How heave occurs | How to deal with heave? | Back to heave
The effects of heave appear to the layman to be similar to the effects of subsidence, but nearly always the cracking is vertical rather than diagonal. Heave can cause diagonal cracking occasionally but it would be orientated differently than in the case of subsidence. As with the investigation of subsidence, boreholes are necessary to prove the moisture profile at the location in question and remote from it. In addition, aerial photographs may show that there was once a tree growing close to the centre of the heave. There are many other ways of detecting this effect.
When designing underpinning, one has to bear in mind that in future, the dry clay could expand back to what it was. The piles have to be designed to reduce the future uplift effects and to sustain such reduced future uplift. The slab or beams usually have some void formed under them so the clay can expand upwards, crushing the void former without overloading the slab or beam. In order to resist both the weight of the wet concrete plus impact loading during pouring wet concrete, the void former must be strong enough to withstand the impact loading of the wet concrete but weak enough not to transfer in the future too much heave force upwards into the slab or beam. As the anti-heave material can only transfer a prescribed uplift pressure, the designer of the slab knows how much upward load to allow for.
To have your problems efficiently sorted out, we recommend that you should engage the services of a chartered civil or structural engineer, who has had formal training in the subject of soil mechanics and who has had practical experience of analysing soil and designing foundations.