Methodology for Determining Laboratory and In Situ Hydraulic Conductivity of Asphalt Permeable Friction Course

The permeable friction course (PFC) is a layer of porous asphalt pavement overlain on conventional impervious hot-mix asphalt or portland cement concrete. The drainage properties of PFC are typically considered to be governed primarily by two hydraulic properties: hydraulic conductivity and porosity. Both of these hydraulic properties change over the life cycle of the PFC layer due to clogging of the pore space by sediment. Therefore, determination of the hydraulic conductivity and porosity of PFC can be problematic. Laboratory and particularly field tests are necessary for accurately determining the hydraulic conductivity of the PFC layer. Taking multiple measurements over the life of the pavement shows how these hydraulic characteristics change with time and the varying roadway conditions at which they are evaluated. Constant head laboratory testing has shown that PFC experiences a nonlinear flow relationship as described by the Forchheimer equation. In addition to the laboratory analysis of the hydraulic characteristics, a falling head field test is recommended to determine the in situ hydraulic conductivity. This incorporates the modeling techniques used in the laboratory testing and applies them to the falling head conditions used in the field. The result is a nondestructive test procedure for determining the in situ hydraulic conductivity which is necessary for estimating the extent to which the benefits associated with the drainage characteristics of the PFC layer will persist.     

Effects of a Permeable Friction Course on Highway Runoff

This project documents the impact of a porous asphalt overlay on the quality and quantity of highway storm-water runoff. A permeable friction course, also known as open graded friction course, is a layer of porous asphalt approximately 50?mm thick, which is often applied on top of conventional asphalt or concrete highways to enhance safety and reduce noise. Storm-water runoff from a four-lane divided highway in the Austin, Texas area was monitored at two sites before and after the installation of a PFC. Observed concentrations of total suspended solids and pollutants associated with particulate material were much lower in the runoff from the PFC than that derived from the conventional asphalt surface. Concentration reductions were observed for total suspended solids, total lead, total copper, and total zinc at both monitoring locations. In addition to the above-mentioned constituents, concentrations of chemical oxygen demand and total Kjeldahl nitrogen were also lower in the runoff from the PFC at a site collecting paired samples from both pavement types. Concentrations of dissolved constituents were not significantly different and concentrations of polycyclic aromatic hydrocarbons were below the detection limit for both pavement types. The runoff coefficient for the PFC appears to be higher than for conventional pavements.     

Evaluation of Four Permeable Pavement Sites in Eastern North Carolina for Runoff Reduction and Water Quality Impacts

Four permeable pavement applications in North Carolina’s Coastal Plain were constructed and monitored to determine their effectiveness of reducing runoff quantity and improving water quality. Sites were either constructed of permeable interlocking concrete pavers (2), porous concrete (1), or concrete grid pavers (1). One site of each pavement type was monitored for runoff reduction for periods ranging from 10 to 26 months. Measured runoff depths from rainfall events over 50?mm were used to determine permeable pavement equivalent curve numbers for the sites, which ranged from 45 to 85. Only the two permeable interlocking concrete pavement (PICP) sites were monitored for water quality. Runoff and exfiltrate samples were intended to be collected, in addition to runoff monitoring, from the Swansboro PICP site. However, no runoff was produced during this study from the Swansboro PICP site for rainfall events up to 88?mm. From exfiltrate concentrations, nutrient retention was estimated to be 3.4 and 0.4?kg/ha/year for total nitrogen and total phosphorus, respectively. For the Goldsboro PICP site, water quality of asphalt runoff and PICP exfiltrate were compared. Analysis of water quality samples from the second site determined that concentrations of total Kjeldahl nitrogen, ammonia, total phosphorus, and zinc were significantly (p ? 0.05) lower in permeable pavement exfiltrate than asphalt runoff.     

Degradation of a Granular Base under a Flexible Pavement: DEM Simulation

Flexible pavements are composed by an asphalt concrete layer, granular base and subbase layers, and a natural subgrade. The granular materials forming part of the granular layers are subjected to static and dynamic loads during their engineering life. As a result of these loads particle crushing may occur depending on the strength of the particles forming the granular layers. Particle crushing is important since it is associated with several detrimental effects such as settlements and a reduction in hydraulic conductivity. A computer simulation using the discrete element method (DEM) is presented in order to understand and visualize how crushing initiates and develops inside a simulated pavement structure.     

