PURPOSE The purpose of this manual is to describe the various procedures for subsurface investigation applicable to the transportation field. An outline of a sequence of operations for conducting an investigation is presented. Data obtained at each operational step should be interpreted and the findings applied to optimize each successive work step. These geotechnical data should be considered as influential or even critical in all planning@ design and construction stages of the project. The manual discusses the increasing demand for detailed geotechnical information which has initiated extensive and costly subsurface explorations. The level of investigation appropriate to a particular project must be given careful consideration. Though the additional information will generally decrease possible unknowns and construction risks@ a balance must be maintained between the costs of the exploration program and the level of information which will be produced. Throughout the manual@ mention is often made of the fact that no standard approach for subsurface investigation has been adopted. Widely diverse geologic environments@ local equipment@ personal preferences and time and budget constraints have all contributed to the development of different approaches. It has been found that subsurface exploration procedures cannot be reduced to a few guidelines that fit all conditions. The effects of specific geologic conditions on the type of proposed facility must be evaluated for each project. The viewpoint taken in this manual is that the selection of individuals to direct the investigation@ interpret the information and present the conclusions in a concise and usable form to those responsible for design and construction is of primary importance in any subsurface exploration program. An area mentioned only briefly@ but which will probably become more significant@ is the importance of subsurface investigation and geotechnical participation in maintenance and rehabilitation projects. Subsurface exploration should not only be seen as important in the planning and designing of new projects@ but in the maintenance and rehabilitation of existing transportation facilities as well.