| With the booming development of high-rise buildings,municipal engineering and big bridges, more and more rock socketed drilled shaft with large diameters have been widely used because of its advantages of high bearing capacity, small settlement, etc. Shenzhen Pingan Center project has adopted drilled shaft with 16 diameter 5.7 m(bell bottom diameter 7.0 m) and 8 diameter 8.0 m(bell bottom diameter 9.5 m). No application of precedent in home and abroad and are considered the largest drilled in the word.However, when come to the design of large diameter drilled shaft, most design standard in different countries are still adopting the design methods used for small drilled shaft. Due to the high bearing capacity of rock socketed large diameter drilled shaft, the traditionally load test could not be applied. Due to the lack of field monitoring data, the force state of large diameter drilled shaft is unclear and this easily causes the waste. The research of large diameter drilled shaft is the requirement of theory and engineering field.This dissertation focus on the research of vertical bearing characteristics of rock socketed drilled shaft with large diameter and bell bottom by using field testing, similarity model test, theoretical analysis and numerical calculation.The surrounding rock of the large diameter drilled shaft bottom is usually high stiffness rock. Traditional artificial mechanical excavation could barely be used so that blasting excavation is the only option. Surrounding rock would be disturbed due to blasting, so it is extremely necessary to design the blasting construction plan. By adopting two times micro blasting method, the effect on surrounding structure and pipes will be minimized. When facing the problem of small operation space, the installation of steel reinforce gage could use inversing traditional method and boost the construction efficiency. The high amount of concrete pouring leads to hydration heat so how to control the temperature is key step of construction. By adding ice and optimizing mixture ratio, the continuous concrete pouring is guaranteed.The theoretical analysis of bearing capacity of rock socketed large diameter drilled shaft was carried out and the results indicated that:(1) After fully considering the weight of wall protection and drilled shaft aas well as character of pile-rock interface, the skin friction of large diameter drilled shaft studied. The result reveals that the surrounding rock's type, operation turbulence, et al. would have a great impact on the skin friction.(2) The calculation method of end bearing capacity is proposed based on RQD. 77 field tests results were collected in home and abroad. The nonlinear static analysis was carried out and brought to the conclusion of calculation method end bearing capacity of large diameter drilled shaft. By considering the rock quality at the end of the pile, the end bearing capacity of large diameter drilled shaft was easily gained. This method was used for the project in Shenzhen PingAn Center and the results were compared with results that gained from standards. This method is more reasonable and could also guarantee the safety factor.The similarity tests were carried out for further study of large diameter drilled shaft, the results revealed that,(1) Based on similarity theory, the similarity criterion was deducted. By choosing different mix proportion of sand, cement and plaster, the similarity materials were selected. Based on the Shenzhen PingAn Center's drilled shaft with diameter 5.7 m and 8.0 m, the model test was designed. It is available to test the vertical bearing capacity of by using similarity test.(2) The urban model test box was upgraded so that the similarity test was more convenient to operate and easily to observed at the process of test. By combining with the loading system, it could be used to add the load for two piles with diameter 160 mm at the same time.(3) The loading-settlement curves of pile top were all slowly changed. In the range of design load, the loading-settlement curves were all linear. By comparing the similarity results with site monitoring results considering similarity ratio, the results met perfectly. This indicated that the similarity results were reasonable. By adding load in the range of design load, the loading- settlement curves of equal diameter and model pile with belled bottom, no difference was observed. However, as the load increased, at the same loading level, the settlement of model pile with belled bottom was smaller than equal diameter model pile. In additionally, it was more obvious as the load increased. By designing bell bottom at the end of drilled shaft could control the pile top settlement effetely. On the other hand, when using pile top settlement as the standard of drilled shaft bearing capacity, the belled bottom could increase the vertical bearing capacity of drilled shaft.(4) Both the axial force and skin friction increased as the load on the top of the pile increased. At relatively small load, the skin friction at the range of strongly weathered rock developed to the limit. The skin friction was strongly effect by the surrounding rock strength. The pile transferred load in the range of weak rock layer. As the load increased, the skin friction of equal diameter pile was larger than model pile with bell bottom.(5) The end bearing capacity occupied most loads. Model pile with bell bottom took more loads at the end of the pile compared with model with equal diameter. So the rock strength at the end of drilled shaft strongly affects the vertical bearing capacity of the drilled shaft. In additionally, the skin friction at the range of moderately weathered rock and slightly weathered rock should not be ignored, especially if the drilled shaft was in the thick layer of moderately weathered rock or slightly weathered rock. It is not reasonable to ignore the skin friction when design the drilled shaft.Numerical analysis was also used to study the bearing capacity of large diameter drill shaft. Results were as follow,(1) The Hoek-Brown criterion is used for decided the parameter of the surrounding rock. By comparing the site monitoring results and numerical calculation results, the numerical analysis was accurate. The results could be use to evaluate the bearing capacity of large diameter drilled shaft.(2) For large diameter drilled shaft and diameter length ratio less than 5, the load-settlement curves were linear. The rock socketed length had great impact on the pile top settlement. When adding the load till the design load, the end bearing capacity and skin friction share the load the top. As the load increased, the end bearing capacity carried almost load.(3) Due to the reason that the enlargement of pile section at the bell bottom, there was negative skin friction observed at the bell bottom. This was usual for small rock socketed pile and special attention must be paid.In order to study the pile quality and vertical bearing capacity, the site tests were also presented as follow,(1) The traditional loading test could not afford enough loads for large diameter rock socketed drilled shaft. By using low strain dynamic testing, ultrasonic test, O-cell test and core drilling test, the quality and bearing capacity of rock socketed drilled shaft were been evaluated. The comprehensive assessment could make up the fact that large diameter drilled shaft couldn't use the loading test method to evaluated the bearing capacity.(2) For the drilled shaft with diameter smaller than 2.5 m, the low strain dynamic testing could accurately detect the flaws in the drilled shaft. The method is well applied for drilled shaft with diameter less than 2.5 m and diameter length ration larger than 5. For drilled shaft measurement within this range, the high frequency has fewer disturbances of the results. By inbuilt several sounding pipe, the ultrasonic test method could be used to detect the flaws for drilled shaft with diameter larger than 5 m. And the results will not affected by the measurement of the drilled shaft. O-cell test could gain both the positive and negative skin friction of the drilled shaft. On one hand, this method could evaluate the bearing capacity wet herd justified design requirement; on the other hand, this method could provide evidence for further research. The core drilling method used to test the strength of the drilled shaft and rock beneath the drilled shaft. Special attention needed to be paid when drilled core from the drilled shaft.For high rise buildings, the uplift pile is also been widely used to resist the load such as wind load, by analysis the bearing capacity of the uplift drilled shaft, the conclusions are as follow,(1) On the same load, if no evident difference of pile diameter, the displacement of the pile is decreased and the capacity is higher as the length of the pile increased. For the site of Shenzhen Pingan Center, the effect of increase the diameter of the pile is relatively small.(2) As the load on the uplift pile increased, the skin friction of the equal diameter and belled bottom both increased. The load portion of the belled bottom also increased. In addition, on the limit condition, the belled bottom takes almost 50% of the load.(3) The parametric study indicates that the diameter ratio of equal diameter part and bell bottom part has more impact on the uplift bearing capacity. On the contrary, the elastic modules and the bell bottom height have relatively small impact on the bearing capacity. |
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