Advanced Front Technique(AFT) is widely used in Automatic mesh generation which is the important ingredient of adaptive finite element method. How to model the irregular surface is one of the unsolved problems in AFT, to overcome the problem, the spline-blended surface interpolation is adopted. It is found the method is suitable for the representation of the large scale surface which usually has no adequate geometric information, such as ground surface, the fault surface which are very common in geotechnical engineering. In the second part of the paper the Alternative Digital Tree(ADT) is used in geometric searching of AFT procesure, which can reduce the computation time in large magnitude. In the end of the paper, a tunnel excavated under a rock slope is modeled by tetrahedral elements, the example demonstrate the validity of above methods.