Study On Settlement Of Super-lone Bored Piles With Experimental And Theoretical Methods

Settlement of super-long bored piles is studied in this paper based on in situ static tests and load transfer methods. The research work and its results are summarized below:Results of in situ static load test on super-long bored piles of the268m high building, Zhejiang Fortune Finance Center, reveal several characteristics of settlement of super-long bored piles. Firstly, pile shaft compression accounts for a dominant part of total pile head settlement, the percentage of which is92%under working load and68%under the maximum test load, respectively. Secondly, side friction at different depth develops asynchronously, with side friction of upper soil layers functioning earlier than side friction of lower soil layers. Moreover, when the load increase to the maximum test load, side friction of upper soil layers has already fully developed, but side friction of lower soil layers has not yet. Finally, there is a limit value for pile-soil relative displacement and after pile-soil displacement exceed this value, sliding occurs between pile shaft and side soil, leading to shaft resistance degradation.Based on statistical analysis on in situ static load test results of about200bored piles embedded in middle-weathered bedrocks in Zhejiang, two empirical formulas are developed to calculate pile compression factor, ζ, and the percentage of pile shaft compression in total pile head settlement, a. Then a simplified empirical formula for pile head settlement calculating is developed. Through comparisons between measured values and calculated values of pile head settlement, validity of the proposed simplified formula is verified.As pile bottom sediment usually weakens pile end soil, a tri linear model is employed to model the load transfer mechanism of pile end soil with pile bottom sediment and an elasto-plastic model is used to model load transfer mechanism of pile side soil. An analytical solution for calculation of load and settlement of bored piles is deduced to realize the calculation of their Q0-S0curves. Comparisons of calculated Q0-S0curves and measured Qo-So curves from literatures illustrate that when pile head load is small, both calculated curves by load transfer methods using bilinear model and tri linear model to pile end soil agree well with measured curves, but under large pile head load, the calculated curves of the latter one are in better agreement with measured curves. Therefore, it can be concluded that when pile end soil is weakened by pile bottom sediment, the tri linear model can better describe the load transfer mechanism of pile end soil.Based on laboratory experiments, observation of grouted soils by Scanning Electron Microscopy (SEM) and static load tests on grouted piles, it is discovered that pile-end post grouting can improve mechanical properties of both pile side soils and pile end soils. Specifically, pile-end post grouting increases elastic shear stiffness,λ1, of pile side soils, elastic vertical compression stiffness, k1, and hardening (softening) vertical compression stiffness, k2, of pile end soil. Load transfer mechanism of grouted pile end soil turns out to obey bilinear hardening model. Then an analytical solution for calculation of pile-end post grouted bored piles is deduced.The effects of parameter changes are studied with the load transfer methods above and the following conclusions are drawn. Firstly, solely increasing λ1, k1or k2can make the calculated Q0-S0curves more gentle inclined. Among these three parameters, λ1has the most significant influence on Q0-S0curves. While the effect of changing k1or k2is less obvious and the effect only begin to function when the pile head load is in a high level. Secondly, increasing λ1,k1and k2simultaneously can make the Q0-S0curves remarkably less inclined, which means pile-end post grouting can contribute a lot to increase of bearing capacity and reduction of settlement of bored piles. Calculation of Q0-S0curves from literatures of a grouted pile and a pile without grouting from literatures illustrates that the calculated Q0-S0curve of the grouted piles is far less inclined than that of the pile without grouting. Furthermore, the calculated curves are well in agreement with measured ones. Therefore, pile-end post grouting can contribute a lot to increase of bearing capacity and reduction of settlement of bored piles, and the calculation results of load transfer methods in this paper are reliable.