Analysis On Uncertainty Of The Pullout Capacity Of Helical Anchor In Clay

Helical anchor is a kind of geotechnical foundation widely used in engineering practice.Generally,the helical anchor is made up of a central steel shaft and helical bearing plates welded to the central shaft.The major advantages of helical anchors include being able to withstand tensile and compression loads,rapid and easy installation,minimal site disturbance,etc.The early application of helical anchors is mainly limited to on-shore construction.In the past two decades,it has been extended to marine engineering,such as as the anchoring foundation for offshore wind turbines and aquaculture equipment.At present,most studies on the pullout capacity of helical anchors in clay are deterministic analysis.However,the reality is that there exist uncertainty in almost all of the geotechnical engineering.The uncertainty in geotechnical engineering can be roughly divided into two categories: subjective uncertainty and objective uncertainty.The main source of subjective uncertainty is the lack of a clear understanding of the failure mechanism of rock and soil,which inevitably leads to the simplification,hypothesis and approximate treatment of the mechanical analysis model and the numerical simulation model.The sources of objective uncertainty mainly include external loading environment,geotechnical parameters,construction technology and technology level,etc.In this paper,the coupled Eulerian-Lagrangian finite element calculation method is used to analyze the uncertainty of the pullout capacity of helical anchors in clay.The main contents of this research are as follows:In terms of subjective uncertainty,the model uncertainty analysis is carried out for predicting the pullout capacity of helical anchors in clay adopting the cylindrical shear method,and the ratio of the measured pullout capacity of the test to the calculated pullout capacity is defined as a model factor to characterize the model uncertainty.In the comparison of cylindrical shear method and finite element analysis,a correction factor is introduced,regression analysis is performed to eliminate the correlation between the correction factor and input parameters,and the regression equation obtained is substituted into the original cylindrical shear method to constitute a modified cylindrical shear method.The results show that the modified cylindrical shear method obtained in this paper can effectively improve the prediction accuracy of the pullout capacity.In terms of objective uncertainty,a large deformation random finite element study is carried out for the pullout process of helical anchor in spatially variable soil to analyze the influence of the spatial variability of undrained shear strength of soils on the failure mechanism and pullout capacity of helical anchors.The results show that in the spatially variable soil,there are three different failure modes in the soil around the helical anchor;the pullout capacity under different combinations of undrained shear strength variability parameters is compared and it is found that the coefficient of variation and the vertical correlation length of soil strength have significant effects on the pullout capacity;based on the probability distribution of the random pullout capacity model factors,the relationship between the failure probability and the factor of safety is established,and it is found that the factor of safety of the embedded anchor in the existing standard cannot meet the requirements of the failure probability.