Building close to trees presents its own challenges for foundations as highlighted in a previous article, but ground heave is the focus here.
What’s Ground Heave?
It refers to the upward movement of the ground typically associated with the expansion of clay soils that swell when wet. Since the ground is unable to expand either sideways or downwards, the result is that the upper surface of the ground, which is exposed rises up.
The impact of ground heave is opposite to the effect of ‘subsidence’, which refers to a situation where the ground is unstable and sinks downwards, or ‘settlement’, which is a result of the building’s weight.
Displacement is typically below 150mm, but even this is enough to cause serious structural damage to the foundations and fabric of the building. It can happen in soils of certain types when a tree or trees that were previously absorbing large volumes of water are removed.
Heave may also occur in soils that are vulnerable to frost, water supplies, and even leaking drains. One tree is capable of removing as much as 150 liters of water every day. The type, size, and maturity of the tree, the absorbency of the clay soil, and the water table can all affect the amount of ground movement after removing the trees.
It is difficult to determine when heave might occur – it could be after several rainy months or after years, but you should always take it into consideration.
Common Signs of Ground Heave
- Doors sticking since their frames become out of square.
- Cracking to windows and brickwork: The cracking will probably be vertical while diagonal cracking is most often associated with subsidence cracking.
- Lifting of patios and paths surrounding buildings.
- The remedial costs associated with ground heave can be substantial.
Top Ways to Reduce the Effects of Ground Heave
- Perform analysis of ground conditions before you start work
- Investigations may include historic research, visual inspection, and drilling trial holes or boreholes to determine a soil classification as well as its plasticity index i.e. how shrinkable it is.
Consider the possibility of heave in the future
If a tree or trees are retained close to or within your property, or has been removed before commencement of the work, chances are the soil will be drier before the foundations are laid. Often, you have no control if they die off or are removed later and the soil will then rehydrate.
In areas where trees have been removed, the ground takes anywhere from 12 to 24 months to rehydrate unless there are sustained droughts, after which time the likelihood of heave reduces.
Carefully choose your type of foundation
The safest option would be to form the foundation under the area affected by trees, which may mean having to dig deep. The required depth ultimately determines which is the most suitable foundation – trench fill foundations for depths of up to 1.5 meters or pile foundations with ground beams if going further than 2 meters.
If the depth of the foundation exceeds 1.5 meters or heave is likely to be an issue, cellular structures need to be installed against the foundations as well as beneath the floor slabs to reduce the upward force of heave from being transmitted to the structure above. Products such as Heaveguard, Clayboard, and Claymaster have all been designed to create a compressible void that allows for ‘swelling’ of the ground without causing damage to the building structure.
Get the proper foundation depth for your ground conditions
Avoid guesstimates – use the foundation depth calculator that considers the soil type, the proximity of the tree root near buildings, among other factors to calculate the most suitable depth needed up to a maximum depth of 2.5 meters.
If you need to dig deeper than 2.5 meters, find an experienced and qualified professional to assess your site. Even if piling ends up being the most suitable option, it is still important to include protective compressible void formers beneath edge beams and around piles.
Consider a suspended floor
A suspended floor made of beams and blocks may be a cost-effective solution compared to the cost associated with accommodating the heave movement.
Don’t forget about the drains
Drains are equally vulnerable to heave, which is why they need to be protected. If heave is likely, minimum gradients need to be avoided and instead steeper gradients with more granular material of up to 150 mm thickness should be used.