The research on subsurface ground movement has a great importance in terms of the evaluation of the risks to the subsurface structures and pipelines influenced by tunneling. However

this is a topic about which is little known. According to the research by Mair et al (1993) who analyzed the subsurface data from various tunneling projects and centrifuge tests in clays

subsurface settlement profiles could also be reasonably approximated by the form of a Gaussian distribution in the same way as surface settlement profiles. Their equation based on this observation is widely used. However

a fixed value for surface settlement trough-width parameters K is used for clay in the equation

which causes considerable errors for different geological conditions. Also

because of the data they used， Mair equation is only applicable to clays. Based on field data analysis

a normalized ratio ?d is suggested in this paper to account for the different geological conditions; and a modified Mair equation is established which is not only suitable for the cases surface settlement trough with different width parameters

but also can be used for sands. Thus this modified general expression can be used to predict the subsurface ground movement with a broader adaptability compared with the original Mair equation.