Subgrade Engineering Design Of Field Roads In Purple Soil Hilly Region

Field road, as a servicing for production,is a channel crossing to the field from the residential area. It is used for transporting goods, transfering the farm machine to the field and adding water or oil to it. With the development of national economy, the road construction in China has rapidly progressed, and was gradually extending to the undeveloped countryside of purple soil hilly area. Owing to the complexity of the purple soil hilly area, there are various regions with different land use patterns, different engineering geology and hydrological conditions along the field roads, which contribute to the road quality of these areas. Just because of all these factors, it is very important to analyze and design the subgrade stability of field roads. At present, the studies on the analysis and design of the subgrade stability from home and abroad mainly focus on those with large investment and strong technology such as railway subgrade and graded highway subgrade, while there are fewer studies on the rural road subgrade with a relatively small investment in small areas but broader scopes. In this dissertation, we analyze the subgrade stability of field roads in purple soil hilly area with different types of land utilization, engineering geology and hydrological conditions, and put forward the following experience of design.(1) The design of the field roads subgrade should differentiate as the three types of 1 land use patterns, the valley paddy field and the slope paddy field as well as dry land. The valley paddy field with low-lying topography are long immersed in water because of the abundant groundwater or surface water, leading to the soil softening, organic deposition, low dynamic quality, high water content, weak shear strength and easily thixotropic. This kind of soil is called High Liquid Limit Soil. Therefore, constructing new subgrade on the valley should change the soil, that is, first get rid of the surface silt with high water content and then fill in sand or macadam with good engineering characters and compact layer by layer. The slope paddy field, with higher topography and lower soil softening than valley, has low dynamic quality and weak shear strength. Its natural soil contains high water content, difficult to evaporate, but after drying in the sun, it would get to the optimum moisture content. In contrast, the soil with low water content is like the natural soil. As a result, constructing new subgrade on the slope paddy field should first discharge the water inside the land, and then smash the upper consolidating soil and compact after drying. While the dry land has small organic deposition and good dynamic quality as well as stronger shear strength, and its water content was close to the natural water content. In the state of compaction, the structure of the soil is tight. So, constructing new subgrade on the dry land only need to clean the surface layer in order to compact. At the same time, the extent of the subgrade compaction with the three types must control up to more than 93%.(2) The geologic conditions along the road, such as the rocks'kind, cause, joints, degree of weathering and fracture cases, the track, trend, obliquity, texture and the thickness of terrane, with or without interlayer or the interlayer softened after meeting water and with or without faultage or other bad geologic phenomena (karst, glacier, debris flow, earthquake and so on) will influence the subgrade stability to some extent. The subgrade design should be treated differently as two different geological conditions of excellently hard sandstone and soft mudstone. When meeting the hard terrane, we should dig road cutting in order to prevent the subgrade instability resulting in sliding; as regards as the soft mudstone, we should also try our best to dig road cutting. If the subgrade height is higher than the ground, embankment can also be designed, but there should be strengthening measures at both sides of the embankment, e.g. retaining wall. After digging road cutting, subgrade rolling is necessary, and the times of rolling should be more than three in order to obtain the compaction extent of 93%. After rolling, we can pave the based layer of road. In addition, the proportion of side and slope should be controlled to more than 1:1 and the defense of side and slope was also necessary preventing the fall of rubble on the slope to affect traffic.(3) Two main methods that improving the design of embankment elevation and the construction of proper drainage facilities could reduce the impact of hydrological factors on the stability of roadbed. Specifically, improving the design of embankment elevation can make the sub grade soil maintain moderate moisture which keep the strength and stability of the roadbed even when the edge of hard shoulder is above high the ground water on both sides of roadbed, and the effects from the groundwater, the role of capillary water and frozen are considered. The raised height of the design of embankment elevation depends on ground's situation:when surface drainage in good condition, the height coud be equal with the ground surfaces or slightly above. When ground's surface is often in water or underground water level is high or the pavement needs to be filled, the height of fill depends on the different kinds of soil selecting between the 0.5m to 2.5m. Considering another method that the construction of proper drainage facilities, on the one hand, the main drainage channels for field road which across the dry field are the roadside ditch. The culvert should be built in the right place to lead the slope water to drainage receiver when the ground is steeply. On the another hand, the culvert should be built for the field Road crossing the paddy field.the location and size of culvert depends on the size of the paddy field in the upstream and water. It needs to build the large size of the box culvert to meet the demand for paddy field drainage and protecting embankment from erosion in excessive runoff situation.The results of this paper was based on a large number of investigations to field roads in the purple soil hilly area of Chongqing and analysis of a lot design results. The character of this thesis is the analysis on a case of the design of field roads in purple soil hilly area of Chongqing under the guidance of theory. So the results can reflect the situation of field road construction in hills and mountains with purple soil. It will help to correctly guide the design of roadbed, and to improve the quality of field road projects.