The Meso-Structure And Compression Behaviors Of Malan Loess

The loess plateau is characterized by complex topographic conditions,fragile geological environment and frequent geological disasters.In recent years,the contradiction between frequent geological hazards of loess and lagging basic studies of disaster-causing mechanism surge increasingly with the development of the Belt and Road strategic plan,which has seriously restricted the economic and social development in loess plateau region.The typical types of geological hazards for loess includes crack-slideing,collapse and topple failure.Different from near-circular arc shear failure surface common in homogeneous soil landslide,the formation of collapse and topple in loess is normally associated with vertical deep cracks developed at the top of the slope.The vertical deep cracks in the Malan Loess can be extend to a depth of several meters,with some reaching as far down as ten meters and have separate loess columns and/or walls from the slope.When either the loess column or wall slides along an inclined plane or rotates outward around a point or an axis at its base,geohazards often occur.In a state with natural water content,the static lateral pressure coefficient of loess is much lower than that of other soils.Thus,unconfined compression failure is one of the potential reasons for the formation of vertical deep cracks in the slope of the Malan Loess.Interpreting mesostructure characteristics of the Malan Loess,ascertaining compressive behavior of the Malan Loess and clarifying the influence of mesoscopic structure in the Malan Loess on its mechanical behavior,can contribute to make more demonstrate the development mechanism of geological hazards,providing theory or method guidance for effectively preventing geological disasters of loess slope and theoretical basis for prevention and control of such geological disasters.Physical experiments,characterizations,real-time monitoring,image processing,numerical simulations and other methods were adopted to solve above problems.The main contents and research results are as follows:Based on the reconstruction theory for three-dimensional structure,and the definition for pore structure characteristic parameters,a quantitative analysis system for pore structure characteristics of loess was developed by applying image processing technology and combining the maximum entropy method,thinning algorithm and structural feature extraction algorithm.The PSPE analysis system integrates the pore identification,reconstruction of three-dimensional porosity structure model,and the extraction of two-dimensional and three-dimensional pore structure characteristic parameter.It can extract the three-dimensional pore structure characteristics of loess by quantitatively analyze of CT scan images.The CT scanning of the Malan Loess specimens was carried out,and the pores of the Malan Loess were identified by using the PSPE analysis system.In addition,the three-dimensional structure model of the pores in the Malan Loess was reconstructed,and their two-dimensional and three-dimensional structure characteristic parameters were extracted.Meanwhile,permeability test and numerical simulation experiment of permeability for the Malan Loess were carried out,and the spatial distribution and correlation characterization of pores in the Malan Loess were analyzed.The results show that the Malan Loess has a complex and dense pore network.Compared with the pipes arranged in the horizontal direction,vertically arranged pipes show better connectivity and have a larger radius,a longer length,and a smaller curvature.When testing Malan Loess specimens by ultrasonic testing equipment,the results show that the skeleton structure of the Malan Loess displays the obvious anisotropy.Combining with the pore structure characteristics of the Malan loess,it is found that Malan loess is characterized by vertically aligned strong units coupled with vertically aligned weak segments.We propose this structure for the Malan loess,and denote it as “vertiloess” structure.Unconfined compressive strength tests of the original Malan loess was carried out.By the assistant of acoustic emission detection and surfical strain monitoring system,the compression failure process of the Malan loess was monitored.Besides,the in-situ CT scan test was conducted.In combination with the digital volume correlation and PSPE analysis system,the evolution of the internal structure and body strain of the Malan loess specimen under unconfined compression was investigated.The results show that the deformation and failure process of the Malan Less in unconfined compression test includes five stages:compaction,elastic deformation,plastic deformation,failure and post-failure stage.Especially,the micro-cracks in the specimen are mainly formed in the stage of plastic deformation,whose formation is accompanied by the slow release of elastic energy.When the stress reaches the peak point,brittle failure occurs in the structure of the specimen,the failure surface penetrates the specimen along an axial direction accompanied by a rapid release of elastic energy in a short time.Obviously concentrated for the axial and radial strain around the failure surface and relatively uniform for the strain distribution in other areas,the specimen presents a typical compression-induced tensile failure mode.According to the micro-structure of the Malan Loess obtained by CT scan,the unconfined compressive numerical simulation experiment of the undisturbed and reconstructed loess was carried out.It is found that under the effect of overlying load,the top and bottom of the vertically alighned weak segments are the main development area for micro-cracks.Such micro-cracks mostly develop along weak structural units,gradually expand and penetrate with other weak structural units arranged vertically to form tensile cracks,which results in the compression failure of the whole specimen.