The use of trenches as vibration mitigation action is widespread in many countries. The complexity of the intervention design process depends on the high number of parameters that influence its efficacy, such as trench geometry size (width, depth, length), section shape, used in-filled material, distance among source-trench-receiver and frequency content of the source. This paper focuses on the study of the mutual influence between trenches length and in-filled material type considering several fixed observation points. The analysis was realized using a finite element model that was calibrated through comparison with in-situ measurements, and the evaluation of the effectiveness of each configuration taken into account was performed calculating the amplitude reduction index Ar and analyzing the vibration attenuation capacity both in time and frequency domain. As found by several other authors, it was found that increasing trenches length, a better isolation effect is performed, but the improvement amount seems to be strongly influenced by the kind of material to be used to fill trenches. This aspect must be taken into account in the optimization process of vibration isolation actions performed using trenches.