p-y曲線代表每單位長度樁身的土壤阻抗力與土壤位移的函數，經常被用於模擬樁與土之間非線性互動作用之行為。許多研究已提出適用於不同土壤的p-y曲線以模擬樁土互制之行為，然而在液化砂土中應如何模擬樁土行為，仍存在很大的不確定性。本文針對福海離岸風場之土層進行案例研究，嘗試使用各種砂土模型與兩種不同修正方式，模擬海床砂質土壤在液化前、中、後之p-y曲線，以及整體樁土反應，將其結果相互比較，以期得到較合理之樁土互制行為。修正超額孔隙水壓影響之方法有兩種：(1)使用傳統砂土之下凹型p-y曲線，並乘上線性p倍率模擬土壤強度弱化現象；(2)使用完全液化砂土之上凹型p-y曲線，並乘上系統性p倍率與y倍率，同時模擬強度弱化與變行放大效應。使在超額孔隙水壓力激發程度不同之土壤，其弱化狀態採用不同的修正後p-y曲線可更合理地模擬可液化海床砂質土壤之樁土互制行為。

SUMMARY

p-y curve that represents the soil resisting force per unit length of pile as a function of soil displacement are generally used to model the interaction behavior of soil and pile. This study focuses on the case study of Changhua Fuhai offshore wind farm with using two different models to simulate the behavior of soil-pile interaction of liquefiable seabed sand:
(1) with multiplying the traditional p-y curves (concave down) by a p-multiplier to simulate the soil strength weakening effect of soil.
(2) with multiplying the p-y curve of fully liquefied sand (concave up) by p-multiplier and y-multiplier to simulate the soil strength weakening and soil deformation enlarging effect. This two methods enable a more reasonable assessment of pile-soil interaction in the liquefiable seabed sand under various level of weakening effect excited by excess pore water pressure using different modified p-y curves. According to simulation result, when ru less than 20%, using Chang and Hutchinson method will get an over-amplification soil resistance, as the result, recommending adopt Liu and Dobry method when ru less than 20% to avoid overestimate soil resistance.

INTRODUCTION

From the borehole record of Fuhai offshore wind farm, it shows that both wind turbine No. 1(WT1) and wind turbine No. 2(WT2) are located in sand and soft/hard clay. Two types of p-y curves were used in the sand layer to simulate their behavior, namely Reese, et al. (1974) and API (2005). The soft and hard clay layers were simulated with Matlock (1970) and Reese, et al. (1975) p-y curves, respectively. For liquefied snad, the p-y curve suggested by Rollins (2005) is used. As for sand that excited excess pore water pressure, two different excess pore water pressure modification methods were used to correct the p-y curves used, which are Liu & Dobry (1995) and Chang & Hutchinson (2013), to obtain the pile body reaction under different levels of soil liquefaction.

With the combinations of the different p-y curves and modification methods described above, with the aid of LPile2013 computer software, which uses finite-difference method to evaluate the interaction between soil and pile body in various depth, pile behaviors under different conditions can be obtained. These results were than compared and discussed.

MATERIALS AND METHODS

From the borehole record of Fuhai offshore wind farm, it shows that both wind turbine No. 1(WT1) and wind turbine No. 2(WT2) are located in sand and soft/hard clay. Two types of p-y curves were used in the sand layer to simulate their behavior, namely Reese, et al. (1974) and API (2005). The soft and hard clay layers were simulated with Matlock (1970) and Reese, et al. (1975) p-y curves, respectively. For liquefied snad, the p-y curve suggested by Rollins (2005) is used. As for sand that excited excess pore water pressure, two different excess pore water pressure modification methods were used to correct the p-y curves used, which are Liu & Dobry (1995) and Chang & Hutchinson (2013), to obtain the pile body reaction under different levels of soil liquefaction.

With the combinations of the different p-y curves and modification methods described above, with the aid of LPile2013 computer software, which uses finite-difference method to evaluate the interaction between soil and pile body in various depth, pile behaviors under different conditions can be obtained. These results were than compared and discussed.

RESULTS AND DISCUSSION

From the comparison of the results between wind turbine No. 1 and wind turbine No. 2, it shows that the trend obtained from both soil layers were similar, and were inferred that both wind turbine are in the same site so that only small differences were observed. These differences do not affect the interaction between soil and pile body.

CONCLUSIONS

1. From the simulation and analysis results, when the Liu & Dobry modification method was used, the pile body behavior (including pile body displacement and moment) is in the non-liquefaction side, however, when the Chang & Hutchinson modification method was used, the resulting pile body behavior is in the liquefaction side. It is suspected that it is due to the different basis function used in both modification methods.
2. When the excess pore water pressure ratio is under 20%, the p multiplier obtained from Chang & Hutchinson modification method is greater than 5, which over-magnify the ultimate impedance of sand and makes it become larger than the non-liquefaction ultimate impedance, which is not reasonable. Therefore, in the conditions of ru under 20%, it is suggested to avoid using the Chang & Hutchinson modification method and chose Liu and Dobry modification method.
3. When the Liu and Dobry modification method was used, the pile head displacement and maximum pile body moment increased with excess pore water pressure ratio proportionally. The results of using Chang & Hutchinson modification method, on the other hand, has higher sensitivity when the excess pore water pressure is between 20% and 40%.
4. Under the soil conditions of Fuhai offshore wind farm, the ultimate impedance of the p-y curves of sand and clay is reduced by 50% due to the application of cyclic loading. Furthermore, from the simulation results, the effect of cyclic loading on pile body moment is far greater than on pile body displacement.

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