The Experimental Research On The Vertical Bearing Behavior Of Super-long And Large Diameter Bored Piles In Lake Dongting

Nowadays, Super-long and Large-diameter Bored Piles are widely used in high-rise buildings and bridges with super-long spans, but man's acquaintance with the mechanism of load transfer about this type of piles is slight. The theory is lagged behind the practice. Its design method is according to that of common pile yet. So it is necessary to study the Bearing Behavior of Super-long and Large-diameter Bored PilesBased on the tests of several piles of four bridges, which are being constructed in the area of Lake Dongting, the mechanism of load transfer and the characteristic of bearing capacity are analyzed. Self-balanced Testing Method was performed to several piles of three bridges and the conventional anchored pile method to another respectively.The experiments show that the bearing capacity of Super-long and Large-diameter Bored Piles is mainly contributed by side friction, especially by their upper and middle parts, even under the bearing limitation, the ratio of base resistance to the total load is very low because of the soil residue at the bottom of piles, so this type of piles is friction ones. Considering these features of Super-long and Large-diameter Bored Piles, bearing capacity design for common pile is not applicable to this kind of piles. It is suggested that the design of vertical bearing capacity should be in terms of permissible settlement of piles, or according to the real side friction distribution of piles.In addition, the value of side friction by test in-site is large than that by surveying or suggesting of codes. That is to say, these designs are conservative.Self-balanced Testing Method has advantages over conventional testing method in safety, economy and high load capacity. What's more, it is effective way to study the negative friction, the side friction of pile-soil, because its O-Cell can be placed any part of the pile.But Self-balanced Testing Method is incapable of acquiring the conventional load- settlement curve, Q-S curve, several transformation method have been developed. Since the work condition of real piles is different from that of testing piles as well as the complicity of mutual effecting between soil and pile, the position of O-Cell is indefinite, leads to inaccurate results. Author developed some ideas to improve Self-balanced Testing Method.