The working behavior of rigid pile composite ground with mat is analyzed. With considering the negative skin friction of the pile

the distribution of side skin friction resistance is discussed. A simplified linear model of this side skin friction resistance is then proposed. Moreover

the interaction between the pile and the mat is analyzed and the penetration magnitude of the pile into the mat is calculated. A theoretical differential equation of pile displacement is then derived. According to this equation and the definition of the neutral plane

a new method is proposed to calculate the depth ratio of the neutral plane. The influencing factors and their value ranges are also discussed. Finally

a case study shows that the calculation results agree well with practical experiences.