Tabu Search based solution methods for scheduling log-trucks

We present Tabu Search based solution heuristics for the Timber Transport Vehicle Routing Problem (TTVRP) that differ with respect to solution space. The TTVRP is characterized as follows: a fleet of m log-trucks which are situated at the respective homes of the truck drivers has to fulfil n transports of round timber between different wood storage locations and industrial sites. All transports are carried out as full truck loads. Since the full truck movements are predetermined our objective is to minimize the overall distance of empty truck movements. In addition to the standard VRP we have to consider weight constraints at the network, multi-depots, and time windows. Altogether four Tabu Search variants have been developed for the TTVRP and are presented here. The first one computes with the full neighborhood of a solution in each iteration step, whereas the second variant reduces the neighborhood and therefore accelerates the search process. The third variant uses an oscillating neighborhood size depending on the number of the iteration step. The fourth variant was developed to solve big problem instances within acceptable computing times. It is based on the second variant but uses an additional technique to broaden the search process. A post-optimization heuristic was developed and can be applied to all Tabu Search variants. The presented methods were verified with extensive numerical studies.