Test and analysis on optimal design of super-long bored piles for super high-rise buildings in soft soils

The building height of a project is 230 m. Large load of superstructure means high requirements for foundation bearing capacity and settlement control. The site stratigraphy is mainly composed of chalky clay and silt, interspersed with layers of chalk and fine sand. It is proposed to adopt super-long bored piles foundation, the design and construction of pile foundations is difficult. Two groups of pile tests were carried out. The pile diameter of pile A was 1 m. The bearing layer was fine sand layer and its burial depth was 93 m. The pile end and 6 pile side grouting were used. The diameter of pile B was 1 m. The bearing layer was silty clay layer and its burial depth was 78 m. The pile end and 2 pile side grouting were used. The field test results show that pile A and B both meet the requirements of bearing capacity and settlement control, but pile B is more economical and reasonable. Pile B is more favourable than pile A for pile end resistance. The enhancement coefficient of lateral friction resistance of piles above 23m depth is less than the recommended value in the investigation report, but the enhancement coefficient of lateral friction resistance of piles with burial depth of 40 ~ 80 m is in good agreement with the recommended value in the investigation report. The resistance enhancement factor at the end of pile A is significantly less than that recommended in the investigation report, while the factor of pile B is significantly higher than that recommended in the investigation report. The softening of the pile lateral friction resistance caused by mud skin and the enhancement of the pile lateral friction resistance and end resistance after grouting should be considered in the design of the super-long bored pile. © 2023 Science Press. All rights reserved.