Basal Anti-Heave Stability Of Foundation Pits In Unsaturated Soils Under RB Mode Of A Retaining Wall

The basal anti-heave stability of foundation pit is of great significance to the design,construction and safety control of foundation pit.The existing research is mostly aimed at saturated soil foundation pit,without considering the actual situation of unsaturated soil around the foundation pit due to dewatering measures and the influence of retaining wall displacement mode on the stability of foundation pit bottom.Based on the shear strength theory of unsaturated soil,this paper establishes a mechanical model for anti-heave analysis of unilateral sliding rough foundation.Considering the displacement mode of retaining wall,water level change,nonlinear strength of unsaturated soil and other factors,the solution of anti-heave safety factor of unsaturated soil foundation pit under RB mode of retaining wall is established,and comparative verification and parameter analysis are carried out.The main research contents and conclusions are as follows:(1)In view of the fact that the soil around the foundation pit is often in unsaturated state under low water level and the retaining wall rotates around the bottom of the wall(RB displacement mode),the bearing capacity of the single-side sliding of the strip foundation at the bottom of the wall and the vertical anti-sliding force formula of the wall-soil under the RB displacement mode are derived.The anti-heave safety factor solution of unsaturated soil foundation pit under uniform and linear suction is established.Combined with the theoretical solution of literature,the application and verification of foundation pit engineering examples and the discussion of retaining wall displacement mode are carried out.It is concluded that the calculation of retaining wall as static state will overestimate the anti-heave capacity of foundation pit and be unsafe.(2)According to the depth of water level outside the pit,the anti-heave calculation of foundation pit is divided into three cases:high water level,medium water level and low water level.The application range of the anti-heave safety factor calculation formula is extended from unsaturated soil to saturated soil and unsaturated soil coexistence.The anti-heave safety factor solution of foundation pit is established,which comprehensively reflects the factors such as water level change,RB mode of retaining wall and matrix suction.The correctness and applicability of the obtained formula are verified by the comparison of engineering examples and the upper limit method of literature.The results show that the water level has a significant influence on the safety factor of foundation pit uplift resistance.The deeper the water level,the greater the safety factor and the more obvious the influence of unsaturated characteristics.(3)Aiming at the nonlinear variation characteristics of shear strength of unsaturated soil,the soil-water characteristic curve parameters of four kinds of soil and five nonlinear shear strength models are introduced to obtain the calculation formula of anti-heave of unsaturated soil foundation pit considering different adsorption strength forms under RB mode of retaining wall.The results show that when the matric suction is greater than the air-entry value,the anti-heave safety factors of all kinds of soil foundation pits show nonlinear changes.However,due to the differences in air-entry value and residual suction value,the sensitivity intervals of anti-heave safety factors to matric suction are different.(4)The anti-heave safety factor ignoring the unsaturated characteristics is the smallest,and a reasonable nonlinear shear strength model should be selected considering the nonlinear characteristics of unsaturated soil strength.With the increase of water level depth,matrix suction,suction angle and retaining wall insertion depth,the anti-heave safety factor increases obviously,and the uniform suction solution is slightly larger than the linear suction solution under the same conditions.There is an optimal value for the insertion depth of the retaining wall.If the insertion depth is too large,the contribution to the uplift resistance of the foundation pit is not obvious and it is easy to cause engineering waste.