Abstract: Geotechnical site investigation with appropriate planning and management consider significant to gain adequate and correct site information for designing a foundation of the building in a timely manner and with minimum cost for the needed effort. The important of this study is to analyze the factors effecting on soil investigations and its impact on the cost of the projects as there is strong relationship between the soil investigation cost and the cost of the projects and more importantly. The methodology of the study divided in two part, questionnaire and the use the techniques of data mining including fuzzy decision tree. The soil investigation consider very important in the construction projects have direct impact on the cost of the project as lack of topographic and geological maps of the region lead to misleading of what type of soil we dealing with hence thus will lead to inappropriate selection of the toll and equipment. The fuzzy decision tree show the higher accuracy and the variable importance flow show that the personal relationship has direct impact on cost.