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What are the different classes of soil reactivity? There are six main classes of soil reactivity. There is more likely to be a significant amount of ground movement with certain classes. – Class A Stable soil where there is likely to be no movement, or only a small amount. This type of soil often contains sand or rock. – Class S A small amount of movement may occur in these soils which often contain clay. – Class M Moderate movement is possible in these soils where clay and silt is often found. – Class H High levels of ground movement can happen in these soils which are often clay based. – Class E Ground movement may be extreme. – Class P Problems with ground movement could be extreme and the expertise of a structural engineer is needed before any building work can start. You should never try and judge soil for yourself. An expert needs to carry out soil testing and produce a report for you.