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The significant impact of the truck traffic associated with the shale energy developments on local roads, including many unpaved gravel roads has been a pressing issue while the regional economy benefits from the recent shale gas boom. In many cases, roads had to be reconstructed over a soft soil subgrade treated and stabilized by cement or lime. As an alternative, geosynthetics offer a potentially more cost effective solution for reinforcing/stabilizing roads on soft soil subgrade. This paper presents a study using a full-scale accelerated traffic load testing to evaluate two recently emerged geosynthetic products, a triaxial geogrid and a high-strength woven geotextile in unpaved roads built over soft soil subgrade. A total of six full-scale test sections were constructed, among which two sections were reinforced by one and two layers of triaxial geogrids, respectively; while high strength geotextiles were used to reinforce two of the other sections with different aggregate layer thicknesses. The rest of the two sections were left as control sections, of which one section was constructed over 30-cm thick sand embankment. The test sections were subjected to a full-scale moving wheel load applied by the Accelerated Loading Facility (ALF). A variety of instrumentations were used to measure the load-associated and the environment-associated responses and performance of the unpaved test sections. Results of the full-scale testing on the unpaved test sections demonstrate the benefits of geosynthetics in reducing the permanent deformation/rutting.

In the state of Louisiana, roadways often have to be built over a weak subgrade due to the soft nature of Louisiana soil, which generates many design and construction difficulties. As an alternative to the traditional method of treating the soft soil with lime or cement, two geosynthetic products, a triaxial geogrid and a high-strength woven geotextile were evaluated for reinforcing unpaved roads constructed over natural soft subgrade. A total of six full-scale unpaved test sections were constructed, among which two sections were reinforced by one and two layers of triaxial geogrids respectively while high-strength geotextiles were used to reinforce two of the other sections with different base thicknesses. The remaining two sections were left as control, of which one was constructed over sand embankments as is common practice in southern Louisiana. The test sections were subjected to a full-scale moving wheel load applied by the accelerated loading facility (ALF). A variety of instrumentation was used to measure the load-associated and environment-associated pavement responses and performance. Results of the full-scale testing demonstrate the benefits of geosynthetics in reducing the permanent deformation in the pavement structure.

This research study was performed to evaluate the performance of cement treated/stabilized very weak subgrade soil specimens molded at high water content condition. Three soil types of different plasticity indices were considered in this research study. Three different wet of optimum moisture contents producing raw soil strength of 172 kPa (25 psi) or less were chosen for treatment/stabilization. The percentages of cement content were determined to achieve target seven-day strength values of 345 kPa (50 psi), treatment for working table application, and 1,034 kPa (150 psi) stabilization for subbase application. The repeated load triaxial (RLT) tests were performed on the laboratory molded treated/stabilized specimens to evaluate their resilient modulus and to study their deformation behavior under cyclic loading. The test results indicated that the proper selection of cement content for very weak and wet subgrade soil can substantially improve their performance in terms of resilient modulus and permanent deformation for working table and subbase applications. A good correlation was observed between the water/cement ratio and the resilient modulus/permanent deformation, with the soil specimens compacted at low water/cement ratios showing better performance than those compacted at high water/cement ratios. For heavily stabilized subgrade soils for subbase application, the permanent deformation of the stabilized layer can be neglected in the pavement design.

The influence of moisture content on unbound aggregates' resilient modulus has been well recognized, but accurate prediction of this influence remains a challenge. With prediction models incorporating matric suction, great improvement has been achieved in understanding and modeling moisture-dependent resilient modulus. Nevertheless, matric suction, in the sense of soil water tension, cannot be measured directly. The actual measured matric suction in geotechnical literature is water potential, which leads to a conceptual inconsistency. In addition, difficulties associated with determining the effective stress parameter χ may limit the general applicability of matric suction-based prediction models in practice. On the basis of suction stress concept, which is lumped macroscopic effect of various interparticle physicochemical forces, a prediction model for resilient modulus is proposed in this paper. The feasibility of such a model has been confirmed and illustrated by laboratory testing results.

