Abstract: The soft soil improvement project combined with vacuum preloading and surcharge is summarized in this paper. The conventional pumping termination criterion in the preloading project is the settlement rate equal or less than 5 mm per month. The results in this case study indicate that the criterion is excessively high and is difficult to achieve or at very cost if it can be achieved. This study modifies this criterion by considering the unloading and reloading characteristics of soft soils and the over-consolidation phenomena induced by the removal of the surcharge.  The pumping termination criteria can be obtained by considering the allowable post construction settlement of the roadway, even if the settlement rate prior to the pumping termination is greater than the conventional 5mm per month.

Key words: Soft soil unloading, Over-consolidated characteristics, Pumping termination criterion, Settlement rate, Pumping termination