Field Trial for Asphalt Pavements Reinforced with Geosynthetics and Behavior of Glass-Fiber Grids

This paper presents details of a large field trial and some observations conducted to evaluate the practical efficiencies of geosynthetically reinforced asphalt pavements in Shanxi Province, China. Three glass-fiber grids (LB2000 II, TGG-8080, GGA 2021), one plastic grid (Tensar AR1), two geotextiles (nonwoven needle-punched and nonwoven heat-bonded), and one geocomposite (Tensar AR-G) application were selected for evaluation. These geosynthetics were installed in the interface between new asphalt pavement layers (APL) and new cement-stabilized gravel–sand base courses coated by emulsified asphalt or within new APL in the reconstruction of asphalt pavement sections (Program I), or in the interface between old APL and new overlay layers in the asphalt overlay pavement sections (Program II). In each program, reinforced sections with different geosynthetics were compared with each other and with nonreinforced sections to determine relative performance. Inspections after construction showed that the integrated damage ratio and deflection in the pavement sections reinforced with glass–fiber grids were less than other pavement sections. Furthermore, after about 4?years of service, glass-fiber grids were dug out and no breaking and node movement were discovered. Nevertheless, observations indicated that geosynthetics may not be effective, if bearing capacity of the base course/subgrade is inadequate, or if the overlay thickness is too thin, or if preconstruction repair of distressed old pavement is incomplete.     

Sudden Failure of a Heavily Loaded Container Pavement

The case history of a sudden and unexpected failure in a pavement designed for 82?Mg axle loads at Port Botany in Sydney, Australia has been prepared using data derived from investigation of the failure. Failure of the pavement, comprised of an asphaltic concrete surface, unbound granular fine crushed rock base, crushed sandstone subbase, and sandy subgrade, and designed using the rational method—CIRCLY, occurred within days of being put into service. The failure resulted from a 20–30% increase in base course saturation levels following compaction that led to partial liquefaction under repeated heavy loading. There was a general failure throughout the storage area where trafficking was most intense and the pavement remained intact in lightly trafficked areas. The intact areas recovered over time without intervention through a moisture equilibration process as evidenced by an increase in measured pavement stiffness and loss of moisture within the pavement profile.     

Dynamic Response Solution in Transient State of Viscoelastic Road under Moving Load and Its Application

In order to study the dynamic response of an asphalt road, a dynamic model of the road under a moving load is proposed, in which the viscoelastic characteristics of the base and pavement are all considered, the pavement is regarded as an infinite beam on a Kelvin viscoelastic base. By using Green’s functions, Laplace transforms, and Fourier transforms, the analytical solution in transient is deduced. As the viscosity of the pavement is included in the model, the analytical solution can be used to investigate more of the factors that affect the dynamic response, such as vehicle speed, temperature, and road material properties. Using this analytical solution, some numerical calculations are given to illustrate the effects of vehicles’ speeds and different damping on the deflection with the displacement.     

Permeable Pavement as a Hydraulic and Filtration Interface for Urban Drainage

Permeable pavement, specifically cementitious permeable pavement (CPP), is a passive structural low impact development material and green infrastructure system for rainfall-runoff control. Multifunctional engineered CPP can fulfill requirements as a load-transmitting surface while serving as a more environmentally conscious infrastructure material that functions to restore the in situ hydrology while also functioning as a passive treatment unit operation and process. As an infrastructure material; CPP reduces runoff, filters, and treats infiltrating runoff, reduces thermal pollution and temperature, provides the load-carrying capacity of conventional rigid concrete pavement, and leaches environmentally beneficial Ca and alkalinity, as compared to flexible asphalt. In this study, pore characteristics of CPP, including effective porosity (?e), pore size distribution and tortuosity (τ) are examined using x-ray tomography and gravimetric-geometric methods. Total porosity (?t) is measured and utilized as a comparative index. Results provide insight when modeling CPP as an infiltration and evaporation interface, a conveyance/storage medium for liquid and gas, and with admixtures or coatings an adsorptive-filtration medium for pollutants such as phosphorus and metals. Results indicate that high particle separation and significant hydraulic conductivities can be achieved with CPP. The effectiveness of CPP for particle removal also requires that these particle deposits are managed on a periodic basis through practices such as pavement cleaning.     

