By René B. Laprade and John M. Lostumbo
Moisture within pavement layers is a principal cause of pavement deterioration (Cedergren 1994, Christopher and McGuffey 1997, Henry and Holtz 2001). Specific problems associated with moisture include (1) reduction in pavement strength caused by excess water in the base course layers and/or the subgrade soils, (2) shrinking and swelling of subgrade materials caused by moisture content changes, (3) frost heave and thaw weakening caused by capillary moisture flow beneath pavements, (4) stripping of asphalt pavements and (5) joint displacement in concrete pavements. Moisture-related problems are responsible for decreased pavement life, increased maintenance costs and increased pavement roughness, and they occur throughout all regions and climates around the world. The detrimental effects of moisture in pavements are tremendous. According to Cedergren (1987), a pavement’s service life can be reduced by half if the pavement approaches saturation just 10% of the time.
Geosynthetics have been effectively incorporated into roadways for several decades to improve the performance and longevity of these and other civil structures. The principal functions to which geosynthetics contribute in these applications include separation, filtration, confinement, reinforcement and drainage. Drainage is one of the most important functions to maintain pavement performance and, historically, has been addressed with passive systems in roadway applications. These passive systems rely on gravitational flow in order to provide drainage. This can provide limited benefit because most water flow in pavement systems is unsaturated through capillary movement. This unsaturated flow cannot be drained effectively by conventional geotextiles if a capillary barrier is formed. When two porous materials with differing hydraulic conductivities are in contact with one another, a capillary barrier develops; the capillary barriers increase the moisture storage around the contact area by forming a temporary barrier at the interface of the two materials (Zornberg et al. 2010).
The development of an enhanced lateral drainage geosynthetic (ELDG) has provided an important tool toward the management of moisture in roadways. The ELDG’s unique ability to remove moisture in saturated and unsaturated conditions has proven to have quantifiable benefits to roadways and other civil structures.
Introduction to ELDG
In unsaturated soils, suction, or capillary forces, between soil particles holds moisture. Conventional geotextiles and drainage nets cannot provide drainage to the moisture within base courses or subgrades under unsaturated conditions. The ELDG with wicking capability was developed to remove water from soil via capillary action. This woven geotextile provides both mechanical and hydraulic stabilization to roadways. It is a two-component geotextile that includes high-modulus black polypropylene fibrillated yarns in the machine and cross-machine direction as well as blue deep-groove nylon fibers in the cross-machine direction of the geotextile. The hydrophilic and hygroscopic nature of the nylon fibers induce high capillary forces to remove moisture out of unsaturated soils. See Figures 1 and 2.
The University of Alaska Fairbanks: Rainfall infiltration soil column tests (2009)
In 2009 the University of Alaska Fairbanks conducted some rainfall infiltration soil column tests. In these tests, individual 8-inch (200-mm) columns of initially saturated silt were placed on top of different geosynthetics. These included (1) a high-modulus woven geotextile without the deep-groove fibers, (2) a woven monofilament geotextile, (3) a drainage composite made from high compressive strength cores and combined with nonwoven filter fabrics and (4) the ELDG. Each configuration was then underlain with an impermeable membrane to encourage lateral drainage, thus preventing any moisture from escaping directly underneath the geosynthetic, as seen in Figure 3.
Results from these tests indicated that the ELDG removed 3%–4% more moisture from the soil column than any of the other geosynthetics, as shown in Figure 4.
According to Budhu (2010), Equation 1 can be used to compare the undrained shear strengths of two samples of the same soil based on the differences in moisture content.
su = undrained shear strength
Gs = specific gravity of soil (~2.7 for silt)
w = water content
λ = compression index (0.15 typical
In this test, a 3% reduction in moisture content leads to q ~70% increase in undrained shear strength of the silt material.
“Since all tested geosynthetics are permeable under saturated conditions and the heights of the soil columns were small (8 inches [200 mm]), gravity induced water flow will not cause differences in moisture content distributions in the soil columns. Therefore, the differences in moisture content distributions in the soil columns are caused by unsaturated water flow induced by differences in suction head” (Zhang and Belmont 2009).
