Failure Modes And Design Calculation Methods Of Geotextile Bag Sand Embankment On Soft Soil Foundation

Geotextile bag sand embankment has been used widely in the practice of offshore engineering because of its high stability,speed mechanized construction,and strong adaptability.However,the failure mode and design method for the large geotextile bag sand embankment have not been established.Due to the lack of understanding and inadequate design,there have been failures observed at the construction sites.To solve these problems,this paper describes an extensive investigation for the behaviours of a large geotextile bag sand embankment over deep soft soil layer.The study focuses on the failure mechanisms and the design calculation methods of the geotextile bag sand embankment on soft soil foundation.Small scale model tests,finite element analysis,and theoretical study are carried out in this study.The main work and conclusions of this study include five points:(i)Depending on the finite element results of this study,it is found that large geotextile bag sand embankment over deep soft soil has two failure modes,local and global failure,with the various parameters such as the geometry of embankment,the strength of soft soil,and the strength of geotextile.The global failure is caused by the lack of the bearing capacity of the foundation.Excessive settlement of whole embankment,symmetrical failure surfaces,and center point of intersection of the symmertrical failure surfaces are the key principal characters of the global failure mode.The local failure is caused by the partial failure of the soft soil under the slope toe of the embankment,which has two separated failure surfaces different from the global failure.For the embankments with the local failure,their failure mode may be transformed into the combine sliding failure of the embankment and underlying soil while the strength of embankment is not strong enough.(ii)A small scale embankment model test was carried out to explore the failure mechanism of the geotextile bag sand embankment over soft soil foundation.Depending on the results of the test,the deformation characteristics and failure mechanism of the foundation can be obtained.It can be concluded that the failure mode of the soft soil under the geotextile bag sand embankment is global failure due to the small scale of the embankment test model.In this failure mode,the deformation of embankment can adopt to that of foundation,which means that the contact surface between the embankment and the underlying soil is flexible foundation.The failure mechanism of the soft soil under the geotextile bag sand embankment obtained from the result of model test is different from that for the soil under rigid foundation which was obtained in the traditional theory of bearing capacity,which is consistent with the finite element results.Based on these findings,a new definition of failure zones of foundation is proposed refer to the definition of active zone of the soil under rigid foundation.(iii)A new equation depending on the new failure mechanism is derived using limit equilibrium method to calculate the bearing capacity of the foundation under geotextile bag sand embankment.For the same foundation,the bearing capacity of the foundation under the geotextile bag sand embankment is higher than that under rigid foundation.But the deformation of the foundation under the geotextile bag sand embankment is also larger than that that under rigid foundation.(iv)A series of parametric studies using the Finite Element(FE)Method are conducted to explore the failure mechanisms and the limiting fill height of the embankment.From the FE results,it is discovered that the geotextile tensile stiffness,the soft soil shear strength,and the width of the embankment base are the main influencing factors on the large geotextile bag sand embankment design.The effects of the sand fill internal frictional angle,mat thickness,interface friction between different materials in contact,and embankment slope are minimal.Based on the FE results,the critical conditions for distinguishing different failure modes are defined.Under each failure mode(or mechanism),some design formulas to assess the limiting fill height and the safety factor of the embankments are proposed for the design framework.Two field cases where failures were observed during constructions are used to validate this design framework.(v)From the results of the FE analyses,it can be found that the bottom geotextile firstly fractured which caused the embankment to collapse,while the geotextiles close to the top are still working in good condition.Therefore,two optimal designs of geotextile bag sand embankment are proposed in this study to improve the traditional geotextile arrangement of the geotextile bag sand embankments with uniform reinforcement,which named non-uniform geotextiles reinforcement and ship-shaped reinforcement.In the non-uniform geotextiles reinforcement,non-uniform geotextile layer height is adopted,hence higher capacity is obtained due to smaller height of the base mats.In the design of the ship-shaped reinforcement,a reinforced geotextile membrane with much stronger material is adopted for the base mat to make the base mat have higher bearing capacity to carry the up mats.The base mat with stronger geotextile behave as the “hull” of the embankment that placed on the soft soil foundation,hence the embankment can avoid self-destruction even the excessive settlement of the foundation occurs.