Jiangxi Shuanggang Shipping & Navigation Project attempted to use the saturated silty clay with soft clay dredged from the open diversion channel to build the cofferdam. During construction, the dredged saturated silty clay was dumped by boats above the water, resulting in disturbed and non-homogenous silty clay with soft clay. This results in the difficulties to form the cofferdam with the desired gradients and failure of the cofferdam during operation. To assess the cofferdam stability during construction and operation with water level fluctuations, extensive numerical modelling using the finite element program PLAXIS was carried out. Further considering the non-uniformity of the dumped saturated silty clay and the variation of its strength with rainfall and evaporation, sensitivity analyses on the soil strength parameters were carried out. Based on the maximum lateral deformation curve of the cofferdam with the factor of safety close to 1.0, the yellow and red alarm levels were proposed for the remote real-time monitoring. The results indicate: the cofferdam with a slope of 1:4-1:5 and berms is stable at normal water levels; the reduction in strength of the dumped soil would induce instability of the downstream cofferdam slope from the toe to the top; due to the non-uniformity and low strength of the dumped saturated silty clay, remote real-time monitoring shall be carried out on the cofferdam lateral deformation, seepage pressure, and rainfall; based on the deformation curve with the factor of safety, the red and yellow alarm levels are 1.5-2.0 mm/d and 1.1-1.5 mm/d respectively.