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Subgrade fines migration, also known as pumping, negatively affects the performance of pavements due to decreased drainage capability, reduced pavement stability, increased permanent deformation, and gradual creation of nonuniform foundation support conditions. The issue of infiltration of fines and the resulting base course contamination have become even more problematic considering the age of the roadways and the increasing levels and magnitude of traffic. Therefore, pumping is considered an important failure mechanism of pavements and should be taken into consideration when designing the pavement or monitoring it during its service life. This paper presents the results and findings from research studies undertaken to determine magnitude and rate of migration of subgrade soil particles into subbase and to evaluate the effectiveness of geotextiles separation and filtration in reducing such migration, in both flexible and rigid pavement systems. A one-third scale Model Mobile Load Simulator (MMLS3), an Accelerated Pavement Testing (APT), was used to apply simulated traffic loading on pavement sections. Analyses were conducted to identify the thickness of stress-equivalent scaled pavement sections, such that the vertical compressive stress at the top of the subgrade (i.e., subgrade and subbase interface) under the wheelpath of MMLS3 were equivalent to the stress in corresponding full-scale pavement sections. Pavement sections were constructed on a bed of non-plastic saturated silt as subgrade and partially saturated aggregate subbase. Earth pressure cells and dynamic pore water pressure cells were installed at different locations of the pavements to measure total stress and excess pore water pressure generated by cyclic traffic loadings and to validate the stress equivalency. A total of six sets of MMLS3 tests were carried out to quantify the magnitude and rate of pumping for flexible and rigid pavement on typical collector roads and interstate highways. The experiments were repeated by using a geotextile separator and filter layer at the interface between subgrade soil and aggregate subbase. The results revealed that due to cyclic traffic loading, considerable subgrade fines migrated into subbase. The migration was affected by the traffic-induced downward pressure at the interface, which varied in different pavement types (flexible, rigid) and classes of roadways (collector and interstate). Pumping was also affected by the number of traffic cycles. It was observed that geotextile effectively reduced pumping in pavements. This is because of a significant decrease in the pore water pressure (a key erosional force) developed at the subgrade-subbase interface due to presence of geotextile. This study was supported by Geosynthetic Institute (GSI), PennDOT, and FHWA. The outcomes are anticipated to lead to revised policies for the use of geotextiles for subgrade-subbase separation, leading to improved performances and service life of pavement systems.