【正文】 tion ,100%. To ensure the sufficient strength and stability of the flexible base， its thickness is not less than 15 cm， the thickness of the aggregate subbase is not less than 20 cm， A semirigid base usually has a good bearing capacity For the rural roads with Ne)500 000， or those with low traffic volumes but relatively,the minimum thickness of semirigid base or subbase is 1618 cm 5 Calculation of the Thickness of Road Surface Deflection (1)Road surface deflection 6 Road surface deflection is a vertical distortion caused by vertical load on the road surface. It not only reflects the whole strength and stiffness of asphalt pavement structure and roadbed， but also has a close internal relation with the service condition of the pavement. (2)Design deflection The design deflection is the index representing the stiffness of the pavement structure. It is also the deflection of the pavement which is established according to the accumulative equivalent axle load estimated to pass over a lane in the expected design life, road types, road classification， and the types of road surface and roadbase. The design deflection is not only the main basis for the design thickness of the pavement structure， but also the necessary index for the examination and acceptance of the project. Through theoretical analysis and experimental study， formulas for the design deflection value which are applicable to the pavement structure design of lowcost rural roads are as follows: semirigid base: flexible base: where A, is the type coefficient of the road surface. The type coefficient of asphalt concrete road surface is 。 Ns refers to the equivalent axle load action frequency in the designed traffic lane in the beginning operation period of rural roads。 the axle weight of standard axle in 100 kN and the action frequency。 Chen Rongshen(陳榮生 ) 1. College of Transportation, Southeast University, Nanjing 210b9b, China 2. College of Civil Engineering, Nanjing University of Technology, Nanjing 210009, China In developing countries， rural road construction is mostly cumbered by shortage of funds. Engineers concerns most in rural areas is how to build roads which not only cost less but also meet the traffic demands. Especially in vast rural areas of China, there are a great variety of transportation patterns, and the traffic position is very plex. Compared with other countries，the traffic position in China rural areas have its own features. Therefore， there is no experience about the rural roads construction for reference. In recent years， the central government of China has increased the strength for rural road construction. At the same time， a lot of researches about rural road construction have been done by researchers in China, and some conclusions about china rural roads have been made. In the authors39。 1 外文文獻 structural Design of Asphalt Pavement for Low Cost Rural Roads Yuan Goulin(袁國林 )139。239。 opinion， the selection of the pavement structure material is the key measure to reduce the construction cost of rural roads after the route has been determined. Compared with concrete pavement， asphalt pavement relatively costs less and is the first choice for rural roads in China. And then， according to the research achievements about rural roads construction， the authors have done some preliminary researches on the structure design for lowcost asphalt pavements for rural roads. 1 Traffic Composition of Rural Road Rural roads include county roads， town roads and village roads. The traffic on rural roads is usually mixed. On a county road, traffic volume is between 300 to 1500 veh/d in average， and in a county with a developed economy， it reaches 1000 to 2 000 veh/d. The traffic volume between county and town is 100 to 300 veh/d， and the traffic volume between towns is usually less than 100 to 300 veh/d. In a mixed traffic flow， trucks 2 account for 40% to 70% of the traffic volume, which are mainly light trucks carrying less than 2. 5 tons(including agricultural vehicles such as electrotricycles， walking tractors etc.)and mediumsize trucks of 2. 5