Research on Seepage Characteristics of Asphalt Pavement Structure SHEN Weijun 1, LIU Jianjun 2 1. Haijian Urban and Rural Planning & Design Institute of Zhanjiang CO., LTD, China, 524000 2. School of Civil Engineering and Architecture, Southwest Petroleum University, Chengdu, 610500 1057505008@qq.com Abstract: Both foreign and domestic research on the asphalt pavement conditions and deterioration shows that water damage is an important factor contributing to asphalt pavement structure destruction. In this paper, the destructive mechanism of water damage of pavement was analysed based on the asphalt pavement characteristics, and the mathematical model for seepage analysis of pavement damage is established. The numerical simulation for the pavement seepage of some typical asphalt highway is carried out using the finite element software of SEEP/W. This paper analyses the regular patterns of the asphalt pavement seepage in different conditions of rainfall intensities, rainfall durations and groundwater levels from simulations, and will provide technological support to asphalt pavements design and maintenance. Keywords: asphalt pavement, rainfall infiltration, numerical simulate 1 Introduction Since the 1950s, asphalt pavement construction has developed rapidly all over the world. With the growing demand of modern pavement transport which can be summarized as large traffic flow, quick speed and heavy axle load, high stability and high strength asphalt pavement have been required. Nevertheless, the existing methods of pavement design conventionally do not take the drainage and effective waterproof layers into account. Once the rainfall penetrates deeply into the pavement structure through the cracks, seams, surface and shoulder, seepage destruction of asphalt pavements cannot be avoided [1]. Study of seepage characteristics of asphalt pavement structure is of great theoretical and realistic significance. There have been many studies on seepage destruction of asphalt pavements. The U.S. Army Cold Regions Research and Engineering Laboratory puts forward an method for pavement design in the seasonal fog regions. Ariza [2] uses the software SEEP/W to numerically simulate for the unsaturated pavement seepage of three highways in Minnesota. Mahboub [3-4] goes on steady-state seepage simulation for asphalt overlay on cement concrete pavement. At the moment, pavement design methods in China do not consider the internal structure (or layer) of floor drains, and the phenomena of seepage destruction has occurred frequently. The situation has aroused extensive concern in the domestic community. A famous expert in pavement engineering, Sha Qinglin, a academician of CAS, published his work "Failure and Prevention of Premature Damage of Asphalt Pavement in Expressway" and "Water Damage and Prevent of Asphalt Pavement in Expressway". A researcher from the Ministry of Communications Highway Research Institute, Shen Jin an [5], published "the Technical Approach of Solving Premature Water Damage of Asphalt Pavement in Expressway". The methods of the on-site investigation and experimental analysis have been mostly used to study the destruction of the asphalt pavement in the domestic community, while there were few studies on the seepage patterns in asphalt pavement. In fact, only if we intensively study the seepage characteristics of asphalt pavement structure from the theory of seepage, can we come up with better drainage engineering measures, so as to offer theoretical guidelines for the design and maintenance of asphalt pavement. Based on the theory of seepage mechanics and the finite element numerical simulation, this paper studies asphalt pavement structure and the pavement seepage patterns for rainfall infiltration under rainfall condition. Consequently, we can put forward better drainage engineering measures, so as to offer theoretical guidelines for the design and maintenance of asphalt pavement. 812
2 Theory of Seepage Analysis of Asphalt Pavement Suppose the asphalt seepage follows Darcy s law, the connection of permeability velocity to hydraulic gradient can be expressed as v= K J (1) where v is the permeability velocity, J represents the hydraulic gradient, K denotes the permeability coefficient of asphalt pavement. The asphalt pavement permeability coefficient is related to several factors, such as material of structure layers, soil type, gradient of soil grain, soil density, coefficient of dynamic viscosity of liquid penetration and temperature etc. Combining the Darcy s law and mass conservation equation, the mathematical model for asphalt seepage is given by H ( kx ) + ( ky ) + ( kz ) = Ss (2) t where k x, k y and k z are the permeability coefficients of in the direction of x-axis, y-axis and z-axis, Ss is the unit storage, H is total head value. Based on Richard s saturated-unsaturated seepage theory [8-10], the saturated-unsaturated seepage equation of asphalt pavement is derived as θ = [ K( θ ) ] + [ K ( θ ) ] + [ K ( θ ) ] (3) t where K (θ ) denotes the permeability coefficient which is the function of moisture content θ. 3 Numerical Simulation of Seepage for Asphalt Pavement Structure 3.1 Mathematical Model The study focuses on a first-grade highway somewhere in southern area of Yangtze River. The total width is 32 meters, which includes six 3.75-metre roadway, a 3-metre isolation and two 3.25-metre shoulders. The transverse slope between roadway and hard shoulder is 2%, and it is 4% for soil shoulder. For the pavement structure, the surface layer is consist of 4 centimeters mid-asphalt concrete, 5 centimeters coarse asphalt concrete, 6 centimeters asphalt macadam, grass-roots level by the 37 centimeters stable cement gravel, cushion with the 20 centimeters natural gravel, the median separator filled with silty sand, the earth foundations covered with silty clay. In order to describe the pavement seepage field accurately, the section extend three meters below and one meter on either side of pavement. The section of pavement structure is shown in figure 1. Figure 1 The section of pavement structure Through the literatures both home and abroad, specification of asphalt pavement Structure for the simulation is shown in table Ⅰ. 813
