Basal Heave Stability Analysis Of Deep Founation Pits In Anisotropic Soft Clays

Soft soil foundation is common in the eastern inshore alluvial plain areas in our country where soil strength low and underground water level high. Instability accidents happened frequently in soft soil foundation pits, and basal upheave is one of the most common form of destruction. Basal heave stability computation is of great value in evaluating the security of foundation pits, and it is an important part of excavation engineering design for excavations located in soft soil places.Nowadays, there are a great many of basal heave stability analysis methods, and they can be classified into four types according to. the computation theory:limit equilibrium method, limit analysis method, nonlinear finite element method, and reliability analysis method. At present, the current basal heave stability approaches have many disadvantages in the comprehensiveness of reflecting influence factors and the flexibility of practical engineering application. Those disadvantages lower the accuracy of basal heave stability analysis seriously. First, the current situation at home and abroad on basal heave stability analysis was reviewed and the urgently need to consider the anisotropic and time-space effect influence was pointed out. After that, the failure mechanism of basal heave was analyzed, the key influential factors were determined. At last, a calculate formula was deduced from limit equilibrium theory based on CASAGRANDE anisotropic strength theory and artificial circular sliding failure mode, then the formula was modified according to time-space effect induced by excavation. The effect of each factor on basal heave stability was studied from three aspects of parameters variation:soil parameters, supporting structure parameters, and time-space effect parameters. The main results for the research are as follows:1. The formula, deduced from limit equilibrium theory based on soil anisotropic strength theory and artificial circular sliding failure mode, is appropriate for basal heave stability analysis considering three aspects of influence factors:soil parameters, supporting structure parameters, and time-space effect parameters. The proposed formula is comprehensive and straightforward while taking account of all factors that effecting the upheave stability, and also shows high accuracy through practical engineering application2. The basal heave stability safety factor will increase as the unit weight y losing, cohesion c increasing, or internal friction angleφenlarging, and the internal friction angleφchange has the greatest impact on basal stability among the influence factors of soil parameters mentioned above. The stability safety factor will also increase as retaining wall insertion ratio D/H enlarging, lowest strut height H' lowing, ground overload q reducing, or pit bottom soil layer reinforcement.3. The calculated safety factor of basal heave stability analysis is obvious differences between anisotropic soft clays and isotropy soils under the conditions that one or more of the following factors value is rather small:soil unit weightγ, internal friction angleφ, retaining wall insertion ratio D/H, compressive strength qu after bottom reinforcement, and reinforcement thickness d, and one or more of the following factors value is relative large: cohesion c, lowest strut height H', ground overload q. Therefore, basal heave stability analysis should be carried out based on anisotropic strength theory in the aforementioned situation.4. The result of basal heave stability analysis will influenced by time-space effect characteristics in soft soils foundation pit engineering if excavation sharp regular and exposure time long.