Performance and Effectiveness of a Thin Pavement Section Using Geogrids and Drainage Geocomposite in a Cold Region

The objective of this study was to evaluate the performance of a flexible pavement system constructed using geosynthetics. Although the benefits of geosynthetics in unpaved roads has been studied and proven, few studies have investigated their use in flexible pavement systems with subbase layers as thick as those used in Maine. The majority of previous studies used subbases ranging in thickness from 50 to 365 rnrn (2 to 14 in.). Only one previous study, by the Maine Department of Transportation (Fetten and Humphrey, 1998), examined reinforcement and drainage applications of geosynthetics with subbases up to 640 mm (25 in.) thick. This study continues to investigate reinforcement and drainage applications of geosynthetics with subbases up to 600 mm (24 in.) thick. In addition, drainage applications with subbases consisting of pavement grindings were investigated. Route 9 in the towns of Monrnouth, Litchfield, and West Gardiner is underlain by areas of very poor subgrade soils. These soils are classified as AASHTO A-2-4, A-4, and A-6 and are highly frost susceptible. Several of the subgrade soil samples taken for the proposed reconstruction of Route 9 had blow counts as low as 7 and natural water contents approaching the liquid limit. The low shear strengths of the subgrade soils combined with the inadequacy of the existing dramage made this a suitable site for testing geosynthetics for reinforcement and drainage applications. Route 9 was reconstructed during the fall of 2001 and summer of 2002. During this reconstruction, several test sections were constructed and instrumented. Four types of test sections were used: reinforcement, drainage, drainage with reinforcement, and control. Test sections using reinforcement geogrid are equipped with strain gages that are used to deduce tensile loads. Drainage test sections have vibrating wire piezometers to monitor porewater pressure in the subgrade and subbase course. The drainage sections use a 100-rnrn (4-in.) diameter underdrain pipe to collect water from the drainage geocomposite. The outlet of each underdrain pipe is equipped with a flow meter to measure the amount of water coming from the drainage geocomposite. Each of the test sections has thermocouples that measure frost penetration. The reconstructed road is designed to carry 500,000 equivalent 80-kN (1 8-kip) single axle loads. The strain gauges attached to the geogrid indicate that the majority of the force in the geogrid was developed during compaction of the first lift of overlying subbase material. In general, subsequent increases in force were small and may be the result of creep strain in the geogrid. The maximum force recorded in the geogrid was only 11 % of the geogrid\u27s ultimate tensile strength. Piezometer and flow meter data shows that the drainage geocomposite placed on subgrade helped to remove water from the overlying subbase. The effect of the drainage geocomposite on subgrade porewater pressures was unclear. The data also indicates that the drainage geocomposite\u27s ability to remove water from the subbase and subgrade may be limited in sections constructed using a subbase of pavement grindings