The Field Test And Numerical Analysis On Negative Skin Friction Of Bridge Pile Foundation Under Large Scale Vertical Surcharge Loads

The soil of coastal reclamation area is much poor, and the silt is very thick. Because this kind of soil has characters of long-time consolidation, highly sensitivity and low intensity, the soil will produce great consolidation settlement under the effect of late filling soil, which makes pile foundation of bridge produce negative skin friction easily. It will increase the load of bridge pile foundation, and reduce the bearing capacity and increase the settlement of pile foundation. On the other hand, serious damage of projects caused by negative skin friction often happen in coastal reclamation area, which cause highly cost of remedy measure. However, there are little special researches on this kind of negative shaft friction, especially about the field test. Therefore, it is very important to conduct the research of negative skin friction in coastal reclamation area. In this paper, combined with the Taizhou Bay Bridge Project Construction, three piles are chosen to study the effect of coastal reclamation area late filling to negative skin friction resistance of pile foundation of bridge. In addition, it combines with different theoretical calculation methods to analyse negative skin friction. Meanwhile, depended on the software of FLAC 3D, negative shaft friction would be studied in the different surcharge loading conditions of area、height and its sharp. Main contents and conclusions are as follows:(1) Depended on the research of three piles under balanced surcharge loading to approximately simulate the influence of late large scale filling to pile foundation of bridge in Taizhou Bay Bridge Project, it shows that, when the height of loading test arrives to 4m, the total negative skin friction is almost up to 2687 kN, and the depth of neutral point is about 29.5 meter. Besides, the development of negative skin friction changes over the time. So it is very important to consider negative skin friction in practice.(2) Depended on the calculation method from Bjerrum、code for design of Ground base and foundation of highway bridge and culverts、method from Garlanger et.al、specification of building structure foundation design from Japan and layered soil iteration design method, negative skin friction of pile was calculated. Results show that the calculation result from Bjerrum method is similar with the actual test result, and it can be utilized in similar project design in this area. Nevertheless, results of other methods are different from the actual test result.(3) The effection of filling on bridge foundation is simulated by using FLAC 3D software. Considering limited height and area of the field filling test, the effection of filling on bridge foundation is researched under the conditions of different filling area and different loading height. The results show that, when filling area is small, negative skin friction will increase dramatically with the increase of area; when filling area is large, settlement of piles will increase dramatically with the increase of area; but when the filling area is too large, negative skin friction increased hardly with the increase of area. While comparing with the large filling size, the filling area in field test has limitation, and the depth of negative skin friction and Neutral-point is small. When simulating the effection of loading height, results show that settlement and negative skin friction of piles will increase nonlinearly with the increase of height.