The University of Alaska Fairbanks: Preventing frost boils in the Dalton Highway Beaver Slide area, Alaska (2012)
Every year, the Beaver Slide area of the Dalton Highway suffered severe road damage through (1) frost heave and subsequent thaw weakening and (2) upward pressurized water flow to the road surface during lengthy rain events. (Figure 5)
In 2012 this instrumented test section was monitored for temperature and moisture content through the placement of sensors in 22 locations within the road cross section.
The moisture content values on the right side of the section measured on September 4 are significantly lower than the readings taken on August 19. The damage caused to this section by frost heave and thaw weakening as well as from significant rain events had been an ongoing problem on the Dalton Highway for years. After the installation of the ELDG, the treated sections no longer suffered from these moisture-related damages (Figure 6). According to Zhang and Presler (2013), the ELDG eliminated the frost heave and thaw weakening within the pavement, and the ELDG also prevented the soils from reaching saturation in the test section.
Mitigating edge cracking on low-volume pavements in the Yukon with ELDG: FPInnovations (2018)
In 2015 the government of Yukon, Department of Highways and Public Works and FPInnovations installed two instrumented sites to study the effectiveness of the ELDG to mitigate edge cracking and control roadbed moisture in northern highways. The final report was issued in 2018. These sections were monitored for temperature and moisture content. Each site consisted of an untreated control section and a section treated with ELDG, which was placed between the base course and the subgrade. Figure 7 shows the edge cracking that was present in the spring of 2017, two years after construction, in one of the control sections. The photograph in Figure 8, also taken in the early spring of 2017, shows very little edge cracking in an area treated with the ELDG. The author concluded that “the ability of the [ELDG] to drain moisture has a direct impact on the increase of bearing capacity of the road matrix.” It has been proven in this study that the geosynthetic decreased roadbed moisture by 1.5% to 2.0% on average. This allows road materials, which are at a partially saturated state right after spring thaw or right after an episode of summer precipitation, to get back to their natural in situ state, which is closer to the optimum moisture content.
Development of design method for ELDG in pavement structures: University of Alaska Fairbanks and University of Kansas (2016)
Several studies were conducted by Lin and Zhang (2017) to quantify the hydraulic benefits of the ELDG. One study focused on the base course resilient modulus tests at various moisture contents and determining the geotextile air entry value. The researchers measured the resilient modulus of the aggregate base at 2% increments over moisture contents ranging between 0% and 12%. All samples were initially compacted at the optimum moisture content of 8.5% to ensure that each specimen had the same microstructure. Specimens were then either dried or hydrated to the target moisture content and tested. Resilient modulus testing was conducted according to AASHTO T307-99.
Results of the resilient modulus testing showed that a 2% increase in moisture content of the base course aggregate from optimum caused a 60% decrease in resilient modulus and a fivefold increase in permanent deformation (Figure 9). Conversely, a 2% reduction of the moisture content from optimum by the ELDG results in a 200% increase in resilient modulus and a 40% reduction in permanent deformation (Figure 10). The benefit of the ELDG in reducing moisture levels at or below optimum are clearly shown in this research.
Instrumented test using ELDG in expansive clays: University of Texas at Austin (2015)
The development of longitudinal cracks associated with the presence of expansive clay subgrade soils is a moisture-related pavement distress. These environmentally induced longitudinal cracks develop toward the pavement shoulders because of moisture content variations in the subgrade. Environmental moisture variations such as wet or dry periods have higher changes in moisture content at the shoulder of the pavement compared to the center of a pavement. Consequently, the edges of the pavement will shrink and swell more significantly during dry and wet periods respectively. During particularly dry seasons, the flexion of the pavement will result in tension toward the surface layer, leading to significant longitudinal cracks (Zornberg et al. 2017).
A 6-mile (10-km) section of Texas State Highway 21 (SH21), just north of Bastrop, Texas, was constructed on an expansive clay subgrade and in 2013 was rehabilitated by the Texas Department of Transportation (TxDOT). Prior to this rehabilitation, the road had received regular attention from the TxDOT maintenance operations group. Despite this regular maintenance, performance was deemed to be inadequate due to the extensive network of lateral shoulder cracking present. The 2013 rehabilitation plan included milling the top 3 inches (75 mm) of the pavement and expanding the shoulder.
