岩石邊坡LRFD部分係數率定探討

Abstract

針對破壞模式屬於岩石邊坡平面破壞的設計，本文擬定了一套可率定載重與阻抗係數設計法（Load Resistance Factor Design）中所需部分係數的流程。傳統安全係數設計（Factor of Safety）其實並無可明確對應之可靠度，本研究之率定流程因而改以要求可靠度之思維出發。礙於所能收集之資料不足，本研究以一系列之虛擬設計案例用於率定部分係數。各種參數的隨機分佈大多簡化假設為常態或對數常態之分佈型態，地震力則引用內政部營建署建議之地震力係數，以仿靜態的分析方式考量之。本文之部分係數率定是以控制可靠度（控制破壞機率）為前提，因此先將率定案例以可靠度設計調整至目標可靠度，再進行部份係數的率定。其中的不確定性方析與部分係數之決定方式以改良一階二次矩（Advance First Order Second Moment）法完成。由於設計中影響平面滑動之參數多，且受設計條件與設計型式所影響，分析的部分係數資料不免較為散亂。為使部分係數之關係有較好之掌握，本研究將天然阻抗合併考量，取不同弱面傾角分別回歸與探討。最後結果包含率定相關問題之探討、建議用於設計之部分係數形式、傳統設計與部分係數設計之結果比較。本文之研究顯示，對特定安全性之部分係數率定是可行的；但設計結果的均一性，則受部分係數採用形式的影響。

This thesis developed a methodology to calibrate the required partial factors of the “Load Resistance Factor Design (LRFD)” for rock slope design against plane-slide failure mechanism. Conventional slope stability design based on factor of safety does not necessarily provide a specific level of reliability. This study, on the contrary, attempted to calibrate partial factors of LRFD so that a targeted reliability can be obtained. A series of artificial design examples were utilized to calibrate the LRFD partial factors. Design parameters are assumed in normal or log-normal distribution. Seismic effect is taken into account by pseudo-static analysis by adopting a horizontal acceleration coefficient corresponding to a specific return period. The partial factors were calibrated using the method of advanced first order second moment reliability analysis against a pre-calibrated or a pre-determined reliability index. To begin with, all the artificial design cases were iteratively adjusted until their corresponding reliability index were satisfactorily close to the target reliability index. Afterward, those adjusted artificial design cases were used for the calibration of partial factors corresponding to the targeted reliability index. Partial factors are assigned to resistance instead of strength parameters (c and f). The resistance term is divided into the natural part and artificial part (from rock bolts), each with a separate partial factor. The partial factors were calibrated for various dip angle of weak plane. The designed values of partial factors for LRFD of rock slope were suggested and discussed. The reliability scatterings of designs using FSD and LRFD were evaluated and compared. The comparison shows that the calibration towards a targeted reliability is feasible.