Geomechanical Analysis of Unbound Pavements Based on Shakedown Theory

A procedure for analyzing the mechanical response of an unbound pavement to the repeated loading of traffic is presented. The pavement is modeled as a layered elastic/plastic structure, and its response is described by the concepts of shakedown theory. A critical shakedown load is identified as the key design parameter. Pavements operating at higher loads will eventually fail, and those operating at loads less than critical may initially exhibit some distress but will eventually shakedown to a steady state. Estimates of this critical load, for different types of pavement, are found by studying various types of failure mechanisms, such as rut formation and subsurface slip. Optimization procedures are then used to determine the most likely form of failure for a particular pavement. The effects of self-weight, dual loads, moisture content, relative strengths of the various layers, and nonassociated plastic flow are studied. Some preliminary implications for pavement design are discussed.     

Unified DSC Constitutive Model for Pavement Materials with Numerical Implementation

Need for unified and mechanistic constitutive models for pavement materials for evaluation of various distresses has been recognized; however, such models are not yet available. There have been efforts to develop unified models; however, they have been based usually on ad hoc combinations of models for special properties such as elastic, plastic, creep and fracture, often without appropriate connections to various coupled responses of bound and unbound materials, they may result and in a large number of parameters, often without physical meanings. The disturbed state concept (DSC) provides a modeling approach that includes various responses such as elastic, plastic, creep, microcracking and fracture, softening and healing under mechanical and environmental (thermal, moisture, etc.) within a single unified and coupled framework. A brief review is presented to identify the advantages of the DSC compared to other available models. The DSC has been validated and applied to a wide range of materials: geologic, asphalt, concrete, ceramic, metal alloys, and silicon. It allows for evaluation of various distresses such as permanent deformations (rutting), microcracking and fracture, reflection cracking, thermal cracking, and healing. The DSC is implemented in two- and three-dimensional finite-element (FE) procedures, which allow static, repetitive, and dynamic loads including elastic, plastic, creep, microcracking leading to fracture and failure. A number of examples are solved for various distresses considering flexible (asphalt) pavements; however, the DSC model is applicable to rigid (concrete) pavements also. It is felt that the DSC and the FE computer programs provide unique and novel approaches for pavement engineering. It is desirable to perform further research and applications including validation with respect to simulated and field behavior of pavements.     

考虑固液分子作用的多孔介质动态网络模型

根据质量守恒定律,从固液分子力作用出发,结合岩心的孔喉分布特征,建立考虑固壁与流体分子作用的动态网络模型.利用模型模拟得到不同时间步的孔隙压力变化,分析了固液分子间力、喉道半径、润湿性等因素对剩余油饱和度分布和水驱相渗曲线的影响.研究结果表明,考虑固液分子间力作用时剩余油饱和度增大,岩石骨架与流体间的分子作用在多孔介质的孔隙流动中不可忽略.      

Field Testing of Stabilized Soil

Remediation of a Superfund site in Stratford, Conn., involved stabilization of the subgrade with portland cement. Part of the remediation site was to be used as a parking area. The stabilized soil was to be covered with natural base∕subbase course materials and capped with an asphalt concrete cover. During the course of the remediation, a base-course layer could not be placed prior to the onset of winter. A field study was conducted to quantify any changes in the mechanical properties of the open stabilized subgrade subjected to freeze-thaw cycling during the winter of 1996–97. Field evaluation was conducted with pavement industry tools: the Clegg impact hammer and the dynamic cone penetrometer. Evaluation results show the viability of the Clegg hammer as an instrument for quality assurance and also show that there can be up to 50% loss in compressive strength of the subgrade within the uppermost layer of the material caused by freeze-thaw cycling.     

Performance of Geosynthetic-Reinforced Asphalt Pavements

This paper describes the performance of geosynthetic-reinforced asphalt pavement under monotonic, cyclic, and dynamic loading conditions. The study differed from current practice where geosynthetics are typically used as separators or to improve the bearing capacity of the subgrade. A geogrid layer was installed at the bottom of the asphalt concrete layer, along the asphalt-subgrade interface, to function as tensile reinforcement. The load was applied to the surface of the asphalt concrete layer using a rigid rectangular footing under plane strain conditions. The strains that developed along the geogrid over time and at different load levels were monitored. Two different types of geogrid reinforcements were used, and their restraining effects on the layered system were compared. The study showed that geosynthetic reinforcement increased the stiffness and bearing capacity of the asphalt concrete pavement. Under dynamic loading, the life of the asphalt concrete layer was prolonged in the presence of geosynthetic reinforcement. The stiffness of the geogrid and its interlocking with the asphalt concrete contributed to the restraining effect.     

