Key points are not available for this paper at this time.
S. Salhi, G. Nagy, A Cluster Insertion Heuristic for Single and Multiple Depot Vehicle Routing Problems with Backhauling, The Journal of the Operational Research Society, Vol. 50, No. 10 (Oct., 1999), pp. 1034-1042
Salhi et al. (Fri,) studied this question.