Study On The Influence Factors Of Stability Of Sandbag Cofferdam On Double-layered Foundation

With offshore engineering's fast development,the construction of cofferdam should meet higher demands in some respects.Sandbag cofferdam is widely applied to engineering practice due to its integrity and convenience in construction.Since relevant study still lags behind engineering practice,cases of instability occurred occasionally.Stability of sandbag cofferdam is fundamental to whole reclamation project.Previous study only focused on uniform soft soil foundation,regardless of inhomogeneity of geology.The form of foundation influences stability of sandbag cofferdam significantly.Case of failure of sandbag cofferdam on soft-overlying-stiff clay foundation was introduced and factors influencing the stability of sandbag cofferdam on double-layered foundation(soft-overlyingstiff clay foundation and stiff-overlying-soft clay foundation)were studied by numerical simulation in this study.The main work and results of study are shown as below:(1)Limiting fill height and failure mode of cofferdam constructed on uniform soft soil foundation and soft-overlying-stiff clay foundation were contrasted.On soft-overlying-stiff clay foundation,higher cofferdam can be constructed,and failure mode is also different significantly.(2)A detailed comparison which involved development of plastic zone,displacement of feature points and position of slide surface of two types of failure mode(overall failure of foundation and sliding of the slope)was given.When overall failure of foundation occurs,settlement of central point of cofferdam and horizontal displacement at the slope toe will increase rapidly.However,when sliding of slope occurs,only horizontal displacement at the slope toe will increase rapidly.(3)Influence of parameters of cofferdam and foundation on stability of cofferdam constructed on soft-overlying-stiff clay foundation was discussed.Overall failure of foundation and sliding of the slope could occur when cofferdam is constructed on soft-overlying-stiff clay foundation.The wider the cofferdam is,the more likely sliding of the slope occurs.However,the higher the thickness of soft soil,shear strength of soft soil and tensile strength of geotextile are,the more likely overall failure of foundation occurs.Formulas of critical bottom width of cofferdam distinguishing overall failure of foundation from sliding of the slope,limiting fill height and factor of safety were presented.(4)Influence of different parameters on failure mode and limiting fill height of sandbag cofferdam constructed on stiff-overlying-soft clay foundation was studied.Only overall failure of foundation will occur when cofferdam is constructed on stiff-overlying-soft clay foundation.Formulas of limiting fill height and factor of safety were fitted and presented.(5)Traditionally,sandbag cofferdam is composed of sandbags with equal thickness,which cannot make good use of strength of geotextile.An innovative arrangement method was proposed to compensate the shortcoming mentioned above,in which the thickness of sandbags increased form bottom layer to top layer to enhance the strength and stiffness of the bottom of cofferdam.It was proved that when innovative arrangement method is adopted,the distribution of stress of each layer of geotextile tends to be more even and cofferdam tends to be more stable accordingly.