軟弱黏土地盤基礎之土壤反力和基樁勁度研究

本研究針對軟弱黏土地盤之土壤反力係數和基樁勁度進行觀察，並能夠建議距離地表 8~12m 開挖深度的筏基下方土壤彈簧勁度以及基樁彈簧勁度，可供基礎工程設計做參考。其考量之設計影響參數包含土壤性質、筏基尺寸(長、寬)、開挖深度、連續壁深度、結構荷重、基樁間距(S/d)、基樁長度(l/d)。本研究範圍包括筏式基礎以及樁筏基礎，共144個數值模型案例。本研究係以三維有限元素之Midas GTS NX軟體，分析將以階段施工方式進行，將觀察開挖、連續壁施作、基礎完工營運下的基礎沉陷、反力分配、土壤彈簧和基樁彈簧勁度之分布特性，並以無因次化值之形式設計出圖表已提供給學術及實務做參考。研究觀察顯示：1. 筏基下地盤反力係數在自償性基礎時約為3000~26000 kN/m3，當荷重增加為挖方重1.5 倍時，其ks值範圍為1500~12000 kN/m3。軟弱土壤、增加筏基尺寸和荷重都會使得地盤反力係數減少，荷重影響尤其明顯；其趨勢皆為兩側支持力最高，中心最小；挖深增加和連續壁加深皆可使地盤反力係數增加。2. 基樁彈簧勁度kp在6830~106760 kN/m間，基樁彈簧勁度將隨土壤勁度、樁距樁徑比及基樁長度增加而增加；版寬、開挖深度增加(荷重亦增加)其基樁彈簧勁度會減少，版寬影響相較於其他影響因子更為明顯。3. 本研究建議可藉筏式基礎地盤之最佳化無因次化參數-位移γss/P、地盤反力q/P和地盤反力係數q/γss對應於2D/B關係圖, 以及樁筏基礎之最佳化無因次化參數-基樁位移γcs/Ec、基樁反力p/aEc和基樁彈簧勁度p/aγcs對應於2D/B關係圖做為搜尋設計案相關參數的基本參考資料。

關鍵字

黏土地盤；樁筏基礎；土壤彈簧；基樁勁度；豎向載重；有限元素分析

並列摘要

This study was made to investigate numerically the coefficients of subgrade reactions and the pile stiffness for piled raft foundations located in undrained soft clays. 3D FE analysis was used for the investigations. The design factors such as the soil stiffness and strength (Vs=100, 120 and 140 m/s), the embedded depth of the raft (8m and 12m), the raft width (16-36 m), the depth of slurry wall (2 and 2.5 times the excavation depth), the superstructure loads (1 and 1.5 times the weight of excavated soils), the pile-to-pile spacing distance (S/d=3 and 5), and the pile length (l/d= 20 and 30) were taken into account respectively for the single raft and piled raft foundations. The foundation displacements, soil reactions, coefficients of subgrade reactions, pile reactions, and pile spring constant were monitored and reported. It was found that, 1. The coefficients of subgrade reactions underneath the single raft are around 3000-26000 kN/m3 should the structural load approximate the total weight of excavated soils. For structural load approximates 1.5 times of the total weight of excavated soils, the coefficients of subgrade reactions underneath the single raft would become 1500-12000 kN/m3. Their values will be reduced by soft soil, and the increments of load and raft size. Increasing the depth of slurry wall and the embedded depth of the raft would increase the coefficients of subgrade reactions. 2. The pile spring constants were found around 6830-106760 kN/m. Their values will be increased with the increase of soil stiffness, the ratio of S/d, and the length of pile. Oppositely the increase of raft size and the embedded depth of the raft, the pile spring constants will be reduced. 3. Non-dimensional design parameters were studied for foundation displacements, the soil reactions and pile loads, and the coefficient of subgrade reactions as well as the pile spring constants. The optimized relations between the non-dimensional parameters were found to be γss/P, q/P, and q/γss with respect to 2D/B for single raft, and γcs/Ec, p/aEc and p/aγcs with respect to 2D/B for piled raft foundation. They were verified by numerical solutions on different foundation cases.