Effect of hydrostatic pressure on deformation,damage evolution,and fracture of iron with various initial porosities

This study evaluated the effects of superimposed hydrostatic pressure (138 to 1104 MPa) on densification and plastic flow behavior of porous iron (0.3 to 11.1 % porosity). Pressurization alone caused densification of the porous iron with the effect being most pronounced when the porosity was greater than 3.7% and the pressure above 276 MPa. For the porosities studied, densification as a result of pressurization increased with hydrostatic pressure and initial porosity. The 0.3% porosity iron was the only one whose density did not increase with pressurization or deformation under pressure. The effect of hydrostatic pressure on the flow stress of porous iron was small when densification resulting from pressurization was not a factor. The ductility was found to increase linearly with pressure and the effect of pressure on fracture strain increased with the initial porosity of the iron. Evaluation of the effect of hydrostatic pressure on development of porosity and growth during tensile deformation was limited to hydrostatic pressures of 138 and 276 MPa and iron compacts with initial porosities of 0.3, 1.5, and 3.7% because of the pressurization effects. It appeared that the porosity at fracture was similar in these compacts at both pressures but it was much larger than that observed at 0.1 MPa. The greater ductility of the iron compacts tested under hydrostatic pressure results from a decrease in the growth of pores with deformation and from a greater damage tolerance prior to fracture. As observed for porosity, the average maximum pore diameters at fracture for the compacts tested under pressure were similar but larger than those observed at 0.1 MPa. It appears that a general model of ductile fracture for porous materials cannot be based solely on a critical degree of dilation or on maximum pore extension as a fracture criterion.     

基于核磁共振的不同含水状态砂岩动态损伤规律

为研究不同含水状态岩石的动态损伤特性,制备干燥、半饱和、饱和3种不同含水状态的砂岩试样.采用分离式霍普金森压杆（SHPB）,以4种不同的低入射能对岩石进行损伤冲击试验.通过核磁共振测试实验对岩石试样进行孔隙扫描,获取岩石孔隙的T₂谱曲线、孔隙度以及孔隙成像等数据.通过试验发现:（1）冲击能量的增加导致岩石的平均应变率和强度的增大;（2）不同含水状态的岩石受到冲击后,孔隙度与孔隙度变化率均有不同程度的增加;（3）与冲击前相比,岩石的T₂谱曲线有明显右移趋势,同时出现谱峰增加的现象,而且冲击能量越大,孔隙谱峰增加越明显;（4）核磁共振成像显示岩石孔隙数量和尺寸有明显的增加,展现出岩石内部孔隙扩展和演化的过程.      

Implementation of a Critical State Two-Surface Model to Evaluate the Response of Geosynthetic Reinforced Pavements

A finite-element model was developed using ABAQUS software package to investigate the effect of placing geosynthetic reinforcement within the base course layer on the response of a flexible pavement structure. A critical state two-surface constitutive model was first modified to represent the behavior of base course materials under the unsaturated field conditions. The modified model was then implemented into ABAQUS through a user defined subroutine, UMAT. The implemented model was validated using the results of laboratory triaxial tests. Finite-element analyses were then conducted on different unreinforced and geosynthetic reinforced flexible pavement sections. The results of this study demonstrated the ability of the modified critical state two-surface constitutive model to predict, with good accuracy, the response of the considered base course material at its optimum field conditions when subjected to cyclic as well as static loads. The results of the finite-element analyses showed that the geosynthetic reinforcement reduced the lateral strains within the base course and subgrade layers. Furthermore, the inclusion of the geosynthetic layer resulted in a significant reduction in the vertical and shear strains at the top of the subgrade layer. The improvement of the geosynthetic layer was found to be more pronounced in the development of the plastic strains rather than the resilient strains. The reinforcement benefits were enhanced as its elastic modulus increased.     

