Meso Mechanical Study On Stress Distribution And Failure Mechanism Of Soil Anchorage

Rock and soil anchoring technology,a key branch of rock and soil engineering,is closely related to various fields of infrastructure construction,such as civil engineering,transportation,mining,water conservancy,and national defense.A large number of studies have been carried out,and various improvements have been made to the mechanical characteristics of rock anchoring systems,but few have focused on soil anchoring systems.Due to the complex mechanical characteristics of soil,soil anchoring systems are considerably different from rock anchoring systems in terms of both the anchorage mechanism and failure mode.At present,the research on soil anchorage system is still lagging behind,and the design of soil anchorage system is still in the experience analogy method.With the development of anchorage technology,the current design method is no longer able to meet the requirements.Therefore,it is of great significance to study the load transfer mechanism and mechanical effect of soil anchorage.This study has obtained the distribution and variation law of the displacement field and strain field of the surrounding soil with the increase of drawing load based on the mathematical description of the geometrical characteristics of the soil anchorage,as well as the visualized meso mechanical test system of the soil anchorage.On this basis,the load transfer law,interface layer constitutive relation and interface element for numerical simulation analysis of soil anchorage were obtained,well the stress distribution law and the failure mechanism of soil anchorage were revealed through theoretical analysis,numerical simulation and experimental verification.As application of this result,by expanding the anchorage system as the research object,the soil expanding anchor in load displacement field and strain field distribution was revealed by means of digital speckle meso mechanical experiment and numerical simulation analysis.The calculation formula of bearing the anchorage ultimate load of expanding section was derived based on the plastic limit analysis method,and its influencing factors were also discussed.The achievements of relevant research include the following:1.Through a macroscopic pull-out test of undisturbed soil,the failure mode and ultimate pull-out capacity of both tensile-type and pressure-type anchor rods of different lengths were analyzed,which allowed us to compare the failure mechanism of the two types of anchor rods.We studied the changes in load and displacement of soil anchors during the pull-out test using the load-displacement curve.2.We here designed a surface profile measurement device for anchors.We used fractal dimensions to quantify the surface geometric characteristics of the anchor.The anchoring force between the anchor and soil was found to be influenced by a variety of parameters(e.g.,soil compactness,cement-to-sand ratio of the grouting body,and sand size).The fractal index of the anchor solid is in fact a quantitative description of these parameters.The correlation between these parameters and fractal dimension was investigated through a large number of experiments.3.A semi-cylindrical visualization apparatus was designed to allow observation of the interface mechanics.We performed micro-scale mechanical tests using the digital speckle correlation method(DSCM)and so observed variations in the stress field and movement of soil surrounding anchors.These tests also showed the formation and evolution of the shear band,as well as its progressive failure.The design of the test apparatus and the testing method may provide reference material for visualization studies of other axial-symmetry problems in geotechnical engineering.4.According to the axial-symmetry features of anchors and the test results,the flow law related to the Mohr-Coulomb yield condition was used to establish the constitutive relations between the anchor and the soil interface layer.In this way,we derived the analytic expression of stress distribution under pull-out loads.We analyzed the distribution of shear stress within soil anchors and the factors influencing that distribution by comparing the theoretical and experimental results.5.Based on the theoretical and experimental results,a new type of compound interface element for soil anchors was proposed.This element is composed of a contact element and a solid element.The contact element was used to describe the sliding friction between the anchor and the soil.The solid element was used to describe the dilatancy of the interface soil.Based on the compound interface element,the stress and displacement of the anchor during pull-out test were studied using numerical simulation software.The results produced by the simulation were then compared with the theoretical and experimental results.6.Using under-reamed anchors as an application subject,the experimental and simulation methods described above were used to investigate the mechanical characteristics and the failure mechanism.Based on the limit analysis theory,a mechanical analysis model was created for under-reamed anchors,from which the ultimate load of the expanding section was derived.The influences of various parameters on the ultimate load were analyzed.The theoretical and experimental analysis performed in this study provide a basis for theoretical analysis of soil anchorage systems and the design and parameter selection of anchorage projects.