making structures unsafe.
How can you prevent foundation failure?
You probably know the old saying about prevention being better than a cure – and the same applies to foundation failure. There are techniques that can be used to repair less severe damage from weak foundations, but it’s better to stop this from happening in the first place by planning and preparing all structural work thoroughly. When it comes to safety and longevity, you can’t skip over any steps.
To help you with this, you should bring qualified professionals on board for your project. It’s even better if they’re local, as they’ll be familiar with specific regional rules as well as national laws. For example, you can find experts on foundation failure and safe construction in your area by searching for them online – e.g. ‘structural engineers in Manchester’ or ‘structural engineering in Liverpool’.
If foundation failure has already occurred, or is currently developing, then you should still contact a structural engineer. They can carry out an inspection of your property to assess the extent and cause of the damage, and suggest which actions to take to fix the issues before they get any worse.