Study On Mechanism And Resistance In The Process Of Driving Pretest Piles

In recent years, great attentions have been attached to the building reinforcement and reconstruction in and out, and it has developed into a new subject. However, in treating most of existing buildings which needs function reconstruction,story-adding reconstruction, or appearing some problems like settling, cracking, leaning etc are related to the treatment and reinforcement of the ground and foundation. Practices showed that the pretest underpinning piles is obviously an effective reinforcing method for ground and foundation, and it is especially available in northwest collapsible loess areas of China. But in present, there are still many shortcomings in theory researches, and many construction technology need to be improved, but the mechanism and the resistance in the process of driving pretest piles is a problem needs solved badly among them.The mechanism and resistance in the process of driving pretest piles have been systemic researched and analyzed according to Hole expansion theory, adopting numerical analysis method and verified by field test, and obtained five conclusions: 1 Soils beside the pile were suffered shearing stresses and side extrusion effect in the process of driving pretest piles, soils below pile tip were squeezed to downwards and around at the same time, and the soil structure beside the pile were deadly damaged. This mechanism made it is possible to keep the pile driving downwards, in addition, soils around the pile were affected by disturbance greatly; 2 According to Hole expansion theory and considering to the initially additional stress, obtained formula for foundation soil's elastic and plastic stress, deformation, radius of plastic zone and pile tip resistance in the process of driving protest pile. More over, rendered simplified theoretic method for calculating pile tip resistance, and verified by field test; 3Soils around the pile between (5-6)d is the main affected region in the process of driving pretest pile. In the process of driving pile, the soils around the pile uplifted, and weakened rapidly along the radial direction, also the amount of uplifted soils declined slowly with depth; 4 The main effective factors on pile tip resistance are additional pressure under the basement of existing building, soil cohesion, and the normal stiffness of the pile-soil interface. The pile vertical deviation should be controlled in 0.003L1(L1 represent the length of each pile extension); 5 Rendered value range of correction coefficient ,using adjustment coefficient method to calculate the resistance of driving pretest pile in xi'an area.The research results above have some reference value for further study on mechanism of pretest underpinning pile, bearing capacity, the resistance of driving pretest pile, and construction technology. Moreover, it could be referenced in engineering practices.