Abstract: Complex foundation pit has explicit three dimensional effect, and the three dimensional stability of the pit cannot be obtained exactly with present computational methods. The space failure range also cannot be defined easily. The Bezier bicubic curved surface is adopted to fit the displacement isosurface which passes the nodes; and it is assumed that the displacement isosurface is a local failure surface of the pit slope. Then the normal stress vertical to failure surface and the shear stress along the slip direction are calculated. A new point safety factor is defined as the ratio of shear strength at the point and shear stress along the slip direction; and the whole safety factor is the weighted average value of the point safety factor according to the area. The stability of pit slope in a normal university is analyzed. The results show that point safety factor has the characteristics of small values in the middle and higher values at two ends along the profile. The region with the values of point safety factor less than 1.06 extends from the bottom to the top of the slope, which indicates the location of slip surface. The dangerous slip surface is the displacement isosurface with values of 15 mm. The consistency of computational results along the specified profile by the methods of rigid equilibrium method and point safety coefficient has also verified.

Key words: foundation pit, slope, three-dimensional stability, point safety factor