以分離元素法探討摩擦角及錐尖阻抗之空間關連性

Abstract

目前在工程分析及設計上主要使用現地取樣所作的三軸試驗的摩擦角作為設計的依據，而作三軸試驗相當費時及昂貴，相對於三軸試驗，圓錐貫入試驗則具有省時、方便、經濟的優點。因此此研究的目的在於找出圓錐貫入試驗所得到的錐尖阻抗值及三軸試驗的摩擦角之間的空間變異性之關連，若能明確的得到兩者之間的關係，則可藉由錐尖阻抗值的空間變異性推求工程設計上所需要的摩擦角之空間變異性，未來應用於現地上，則可為工程界節省下許多資源。 定義空間變異性中重要參數為變異數及相關聯性長度(SOF)，因此我們必須明確清楚參數各別在錐尖阻抗值及摩擦角之間的關聯性，例如:錐尖阻抗值的關連性長度與摩擦角的關連性長度，兩者間的關連性。如此才能由圓錐貫入試驗得到的錐尖阻抗值空間變異性推求三軸試驗得到的摩擦角的空間變異性。 由於圓錐貫入試驗屬於變形較大的試驗，因此連續體分析較不適用於此研究，因此採用分離元素法軟體作為主要的研究工具，本研究所使用的分離元素法軟體PFC3D，利用數值方法能產生句空間變異性的試體及重複試驗相同試體的特性來得到數據探討本研究想探討的問題。 分析結果顯示，若假設圓錐貫入實驗所得到之錐尖阻抗值是不發生強度的平均效應，而由於三軸試體相對於圓錐貫入器所接觸的試體來的大，故三軸試驗時，試體的強度將發生平均效應，而平均效應的型式為破壞面上的3~4點平均效應，而非面平均效應及體平均效應。

In engineering analysis and design, the value of soil friction angles used were mainly obtained from triaxial test. However, considering the cost and time-consuming of triaxial tests, cone penetration tests have its advantage of cost and time-saving, and convenience. Hence, this research focuses on the relationships between spatial variability of friction angles acquired from triaxial tests and cone penetration tests. The clarification of this relationships helps to make it possible to seek the spatial variability of design friction angles through the spatial variability of cone resistance. Variance and scale of fluctuation (SOF) are two important parameters to define spatial variability. Thus, it is crucial to understand the relationships, for example, the relationships between SOF of cone resistance and SOF of friction angle, in order to obtain one from another. Since the deformation during cone penetration tests is relatively high, continuum analysis may not be a suitable method, a distinct element method PFC3D were taken throughout this research. Specimen with spatial variability were created and repeated to generate sufficient data. The results show that, it is reasonable to assume that the cone resistance does not induce average effect of soil strength. Yet average effect would occur during triaxial tests for the contact area is larger with respect to cone penetration tests. The soil strength of a triaxial test is the average of 3 to 4 points on failure surface, rather than surface average or volume average