Ground Heave – Mitigating
Ground heave can be a major problem, especially on clay ground (see Clay Ground and Hard Landscaping) and also on sites with a high-water table, where hydrostatic pressure can lift long established areas of tarmac driveways, albeit on a temporary basis, until the water subsides and the ground returns to normal.
Unfortunately, when ground heave occurs under paving projects, the resultant damage can mean a complete rebuild, from sub-base upwards, as the paving will have become loose from its laying bed, and all jointing may be cracked.
It is advisable, if you suspect that a site may be susceptible to ground heave – caused by water pressure and subsequent soil expansion – to arrange for a site survey, showing the likely movement of water across the localised area surrounding the garden. (See Hydrological Surveys) as the information shown will predict the likelihood of heave potential under your work. Expand these surveys to include neighbouring sites for a hundred metres or more to gain a wider picture of the water movements.
Knowing that there may be a problem, steps may be taken to alleviate or divert excess ground water away from the paved areas, keeping the water on the surface, instead of creating upward pressure, by means of gullies, channels, French drains and swales, designed in as part of the scheme. Once a client understands the evidence indicated by the hydrological survey, they will appreciate the requirement for additional works below ground level i.e. unseen and therefore unappreciated, when costing the project.
Water will always take the route of least resistance, and by allowing it to traverse the site, you can at least attempt to mitigate the potential for future problems caused by excess underground water likely to cause ground heave.
Some sites may require a submersible pump to be installed in an underground chamber, with a float switch that only operates once the chamber fills with ground water, which is then pumped to a suitable surface drain. Soakaways are not suited to sites prone to ground heave.
