Objectives: Engineering geological investigation, as one essential basis of geoengineering design and construction, is supported by advanced theories, technologies and the experiences from previous typical examples. While in complicated geological conditions, the engineers have to adopt not only those procedures conducted by manpower and instruments, but also the empirical method which is mainly based on engineering geological analysis, in case of potential construction induced geohazards. That is what the so called thirtypercentinvestigation and seventypercentexperience approach. How to scientifically understand the meanings of this approach and to accomplish engineering geological investigation in a feasible way? Methods:Reviewing and summarizing the theories in engineering geology, and comparing them to the TCM (Theory of Chinese Medicine), this paper introduce the idea of normal deducing plus back analysis, the former is the general process of summarizing, and the latter refers to the process of hypothesizing and analogic analyzing. Moreover, a slope failure case at an expressway in North China was analyzed. Results:The case example analysis reminds the importance and exemplifies the operation of the methodology combining the engineering geological investigation with empirical analysis under complicated geological conditions. And the measurements could be adopted step by step when combining them. Therefore, the results provide a reference to the check of data and case analysis corrections, to the rationality of the workload of the investigation, the basis of representation, the correctness of the experience of experts and regional constraints.