Investigation of Heaving at Holloman Air Force Base, New Mexico

Heaving of pavements and a building foundation became progressively worse on a project at Holloman Air Force Base (AFB), N.M. The cause of the heaving was identified as sulfate attack on recycled concrete used as fill and base course below the buildings and pavements. This recycled concrete came from sulfate-resistant airfield Portland concrete pavement that had existed for decades at Holloman AFB without distress. However, severe sulfate exposure conditions, ready availability of water, the more permeable nature of the crushed recycled concrete, less common thaumasite attack, possible soil contamination as a secondary source of alumina, or some combination of these factors allowed sulfate attack to develop in the recycled material even though it had not in the original concrete pavement.     

Wave Attenuation over a Rigid Porous Medium on a Sandy Seabed

In this study, an analytic solution of wave interaction with a rigid porous medium above a poro-elastic sandy bottom is derived to investigate the attenuation of the surface wave and the wave-induced soil response. In the model, both inertial and damping effects of the flow are considered in the rigid porous region using the potential theory, while the consolidation theory is adopted in the sand region. A new complex dispersion relation involving parameters of the rigid porous and the poro-elastic medium is obtained. The analytic solutions are verified by some special cases, such as wave interaction with a porous structure over an impermeable bottom or wave interaction with a poro-elastic medium only. Numerical results indicate that the wave attenuation is highly dependent upon the thickness of the rigid porous layer, the soil stiffness, and their respective coefficients of permeability. Increasing the thickness of the rigid porous layer will shorten the wavelength of the surface wave regardless of the sand coarseness. The pore pressure in fine-sand is larger than in coarse sand, with both decaying with wave progression. It is also found that increasing the thickness of the rigid porous medium will effectively reduce the pore pressure in the sand. For the applications, an extended hyperbolic mild-slope equation is finally obtained, based on the basic analytic solutions. Examples of the wave height transformation over submerged permeable breakwaters on a slope sandy seabed are given. The simulated results show that the wave decay of the coarse sand seabed is larger than those of fine-sand and impermeable seabeds when waves pass after the submerged porous breakwater. The wave damping versus the friction factor for various height of the submerged breakwater is discussed.     

三轴受压状态下岩石试块变形破坏过程中孔隙水压的变化特征及其临界破裂前兆

为讨论饱水岩石中孔隙水压与损伤变形过程的关联性,探究岩石破坏的孔隙水压前兆特征信息,采用三轴压缩试验、渗流实验相结合的方法,分析了在不同孔隙水压和不同围压条件下饱水岩石变形破坏过程中损伤扩展与孔隙水压变化的内在联系.研究表明:(1)开放饱和岩石单元体中,岩石在受荷破坏全过程中的初始压密、弹性压缩、塑性变形和破坏失稳阶段,孔隙水压呈现增高、稳定、逐渐减小和锐减变化;(2)岩石破坏的孔隙水压前兆特征明显,在主破裂前夕,内部损伤加剧,裂隙贯通,孔隙水压将失去稳定状态,孔隙水压由稳态锐减可作为岩石破裂的前兆信息;(3)在应力变化大、高孔隙水压以及高围压条件下,孔隙水压对损伤发展更为敏感,响应也更明显,孔隙水压与损伤发展相互影响,互为关联,损伤扩展造成孔隙水压降低,孔隙水压降低又促进损伤发展;(4)可以尝试在深部开采工程中监测岩体内部孔隙水压的变化来预测岩体失稳的突发性灾害.      

Normalizing Behavior of Unsaturated Granular Pavement Materials

One of the important components of a flexible pavement structure is granular material layers. Unsaturated granular pavement materials (UGPMs) in these layers influence stresses and strains throughout the pavement structure, and have a large effect on asphalt concrete fatigue and pavement rutting (two of the primary failure mechanisms for flexible pavements). The behavior of UGPMs is dependent on water content, but this effect has been traditionally difficult to quantify using either empirical or mechanistic methods. This paper presents a practical mechanistic framework for determining the behavior of UGPMs within the range of water contents, densities, and stress states likely to be encountered under field conditions. Both soil suction and generated pore pressures are determined and compared to confinement under typical field loading conditions. The framework utilizes a simple soil suction model that has three density-independent parameters, and can be determined using conventional triaxial equipment that is available in many pavement engineering laboratories.