Research On The Failure Mode And Bearing Behaviours Of Soft And Hard Interbedded Mudstone In Nanning Basin

In recent years,with the establishment of the China-ASEAN Expo in Nanning and the establishment of the China-ASEAN Free Trade Area,the development plan of the Beibu Bay Economic Zone in Guangxi has become a national strategy.The high,large,heavy and deep structures in Nanning basin develop at a high speed on the scale of tens of billions.The character of argillaceous rock is more and more restricted to the project construction.Researchers at home and abroad have carried out some researches on the mechanical properties,water rock action,microstructure and bearing properties of mudstones,and achieved some results.The study of the soft-hard interlayer mudstone strata in Nanning basin is rarely involved,especially in the study of mechanical properties,bearing characteristics and failure patterns.To study the mechanics,load bearing characteristics and failure modes of soft and hard interlayer mudstones can not only enrich and perfect the research results of mudstone characteristics in Nanning Basin.It can also provide valuable reference and guidance for engineering design and construction of such soft rock areas.Therefore,it is of great practical significance and practical value to study the nature of soft and hard interlayer mudstone in Nanning Basin.Based on the existing research,the mechanical characteristics,failure patterns and bearing characteristics of soft-hard interlayer mudstones in Nanning basin are systematically studied in laboratory model test,finite element numerical simulation test and field static load test.The main studies and conclusions are as follows:(1)The argillaceous rock samples of hard-soft-hard interlayer,soft-hard-soft interlayer and soft-hard interlayer were designed to manufactur.462 samples were completed to be tested.The test includes uniaxial compression test and triaxial shear test.The strength variation and failure characteristics of soft-hard interlayer mudstone are analyzed.The concept of the loss rate of interlayer strength is proposed and used to evaluate the influence of soft layer on the strength of interlayer mudstone.The results show that the thickness and distribution of soft layer have a great influence on the strength of interlayer mudstone,and the loss rate of strength is the largest when the homogenous hard layer mudstone is transformed into soft-hard interlayer mudstone.The shear strength index of interlayer mudstone decreases with the increase of the thickness of soft layer.(2)Three failure modes of soft-hard interlayer mudstone are proposed:brittleness-expansion failure,expansion-penetration failure and plastic expansion failure.(3)Using the finite element software Abaqus,uniaxial compression test model of soft-hard-layer interlayer mudstone and soft-hard-layer interlayer mudstone is established.The results show that the soft layer mudstone is mainly subjected to compressive stress and plays a stress dissipation role.The soft layer is the main factor that causes the strength decrease,deformation increase and failure of interlayer mudstone.(4)The softening test of mudstone in engineering years was designed and the physical model of mudstone softening was established.The static load test of hard layer foundation softening was completed.The results show that the influence of the environment change on the mudstone is very significant.The hardness softening reduces the bearing capacity of the foundation by more than 50%,and increases the settlement deformation of the foundation.(5)Based on Abaqus finite element software,a numerical model of bearing capacity of soft-hard interlayer mudstone foundation is built.The influence of layer thickness and strength on the bearing capacity of the interlayer mudstone foundation is studied.The formula of ultimate bearing capacity of soft-hard interlayer mudstone foundation is established.(6)In the typical soft-hard interlayer mudstone stratum,the 12 static load test is designed,and compared with the numerical simulation of the bearing capacity of the interlayer mudstone foundation,it is proposed that the reduction factor of 0.85 times the calculated value of the simulation formula can meet the need of engineering safety.