Research Of Pullout Failure Mechanism And Computational Method Of Shallow Plate Anchors And Pipelines Based On PIV Technology

Buried strip structures are frequently subjected to soil restraints resulting from the pullout loading and differential soil-structure displacement.Among these structures,the plate anchors with rectangular section and the pipelines with circular cross section are the most typical ones.Specifically,plate anchors are widely used in onshore and offshore structures such as transmission towers,retaining walls,offshore drilling platform and dry docks to provide pullout resistance against tensile forces arising from wind load,buoyancy effect,wave action and earth pressure.Similarly,the buried pipelines may subject to surcharge soil pressure due to the thermal expansion,urban excavations,slope failures and earthquake-induced faulting.Investigations on the anchors/pipes-soil interaction mechanism and prediction of the pullout resistance are of great significance to the stability and security of relevant engineering.Plenty of previous studies have shown that the strip anchors and pipelines are similar in pullout behaviour.Because of the complexity of anchor/pipe-soil interaction mechanism and the limited measurement of failure mechanism due to limitations of the conventional displacement measurement technology,many problems still exist with the calculation theory and methodology of anchor/pipelines pullout resistance:? many of the current investigations on this topic focused on the load-displacement curves during loading,whereas the deformation and failure mechanism especially the systematic studies on the pullout behaviours of plate anchors and pipelines in the V(vertical)-H(horizontal)plane is limited,and the comparative study on mechanisms between pipes and anchor is still not deep enough;? many studies have been performed by considering factors such as burial depth,anchor/pipe geometry and soil parameters,research on effect of anchor/pipe-soil interfacial strength on pullout resistance and failure mechanism is limited;?the prediction of pullout resistance from different models may vary greatly as most of the commonly used theoretical methods are developed based on the assumed rupture surface patterns,and the theoretical methodology which is suitable for prediction of pullout resistance in the V-H planes is limited.In view of the facts mentioned above,this paper performed a serious of investigations on pullout behaviours of plate anchors and buried pipelines by using a nondestructive displacement measurement technology(i.e.Particle Image Velocimetry,PIV)and the finite element limit analysis(FELA).Based on the measured failure mechanism,the theoretical methods which considers the interfacial strength for predicting pullout resistance of anchors and pipelines in V-H plane are developed.The research contents and achievements in this study are as follows:The formation and development of pullout resistance of anchors and pipes during pulling are investigated using plane strain model tests.By incorporating the PIV technology,the underlying soil deformation and failure mechanism are captured,through which the influence of the several factors such as burial depth,soil density,inclination and the anchor/pipe-soil interface strength are investigated.Typical characteristics of the failure mechanism and rupture surface geometry are summarized based on the measured soil deformation,by which the comparison of failure mechanisms between the plate anchors and pipelines are performed.The results of failure mechanisms provide the possibility to develop associated theoretical methods and verify present finite element limit analysis models.A limit equilibrium model for the prediction of pullout resistance of vertical plate anchors under plane strain conditions developed based on observed failure mechanism and the limit equilibrium theory.Then the limit equilibrium solution which considers the effects of sand-anchor interface strength,embedment ratio and sand internal friction angle is inspired by the global equilibrium analysis of failure soil wedge in front of plate anchor.By incorporating the limit equilibrium solution of horizontal plate anchors of White et al(2008)and the anchor inclined factors,the theoretical method for prediction of pullout resistance of plate anchor under oblique loading in the V-H plane is developed.The two-dimensional limit equilibrium solution was applied for the estimation of ultimate pullout capacity of rectangular plate anchors with varied anchor height and confining stress by a combination use of the shape factors proposed by Ovesen and Str(?)mann(1972)and Bolton's(1986;1987)empirical correlations which captured the influence of sand relative density and stress level on the sand frictional strength,respectively.The accuracy of the new solution is validated by using a database of plate anchors and other commonly used solutions.The finite element limit analysis of soil-pipeline interaction is performed to investigate the effect of sand-pipe interface strength,sand internal friction angle and burial depth on failure mechanism,and the empirical correlation which relate the influence factors mentioned above to the distance of initial points of passive failure surface are developed.Based on the geometrical similarity of failure mechanism between plate anchor and buried pipelines,the limit equilibrium solution of White et al(2008)for horizontal plate anchors and limit equilibrium solution for vertical plate anchor in this study are modified to predict the pullout resistance of pipelines.The prediction of pullout resistance of pipelines under oblique loading in the V-H plane is achieved by incorporating the modified inclined factor,and the reliability of modified solution is compared with model test results in sand.The finite element limit analysis which considers the coupling effects of anchor/pipe-soil contact condition(i.e.immediate breakaway(BA)and no breakaway(NBA)),anchor/pipe-soil interface strength,embedment ratio,overburden pressure,nonuniform distribution of the soil strength and loading direction are conducted to investigate the failure mechanism of plate anchors and pipelines in clay.The calculation method for prediction of pullout resistance of horizontal and vertical plate anchor/pipes in weightless clay is developed first and then the failure envelopes for pipes under oblique loading in the V-H load space are inspired.A design procedure which considers the effect of overburden pressure and based on the data-fitting equations of the present finite element limit analysis solutions is also presented for the prediction of pullout resistance of BA and NBA pipelines buried in clay with uniform and nonuniform distribution of the soil strength.