Experimental Study On Water Retention Capability Of Bentonite In The Dredged-silt Deposition Area Of The Lower Yellow River

Under the guidance of major national strategies for ecological protection and high-quality development planning of the Yellow River basin,the ecological greening of the dredged-silt deposition area on both sides of the lower Yellow River has gradually been emphasized,and the Yellow River ecological landscape belt and green landscape promenade have been developed and constructed.In recent years,in response to the problems of weak water retention capacity and poor growth of ecological forest in the dredged-silt deposition area,the construction of dredged-silt deposition area is often done by setting loamy soil water retention layer to achieve the effect of water retention.In view of the difficulty in finding soil sites,the shortage of soil materials and the damage to land and the environment caused by soil extraction sites,the problem of difficulty in using loamy soil for water retention layer construction is becoming more and more prominent in silt release and dike consolidation projects.Therefore,it is necessary to find water retaining materials that are suitable for the characteristics and soil conditions of the lower Yellow River dredged-silt deposition area.Through reasonable test programs and technical methods to test and study the new materials in terms of water retention effect,we try to propose a scientific and economic solution for improving water retention in the dredged-silt deposition area.In this paper,bentonite was used as water retention material to set up water retention layer scheme(pure bentonite thin layered water retention layer,bentonite-sand mixed water retention layer,no water retention layer)for indoor tests and field tests respectively.In order to provide guiding ideas and data support for field tests,indoor cylindrical simulation tests were used to study the water retention effect of bentonite water retention layer.Through dynamic monitoring of several indicators in field trials,the effect of different bentonite water retention layers on soil water content and vegetation growth properties would be studied.By establishing a multi-indicator evaluation system,the actual water retention capacity and ecological benefits of different water retention layer placement schemes in the dredged-silt deposition area of the lower Yellow River were evaluated comprehensively by using grey correlation degree-TOPSIS.Then a scientific and feasible bentonite water retention layer preference for the lower Yellow River dredged-silt deposition area would be screened out,and it could provide technical support for the future construction,transformation,application and management of the dredged-silt deposition area.The main research results and conclusions that have been obtained are as follows:(1)The selection of bentonite as a material for water retention layer placement is feasible to effectively enhance the water retention capacity of the soil in the dredged-silt deposition area of the lower Yellow River.Bentonite water retention layer can significantly increase the average water content of soil by 4%～12%.It also can improve the physical and chemical properties of soil,enhance the stability of soil water retention,and reduce the fluctuation of soil water content between seasons.Compared to the control group,all growth indexes of the tree plants increased by 4%-17%,and the growth quality was significantly improved.(2)The results of the indoor test show that,taking into account the actual construction conditions and cost control,bentonite water retention layer recommended admixture ratio of12%(with 10 cm thick silt sand mass ratio)under the precondition of the optimal moisture content and maximum dry density.Compared the mixed and thin bentonite water retention layer with the same conditions of bentonite application amount,we can see that the thin laying method is more effective in slowing down the rate of downward movement of wetting fronts,which obviously plays a role in interception and slow release.However,the infiltration rates and cumulative infiltration volumes of the two bentonite water retention layer methods in the same time period do not differ significantly,it shows that the pure bentonite thin layer test group which can retain water in the soil for a long time is better than the bentonite-sand mixed layer test group overall.(3)The results of the field test show that: as evidenced by the average vertical distribution of soil water content and the general information of vegetation growth in the test area,pure bentonite thin layer test section > bentonite-sand mixed water retention layer test section > no water retention layer control test section.Compared with the control group,the soil water content and interannual soil water holding capacity variation are significantly increased in the test section,all the growth indexes of the trees showed an increasing trend and the growth quality was significantly improved.Considering the comparison,it is clear that Pure bentonite thin-layered water retention layer design scheme is a more preferable bentonite water retention layer design scheme for the lower Yellow River dredged-silt deposition area.At the same time,the indoor test and the field test show extremely high similarity in the soil water content change pattern in different bentonite water retention layer design schemes,which can be compared and verified.The fact that can indicate that the credibility and accuracy of the two tests were guaranteed.(4)The results of the comprehensive multi-indicator evaluation method show that: as evidenced by the soil improvement effect of the dredged-silt deposition area in the lower Yellow River,the thin water retention layer scheme > the mixed water retention layer scheme > the no water retention layer control scheme.That means the preferred scheme of bentonite water retention layer in the dredged-silt deposition area of the lower Yellow River is pure bentonite thin layered water retention layer scheme.Pure bentonite thin layer scheme can obtain more water retention and ecological benefits,and also shows greater advantages in later water retention capacity such as gradually improve the supply and storage capacity of soil moisture.Combined with the construction and economic considerations,the construction cost of thin pavement water retention layer program is much lower.And it has the best comprehensive benefit.In addition,from the perspective of climate and soil conditions,the thin bentonite water retention layer will have good water retention for the lower Yellow River dredged-silt deposition area as a whole,which means that it has the highly value of promoting practical use.