場鑄樁於排水土層受側向載重之p-y曲線法評

Evaluation of p-y Curve for Drilled Shafts in Drained Soils under Lateral Loading

周思安(Ssu-An Chou)

指導教授 ： 陳逸駿

中原大學/工學院/土木工程研究所/碩士(2021年)

https://doi.org/10.6840/cycu202100856

摘要

為抵抗側向載重對建築物之影響，工程設計者通常會設計深基礎以解決此問題，而深基礎之場鑄樁為工程設計者經常所採用。在眾多側向載重分析法中，p-y曲線法常於工程界中所使用且已行之有年。為能對排水土層之p-y曲線法進行評估與改良，本研究蒐集世界各地的現地場鑄樁側向載重試驗資料，評估場鑄樁於排水土層之p-y曲線法以探討側向載重預測之可靠度，並依照分析結果加以改良以提升精確度。
本研究首先依照柔性因數(Kr)之大小，將Kr值大於10-2者視為剛性樁，小於10-2者視為柔性樁分別進行分析與改良。在用原始p-y曲線法對剛柔性場鑄樁的預測結果進行分析後，發現其預測結果有低估之現象，且此低估之現象與樁徑大小成正比之關係。使用原始p-y曲線對剛性場鑄樁於砂土層進行預測後，其量測值(Qm)與預測值(Qp)之差異(ΔQ = Qm - Qp)相較於量測之總體平均差異比值(ΔQ/Qm)為 - 38.1%，其斜率與1.0之平均絕對差值為0.26；而使用原始p-y曲線對柔性場鑄樁於砂土層進行預測後，其總體平均差異比值ΔQ/Qm為 - 49.1%，平均絕對差值為0.33。
以上預測結果皆表示剛柔性之場鑄樁於砂土層以p-y曲線法預測之結果相較現地量測結果存在低估之現象，因此本研究設計兩參數pout與pin對原始p-y曲線進行修正，並以平均差異比值與平均絕對差值分別進行改良。經過對砂土層之p-y曲線法進行改良後，以平均差異比值做為改良依據方面，剛性場鑄樁採用參數pout為2.0及參數pin為1.9，改良後之平均差異比值收斂至0.1%。而柔性場鑄樁建議以參數pout為3.0、參數pin為1.6對其進行改良，改良後之平均差異比值收斂至 - 0.2%。
若以平均絕對差值做為改良依據，剛性場鑄樁建議以參數pout為1.4、參數pin為2.9對其進行改良，改良後之平均絕對差值收斂至0.07；而柔性場鑄樁建議以參數pout為2.0、參數pin為1.4對其進行改良，改良後之平均絕對差值收斂至 0.087。
最後，本研究根據以上剛性樁及柔性樁之分析結果提出相關之初步設計建議，供爾後分析之參考。

關鍵字

場鑄樁 ； 側向載重 ； 樁載重試驗 ； 排水土層 ； p-y曲線法

並列摘要

In order to resist the influence of lateral loads on structures, engineers would usually utilize deep foundations as the building support. Various types of deep foundations are available for use but drilled shafts are often utilized for its flexibility and high load carrying capacity. So as to determine the lateral capacity of shafts, numerous lateral load analysis methods are available, in which the p-y curve method is often used in engineering industry and has been in practice for many years for its simplicity and reliability. But due to its simplicity, improvements should be made to the method in order for it to provide more accurate results. Therefore, to be able to evaluate and improve the p-y curve method to be used for drained soil analysis, this study collected on-site drilled shaft lateral load test data from all over the world and evaluated the reliability of the p-y curve method by comparing the measured values to the lateral load predicted values. This lead to the improvement of the accuracy according to the gathered results.
According to the values of the flexibility factor (Kr), the research considered those with Kr values of greater than 10-2 as rigid piles, and those with Kr values of less than 10-2 as flexible piles. The analysis and improvement of the method were carried out according to the above classification. After using the original p-y curve method to analyze the prediction results of rigid-flexible drilled shafts, this study found that the predictions were underestimated, and that the degree of underestimation is inversely proportional to the size of the pile diameter. After using the original p-y curve to predict the rigid drilled shafts in the sand layers, the predicted value (Qp) and the measured value (Qm) were subtracted to each other and the difference (ΔQ = Qm – Qp) were then normalized by the measured value (ΔQ/Qm). This resulted to a value of - 38.1%. The average absolute difference calculated by taking the absolute value after subtracting the slope (a-value) from one is 0.26 for rigid drilled shafts. The same procedure was done for flexible piles which garnered an overall average difference ratio ΔQ/Qm of - 49.1% with the average absolute difference of one and a-value calculated to be 0.33.
The above prediction results indicate that the results predicted by the p-y curve method for rigid and flexible field cast piles in sand layers are underestimated compared with the on-site measurement results. In order to address this issue, this study designed two parameters, namely pout and pin to modify the equation and to improve the original p-y curve method. After some modifications on the p-y curve method for drilled shafts in sand layers, it is recommended that in order to improve the rigid drilled shafts prediction, the parameters pout = 2.0 and pin = 1.9 should be included in the computation. After including these modifications, the average difference ratio converges to 0.1%. The same was done for flexible shafts with pout = 3.0 and pin = 1.6. This garnered average difference ratios converging to - 0.2%.
In addition, the average absolute difference between 1 and the a-value (Qp/Qm) was taken into consideration. This garnered a result of pout = 1.4 and pin = 2.9 for rigid shafts and pout = 2.0 and pin = 1.4 for flexible shafts. This gathered average absolute difference converging to 0.07 and 0.087 for rigid and flexible shafts, respectively.
As a conclusion, this research proposed relevant preliminary design suggestions based on the above analytical results for both rigid and flexible piles in drained soil conditions as a reference for subsequent analysis.

並列關鍵字

drilled shafts ； lateral load ； pile load test ； drained soils ； p-y curve

參考文獻

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