Research Associate Professor, Louisiana Transportation Research Center, Louisiana State University, Baton Rouge, LA 70808, cefars@lsu.edu Graduate Student, Department of Civil and Environmental Engineering, Louisiana State University, Baton Rouge, LA 70803, jgu3@lsu.edu Boyd Professor, Department of Civil and Environmental Engineering, Louisiana State University, Baton Rouge, LA 70803, voyiadjis@eng.lsu.edu Research Associate, Louisiana Transportation Research Center, Louisiana State University, Baton Rouge, LA 70808, qchen1@lsu.edu

This research investigates the behavior of a geosynthetic reinforced sandy soil foundation using laboratory model footing tests. The model footings were steel plates with dimensions of 152 mm × 152 mm. The parameters investigated in this study included top layer spacing, number of reinforcement layers, vertical spacing between layers, tensile modulus and type of reinforcement, and embedment depth. The test results showed that the inclusion of reinforcement can appreciably improve the soil’s bearing capacity. Higher tensile modulus geosynthetics performed better than others; and that sand reinforced by the composite of geogrid and geotextile performed better than those reinforced by geogrid or geotextile alone. The test results also showed that the reinforcement configuration/layout has a very significant effect on the bearing capacity of reinforced foundation. The results of the model footing tests were compared with the analytical solution developed by the authors in a previous study. The analytical solution gave a good predication of the experimental results of footing on geosynthetic reinforced sand.

In this paper, the results of a series of cyclic plate load tests on geogrid base reinforced pavement sections were analyzed to incorporate the benefit of geogrid within the context of the new AASHTO Mechanistic-Empirical Pavement Design Guide (MEPDG). The structural contribution of geogrid reinforcement was quantified in terms of increasing the resilient modulus of base course layer and/or reducing the thickness of base aggregate layer in pavement structure. With regard to increase in resilient modulus, the effective resilient modulus of base course layer was backcalculated to account for the improvement in the performance in pavement section provided by the geogrid reinforcement. The increase in the base course resilient modulus was then quantified. The benefit of geogrid base reinforcement was also evaluated by the amount of reduction in the thickness of base aggregate layer of required performance defined in terms of Base Course Reduction (BCR) factor. The results show that the value of resilient modulus of the base course layer can be increased by 10-90 percent and that the thickness of base layer can be reduced by 12 to 49 percent for the geogrid reinforced pavement sections tested in this study. The results also indicated that the higher tensile modulus geogrids typically provide better performance.

The Louisiana Department of Transportation and Development has initiated a major effort to minimize the bridge end bump problem. As a result, a new design for the approach slab was proposed, which requires increasing the slab rigidity and the use of reinforced soil foundation (RSF) to support the slab and traffic loads. The Bayou Courtableau Bridge was selected as a demonstration project to evaluate and verify the new approach slab design system. The thickness of the approach slab is 16 inch. The end support of the approach slab is a 4.0-feet wide strip footing with soil underneath reinforced by geogrid. The first static load test was conducted right before the bridge opened to the traffic. The test results indicated that the west approach slab (with new design) remained its full contact supports from the soil while the east approach slab (with traditional design) started losing a portion of its supports from the soil.

This research study aims at investigating the scale effect of shallow foundation on reinforced soils using finite element analysis. The finite element model was first verified by the results of laboratory model footing tests, and then was used to numerically investigate the load-settlement response of different footing sizes on reinforced soil foundations. Two different soil-reinforcement interface models were studied and three different reinforcement configurations were examined. The results of finite element analyses indicate that the scale effect of reinforced soil foundation is mainly related to the reinforcement ratio (Rr) of the reinforced zone. The difference in the bearing capacity becomes negligible as long as the reinforcement depth ratio (d/B) and the reinforcement ratio (Rr) of the reinforced zone remain constant for all footing sizes.