Materials TableⅠPAVEMENT SPECIFICATION Ks / cm s -1 θ s / % / % Mid-asphalt concrete 2.2e -3 0.08 0.046 Coarse asphalt concrete 5.5e -3 0.16 0.046 Asphalt macadam 0.027 0.29 0.05 Stable cement gravel 1.22e -4 0.15 0.065 Natural gravel 1.59e -3 0.15 0.6 Silty clay 1.40e -7 0.18 0.08 Silty sand 3.6e -4 0.39 0.06 Suppose that there is no water interchange or movement between the two pavement interfaces. That is, the boundary flux is zero. The rainfall happens once five years which lasts for 2 hours in the main cities of southern China and the average rainfall intensity is 18.6x10-6 m/s (66.954mm/h). And the boundary conditions are pavement and median separator. 3.2 Numerical Simulation of Rainfall Infiltration for Asphalt Pavement Figure 2 shows the volumetric water content in the three cracks from the near to the distant during the rainfall. The four curves in each figure stand for the volumetric water content of different position cracks from top to bottom. θ r Figure 2 The volumetric water content in 3 cracks from left to right over time As can be seen from figure 2, as time variation, the volumetric water content of three cracks is almost the same. Among them, the third crack has the maximum volumetric water content in the layers, and the first crack has the minimum one. That is because the rainfall flows along the road on both sides, and there will be extra rainfall flowing into the cracks. Hence, the cracks on both sides of the road are almost drowned, which results in lots of rainfall sank into the pavement. 3.3 Analysis of Influencing Factors on Rainfall Infiltration 3.3.1 Effect of rainfall intensity The rainfalls with intensities of 18.6x10-6 m/s and 9.3x10-6 m/s are used for simulation. Supposed that the rainfall happens once five years, and will last for 2 hours. From the result shown in figure 3, we can see that the greater the rainfall intensity, the more infiltration velocity and volume. 814
Figure 3 The volumetric water content in each layer with rainfall intensities of 18.6 10-6 m/s (left) and 9.3 10-6 m/s (right) 3.3.2 Effect of rainfall duration Assume that the rainfall intensity is 18.6 10-6 m/s, and the curve of volumetric water content with rainfall duration of 2 hours and 4 hours are shown in figure 4. As can be seen from figure 4, volumetric water content with 4 hours rainfall duration in all layers is significantly great. Figure 4 The volumetric water content in each layer with the rainfall duration of 2 hours and 4 hours 3.3.3 Effect of water table Comparing with the calculation result in the section where the water table is 3 meter, our simulation is performed 5 meters under the ground. As is shown in figure 5, the low water table will result in the great infiltration velocity and volume. In other words, there will be a greater rainfall infiltration volume in the place where the water table is lower when the rainfalls are equal. Figure 5 The volumetric water content in each layer with the water tables of 3-metre and 5-metre 815
4 Conclusion Based on foreign and domestic literatures, asphalt pavement characteristics and its seepage deterioration is introduced first in this paper. By means of combining theory analysis with numerical simulation, asphalt pavement seepage is analysed and studied in the context of rainfall intensities, rainfall duration and groundwater level. This paper aims to optimize the pavement drainage system to protect pavement structure from seepage deterioration,and lengthen the operation duration of the asphalt pavement. Acknowledgements: This work was financially supported by National Natural Science Foundation of China (Grant No. 50874082), Hubei Excellent Young and Middle aged Innovation Group Project, major project (Grant No.T200603 and No.Z20091801) from the Hubei Education Department, and major project (09ZA139) from the Education Department of Sichuan Province. References [1]. Sha Qinglin. Failure and Prevention of Premature Damage of Asphalt Pavement in Expressway. Beijing: China Communications Press, 2000 (in Chinese) [2]. Ariza P and B Birgisson. Evaluation of Water Flow through Pavement Systems [R]. Civil and Coastal Engineering Department, University of Florida, Florida, 2002: I-125 [3]. Mahboub K C, Liu Yinhui and D L Allen. Evaluation and Analysis of Highway Pavement Drainage[R]. Kentucky Transportation Center, College of Engineering, University of Kentucky, Kentucky, 2003: 1-30 [4]. Mahboub K C, Liu Yinhui and D L Allen. Asphalt Overlay and Subsurface Drainage of Broken and Seated Concrete Pavement [J]. Journal of Transportation Engineering, ASCE, 2005, 131(8): 617-622. [5]. Shen Jin an, Li Fupu, Chen Jing. Analysis and Preventive Techniques of Premature Damage of Asphalt Pavement in Expressway. Beijing: China Communications Press, 2004 (in Chinese) [6]. Sun Lijun. Structural Behavior Study for Asphalt Pavements. Beijing: China Communications Press, 2005 (in Chinese) [7]. Yuan Lianju, Li Zhenshuang, Wu Shenzhong. Application and Mechanics of Engineering Seepage Beijing: China Building Material Industry Publishing House, 2001 (in Chinese) [8]. Zhao Jiang. Seepage analysis and study on slope stability of subgrade pavement under rainfall condition [D]. Kumming: Kunming University of Science and Technology. 2005: 1-40(in Chinese) [9]. Liu Ming. Unsaturated Seepage Numerical Simulation and Analysis of Pavement Drainage Characteristics [D].Hunan: Hunan University, 2007 (in Chinese) [10]. He Yaochao. Unsaturated Seepage Analysis and Performance Evaluation of Drainage for Pavement under Rainfall Condition [D].Hunan: Central South University, 2009 (in Chinese) 816