This study focuses on development of a Geographic Information System (GIS)-based decision support system (DSS) to assist construction engineers in monitoring and controlling the excavation conditions. In this system, layout of the construction site and various instruments are represented in several data layers. Each instrument layer is integrated with a relational database, which receives data collected from the site. Applying fuzzy set theory, the system analyzes the collected data and identifies possible causes of adverse conditions. Through database queries and spatial displays, the position of the extreme instrument readings exceeding threshold limits is identified. The primary features of the system are as follows: (1) development of a knowledge base for planning layout of instruments according to design requirements, site characteristics, or instrument capabilities, (2) provision of a quality control checking list for instrument installations, (3) collection and transmission of measured data to the job site office using automated transmission technology, and (4) analysis of the collected data and diagnosis of possible causes of the adverse conditions. This system improves the instrumentation program by providing a logical and systematic manner to analyze the measured data in a real time basis. Thus, predictions of any adverse conditions and appropriate actions can be taken to prevent construction accidents. Furthermore, as compared to manual methods, the system significantly improves the computational effort and increases data accuracy and consistency.