In this research the effect of cavities on the stability of foundation has been studied using non-linear finite elements analysis through the programs PLAXIS2D, and PLAXIS3D. The study included a number of variables represented by shape, size, sectional area, location, and depth of a single cavity under the base of footings. The effect of the above variables on settlement and stress distribution was studied on isolated square, round and strip footing.
Results showed that the shape and volume of the cavity has an effect on the settlement and concentration of stress under the footing for the chosen cavity sections (Circular, Ellipse1, Ellipes2, Loaf, Square) when the cavity is situated at a depth less than twice the width of strip foundation or 1.5 times the width/diameter of isolated square/circular footings. The study also showed an increase in the values of settlement and concentration of stress at what may be termed as the critical depth under the footing. It was also concluded that there exists a zone that may be called the critical zone below the footing (zone of radial shear and failure plane). If a cavity is situated within this zone then it will have a serious effect. Large value of settlement were recorded for cavities situated within this zone under the footing.

Keywords: Cavity, Settlement, Stress, Finite Element.