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than 15 cm，the thickness of the aggregate subbase is not less than 20 cm， A semi-rigid 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 16-18 cm 5 Calculation of the Thickness of Road Surface 5.1 Deflection
(1)Road surface deflection
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:
semi-rigid base:
flexible base:
where A, is the type coefficient of the road surface. The type coefficient of asphalt concrete road surface is 1.0;that of hot-mix asphalt macadam and that of emulsified asphalt macadam road surface are all 1. 1; and that of asphalt surface treatment road surface is1 .2. (3)Allowable deflection
Allowable deflection is the maximum deflection allowed at the end of the road's service life under lim- iting conditions in poor season. Through thoretical
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analysis and experimental study，the calculation for- mulas for the allowable deflection of road surface which are applicable to the pavement structure design of low-cost rural roads are as follows}2}:
When designing the asphalt pavement structure of low-cost rural roads, we should use formula (6) or (7 ) according to the types of roadbase to determine the thickness of asphalt pavement structure. 5.2 Tensile stress
Because the asphalt pavement structure of lowcost rural roads is not substantial enough and the heavy vehicles are allowed to pass over them, the maximum tensile stress should be checked by computing the stresses of the semi-rigid base and subbase. The tensile stress at the bottom of semi-rigid base or subbase，would be less than or equivalent to the allowable tensile stress of the materials of the semirigid base or subbase , namely，
For the stabilized aggregate base with an inorganic binder-
For the stabilized fine-grained soil base with an inorganic binder:
5.3 Pavement thickness
To make it simple and convenient for engineers to determine the desired thickness of rural road pavement, the curves of the thickness of the roadbase of low-cost rural roads according to typical pavement structures and accumulative frequency of equivalent axle load are shown in Figs. 3，4 and 5. (1)When the accumulative frequency of equivalent axle load is within 500000 times per lane，asphalttreated or asphalt penetrated surfaces with thickness of 1. 5 cm一cm is recommended for road surface. For various
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accumulative equivalent axle loads and the moduli(Eo)of roadbed，the equivalent thickness of roadbase is shown in Fig. 3.
(2)When the accumulative frequency of equivalent axle load is within 500 001)一1 000 000 times per lane，asphalt macadam or asphalt concrete with thickness of 3 cm -5 cm is recommended. For various accumulative equivalent axle loads and moduli(Eo)of roadbed，the equivalent thickness of roadbase is shown in Fig. 4.
(3)When the accumulative frequency of equivalent axle load is within 1000 000-2 000 000 times per lane，asphalt concrete road surface of 5 cm-7 cm thick is recommended. For various accumulative equivalent axle loads and moduli(Eo)of roadbed , the equivalent thickness of roadbase is shown in Fig.S.In Figs.3-5，Ld is the designed deflection, Lo is the representative deflection of roadbed，E, is the modulus of resilience of the roadbase，in MPa , Eo is the modulus of resilience of the roadbed，in MPa ,and H, in cm，is the equivalent thickness of the base (roadbase and subbase)，which can be obtained through calculation and in-site investigation for a trilevel-pavement roads(including road surface，base and roadbed).If a designed road has four layers，i.e. a subbase is added，according to the regression analysis of the extrapolated results of a number of multi-layer flexible systems and the available research findings，the thickness of the roadbase , h,，in cm, can be calculated from the following equation:
6 Concluding Remarks
Compared with concrete pavement, asphalt pavements have a lower construction cost, which is suitable for the roads in relatively underdeveloped rural areas in China. The research in this paper proposed a method for structural design of low cost asphalt pavements. The method is to provide an guideline for the design of asphalt pavement structure in rural areas. References
[1]Yuan G L , Zhang F , Chen S W , et al. Research on technical indexes of rural highway construction in Jiangsu province [ J ].Highway, 2005(6):135一139(in Chinese).
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[ 2 ] Research Institute of Highway , the Ministry of Communications. Final Report on Low Cost Inter-township and Rural Road Construction Techniques〔R].Beijing; Resdarch Institute of Highway, 2003(in Chinese). [ 3 ] Liu Q Q. How to reduce the construction cost of the rural highway [ J ] .Journal of Highway and Transportation Research and Development, 2005(2):41一44(in Chinese).
[ 4 ] JTG B014-97. Specification for design of highway asphalt pavement[ S ](in Chinese ).
[ 5 ] JTG BO1-2003. Technical Standard of Highway Engineering [ S ](in Chinese).
[6] Deng X J. Engineering for sub-grade and pavement[ M].2nd ed. Beijing; People's Communications Press, Beijing, 2004(in Chinese ).
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中文译文
沥青路面结构设计的低成本
农村道路
在发展中国家,农村道路建设主要是因缺乏资金而受阻。工程师最关心的农村问题是如何修建的公路不仅成本低,而且满足交通需求。特别是在广大农村地区,有各种各样的交通方式,交通成分非常复杂。与其他国家相比,在中国农村地区交通组成有其自身的特征. 因此,没有足够经验对农村公路建设提供参考依据。近年来,中国的中央政府增加了对农村公路建设的力量。与此同时, 在中国，研究人员已经完成了关于中国农村道路的大量研究和一些结论。在作者的观点,选择不同的路面结构材料是关键措施，从而降低农村道路的工程造价。相比,沥青混凝土路面相对费用少、是第一个在中国农村道路的选择。然后,根据农村道路建设研究成果,作者也做了一些对低成本沥青路面结构设计的初步研究。 1 农村道路交通组成
农村道路包括县公路、城市道路和村的道路。
农村道路上，交通通常是喜忧参半。一个平常县的道路，交通量平均为的300至1500辆/ 天；一个经济发达的县,已经达到了1000到2000 辆/ 天。通信量县、镇之间是100到300辆/ 天，交通量之间通常低于城镇是100至300辆/ 天。在一个混合交通流、卡车占有40%到70%的交通量,主要为轻型卡车携带小于2.5吨(包括农业车辆如拖拉机等)、行走、中型卡车2.5 - 5吨。大部分的轻或中等卡车超载。重型卡车的比例小于9%。某些道路上的比例对县、超载的卡车是5%至32%,而在一些县公路连接到国家或省级公路的比例躯干,超载的比例通常数量的20%提高到32%。
交通量对农村道路不会很重。然而,考虑到中国的实际情况,以及车辆超载,要满足 100 kN,和 BZZ-100标准轴载作用次数。
人行道挠度和底部的拉应力作为沥青路面设计参数。轴载荷计算方式,依照下面
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