Abstract:Loess is distributed extensively and is considered a special kind of soil; its strength and deformation are often different from those of other soil materials.At present,studies on the strength of structural loess are insufficient and the destruction form of loess and its destructive mechanism are also far from perfect.Owing to a lack of awareness about the structure of soil and test conditions,previous theories pertaining to structural strength criteria that describe the structural strength of soil are inadequate for establishing a law for the structural strength of loess.In particular,the adaptability of these criteria theories remains to be studied in the case of loess strength under complex stress variation.Among the many available strength criteria,the ones used commonly for soil are the generalized Tresca criterion,generalized von Mises criterion,Mohr-Coulomb criterion,and Matsuoka-Nakai criterion,among others.Testing of a large number of rock and soil materials has shown that the generalized von Mises criterion and the generalized Tresca criterion have fairly good applicability despite the introduction of the influence of ball stress on the rock and soil materials; however,these criteria give larger errors.The Mohr-Coulomb criterion has been widely applied in the field of soil engineering on account of its clear physical meaning,ease of setting of parameters,and the rationality and applicability in describing the destruction of rock and soil materials.However,this criterion also has an obvious drawback in that it does not consider the impact of intermediate principal stress on soil strength.Although the Matsuoka-Nakai criterion,which is based on the Mohr-Coulomb criterion,considers the impact of the intermediate principal stress,it is associated with true triaxial tests of sand and its physical meaning is not sufficiently clear.Natural loess is a unique structural and collapsible soil.In this study,based on true triaxial tests of different kinds of structurally intact loess,the influence of structural changes on the strength of the intact loess is analyzed.The Mohr-Coulomb criterion and Matsuoka-Nakai criterion,which are commonly used in geotechnical engineering,are used to analyze the adaptability of different kinds of undisturbed structural loess.The results show that the strength of natural structural loess is closely related to its structure and its property index.When the moisture content is low,the influence of change in the dry density on the structural strength of undisturbed loess is greater than the influence of change in moisture content; in contrast,the influence of the change in moisture content on the structural strength of undisturbed loess is greater than that of change in dry density.In terms of the failure line of structural loess in the PI plane,the greater the strength of structural loess,the closer it is to a rounded crooked triangle.The failure line is close to the Mises circle with structural infinity.Conversely,the smaller the strength of structural loess,the closer it is to a rounded triangle.It follows that with an increase in the structural strength of loess,its failure line in the PI plane gradually changes from a rounded triangle to a rounded crooked triangle,eventually tending toward the Mises circle.For different kinds of structurally intact loess,the use of the Mohr-Coulomb criterion and Matsuoka-Nakai criterion in describing the strength of structurally undisturbed loess has certain drawbacks.Overall,the strength described by low-strength criteria is smaller than the actual strength of structurally intact loess.The Mohr-Coulomb criterion has a larger error than the Matsuoka-Nakai criterion; despite the fact that these criteria are traditionally used for describing weak structural soil,their error increases with an enhancement of structural strength？＂
