Resource-constrained scheduling for continuous repetitive projects with time-based production units

In construction projects of highways, pipelines, and tunnels, labor and equipment continuously move in a linear geographic layout. This class of continuous repetitive projects encounters the resource-constrained problem when there are limits on the availabilities of resources (labor and equipment). Conventional scheduling models divide continuous repetitive projects into space segments. The premise is that crews would maintain the same production rate in each space segment. However, when the length of segment is indivisible by the production rate, this assumption leads to an inefficient schedule which asks the crews to change their production rate, a reflection of their size, composition, and associated equipment, in the middle of a time period. Unproductive time would then be spent in extra preparation and unnecessary warming up. Another drawback is that production units divided in space cannot be directly linked to the time-based payment schedule. In light of these shortcomings, this paper presents a scheduling model to find the optimal set of production rates in different time periods for each crew, considering limited availability of resources. To be practical, the proposed model addresses work continuity while maintaining lead-time and lead-distance between operations. The optimization problem is solved by an evolutionary strategy algorithm, which is easy to program and takes less execution time, with no need for selection and crossover process. A real-life project is used to validate the performance of the proposed model in terms of effectiveness, efficiency, and stability.