Moisture Reduction Factors for Shear Strength of Unsaturated Reinforced Embankments

Abstract

This paper describes the construction and testing of six 1-m-high model embankments constructed at three different gravimetric water content (GWC) values to study their performance and to validate a set of moisture reduction factors (MRFs) introduced by the authors in their recent studies. The earlier MRF values were obtained from a series of pullout and interface shear tests on the same soil and reinforcement materials. The MRF in this study is defined as the ratio of soil-reinforcement interface shear strength at an increased GWC value [e.g., optimum moisture content (OMC) + 2%] to the shear strength at OMC − 2% representing construction conditions. Therefore, MRF values can be used to account for an anticipated reduction in the shear strength of the soil-geotextile reinforcement interface caused by wetting in the stability analysis and design of reinforced soil structures constructed with marginal soils. The embankment models were built using a lean clay (CL) at the GWC values ranging between OMC − 2% and OMC + 2%, which included a single-reinforcement layer near the top of the embankment. This provided a horizontal soil-reinforcement interface subjected to shear sliding of an overriding block of soil caused by surcharge loading of the embankment. Two different woven polypropylene geotextile products were used to build the six model embankments. Each model was instrumented with a total of 67 sensors to measure the soil GWC, matric suction and excess pore pressure, reinforcement strains, earth pressure, and deformations of the embankment model and the test box during the test. Results from the embankment tests in this study indicate that the change in the matric suction and GWC could have a significant influence on the soil-geotextile reinforcement interface strength. Wetting of the soil and the soil-geotextile interface during construction or service life of reinforced soil slopes could considerably reduce their shear strength, resulting in lower factors of safety for their stability. The results of the study showed that within the range of GWC values examined (i.e., OMC ± 2%), the embankment model constructed at OMC − 2% yielded the greatest shear strength and stability when subjected to a strip footing load. The MRF values for the model embankments constructed at OMC + 2% were found to be as low as 0.74–0.79 for models that were reinforced with different woven geotextiles of comparable apparent opening size (AOS) but different ultimate strength values. The MRF results presented in this study, although obtained from soils with different as-compacted GWC values, indicate that the loss of soil-reinforcement interface shear capacity as a result of wetting in reinforced soil structures involving marginal fills could be significant and deserve proper attention in the design of these systems.