Experimental Investigation And Theoretical Analysis Of Time-dependent Loading-deformation Behavior For Ground Anchors

The processes of modernization and urbanization are promoting rapidly,and infrastructure construction is developing vigorously in China.Under this background,anchoring support technology has been employed in civil engineering on an unprecedented scale.Ground anchors are usually designed for a long service time,in particular permanent anchors.In addition,geotechnical materials are characterized by the remarkable rheological behavior.Hence,various time-dependent problems will inevitably appear during the service period of anchoring structures.The time-dependent problems of ground anchors are characterized by their loading-deformation behavior varying with time elapsed,which is directly related to the long-term safety of the supporting system.There are numerous cases of deteriorating bearing capacity or even overall failure of supporting system owing to the time-dependent problems of anchors.However,the long-term bearing mechanism of anchors is extremely complicated and is connected with plentiful influencing factors,which results in the lack of systematic and comprehensive understanding on the pertinent issues of time dependent loading-deformation behavior of anchors.Therefore,a series of test devices with different scales were specifically deve loped in this thesis.A large number of laboratory tests were conducted using the developed devices to obtain the mechanical properties of soil,soil-anchor interface,and large-scale anchors that are subjected to instantaneous(short-term)and long-term loads.In addition,theoretical analyses were also implemented for full-length grouted anchors and prestressed anchors to predict their time-dependent loading-deformation behavior,in which the soil-anchor interface rheological behavior was specially considered.The findings achieved in this thesis are expected to reveal the long-term bearing mechanism of ground anchors and provide theoretical insights for the design and long-term safety assessment of anchoring engineering.The key works and innovative achievements are as follows.(1)Triaxial creep tests were utilized to investigate the long-term shear strength and creep-consolidation behaviors of soil,through which an adaptive neural fuzzy inference system(ANFIS)-based soil creep model was developed to d escribe the combinative influence of multiple factors.Six groups of consolidated drained triaxial creep tests were conducted for soils with various confining pressures and dry densities.The isochronal curve method was employed to determine the soil long-term shear strength.According to the test results,a soil creep-consolidation coupled model was proposed in reference to consolidation theory and rheological theory.Meanwhile,another soil creep model considering the synthetic effect of dry density,conf ining pressure and deviatoric stress was also proposed based on ANFIS.(2)The effects of ground pressure and sample/anchor diameter ratio on the soil-anchor interface mechanical behaviors were investigated via element-scale pullout tests simulating the ground pressure,and then the critical ratio between sample and anchor to eliminate boundary effect was obtained as well as two kinds of interface shear models.A novel element-scale pullout test device was developed,in which the ground pressure and construction process of anchors can be simulated.Twenty groups of interface shear tests were carried out for the element-scale pullout anchoring specimens with different construction processes,diameter ratios,and ground pressures.The soil-anchor interface shear strength was divided into adhesion force and friction force,and the two parts were modeled by rational and exponential functions,respectively.A generalized interface shear model was further proposed along this way.Besides,another interface shear mo del considering the effect of normal stress was also proposed based on disturbed state concept(DSC).It indicated that the influence of boundary conditions on test results can approximatively be eliminated if the sample/anchor diameter ratio exceeds 5.0～6.5.The problem of boundary effect for element-scale pullout test was thus solved,which can provide a basis for the size design of element-scale anchor specimens.(3)Element-scale stress relaxation pullout tests were utilized to investigate the interface stress relaxation behaviors between the soil with different densities and moisture contents and the anchor,and a DSC-based interface stress relaxation model was then proposed.Twelve groups of element-scale pullout anchoring specimens with different soil dry densities and moisture contents were prepared.The red clay –anchor interface shear stress relaxation behavior was measured using a developed stress relaxation pullout test device in a multi-stage loading way.The influence of soil dry density and moisture content on the loss rate of soil-anchor interface shear stress was discussed based on the test results.A novel interface stress relaxation model that can characterize the effect of stress level was established according to DSC theory.(4)The pullout responses of anchors subjected to instantaneous(short-term)and long-term loads were obtained via laboratory model test for large-scale soil anchors.Laboratory pullout model test devices were developed to test the instantaneous and long-term loading-deformation behaviors of three groups of large-scale ground anchors.The physical model tests include full-length grouted anchors subjected to instantaneous and long-term constant(creep)load,and prestressed anchors with long-term constant pullout displacement.The load-displacement curve,creep curve,and anchoring force loss curve at anchor head were measured from the three groups of pullout tests,respectively.Besides,the distributions of axial force and interface shear stress over bonded length at different times were also obtained and discussed.(5)Two theoretical analysis methods of loading-deformation behaviors for anchors subjected to instantaneous load were established by employing the developed generalized interface shear model,including displa cement coordinated method and discrete recursion method.The subroutine of generalized interface shear model was exploited based on ABAQUS,aiming at investigating the loading-deformation behaviors of anchors via finite element simulation.The comparisons between the predictions and measurements from laboratory model tests and in-situ tests demonstrate that the theoretical analysis methods and FEM simulation are all effective and suitable for various ground conditions.Compared with the displacement coordination method,discrete recursive method equips with higher calculation accuracy and efficiency.Parameter study was implemented to investigate the effects of anchorage length and axial stiffness on the bearing capacity of anchors.(6)The full-length grouted anchors and prestressed anchors are studied by theoretical modeling,and hereinto,the former is subjected to long-term constant(creep)load.Merchant model was employed to characterize the soil-anchor interface shear rheological behavior.The theoreti cal analysis methods for the time-dependent loading-deformation behaviors of the two kinds of anchors were further established based on load-transfer theory and finite difference technique.The creep curve of full-length grouted anchors,anchoring force lo ss curve of prestressed anchors,time-dependent distributions of axial force and shear stress over bonded length can be calculated by the two analytical methods.The effectiveness and accuracy of the two theoretical analysis methods were verified by the re sults of laboratory model tests and in-situ monitoring.Parameter study was implemented to investigate the effects of some key design parameters on the creep behavior of full-length grouted anchors and anchoring force loss behavior of prestressed anchors.