Effect Of Rock Fragments On Hydraulic Characteristics Of Purple Soil

Purple soil is a primary fertile soil developed from the purple red sandstone and shale which is rich in calcium carbonate in the subtropical region.It is widely distributed in the Sichuan basin.Because of its short development time,a large number of rock fragments are distributed in the soil,and the rock fragments,as a part of the parent rock,inherit the characteristics of the parent rock perfectly.The existence of rock fragments has a very important influence on the soil characteristics,e.g.the structure,texture,moisture characteristics,water conductivity and soil strengthand so on.However,there are few studies on hydraulic characteristics processesand influenced by rock fragments of purple soil.Therefore,this study took purple parent rock and soil of Feixianguan Triassic Series and Suining Jurassic Series as pioncut,by proportioned 0.25-2mm,2-5mm,5-10 mm rock fragments from 0%,30%,50%,70%,100% simulate soil,through the comparative analysis of simulate soil moisture characteristic curve,moisture diffusion and infiltration,hydraulic characteristics of purple clay rock with soil.The main results are as follows:(1)The rock fragments can reduce the moisture holding capacity of soils.The saturation capacity,field capacity and hygroscopic coefficient decreased with the increasing content of rock fragments,regardless of purple shale simulated soil or purple mudstone simulated soil.For water characteristic curve,with the increasing of the soil suction,the soil moisture content decreased gradually.With the increased size and content of rock fragment,the smaller moisture content.Therefore,the rock fragments can reduce the moisture holding capacity of soils.(2)The rock fragments can improve the diffusivity of soilsThe advance velocity of wetting front slows down as time increased in soils concluded of different content of soil fragments in unsaturated diffusion experiment.For both Feixianguan simulate soil and Suining simulated soil,when the rock fragments diameter ranged from 0.25-2mm,with the content increased,the diffusion volocity increased;when the rock fragments diameter ranges from 2-5mm,5-10 mm,water content of simulation soil was less than that of 0.25-2mm rock fragments soil.At the same time,diffusion rate rises with the increased of the rock fragments proportion,in Feixianguan 5-10 mm simulation soil.while for 2-5mm rock fragments soil,diffusion rate with the increased of water content of 70%>30%>50%>0%.Meanwhile,in Suining simulated soil,when the rock fragments were 2-5mm and 5-10 mm,diffusion rate was significantly higher in 70% rock fragments the proportion of the simulated soil than that of the other three proportion of simulated soil for rock fragments,the proportion of 30% and 50% soil diffusion simulation rate curve was not obviously changed.Therefore,the rock fragments can improve the diffusivity of soils(3)The rock fragments can improve the infiltration property of soilsWith the increased of time,the rate of soil infiltration with different content and size of rock fragments was first to fluctuated and then gradually stabled.Meanwhile,with the increased size and content of rock fragments,the infiltration rate was increased.Therefore,the rock fragments can improve the infiltration property of soils and the infiltration rate has a positive correlation with the content and particle size of rockfragments.(4)The characteristics of parent rock determine the moisture capacity of purple shale rock-derived soils and purple mudstone rock-derived soils.Purple clay rocks are easily weathered into soil.But due to the different sedimentary types of parent rock,the weathering and formation rates of purple mudstone of Suining Formation are significantly higher than that of purple shale of Feixianguan Formation.On microstructure,the purple shale rock of Feixianguan Formation aggregates to form massive or granular bodies with compact structure and small pore area mainly in the form of flaky surface-surface contact.The purple mudstone of Suining Formation forms massive or sheet polymer with a loose structure and large proportion of pore area in the form of flaky edge-edge or surface-surface contact.The grain size of purple mudstone is larger than that of purple shale of Feixianguan Formation.In terms of phy-chemical properties,the content of clay,organic matter in purple shale is higher than purple mudstone's.And the content of rock fragment in rock's soil of purple shale is higher than that of purple mudstone with a harder texture.It is because of the differences in many basic characteristics mentioned above that there is a clear difference in the hydraulic characteristics between purple shale and muddy soils,mainly as follows: Under the same rock fragment size and content,purple shale soils have the characteristics of stronger water holding capacity,poorer diffusivity and water infiltration than these of purple mudstone soils.To sum up,there is a large amount of rock fragments with the form of pre-aggregates in purple soil derived from clay rock.And rock fragments can reduce the moisture holding capacity while improve the diffusivity and hydraulic infiltration of soils.The characteristics of parent rock determine that the moisture holding capacity of purple shale are stronger than that of mudstone soil,while the diffusivity and infiltration are poorer.The results of this study can provide supportive data for soil-water management and water conservation in purple clay rock-derived soils to promote the sustainable development of regional cultivated land resources.