Abstract: The effect of stress dispersion and shear strength of dry crust has been considered, the effect of intermediate principal stress is studied; and the realistic coefficient of lateral earth pressure has been undertaken. Reference has been made to the Flamant formula of polar coordinate system and unified strength theory. Therefore, new formulas of the critical edge pressure and critical loading of natural soft soil foundation are deduced. A calculation method of the critical height of embankment on natural soft soil foundation is also proposed. In accordance to a practical example, this paper also expounds a comparative analysis which has been made on calculation results of the critical height of embankment, different values of coefficients of lateral earth pressure of soft soil layer and dry crust and intermediate principal stress b are taken into consideration. This paper also studies the effect of these factors on the critical height of embankment . It is shown that the changes of and b have significant influences on the critical height of embankment . New formulas can more accurately reflect the essentials of soft soil foundation bearing capacity. It is of great significant to the construction of embankment in the natural soft soil foundation covered with dry crust.

Key words: natural soft soil foundation, critical height of embankment, Flamant formula, unified strength theory, coefficient of lateral earth pressure