Eight 500-foot (152.4-m) test sections were constructed to evaluate the performance of the ELDG, as well as two other high-modulus woven geotextiles and a nonwoven geotextile (Figure 11), in controlling moisture content changes at the subgrade level. Moisture sensors were installed at the center of each test section to monitor changes in moisture content in the subgrade along the road shoulder, where moisture contents were expected to be more variable. These test sections were monitored over a 20-plus month period. Figure 12 shows that the ELDG was very effective in minimizing the moisture content variability over the test period, as it had the lowest spread of the geosynthetics included in these tests. Reducing the difference in high and low moisture content will reduce the pavement damage caused by the regular expansion and contraction of the subgrade. Delgado (2015) stated that the ELDG test section on SH21 showed the least shoulder cracking with none and that results indicate the ELDG can prevent cracking in the shoulder by equilibrating moisture beneath the geotextile through enhanced lateral drainage.
Quantification of hydraulic benefits of ELDG to AASHTO 1993 and mechanistic-empirical design guides: University of Kansas (2018)
A series of laboratory tests, including demonstration tests, water tank moisture removal tests, small box tests and soil column tests, were conducted at the University of Kansas to investigate the hydraulic characteristics of the ELDG. Six large-scale cyclic plate loading tests with rainfall simulation were done to evaluate the effect of the ELDG on the permanent deformations of base courses over weak subgrades. The small box tests, the soil column tests and the large-scale cyclic plate loading tests provided the relationships between base course water content and drainage time. Design guidelines that incorporate the water content reduction benefit of the ELDG geotextile were developed by modifying the 1993 AASHTO Pavement Design Guide and the AASHTO Mechanistic-Empirical Pavement Design Guide (MEPDG) (Guo et al. 2018).
A series of rainfall simulations demonstrated that the ELDG reduced the water content in the base course below its optimum moisture content. Lin and Zhang (2018) demonstrated through their series of triaxial tests that the ability to lower the moisture content in unsaturated materials has highly beneficial implications regarding increasing the base course resilient modulus and reducing its permanent deformation due to the presence of excess moisture.
This research project also showed that the zone of influence of the ELDG on the overlying base course is 8 to 10 inches (200 to 250 mm) during the short evaluation time of the project. This zone of influence is included in the quantification of the hydraulic benefit the ELDG brings to an overlying base course developed as part of this research.
This project led to a method that incorporates the hydraulic benefits of the ELDG in the 1993 AASHTO Flexible Pavement Structural Design Method. Accomplishing this is a four-step process. (1) Incorporate mechanical stabilization within a pavement design. (2) For a predetermined time frame (five years, for example), determine the dates upon which rain events of sufficient intensity to saturate the overlying granular layer occur. (3) For the same time frame, determine the number of days with average temperatures below freezing. The environmental information required to accomplish steps 2 and 3 is available at the National Oceanic and Atmospheric Administration website (https://www.noaa.gov) for the U.S. and at the Environmental and Natural Resources website for Canada (www.climate.weather.gc.ca). (4) Determine the damage to the base course for each of the saturation rain events. Following these steps will lead to a hydraulic improvement factor that can be applied to the 1993 AASHTO Flexible Pavement Structural Design Method. The current MEPDG software includes an enhanced integrated climate model (EICM) to modify the representative resilient modulus of base course for seasonal effects. Based on the water content change measured in the previous tests, modification can be made to the environmental effect model in the MEPDG design software for the design of the wicking geotextile in roadway structures. However, the current MEPDG design software does not allow modification to the environmental effect component. The MEPDG design software does allow input of base course resilient modulus of each month of the year. The base course resilient modulus of each month can be calculated with the same procedure described in the previous section.
The combination of all these different research efforts has confirmed the performance of the recently developed ELDG. Continuous moisture management of pavement systems provides a new function of geosynthetics. The ability of an ELDG to reduce the moisture content of base/soil material in unsaturated conditions provides significant long-term benefits to roadways. These benefits include modulus increase, pavement sustainability, increased performance and maintenance reductions. ELDG can also address frost heave and thaw weakening conditions as well as shrink/swell from expansive soils. These research results have provided guidance to incorporate ELDG into AASHTO 1993 Pavement Design Method and into MEPDG.
René B. Laprade, P. Eng., is technical marketing manager for TenCate Geosynthetics Americas in Aurora, Ont., Canada.
John M. Lostumbo, P.E., is director of technical marketing for TenCate Geosynthetics Americas in East Amherst, N.Y.
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