Performance of geotextiles with enhanced drainage

Abstract

Geotextiles have been successfully used in multiple geotechnical and geoenvironmental applications over the years, and are now commonplace in projects such as waste containment facilities, pavements, and earth retaining structures. While significant information has been documented on the mechanical behavior of geotextiles, information on the hydraulic behavior of geotextiles has been investigated primarily under saturated conditions. Theoretical background, laboratory data, and full-scale measurements have become recently available to understand the interaction between soils and geotextiles under unsaturated conditions. This includes the water retention curve and the hydraulic conductivity function of geotextiles. The mechanisms involved in the development of capillary barriers are relevant to explain the storage of moisture that may develop at the interface between materials with contrasting hydraulic conductivity (e.g. a nonwoven geotextile overlain by a fine-grained soil). This can be problematic in unsaturated soil as the capillary barrier caused by the geotextile may instigate undesirable moisture buildup in the overlying soil and undermine some of the benefits provided by the geotextile. Conventional geosynthetic materials are typically only able to drain moisture under saturated conditions. However, in many instances, unsaturated soil conditions prevail and hinder conventional geosynthetics from properly draining. The main objective of this study is to assess the performance of newly available enhanced drainage geotextile products for their capacity to drain under unsaturated conditions. Various prototype versions of an enhanced drainage geotextile incorporating wicking fibers were developed to help prevent a capillary barrier from forming by promoting cross-plane drainage of any excess moisture from the soil. The unsaturated properties of both woven and nonwoven configurations of these enhanced drainage geotextiles were investigated in an experimental study. The testing program included soil column infiltration tests to assess the development of geotextile capillary barriers, as well as their performance, with moisture monitored using time domain reflectometers. In addition to assessing the cross-plane behavior of enhanced drainage geotextiles, the in-plane enhanced drainage capabilities of the geotextiles were investigated. An experimental test setup involving ultraviolet dye allowed for visualization and quantification of the vertical capillary rise in the wicking fibers. An analytical capillary rise model was developed, which accounts for the tortuous flow path through the fibers. The model predictions were found to match well with the experimental results. A microscopy study incorporating both optical and scanning electron microscopes allowed for observation of the wicking behavior of the geotextiles at a micro-scale level. Complementing the laboratory research, the field performance of enhanced drainage geotextiles was evaluated through several pavement case studies. This study in particular included a field research component, involving construction of an instrumented pavement test section founded on an expansive clay subgrade along a portion of SH-21 in Bastrop, TX. Eight 500 ft long test sections with different types of geotextiles were constructed in order to investigate the possible benefits of utilizing both conventional and enhanced drainage geotextiles within a pavement. Each test section was instrumented with a horizontal and vertical array of moisture sensors, which were monitored to assess the effectiveness of the various geotextiles to remove excess water from the pavement section. Additional monitoring included condition surveys to document pavement distresses and total station surveys to document fluctuations in the surface profile of the pavement due to the presence of expansive clays. Overall, experimental and field results illustrate advantages in both cross-plane and in-plane drainage for the enhanced drainage geotextiles when compared to conventional geotextiles. Furthermore, the woven version of the enhanced drainage geotextile has the potential to perform the additional functions of separation, filtration, protection, reinforcement, and drainage. Offering these multiple functions by a single product could lead to significant cost savings compared to the use of separate products to individually perform each function.