Study On Design And Construction Technology Of Secant Pile Retaining Structure In Soft Clay

Bored secant pill wall is a new bracing of foundation pit type, it was firstly used in the construction of Shenzhen Metro in 1999. Secant piles are two piles: A and B (vertical holes filled with concrete) placed side by side. One pile cuts into the second pile, so the two are in direct contact with each other and form water tight wall. Usually, "A" piles are filled with super slow setting concrete, the initial setting time of which is longer then 60 hours, so that B piles can be freely constructed .Bored secant pile is mostly constructed by bushing pile rig in our country.Currently, bored secant pile wall is mostly used as temporary structure in our country, it is a uneconomic way that the secant pill wall is abandoned after foundation ditch excavation, at the same time, the abandoned pill wall is a big obstruct to the subsequent development of urban underground space. In order to reduce construction costs and protect underground environment, should secant pill wall be used as both temporary and permanent structure at the same time like the diaphragm wall? Based on the previous viewpoints and the testing examples of Shanghai Metro and by means of indoor model test, this paper presents research on the design and construction of the secant piles used as permanent structure.Firstly, the paper introduces the existing conditions of application and research in our country, and they are still in the stage of familiar with and exploring construction methods.Secondly, the paper discusses the action mechanism of super delay coagulant and the influencing factors of super delaying time. Based on these discussions, the paper presents the problems that should be paid attention to, and also the paper presents the items necessary for the composing test. Take the example of super delaying concrete composition, the paper introduces the whole composing process.Then, the paper studies the mechanical properties of secant face surface under the conditions of straining, shearing and bending, and under the condition of coactions with plain piles when being bent. The testing data indicates: the quality of secant face surface is related to the separating time between occlusion of A and B piles. The optimal occlusion time is between 10 and 20 hours. The strength of secant face surface should guarantee co-deforming of plain piles, and based on the developing conditions of cracks, the paper presents calculation sections of different phases.And then, based on the model testing results, the paper includes that thehard/hard secant pile wall can be used as permanent structure and hard/firm as a part of permanent structure, the joints between secant pill wall and the exterior wall of the major structure are also presented. The paper qualitative discusses the additive condition of the load moment and stress on the inner wall and exterior wall and presents the design items needed attention under the different links.At last, the paper discusses the secant pill wall's construction technology and presents the draught of code of acceptance of construction quality of bored secant pile wall.