Study On The Evolution Of Loess Structure And Its Constitutive Model In Acidic Environment

As a typical structural soil,loess is widely distributed in China and many other countries in the world.The primary structural feature of loess,i.e.,macropores,results in its high compressibility and undesirable engineering characteristics,namely collapsibility,seismic subsidence,and liquefaction.These unfavorable features are different from other sediments in the same period and cause serious damage to buildings built on loess areas.The constitutive properties of loess,namely macropores structure,makes it a large compression space.When subjected to special external forces,such as flooding or earthquakes,loess can drastically collapse or be liquefied.Evidently,the poor macropore structure of loess is the main cause of loess-related disasters.The research on its structure is of great importance for understanding the macroscopic physical phenomena of loess and improving its engineering properties.In recent decades,China's economy has been developing rapidly but acid rain p.ollution,urban waste,and industrial wastewater caused serious soil and groundwater acidification,which seriously influenced the sustainable development of the economy of our country.Therefore,it is of great significance for the engineering construction and environmental protection to study the influence of acidic environment on the structural properties of loess(including the passive destruction of loess under natural environment and the active improvement of the structure of loess by human).Based on the above two points of departure,the microstructure and macroscopic physical characteristics of loess in acidic environment were studied.The main contents and achievements of the research in this paper are as follows.(1)The concept of loess structure improvement was put forward in this study.It refers that the collapsibility and seismic subsidence are poor engineering properties for loess,and the elimination of that is essentially the optimization or evolution of the loess intrinsic quality.On this basis,a chemical method to actively improve the loess structure by h,uman was pointed out,i.e.using acid to dissolve CaCO3 to destroy the pore structure quickly and effectively.The structure evolution of collapsible loess under different pressures in different concentrations of phosphoric acid solution was observed and analyzed by scanning electron microscope(SEM).The tests results indicated that the structure of loess was improved due to the phosphoric acid under the pressures below 800 kPa,while the effect of phosphoric acid was not obvious under the pressures above 800 kPa because of the previous complete elimination of the pore structure.In addition,compression tests and collapsible tests results proved that under same pressures the loess treated with phosphoric acid had a smaller possibility and perniciousness of collapse or multiple collapse.Static triaxial tests confirmed that the enhancement of the original cohesion force and curing cohesion force among soil particles improved the shear strength of loess samples treated with acid(LTA).Dynamic triaxial tests showed that,because of a lower void ratio(e)than the natural loess,LTA had a better seismic performance as well as a lower likelihood of liquefaction.(2)Collapsible tests of loess in acid conditions were carried out to investigate the collapsible coefficient of loess,the stability time,the collapsible deformation rate and other sensitivity indexes of collapse under different pressures.Different concentrations of nitric acid and acetic acid were added to the immersion solution of distilled water.In addition,a scanning electron microscope(SEM)was used to observe the differences of the soil samples void space and structure before and after these tests.The results showed that in an acid solution the collapsible coefficient of collapsible loess increased greatly at the loads below 400 kPa,and the stability time of the collapse was longer under all pressures.When collapse,loess had different sensitive pressure sections in distilled water and in an acid solution,etc.When combining the results of these tests and previously conducted research,two indexes that pertain to 90%of the finished time of collapse and the collapsible coefficient were utilized to evaluate the collapsible sensitivity of the loess and the site.Chou Huang Canal,which had a collapsed before,was presented as a case study to demonstrate the scientificality and practicability of this method.(3)The SEM images of the samples were quantitatively pretreatmented and calculated by computer software of MATLAB and Image-Pro Plus 6.0.The Van der Waals force and Coulomb force between the particles of the different soil samples were tested and calculated.From the view of the structural entropy of loess,the mechanism of improving its structural and macro-mechanical characteristics under the action of load.l,water and acid was explained.Quantitative calculation of its structure was made from three aspects,namely,grain size entropy,permutation entropy and energy entropy.The results showed that the fractal dimension of particle size was a damping wave and moves forward with the increase of pressure during the trend adjustment of particle size.This reflected that the adjustment of particles was a periodic transformation process,namely relatively homogeneous-relatively inhomogeneous-relatively homogeneous.This cyclic transformation process was consistent with the structural changes of loess under other conditions(humidification and acidification).With the increase of pressure,water content and acidity,the number and content of micro-pores and micro-pores increased,and that of meso-pores and macro-pores decreased.The pore spaces between soil particles and the particle size of soils decreased accordingly.The void spaces in soils and the particle size of soils decreased resulting in the micro-gravity ratio of van der Waals force and Coulomb force increased.The above results proved that the collapsibility of loess decreased with the increase of overlying pressure,water content and acidity,and the stability of loess increased accordingly.(4)The basic concepts and research objects of binary medium model were briefly introduced,and the passitive damage mechanism of loess in acidic environment was introduced in detail.Based on the abstraction of the microstructure of cemented loess,a method for calculating the volume of cemented and friction elements in loess by means of particle contact(cemented and friction contact)was proposed.Based on the principle of macro and micro energy balance,the damage variate considering the influence of chemical factor was deduced by energy equation.Based on the reaction rate between calcium cementation and acid,and the damage mechanism of loess in acidic condition,an improved binary-medium model was used to predict the stress-strain relationship of the loess samples under acid conditions.In the model,loess was regarded as a composite material comprising bonded elements and weakened elements,which undertook their respective external stresses.Simple expressions were provided to describe the stress-strain relationship of loess bonded elements,weakened elements,and the evolution of breakage.The method of calculating the model parameters was also introduced.The collapsible tests and triaxial tests results proved that this model was suitable for predicting deformation development of loess under acid conditions.