What is heave?
Heave is the opposite of subsidence, referring to an upward movement of the ground surface or the foundation of a structure. Heave occurs when the soil expands or rises causing the foundation of a structure to rise which intern causes cracking and distortions to the structure above.
Photo by Joe Dudeck on Unsplash
What causes heave?
There are generally two causes of heave.
Heave is caused when a property is built on desiccated ground. This occurs when the geology consists of shrinkable clay soils and one or more very large mature trees have been removed and a property is built on the same ground soon afterwards. The very large mature tree/s will have over the years desiccated the ground owing to its/their roots extracting all the moisture from the surrounding area. As the ground dries out it shrinks and sinks forming a depression or bowl, although invariably it cannot be seen with the naked eye. The ground recovers to its normal level as it absorbs moisture from rainfall over the coming years and the ground will eventually recover to its normal level. Any property that was built within the depression or bowl soon after the tree was removed will also rise with the ground levels causing movement to the foundations and cracking to the structure above. This process can take many years to be completed and a ten year cycle is not uncommon.
Alternatively, if the property is built very close to very large mature tree/s which is/are then subsequently removed the same effect as above occurs.
How often does heave occur?
Heave is extremely rare. In the last twenty years I have dealt with only a handful of heave cases compared with thousands of subsidence cases. Heave is often confused with general recovery of the ground from normal seasonal movement. Shrinkable clay soils shrink and expand with the seasons and after a very dry summer where the ground will have shrunk there will be recovery over the winter. However, this is not the same as heave when a property was built on previously desiccated ground following to the removal of large trees. The effects from normal ground recovery over the winter cause only minor damage in some cases and is easily rectified whereas heave can cause substantial damage which is very difficult to rectify.
What is the remedy for heave?
Heave is a very difficult problem to solve. The ground will continue to rise until it has reached its equilibrium with the surrounding area. This can take many years. Invariably the only solution is to install a piled raft which is extremely disruptive and expensive normally costing in excess of £150 k. Once the piled raft has been installed the damage to the superstructure can be repaired.
Photo by Etienne Girardet on Unsplash
How can heave be avoided?
When shrinkable clay soils are present it is important that developers are aware of the history of a plot they intend to build on so that engineers can design the property to accommodate any heave potential where large mature trees have been removed.
Similarly, if a very large mature tree or trees are to be removed near to an existing property and the trees are significantly older than the building a full heave assessment will need to be carried out prior to any tree removal.
The main underlying question in this situation is how old is the tree and how old is the house. Was the tree present before the house and was it of substantial size? In other words was the house built on desiccated ground? If so there is a large risk of heave and a full heave assessment will need to be carried out. If heave is a risk the tree should not be removed or if this is unavoidable substantial foundation stabilization works will need to be carried out prior to its removal.
Author: Tony Waring is a Chartered Building Engineer (C.Build E MCAB), an Associate of the Royal Institute of Chartered Surveyors (AssocRICS) and a member of the Faculty of Party Wall Surveyors (MFPWS). He has over 30 years of surveying experience and 20 years as an engineer.
Comments