崩積層常由由級配分布廣且排列鬆散的土壤所組成，且常為具大顆粒之礫石土壤，陡坡破壞常發生於崩積土層，而進行崩積土層邊坡破壞之穩定分析時，須掌握其剪力強度參數，但受到礫石土中大尺寸顆粒之影響，需以大型試驗儀器進行實驗。受限於目前大尺寸顆粒試體的取樣與重模方法及試驗儀器限制，加上崩積層及礫石層中顆粒的尺寸效應難以評估且受場址所影響，礫石土壤之室內試驗仍是一項艱鉅挑戰。有鑑於此，本研究將以微觀顆粒狀態 (Intergrain state)概念結合單剪試驗，針對不同礫石含量對剪力阻抗的影響進行系統性了解及探討。

本研究先採用離散元素法(DEM)進行量化評估單剪試驗之尺寸與邊界效應，再以均勻及二元混合分佈的鋼球試體進行元素單剪儀試驗以驗證DEM結果。驗證後之模型將用以評估巨觀之應力應變反應，結合微觀之顆粒受力與位移場的分布，量化試體尺寸及最大顆粒尺寸對巨觀反應之影響。

由具跳躍級配及良好級配之砂礫混和重模試體(，進行排水單剪試驗，實驗結果顯示：(1) 顆粒性土壤的剪力強度取決於主導混和土壤行為之基質顆粒堆疊排列狀況，相較傳統之整體孔隙比，粒間顆粒之孔隙比參數更能代表土壤的排列狀況；(2) 鬆散砂質崩積土之排水剪力強度受主導行為之基質砂土控制，含有礫石的剪力強度相較純砂土降低量低於 20% 以下。以跳躍級配及良好級配之基質粉砂土重模試體進行不排水單剪試驗，實驗結果顯示：(1) 緊密堆積下具跳躍級配之礫石粉土其不排水剪力強度取決於基質粉土，含有礫石的剪力強度比純粉土增加少於7%； (2) 具良好級配的砂質粉土之礫石土壤，其不排水剪力強度較無礫石堆積降低約 7% ，並隨著礫石含量增加而增加。綜合上述實驗結果，本研究提出對礫石含量進行適當的調整下，可由細顆粒基質土壤評估崩積土的剪力強度參數。

Shallow landslides in colluvium, which generally consists of wide particle gradation in a loose state, occur in steep-slope terrains. To perform stability analyses, shear strength parameters that represents soil constituents, in-situ stress state, and shearing mode are needed. Due to the presence of oversized gravel particles, block sampling and large-scale testing apparatus are required to determine the shear strength parameters of colluvial deposits. Limited by the sampling methods and testing apparatus, laboratory testing on soils with oversized particles is a challenging task. In addition, the effects of particle size on colluvium or rockfill are difficult to evaluate and could be site-dependent. A comprehensive study is performed to study the effects of gravel content on shear resistances under simple shear conditions on the framework of intergrain state concept.

To quantitatively evaluate the size and boundary conditions of simple shear testing, distinct element method (DEM) were carried out and validated by physical element tests on uniform and mixture steel balls using a simple shear apparatus. The validated models are used to evaluate the influences of specimen dimensions and maximum particle size on macroscopic stress-strain responses and microscopic uniformity on particle force and displacement fields.

A series of drained simple shear tests were performed on reconstituted specimens of gap- and well-graded sands. The results show that: (1) the shear strength of granular soils depends on the packing condition of dominant particles, with the intergrain granular void ratio being a more representative parameter for the soil packing condition than the global void ratio; (2) the drained shear strength of loose, sand-like colluvial soils is dominated by sand matrix properties and the inclusion of gravel content reduces the strength by no more than 20% compared to that of pure sand.

The experimental study on undrained simple shear tests was conducted on reconstituted specimens of gap- graded silt and well-graded sandy silt. The results indicate that (1) the undrained shear strength of dense, gap-graded silt with gravels is dominated by silt matrix properties and the inclusion of gravel content increases the strength by no more than 7% compared to that of pure silt; (2) the strength parameters of colluvium can be estimated from the fine particle matrix with proper adjustment of gravel content.

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