植入PC樁於砂土層之底承力分析模式改良

Improvement of Analysis Model of Tip Capacity for Pre-Bored PC Piles in Sandy Soils

本研究藉由植入PC樁之載重試驗資料，改良底承力分析模式於砂土層之高估現象。首先評估理論底承力公式各項因子之變異性，對變異性較大的有效樁深、覆土壓力因數 （Nq）、及土壤勁度係數 （ζqr） 進行回饋分析，使各因子修正後能可靠的預測樁之底承力。同時為方便爾後工程上使用，底承力之評估則分別採用樁載重詮釋法之上限值、中等值及下限值，以能涵蓋不同的載重階段。經由本研究分析，修正後之Nq及ζqr仍維持其典型與有效摩擦角 （ö ） 之趨勢，即Nq隨著ö 增加而遞增、ζqr則隨ö 之增加而遞減。當分析樁底之有效覆土壓力時，樁之有效分析深度限制於25 B （B為鑽孔直徑） 為最佳。本研究最後並初步建議一套適合植入PC樁於砂土層之底承力分析模式，供工程設計使用。

This study improves the analysis model of the tip bearing capacity forpre-bored PC piles in sandy soils. A large number of pile load test data isused for analysis. First, the variance of related analysis coefficients fortheoretical bearing capacity equation is evaluated; then, the more variablecoefficients, including effective depth of pile, overburden bearing capacityfactor (Nq), and bearing capacity modifier for soil rigidity (ζqr), are modifiedfrom back-calculation analysis of pile load test results. To include differentloading stages, the high, middle, and low interpreted capacities are adoptedfor the back calculation. Based on these analyses, the modified bearingcapacity coefficients retain their typical relation to soil effective frictionangle (ö ), in which Nq increases and ζqr decreases with increasing ö . Thesuggested effective pile length is limited to 25 B (B = bored diameter) for theanalysis of effective overburden pressure at the pile tip. Specific preliminarydesign recommendations for the tip bearing capacity analysis ofpre-bored PC piles in sandy soils are given for engineering practice.