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"The behavior of flexible pavements under traffic and environmental loading can be significantly affected by subsurface conditions. Inadequate support conditions under the surface can lead to excessive pavement deformations, often leading to structural and functional failure. This research effort focused on assessing the effects of base/subbase and subgrade layer conditions on flexible pavement behavior. The results of this study are presented in the form of two journal manuscripts. The first manuscript focuses on utilizing pavement structural and functional evaluation data in making pavement rehabilitation decisions. Visual distress surveys and Falling Weight Deflectometer (FWD) testing are often carried out by agencies as a part of their pavement preservation programs. Although back-calculation of individual layer moduli from FWD data is a common approach to assess the pavement's structural condition, the accuracy of this approach is largely dependent on exact estimates of individual layer thicknesses. Considering the lack of pavement layer thickness information for all locations, this study used Deflection Basin Parameters (DBPs) calculated from FWD test data to make inferences regarding the structural condition of individual pavement layers in conventional flexible pavements. The adequacy of DBPs to assess the structural condition of individual pavement layers was assessed through Finite-Element (FE) Modeling. Subsequently, four selected pavement sections in the state of Idaho were analyzed based on this method to recommend suitable rehabilitation strategies. The second manuscript focused on studying how improvements to subsurface layers can affect the flexible pavement behavior over expansive soil deposits. A recently completed research study at Boise State University investigated a particular section of US-95 near the Idaho-Oregon border that has experienced significant differential heave due to expansive soils. Laboratory characterization of soil samples indicated the presence of highly expansive soils up to depths of 7.6 m (26 ft.) from the pavement surface. Through subsequent numerical modeling efforts, a hybrid geosynthetic system comprising geocells and geogrids was recommended for implementation during pavement reconstruction. This research effort focused on evaluating the suitability of polyurethane grout injection as a potential remedial measure for this pavement section. Laboratory testing of unbound materials treated with a High-Density Polyurethane (HDP) demonstrated that resilient modulus and shear strength properties could be improved significantly. Finite Element modeling of the problematic US-95 pavement section indicated that depending on the treated layer thickness, the differential heave magnitude can be reduced significantly, presenting polyurethane injection as a potential nondestructive remedial measure. ."--Boise State University ScholarWorks.
Following the recommendation of the Virginia Transportation Research Council's Pavement Research Advisory Committee, this project was initiated to determine the effectiveness of including subsurface drainage systems in pavements in Virginia. The researchers sought to determine the effectiveness of these systems by conducting a literature review and by comparing the strengths of pavement sections with and without a subsurface drainage layer in a limited field investigation involving two pavement structures in Virginia. The strength of the pavement structure was analyzed using the falling weight deflectometer. The researchers concluded that the drainage layer appears to affect positively the in-situ subgrade resilient modulus and the in-situ structure number. Further, inclusion of a properly constructed drainage layer does not adversely affect the deflection of a pavement and thus does not introduce a weakness into the pavement structure. However, the condition of the outlet pipes appears to be of high importance. The researchers recommend that tests with additional sites be conducted in the spring when the subgrade moisture is expected to be highest; that the Virginia Department of Transportation (VDOT) develop a maintenance program to maintain functioning drainage outlet pipes; and that VDOT continue the practice of constructing subsurface drainage features on high-priority pavements. In 2005, VDOT anticipates spending approximately $45 million on resurfacing interstate and primary roadways. According to the literature review, the average service life of flexible pavements (time between successive rehabilitation efforts) is approximately 9 years. Including subsurface drainage features offers a 4-year extension of service life (a 44% extension). Thus it can be approximated that the current practice of including subsurface drainage features is saving VDOT approximately $20 million per year. However, the amount of this cost savings may not be fully realized if drainage outlet pipes are blocked or partially blocked. As reported in the literature review, nonfunctioning drains accelerate pavement deterioration and thus may actually shorten the service life of pavement structures.
