mechanically reinforced granular shoulders on soft subgrade: laboratory and full scale studies

a recently completed field study in iowa showed that many granular shoulders overlie clayey subgrade layer with california bearing ratio (cbr) value of 10 or less. when subjected to repeated traffic loads, some of these sections develop considerable rutting. due to costly recurring maintenance and safety concerns, the authors evaluated the use of biaxial geogrids in stabilizing a severely rutted 310 m tests section supported on soft subgrade soils. monitoring the test section for about one year, demonstrated the application of geogrid as a relatively simple method for improving the shoulder performance. the field test was supplemented with a laboratory testing program, where cyclic loading was used to study the performance of nine granular shoulder models. each laboratory model simulated a granular shoulder supported on soft subgrade with geogrid reinforcement at the interface between both layers. based on the research findings, a design chart correlating rut depth and number of load cycles to subgrade cbr was developed. the chart was verified by field and laboratory measurements and used to optimize the granular shoulder design parameters and better predict the performance of granular shoulders.