Levee security assessment is a complex expert assessment process based on several heterogeneous data. In our previous research works, we applied information fusion techniques to characterize flood protection levees. We used the proportional conflict redistribution rule no.6 (PCR6) proposed in DSmT (Dezert Smarandache Theory) framework to combine data from geotechnical and geophysical investigation methods. However, in some cases, this rule can generate non satisfactory results. Indeed, the uncertainty between several hypotheses (lithological materials) is overestimated after the fusion process, which is detrimental to decision making in the end. This result occurs because the PCR6 rule does not preserve the neutrality of the vacuous belief assignment, which can be judged as being a counter-intuitive behavior. To overcome this problem we present an improved rule that preserves the neutrality of vacuous belief assignments in the fusion process. Hence, the redistribution of the partial conflict masses using this new rule does not overestimate the masses associated with partial uncertainties. To illustrate the use of this new fusion rule in a levee characterization problematic, we simulate data acquisition. Two geophysical investigation campaigns (electrical resistivity tomography and multi-channel analysis of surface waves methods) and a geotechnical acquisition campaign (core drillings with particle size analysis) are numerically simulated on an earthen structure. The objective is to compare and discuss the fusion results obtained using this new rule with respect to the methodology based on the original PCR6 rule as well as to demonstrate the enhancement of the levee characterization.