It is not property of soil at a point that is controlling performance of a geotechnical structure but average over a certain line, area or volume (i.e. the local average). This fact has been pointed out by Vanmarcke (1977) where he proposed the variance function. This paper is succeeding and developing this idea, where it tries to specify the effective size of the local averaging of the soil parameters. The soil profile is model by a stationary random field, which is simplification and idealization of the real ground. In the proposed method it was assumed that the effect of spatial variation of geotechnical parameters on the performance of a geotechnical structure can be evaluated by considering the local average over a certain appropriate range of ground. The validity of this approximation method is examined in this paper by taking several typical geotechnical structures as examples. The exact solutions by MCS and those by the approximation method are compared to examine the validity of the method. It was found that the size seems to be much dependent on the types of geotechnical problems under consideration. The paper deals with deformation and stability problems of the shallow foundation to specify the averaging volume. The results suggest that this idea can be extended to many other geotechnical reliability analysis.