On Planning And Engineering Design Of Terrace In Hilly Region, Chongqing

Our country is characterized by a large population with relatively arable land and a shortage of mothball farmland resource. More over, the cultivated land quality has a decreasing tend in many areas on account of soil leanness, salinization, acidification and pollution and so on, which pose a great threat to food safety, people's living quality and social sustainable development of our country. Many scholars, native and overseas, have studied dynamic changes of cultivated land quality and influencing factors. However, soil resource was polluted or destroyed, it can't renew at short time. And the traditional agriculture management measures are hardly to improve cultivated land quality at short time. For the increasing scare for food and ecology safety, how to resolve the decreasing land plantation ability and improve farming plantation ability is becoming a new study hotspot, so modern soil engineering science appeared. At present, the research on modern soil engineering science is still at experience stage, thus, it is necessary to do more researches for realization of quantitative, programmed and standardized development of engineering design and implementation.As a component of modern soil engineering, the project of transformation of slope land into terrace is an important measure. As for hilly region, in Chongqing, it is in favor of increasing agriculture production and developing rural economics, as well as preventing soil and water loss and improving ecological environment. However, at present, study of transformation of slope land into terrace in hilly region, Chongqing were mainly concentrated on fertilization, and there were little relevant literatures on the project itself. So, in this paper, taking the project of transformation of slope land into terrace in Dashi Town, Hechuan District and Houba Town, Kaixian County, Chongqing for two typical examples, the planning and engineering design of terrace in these two regions were analysized, and drainage system, soil properties and rock stratum were discussed. The main results were as follows:(1)The planning of dry land terrace was mainly based on the conditions of topography and slope drainage system. According to critical slope method and subjective classification method, a system of four categories of slope classification was determined, which was 0°-3°,3°-5°,5°-15°and 15°-25°. Furthermore, slope maps of study areas were picked up, and rectangular or trapezoid parcel of rice field were to divide, which designed the orient of longer-side accurately, especially, to ensure the direction of mechanized farming, increase efficiency of mechanization farming, decrease soil erosion rate and enhance crop yield. Meanwhile, the main problem of irrigation and drainage in hilly region, Chongqing were the severely collapse of main trench, scattered distribution of the others, deficiency of drainage ability, and serious soil erosion. Therefore, based on slope maps, water system were designed anew.In the planning of paddy field terrace, topography, irrigation, drainage, mechanized farming and so on were taken account of. The collocation of parcel of rice field in Houba town, Kaixian County was that based on the situation of roads and ditches, an amount of roads and irrigation and drainage ditches were programed, and then a certain scale of cultivation areas were built. Although there existed tasks of controlling ground water, because of the light soil property, the distance of the drainage ditch ranges from 150 to 300 meters, and big discharge section, thus the farming drainage ditches of removing the surface water and controlling the underground water can be used integrated, which was arranged along the contour line or having a small angle with it, and directly discharged the seeper of farmland. The lather drainage ditch should be designed vertically to the counter line, and directly receives the drainage water. The programing of combination the up and low fields in Dashi Town, Hechuan District was based on the existence farming field, the longer side of which approximately along the counter line, but the width was according to the results of field survey. The irrigation and drainage system was based on the natural ditches as main drainage ditch, received the seeper of farming ditches or trenches, which were designed vertical to the counter line, making sure of a smoothly drainage.(2) The relationships between strata obliquity, bed thickness, lithological characters and the formation & utilization of slope land were analyzed in the study area, and the design of the transformation of slope land into terrace was further discussed based on the analysis of rock stratum. At the same time, digital elevation model (DEM) was used to study change of slope, so that the subjectivity of choosing representative farming lands could be avoided. The results showed that:with slope of 0°-5°,5°-15°, 15°-25°, field width was in range of 20m-38m,9m-14m,7m-10m, respectively, which could not only be to adapt to mechanical tillage and irrigation, but also to decrease the amount of earthworks farthest.In the design of level terrace, soil conditions, topography and drainage status were analyzed. The depth of farming ditch was designed 0.6m-1.5m, and distance between farming ditches was 100m-200m.Based on DEM and mechanization level, parcel of rice field in Kaixian County and Dashi Town, were length of 100m-150m, width of 20m-50m and length of 50m-100m, width of 10m-30m respectively.At the same time, the terrace design of paddy had the advantages of large area, integrated management, high machinery degree, high industrial degree, and earthwork calculation was convenient and accurate, compared with terrace design of dry land.Therefore, different methods of earthwork calculation were selected based on the two study areas. By means of DEM, land leveling earthworks was 4,400 m³ hm^-2 in Kaixian County, but the results were relatively large, and the accuracy was not high. At the same time, based on contour method and field observation, land leveling earthworks was 2,700 m³hm^-2 in Dashi Town. The results were accurate, but this method caused the waste of human and material resources. Therefore, we could choose simple, fast earthwork calculation method, and improve the accuracy to achieve the maximum of comprehensive benefit.The design of stone dikes was studied based on strata, topography and engineering properties. What is more, due to non-entirety of field surface and stone dikes in dry land, T-shaped stones of 30×30×100cm were of masonry for stone dikes of 1.8m, in order to meet the need of the anti-gliding stability and bearing capacity of foundation soil. While in the paddy field in Kaixian County, T-shaped stones of 30×30×100 cm were of masonry for stone dikes of 2.25m and 2.5m, accounted for 5.26% and 4.76% of the total amount of stone dikes, respectively. Meanwhile, in Dashi Town, T-shaped stones of one-layer, two-layers and three-layers were built for stone dikes of 1.75m,2.0m,2.25m and 2.5m, respectively, the amount of which accounted for 6.67%,5.88%,10.0%,13.04% of the total amount of stone dikes, respectively. In addition, due to foundation settlement existed at some degree, foundation treatment of concrete cushion was in need, when the stone dikes height was larger than 1.5m. However, the masonry of T-shaped stone enhanced the stability of stone dikes, and eccentricity and bearing capacity of foundation soil of stone dikes has met the need of their stability, without foundation treatment of concrete cushion. Based on Du Yongfeng's reliability theory, the stone dikes of≥1.0m were performed foundation treatment of concrete cushion in Dashi Town, whose thickness was 10cm, in order to reduce instability probability of stone dikes.Meanwhile, land occupation ratio of boulder strip dikes is far less than that of dimension stone dikes by comparative analysis. Gradient angle for each region has its appropriate critical value. For example, in the case of altitude difference of 2m, critical gradient angle of stone dikes of 1.8m was 64°in Dashi Town, so as to guarantee the stability of stone dikes, and decrease land occupation ratio of different field dikes.Taking engineering of the transformation of slope land into terrace in Houba Town, Kaixian County and Dashi Town, Hechuan District as examples, planning and engineering design of terrace were analyzed in the study areas, and the results can offer reference for quantitative, programmed and standardized development of engineering design and implementation in hilly region, Chongqing. However, due to great difference of soil and rock, complex terrain, many factors had association with planning of project of the transformation of slope land into terrace, and the design parameters was difficult to determine. Therefore, more researches are still to do for realization of quantitative, programmed and standardized development of engineering design and implementation in hilly region, Chongqing.