An investigation into Atterberg limits and their suitability for assessing the shrinkage and swelling characteristics of clay soils for foundation design.

Clay soils shrink and swell with changes in moisture content. This can be exacerbated in the presence of trees, and in the vicinity of buildings, the resultant effects can cause structural damage. For foundation design in such circumstances in the UK, reference is often made to guidelines published by the National House Building Council (NHBC), which were primarily written for low-rise residential structures. These guidelines are based on a clay shrink / swell potential assessed using Atterberg Limits and a 'water demand' classification for trees. Atterberg limits are semi-empirical tests that assess a fraction of a soil sample which passes an arbitrary sieve size. The structural features of clay soils, mineralogy, chemistry, prior stress history and cyclic effects all influence the magnitude of volumetric change. Atterberg limits do not directly measure any of these features but have been related to some of them empirically. This paper reviews the processes of clay shrinkage and swelling and the applicability of the Atterberg limits in the assessment of volume change potential. It concludes that modifications to the NHBC guidelines can make designs more site specific. However, because the guidelines are based on a cost - benefit analysis, they will over-design foundations for structures other than low rise residential houses and will not be relevant for others. It is concluded that a more appropriate analysis should be based on a wider overall assessment of all the available soil/site information, in conjunction with a simple assessment of plasticity.