Interface Characterization And Load Transfer Analyses For Anchored Systems

The globally prevailing infrastructure construction has resulted in increasing application of anchors and anchored structures,particularly in fields involving civil engineering,water conservancy and hydropower engineering.Specifically,the embedding media of anchors and anchored structures has extended from rock stratus to clayed soils and soft clays,which are characterized by much more complicated geological conditions.Additionally,the service period of anchors and anchored structures have been gradually increased as they are more frequently used in permanent retaining structures rather than temporary projects.The pullout resistance of anchor depends as an essential part on the shear strength and the area of the anchoring interface.Accordingly,the product of interface shear strength and the entire interface area has been used as a controlling parameter in designing the pullout resistance of engineering anchors.Aiming to determine the value of interface shear strength,in-situ preliminary pullout test or laboratory element interface characterization are needed.As a competitive interface characterization method,element anchor pullout tests are increasingly used in geotechnical community due to its higher controllability on testing conditions.The anchoring section of practical anchor in clayed soils is usually embedded in several layers characterized by different moisture conditions because of the complex hydrogeological conditions of soils with the effect of seepage filed of underground water.Determination of long-term pullout resistance for these anchors requires the characterization of creep behavior and long-term shear strength of the anchoring interface.As a primarily used anchor type in soft soil region,reinforced cement-soil-mixing anchor(RCSMA)has witnessed the development of its application practice exceeding its design computation theory.Specifically,the design guidelines for RCSMA is largely defected by failing to provide bond strength parameters of reinforcement-cemented soil interface,which will probably induce risk to be involved in the safety of design.Based on the above research and practice background,this paper investigated the impact of specimen configuration on the anchoring interface characterization,creep behavior of anchoring interface for grouted anchors installed in clayed soils with varying moisture conditions,the bond characteristic of interface between reinforcement and cemented soil of RCSMA under different cement-soil content ratio and curing time,pullout response and resisting behavior of large-scale reinforcement embedded in cemented soil,and load transfer analyses of the anchor with dual interface resistance.The findings achieved from these works were expected to provide adequate insights suitable to guide necessarily the current design computation theory and engineering practice of anchor and anchored structures.Firstly,element anchor pullout tests and numerical modelling were incorporated to investigate the pullout response of anchor specimens under various configuration conditions,through which the most appropriate configuration for element anchor test was achieved.A series of element anchor pullout tests,which involves 40 groups of pullout specimens with varied ratios between free length and bond length,were carried out by using the previous version of element anchor pullout device to investigate the impact of specimen configuration on the test results in interface characterization.Finite difference numerical modelling on an element specimen with typical configuration was conducted to evaluate the development of soil deformation in vicinity of the anchoring interface and the impact of different ways to maintain the constant interface area on the elementary property of the specimen subjected to pullout loads.Secondly,pullout tests and creep tests on element anchors with varying moisture conditions were used to explore the short-term and long-term pullout response of anchors,through which interface shear model and interface creep model as well short-term and long-term interface shear strengths were obtained.Rapid pullout tests on 18 groups of element anchor specimens with varying moisture conditions based on optimized configuration were completed to obtain the interface shear model and correlation between ultimate interface shear strength and moisture content.With loading strategy confirmed based on ultimate interface shear strength,multi-stage loading was used in creep pullout tests for element anchor specimens which is same with that in rapid pullout tests.Nonlinear superimposition was applied to transforme the time history of anchor head displacement into a group of interface shear creep curves for various shear stress levels.Furtherly,the long-term interface shear strengths for specimens with varying moisture contents were determined by incorporating isochronous curve method and the steady creep rate tangential method.Modified Burgers creep model was adopted to characterize the creep response of anchoring interfaces with different testing conditions and acceptable fitting effectiveness of this model was demonstrated.The influence of testing conditions on the model parameters was explored as well to facilitate the better understanding of this creep model.Additionally,pullout tests on reinforcement elements bonded in cemented soils with varying cement-soil content ratio and curing time were conducted to investigate the impact of the above testing conditions on the pullout responses.Furtherly,reinforcement-cemented soil interface bond-slip model and interface bond strength prediction model were derived.The previously mentioned element anchor pullout testing protocols were extended to carry out interface characterization between reinforcent and cemented soils,with three testing variables in specimen preparation as surface configuration of reinforcement,cement-soil content ratio and curing time.By combining with the results of unconfined compression tests on cemented soil specimens,regression analyses was used to relate the interface strength parameters with cemented soil strength parameters and testing variables adopted in this test.A simplified bond-slip model was established to characterize the bond behavior of reinforcement-cemented soil interface with governing parameters well calibrated by using observations and correlations obtained in tests.Afterwards,specially designed large-scale pullout setup was used to conduct the model test of tensioned reinforcement encapsulated in cemented soils.Detailed descriptions were given to introduce the design,preparation,operation,data reduction and other aspects involved in the model test.Critical concerns in specimen preparation,instrumentation and pullout loading were reviewed by observations and analyses on test results.Better understanding for pullout response and interfacial resisting process of reinforcement in cemented soils were achieved as well.Eventually,modified shear-lag model and interface bond-slip model were combined to develop a theoretical dual interface load transfer model accounting for the coupling effect between interface slip and interface shear stress.By introducing the interface characterization results obtained in above works,this theoretical model is capable of predicting the load-transfer process and pullout response of the in situ engineering anchor and the large-scale reinforcement in aforementioned model test.Based on the designed setup of model test,sensitivity analyses was performed to study the influence of varying principle design parameter values on the pullout resistance.The results were expected to provide adequate insights and guides to the design and construction practice of reinforced cemented soil structures.