Ultra-deep Excavation's Impact On Bearing Capacity Of Single Pile

Since the 1990s, large number of super-high-rise buildings have been completed one after another. Super-high-rise buildings have to set up extra deep foundation pit, extra deep underground works are bound to have deep excavation. It is the cause that makes a great depth and large-scale development of the excavation pit. The depth of excavation has developed from 10m to above 20m, 30m.Because the depth goes far beyond by the deep hole excavated for building foundation which said in the past, it is generally called ultra-deep pit. This will have significant impact on the capacity and settlement of the pile foundation.It may have not great impact on the bearing capacity of piles in shallow excavation that use the method which deduct the skin resistance in the depth of excavation from the routine surface load. But to the ultra-deep pit, there are two problems that must be considered: first, there will be skin resistance along great depth of pile under the pile head because of the strong rebound trend after excavation, then, negative skin friction will appear in lower part of pile to balance the force of pull up. It will impact the load transfer after loading on the pile head. Second, the ultra-deep excavation would mean that the normal soil pressure along the pile will reduce greatly after excavation, which may caused the bearing capacity and the vertical stiffness of piles decline at different rate, therefore, the actual capacity of the pile would be overestimated, this would lead to the lack of security(for major projects, this is not acceptable), and the settlement of support column which use the top-down method may surpass the predicted value(bear much of the load during the construction period).Based on the above analysis, the author has carried on the single pile bearing capacity model test and the finite element model analysis, through the conventional load test, the load test with sleeve and the excavation bottom pile test bearing capacity's contrast, the bearing capacity and vertical stiffness of pile loaded after excavation are reduced. Under the same load the pile affected by deep excavation has a larger settlement and proportion of pile rigid penetration in settlement than that of piles in the other two tests. Deep excavation also leads to a large pile-soil relative displacement when skin friction is total mobilized. In addition, deep excavation can cause tension in pile, which means that the reinforcing steel bars in the lower part of pile should not be reduced at discretion. At last, the author find out the rule of the Variation of pile's bearing capacity as the variation of the excavation's depth and width.