"Expansive soils present significant engineering challenges, with annual costs associated with repairing structures constructed over expansive soils estimated to run into several billion dollars. Volume changes in expansive soil deposits induced by fluctuations in the moisture content can result in severe damage to overlying structures. A flexible pavement section near the Western Border of Idaho has experienced recurrent damage due to volume changes in the underlying expansive soil layer; traditional stabilization methods have provided partial success over the years. The main objective of this research effort was to characterize the problematic soil layer contributing to the recurrent pavement damage and propose suitable rehabilitation alternatives. An extensive laboratory test matrix was carried out to characterize soil samples collected from underneath the problematic pavement section. Laboratory tests showed that the problematic expansive soil deposit was often at depths greater than 6 ft. (183 cm) from the pavement surface. Potential Vertical Rise (PVR) values calculated for ten boreholes strategically placed along the problematic pavement section closely matched with the surface roughness profile observed in the field. Liquidity Index (LI) calculations indicated that the active-zone extended to a depth of least 11 ft. (335 cm) from the pavement surface, and therefore, most of the heaving likely originates from soil layers as deep as 11 ft. (335 cm) from the pavement surface. Clay mineralogy tests indicated the presence of high amounts of Montmorillonite that can lead to significant volume changes. Moreover, high sulfate contents were detected in soil samples obtained from several of the boreholes, indicating a potential for sulfate-induced heaving upon chemical stabilization using calcium-based stabilizers. Based on findings from the laboratory testing, it was concluded that chemical stabilization or shallow treatment alternatives are not likely to be successful in mitigating the recurrent differential heave problems. A mechanical stabilization approach using geocells was proposed as a likely rehabilitation alternative for this pavement section. By dissipating the heave-induced stresses over a wider area, this reinforcement configuration was hypothesized to significantly reduce the differential heave. Finite-element models of the pavement section comprising six alternative geocell-reinforced configurations were prepared using the commercially available package, ABAQUS. Moisture swelling and suction properties for the expansive soil deposit were established in the laboratory and were used in the numerical model to simulate the swelling behavior. Results from the numerical modeling effort established that placing two layers of geocell within the unbound granular base layer led to the highest reduction (~60%) in the differential heave. Placing a single layer of geocell, on the other hand, reduced the differential heave magnitude by approximately 50%. A single layer of geocell was therefore recommended for implementation to achieve the optimal balance between pavement performance and construction costs."--Boise State University ScholarWorks.
Permanent deformation (rutting) is a pavement distress condition visible in the surfacing layer of a pavement. It occurs along the wheel path and results from the accumulation of load-induced permanent deformation developed from all individual pavement layers, including the subgrade. It is one of the major distress conditions in flexible pavements. Plenty of research regarding permanent deformation in flexible pavements exists, but it is mainly focused on asphalt surface layers and granular base, subbase, and subgrade layers. The South African National Roads Agency Ltd (SANRAL) completed the construction of seven flexible pavement sections on the R104, between the east of Pretoria and Bronkhorstspruit, during 2013. In-situ pavement response and environmental related data have been collected from these test sections ever since on a number of occasions. The seven flexible pavement structures include a natural gravel (G4) base, a high-quality graded crushed stone (G1) base, a Foam Treated Base (FTB), an Emulsion Treated Base (ETB), a Cement Treated Base (CTB), a Bitumen Treated Base (BTB), and a High Modulus Asphalt (HiMA)/Enrobe̹s A̳℗ Module Eleve̹ (EME) base. The permanent deformation behaviour of different flexible pavements relative to each other was investigated by processing, validating, and analysing the relevant in-situ pavement response and environmental related data collected from each of the SANRAL test sections. With the focus on total and base layer deformation, it was found that in terms of a short-term loading response and under normal operating conditions, bituminous pavements show superior performance to cement/bitumen stabilised pavements, while the latter performs better than granular pavements. CTB and ETB pavements are very similar with FTB pavements closely behind. The only granular exceptions are inverted crushed stone pavements, which should closely follow bituminous pavements at the top end of the performance range. For permanent deformation behaviour in terms of a longer-term recovering response, it was found that bituminous pavements tend to recover a larger amount of the permanent deformation attained after load application than granular pavements, probably due to the delayed elasticity (visco-elastic properties) of bituminous materials. The possibility of a transfer function for linking the permanent deformation behaviour of a pavement to its structural integrity was also investigated by determining a representative pavement number for each of the SANRAL test sections. It was found that the permanent deformation behaviour of flexible pavements relates relatively well to their structural integrity as a general decrease in permanent deformation (rut rate) was observed with an increase in pavement number. A negative power function for linking permanent deformation behaviour to structural integrity was proposed (y = 76.657x-0.752, R2 = 0.77). Additionally, it was found that post-compaction trafficking has a significant effect on the permanent deformation behaviour of flexible pavements during the initial stages of their life cycle; temperature variations can have a major influence on the in-situ performance and behaviour of bituminous layers, and the permanent deformation behaviour of flexible pavements correlates positively with the corresponding dynamic response as an increase in permanent deformation (rut rate) was observed with an increase in maximum dynamic deflection (positive linear function, y = 0.0361x - 2.5687, R2 = 0.92).
NCHRP Report 583 explores the effects of subsurface drainage features on pavement performance through a program of inspection and testing of the subsurface drainage features present in the Long-Term Pavement Performance SPS-1 (flexible hot-mix asphalt pavement) and SPS-2 (rigid portland cement concrete pavement) field sections.