The Western Transportation Institute (WTI) recently completed a subgrade stabilization study
for the MDT in 2009 (Cuelho and Perkins, 2009). This project was co-sponsored by NAUE GmbH & Co. KG (Germany), and used three of its products along with seven other geosynthetics to construct subgrade stabilization field test sections at the TRANSCEND research facility in Lewistown, MT. The test sections were constructed using a weak subgrade with a California Bearing Ratio (CBR) of 1.7 and with a relatively thin aggregate layer of 200 mm (8 in.). The aggregate layer was designed to carry fewer than 1000 traffic passes for sections without geosynthetics and more than 1000 traffic passes for sections with geosynthetics. The design was performed according to procedures contained in FHWA (1995) and checked against a more recent design method (Giroud and Han, 2004).
The test sections reached a terminal rut depth of 100 mm (4 in.) in less than 40 truck passes. The mode of failure of most test sections was a clear bearing capacity (shear) failure in the subgrade and involved tensile rupture of several geogrid products and pullout of one geotextile product. The results showed that several products that would not meet the previous MDT specification performed better than several other products that did meet that specification for this specific case. It was also shown that the ultimate tensile strength and tensile strength at 2 percent axial strain were relatively important material properties in determining how well the geosynthetics performed under conditions of rapid rut development. These properties were most important because of the large loads the geosynthetic was required to support, which approached, and in some cases exceeded, the tensile strength of the materials.
Subgrade stabilization for roadway construction generally requires that the subgrade geosynthetic-base layer system reaches a stable condition. This condition is typically assessed by observing the deformation of the system under the single pass of a loaded vehicle and observing that this deformation is minimal. Under stable conditions, bearing capacity failure of the subgrade has not occurred. In this operational condition, it is anticipated that other geosynthetic properties that might be more significant for conditions of smaller loads and deformations will be important for determining how well the material performs. The intention of the proposed research is to create these operational conditions in field test sections to determine which material properties are most responsible for showing good performance in subgrade stabilization applications.
The main objective of this project is to determine material properties of geosynthetics that affect
in-field performance of geosynthetics used for subgrade stabilization, so that DOT personnel can
objectively and confidently select appropriate geosynthetics based on material properties and cost for a specific situation, while also allowing competition from different manufacturers.
Scope of Work
To accomplish the stated objective, test sections will be constructed at a controlled test site to investigate the relative benefit to an unpaved road of various geosynthetics available on the market. An
artificial subgrade will be constructed to provide equivalent conditions for each test section;
likewise the gravel surfacing along the entire test bed will be uniform to be able to make direct
comparisons between geosynthetic products. Laboratory tests on the subgrade, base course and geosynthetics, as well as large-scale box tests conducted by a commercial testing laboratory, will be used to determine key material properties. Laboratory tests that will be used to characterize the materials used during this research project.
Controlled traffic loading with frequent rut measurements will indicate performance benefits of each geosynthetic. Five basic measurements are necessary in this research project to quantify and understand the behaviors of the geosynthetic in the field test sections during trafficking: 1) longitudinal rut depth of the gravel surface, 2) transverse rut profile of the gravel surface, 3) displacement of the geosynthetic in the transverse direction, 4) strain in the geosynthetic in the transverse direction, and 5) pore water pressure in the upper layer of the subgrade.
Additionally, post-traffic examination will provide invaluable information regarding the performance and installation survivability of the geosynthetics. Post-trafficking, forensic investigations will be conducted to evaluate damage to the geosynthetic from trafficking, as well as to re-evaluate pertinent soil strength characteristics. Intensive evaluations will take place in areas within each test section that have similar rutting. The base course will be removed from a sample area to carefully expose the geosynthetic. The geosynthetic will then be carefully removed from the area to analyze damage to junctions, rib integrity and material continuity. Samples of the extracted material will be removed to conduct monotonic tensile tests to evaluate changes in tensile strength during construction and trafficking, as well as to determine how much permanent strain was imparted in the material from construction and trafficking. Several DCP, LWD and in-field CBR measurements will be taken on the exposed subgrade surface within each test section. Additionally, the depth of the base course aggregate layer will be measured during these evaluations. The transverse rut profile of the base and subgrade will be measured on each side of the excavated area during these investigations to determine strain in the base course aggregate and characterize movement of the subgrade due to trafficking. Finally, the subgrade will be excavated from this area to comprehensively evaluate soil mixing between the subgrade and base course in the rutted areas, soil shear strength at various depths using the hand-held vane shear, and to facilitate a visual evaluation of the rutted area.
Further analysis will be conducted to illustrate cost savings by optimizing material properties that most influence the design and performance of these materials, thereby increasing the knowledge base, confidence and efficiency for state DOTs to update their specifications.
The MDT would like to hear from those states who are interested in the project but may not be able to commit at this time.