| With the implementation of “YB Conservation and Development” and “The Belt and Road”,the construction of infrastructure,water conservancy projects,ecological restoration projects and other major projects in the loess plateau region have been started.It is urgent to have a thorough understanding of the collapsible mechanical properties of loess,because the special geological conditions of loess collapsible put forward new requirements and challenges to the owners,designing institutions and construction enterprises.A large number of experimental studies and engineering practices show that the collapsibility of loess occurs not only in the natural loess area,but also in the long-term agricultural irrigation area.Therefore,the loess collapsibility can occur many times and repeatedly,and even the remolded loess has collapsibility。As a special geological soil formed by aeolian deposition,loess has complex and changeable material composition and pore structure,so the research on collapsibility deformation mechanism has always been a major scientific problem in the field of loess mechanics.Supported by the national basic research program of China "Study on the mechanics of loess slope failures at multi-scale(micro-meso-macro)under the influence of environment conditions",in this paper,the Malan Loess in Jinjiaya Village,Jinya Town,Lanzhou City,Longxi basin is taken as the research object.The evolution characteristics of loess’ s humidified microstructure and the law of collapsible deformation were studied by means of soil test and field pit flooding test,and SEM,XRD,EDS and other microstructure observation methods were used.From the perspective of macroscopic mechanical behavior and microstructure evolution of loess collapsibility,this paper focuses on and explores the essence of loess collapsibility mechanism,which provides scientific basis for foundation treatment of collapsible loess and the formulation of countermeasures for geological disaster prevention.The main research results of this paper are as follows:(1)The collapsibility test results of 10 groups of loess samples in Jinya Town show that:the self-weight collapsibility coefficient is between 0.008～0.020,and there is no obvious change rule in the vertical direction,and the non-self-weight collapsibility coefficient is between 0.049～0.159.Although there is fluctuation in the vertical direction,it still shows a decreasing trend with the depth.Compared with the research results of natural loess in Lanzhou,it can be seen that irrigation has a certain influence on the coefficient of self-weight collapsibility,but has no obvious effect on the coefficient of non-self-weight collapsibility.The irrigated loess is still a strong collapsible loess,and the potential collapsibility cannot be ignored.(2)For the collapsible loess samples from Lanzhou,the SEM image information of a series of collapsible loess was obtained on the micron scale,and the structure of the loess was finely divided by the microstructural morphology method.The particle morphology was divided into 6 types,namely,point,flake,cake,block,spherical and cubic block;the contact mode was divided into point contact,surface contact,overlapping contact,embedded contact,package contact,aggregate,etc.;the pore types can be divided into 5 types,namely,intragranular pore,intergranular pore,overhead pore,gallery bridge pore and splitting pore.The complex and changeable spatial pattern of the microstructure causes the heterogeneity of the loess structure and determines the instability and variability of the water-affected loess structure.(3)In order to dynamically observe the microstructure of collapsible loess,a set of observation device which can be directly placed in the SEM chamber was designed and modified.With the help of comparative statistics technology,51 groups of loess microstructure images with continuous changes at the same position were analyzed and compared.4deformation and failure modes of loess particles under the action of water,including dissolution,movement,recombination and fracture,were obtained.The movement of particles is the main deformation mode,accounting for 65% of the deformation area in the sensitive area.Only 3%of the samples were fractured,and the dissolution and recombination of the particles occurred in the point particles with the particle size less than 10 μm.The dynamic change process of microstructure in time and space reveals the inherent nature of multi-level collapsible deformation and residual collapse of loess to a certain extent.(4)The microstructure of loess can be divided into water-sensitive area and non-sensitive area under the effect of humidification.The area of sensitive area is related to the times of humidification and water content,which has an additive effect and can reach up to 82.66% after water saturation.The deformation and instability of loess microstructure under water is hysteresis,hysteresis time is related to the degree of dissolution of cementing material and the change of porosity.(5)In order to solve the key technical problems of orientation,high precision and rapid identification of loess mineral composition,an energy spectrum superposition method suitable for micro information extraction of loess minerals is proposed.8 main minerals are identified by using this method.According to the mineral changes after continuous humidification,it can be divided into three categories: easy soluble minerals dominated by chlorite,soluble minerals dominated by dolomite,potash feldspar and calcite,and insoluble minerals dominated by quartz.The easy soluble minerals with the least content is the main component of loess cement,which determines the collapsible deformation process of loess to a certain extent.(6)The large-scale field immersion test and the laboratory variable load humidification and dehumidification cycle collapsibility test have proved that the loess can still collapse again after irrigation.The process of field irrigation collapsibility includes 3 stages: deformation acceleration period,deformation oscillation period and deformation gradual change period.The collapsible rate changes fastest in the deformation acceleration period,and the accumulated collapse amount is the largest in the deformation oscillation period,accounting for 60% of the total collapse amount.The deformation slow change period lasts the longest,but the cumulative collapse amount is the smallest.The laboratory collapsible test under multiple conditions shows that each layer of loess has a potential collapsibility,and the first 3-5 collapsibility accounts for about 90% of the potential collapsibility.The potential collapsibility of loess with different water content in the same layer is the same,which is only related to its own structure and not affected by the collapsing process.(7)The special hydrodynamic properties and microstructure plasticity of loess determine the mechanical behavior of loess collapsibility.After saturated collapsibility,the collapsibility of loess is not completely eliminated,and the loess still has potential collapsibility.Once encountering water,loess will collapse again under self-weight or load conditions.On the macro scale,it shows the decrease of bearing capacity,and the increase of settlement;on the micro scale,it shows the deformation of loess grain,mineral dissolution,cementation weakening,pore restructuring and other forms of microstructure evolution. |
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