| Southern Shaanxi is the water source of the South-to-North Water Diversion Project in China and an important national water resources protection area.The geomorphological feature of the area are mainly mountainous or hilly,the cultivated land type is mainly sloping farmland,and the soil erosion is serious.The slope-to-ladder project has been proved to be a water,soil and fertilizer conservation project,as well as a soil and water conservation project loved by farmers.However,the soil quality in this area is expansive soil with large clay content,which is easy to soften and deform in case of water,which leads to the serious phenomenon of terrace collapse.Therefore,the local use of stone terraces or prefabricated terraces.With the strengthening of our country’s environmental protection consciousness,it is forbidden to open mountains and take stones,the source of stone is short,and the cost of building stone terraces is very high.To this end,we try to use traditional additives to improve the local expansive soil to obtain good applicability in terrace construction.In this paper,four dose gradients of 2%,4%,6% and 8% of lime and cement additives were selected,and the additive types,dosage and tamping process were selected by indoor evaluation indexes such as plasticity,expansibility,anti-collapse and anti-scrubbing.The purpose is to solve the dilemma between the shortage of rammed earth construction materials in southern Shaanxi and the fact that a large number of expansive clay earth resources can not be fully utilized.The main results are as follows:(1)The laboratory test results show that the improvement target is achieved by adding2% lime and cement.Compared with undisturbed soil,adding 2% lime can effectively improve the plasticity,expansibility and brush resistance of expansive soil,but reduce the disintegration resistance of expansive soil: adding 2% lime reduces the plastic index of expansive soil by 18%,and the expansion force and expansion rate decrease to zero.The contrast test results show that adding 2% lime can effectively improve the permeability of expansive soil and make it not fall down for a long time under the scour of thin layer water.With the increase of lime content,the better the plasticity,expansion,erosion resistance and collapse resistance of expansive soil.The indoor test law of cement modified expansive soil is similar to that of lime,but the plasticity index of cement decreases significantly after adding6% dose.The disintegration resistance of 4% cement is much lower than that of 4% lime.(2)Field test results show that the most efficient,economical and environmentally friendly terraced fields are constructed by adding 2% lime in the form of full ridges.The improvement effect of adding 3% lime to the whole ridge is the best,the preservation degree is more than 95%,and almost no collapse occurs.Although there is little difference between the material used for tamping 2% lime and 3% lime in 20 cm soil layer,the improvement effect is higher than that of it.The law of cement improvement expansive soil field collapse is similar to that of lime,but the improvement effect of lime is better than that of cement under the same construction technology and the same amount of content.From the point of view of ecological environmental protection,the vegetation growth on the terrace with 3% lime and cement is inferior to that of plain soil and 2% is not different from that of plain soil.From the economic point of view,the construction way of adding lime to improve the expansion of soil terraces is 250,000 yuan cheaper per hectare than that of Shikan terraces,significantly reducing the cost of compaction.Lime tamping is cheaper than cement.From the point of view of the combination of indoor and field tests,it is feasible to use lime and cement to improve expansive soil to build terrace fields,and the optimal construction scheme is 2% lime full ridge tamping. |
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