Study On Design Method Of Vertical Bearing Capacity Of Rock-socketed Piles Based On Reliability

The rock-socketed piles are an important type of pile foundations,and their bearing capacity generally includes the friction resistance of the overlying soil layer,the friction resistance of the rockinserted section and the end resistance of the rock-inserted section.At present,the design method of vertical bearing capacity of rock-socketed piles in domestic codes is mainly safety factor method.In this method,the load,resistance and geotechnical parameters are all considered as fixed values.Without considering the variability of the pile load and structural parameters,the total safety factor is used to measure the reliability of the pile foundation.However,this empirically determined constant safety factor can not guarantee that all pile foundations have a consistent level of reliabilit y,resulting in unclear failure probability of rock-socketed piles,unclear safety factor values,and inaccurate values for each parameter.Based on the data collected from the rock-inserted piles,this paper systematically studies the reliability of the vertical bearing capacity of the rock-socketed piles,and makes a useful exploration for the reliability analysis of the vertical bearing capacity of the rocksocketed piles.Through the 44 sets of self-balanced method and traditional static loading method of the rocksocketed piles tests,the test data were processed using the mathematical statistics theory and Bayesian optimization estimation.The reliability of the vertical bearing capacity of self-balanced test piles with different self-balanced conversion coefficients was calibrated.The influence of parameters such as live-constant ratio,safety factor,and load effects on the reliability calculation results was analyzed,and the parameters were determined.Based on the calculation results and the American Bridge Load Resistance Design Code,the proposed self-balanced conversion factor ? of the rock-socketed piles meeting the target reliability is finally given as 0.85.By summarizing the calculation methods for the bearing capacity of rock-socketed piles in the current domestic mainstream specifications which are“Code for Design of Foundations for Buildings”(GB50007),“Technical Specifications for Pile Foundations”(JGJ94),“Code for Design of Foundations and Foundations of Highway Bridges and Culverts”(JTG D63)and “Code for Design of Foundations and Foundations for Railway Bridges and Culverts”(TBl0002.5)and comparing their advantages and disadvantages,the reliability of vertical bearing capacity of rock-in-single piles calculated in each specification is compared and calibrated.Finally,the calculation formula of "Road Bridge and Culvert Foundation and Foundation Design Code"(JTG D63-2007)was selected.The Bayesian optimization estimate was used to process the complete data of the 144 rock-socketed piles,65 sets of lateral resistance data and 31 sets of end-resistance data.The analysis of the probabilit y limit state design method was carried out for the partial resistance coefficient of rock-socketed piles,the lateral resistance of the soil,the total sub-item coefficient of the rock-socketed section,the side resistance of the rock-soaked section and the end-resistance partial coefficient of the rock-socketed section.The values of the sub-factors satisfying the target reliability index when the uniaxial saturated compressive strengths which are from 5MPa to 15 MPa of the rock at the pile end were investiga ted respectively.The most unfavorable values of the partial coefficients of the lateral resistance of the upper soil,the lateral resistance of the embedded rock segments,and the end-resistance are recommended.The relationship between the partial factor and the ratio of rock-socketing is established,and the partial coefficient based on the ratio of the rock-socketing ratio is proposed.Combined with the value of the partial factor,the degree of exertion of each resistance part was judged,and the depth of the recommended rock plug was given.Finally,each partial coefficient was introduced into the calculation formula,and the formula for bearing capacity of rock-socketed piles meeting the target reliability index was obtained.To verify the validity and feasibility of the calculation formula,three groups of test piles were selected for verification.The results show that the calculated values are in good agreement with the measured values and can also meet the engineering design requirements.