細顆粒對砂土性質之影響:試驗與結果之分析

Abstract

由經驗得知當砂土中含有細顆粒時，其工程性質與均勻級配之砂土有很大 不同。在相同的相對密度下，均勻級配的砂土可能是膨脹性(dilatant)， 如有細顆粒則可變成收縮性(contractive)。細顆粒對現場土壤試驗，比 如圓錐貫入試驗也可能有很大的影響。我們對於細顆粒如何影響砂土之工 程性質所知有限。本研究進行了一系列試驗以進一步了解細小顆粒對砂土 性質之影響。本文有兩個主要項目：第一、進行不排水之中空扭剪試驗來 瞭解細顆粒含量對砂土剪應力與超額孔隙水壓對剪應變之關係；第二、使 用粉土細料含量從 0 % ~50 %，起始相對密度 30 % 之排水三軸試驗及中 空扭剪試驗來決定細料含量對砂土膨脹性之影響。結果發現細料含量能夠 影響砂土之剪力強度和膨脹性。其影響程度受粒徑比例，圍壓和剪力模式 之控制。 Experience indicates that there are significant differences between uniformly graded clean sand and graded sand with fines. Under the same relative density, a uniformly graded sand could be dilatant and a sand with fines could be contractive. The fines contents can affect results of in situ tests such as cone penetration tests. We have limited knowledge as to the how fines contents affect the behavior of sands. In an effort of enhance our laboratory experiments have been carried out in this research project. There are two parts of the research : the first part includes a series of undrained torsional simple shear tests to evaluate the shear stress and pore pressure response during shearing, and their relationship with contents of finer material. Strain rate controlled drained triaxial and torsional simple shear tests were performed on sand specimens with 30% relative density in the second part of the experiments. The purpose of these tests is to study the effects of fines on dilatancy of sands and its relationship with shearing mode. Silt was used as fines and the contents varied from 0 to 50% in the triaxial specimen. Results indicate that contents of finer material can affect the dilatancy of sands. These effects are closely related to diameter ratio between coarse and fine materials, confining pressure and shearing mode.