Water Infiltration In Soils Containing Rock Fragments In Karst Peak-cluster Depression Area Of Northwest Guangxi

Southwest China is one of the three distribution centres of Karst in the world,and it is the typically fragile ecosystem.The content of gravel in karst peak-cluster depression area is more,and which has significant influence on soil infiltration and evaporation.This study abroad more,recently launched a national study,but on the whole it was still in the exploratory stage.According to the thin soil layer but also with discontinuous distribution and more rock fragments content with obvious variation,and based on the investigation of slope and distribution and composition of gravel in depression,this paper studied the influence of rock on water infiltration in soils by combing tension disc permeanmeter in-situ determination with laboratory simulation test to make clear the relationship between distribution characteristics of gravel in earth-rock slope and water infiltration in soils.The study conclusions are as follows:(1)There was great difference in gravel coverage,distribution density and content of different slope position in slopeⅠ,and it in the uphill and downhill was both bigger than in the midhill. However slopeⅡshowed a increaing trend from the toe of slope to the top of slope.The gravel coverage of different slope position in two slopes was between 4%-20%,and the maximum gravel content could attain 39.49%.The dominated size fraction of gravel caverging in two slopes was 75-250mm.,and size fraction of 20-75mm was secondary.Excepting for size fraction of 5-2mm, the gravel content of other size fraction showed a increaing trend from the bottom to top of slope. The coefficient of variation of gravel distribution,in different slope position and of different size fraction,was bigger.The gravel content of depression was less,and the maximum of it was 11.64%.There was little difference in soil gravel content in the same ground,and the dominated size fraction of gravel was 20-5mm and 75-20mm.The gravel content has more close relation with position and elevation and soil thickness of ground.The coefficient of variation of gravel content of the minimum size fraction was maximum,and it of other size fraction was less.With the depth of soil thickness increasing the gravel content of slope with rock fragments showed a rising trend. (2)There was obvious difference in infiltration indicators of cumulative infiltration,infiltration rate,approximate hydraulic conductivity and sorptivity in slopeⅠand slopeⅡ.The changing trend of water Infiltration of every slope position in slopeⅠshowed:the uphill＞the downhill＞the midhill, however,it has a increasing trend from the bottom to top in slopeⅡ.Fit the relationships among gravel content,steady infiltration rate and approximate hydraulic,and which accorded with the increasing exponential function Y=aebx.The relationships among gravel content of different size fraction,soil stable infiltration rate,and approximate hydraulic conductivity all accorded with Y=aebx.The positive correlation was observed among the gravel content of size fraction 250-75mm, 75-20mm and 20-5mm,soil stable infiltration rate,and approximate hydraulic conductivity. Moreover the gravel content of size fraction 75-20mm has significant promoting effect on Water Infiltration,that means there was promotion of gravel for water infiltration in soil containing rock fragments.(3)With soil depth increasing there was a decreasing trend for infiltration indicators of cumulative infiltration,stable infiltration rate,approximate hydraulic conductivity and sorptivity. However the infiltration indicators of soil containing rock fragments showed:25-52cm＞52-85cm＞7-25cm＞0-7cm.Excepting for the third soil layer the water infiltration rates in soil containing rock fragments were higher than that in soil without rock fragments,that means there was promotion of gravel for water infiltration in cluster depression.(4)The correlative analysis showed that excepting for rock fragments such physicochemical properties indicators as soil specific gravity,general porosity,silt content,day content had remarkable relationship with stable infiltration rate,approximate hydraulic conductivity and sorptivity.The positive correlation was observed in slope area among soil bulk density,ph value, organic matter content,approximate hydraulic conductivity and steady infiltration rate,and it was also observed in depression area among soil bulk density,ph value,claycontent,approximate hydraulic conductivity and steady infiltration rate.(5)According to the in-situ determination conclusion,water infiltration determination was simulated in different conditions.The results showed that there was a trend of "increase-decrease-increase" for soil infiltration when the gravel content increased from 0 to 30%. With the size fraction of gravel increasing water infiltration capacity has a strengthing trend in every period,and the promotion of gravel of size fraction 10-20mm and 20-30m for soil infiltration was the most significantly.Soil infiltration showed a trend from increase to decrease when the soil bulk density increased to 1.4 g-cm-3 from 1,2 g·cm-3.With the soil initial water content increasing soil infiltration decreased.Compared with red soil,the infiltration capacity of lime soil was much larger than that of red soil. (6)According to the orthogonal test conclusion,the effect on stable infiltration rate was initial water content＞gravel size fraction＞bulk density＞gravel content,and the effect on soil approximate hydraulic conductivity was bulk density＞gravel size fraction＞initial water content＞gravel content.The combination of factors beneficial to soil infiltration showed that the soil infiltration capacity was the most strengthing when the soil was composed mainly of gravel of size fraction 20-30mm,and gravel content was 10%,and soil bulk density was close to 1.2 g·cm-3,and the initial water content was close to 10%.