以二元混和物之框架探討砂土於動態荷載後之行為

Abstract

臺灣位於環太平洋地震帶上，每年約發生一萬六千餘次規模大小不一的地震，而土壤液化往往伴隨著地震的發生而形成，進一步使坐落於地表之結構物傾倒，而近年來，許多研究皆在探討在非塑性土壤在地震過後超額孔隙水壓消散之行為。 現地之土壤通常是不同粒徑大小組成，而顆粒大小之排列會進一步之影響到土壤之行為，自1990年前後，許多學者也提出了二元混合物排列所產生變化之概念，並在近幾年廣泛應用在大地工程中。 根據前人的研究結果，有效粒徑比為9.66的越南石英砂之細顆粒含量約為35%時，土壤可以達到極限值（最緊密的狀態）。此研究為了探討不同細顆粒含量之砂土（15%、35%）在動態荷載後之行為，以二元混和物之框架解讀此兩種細顆粒含量值皆位於極限值之左側。本研究藉由控制相同孔隙比施作了三種不同形式之三軸試驗，包含：靜態均向壓密排水試驗、靜態均向壓密不排水試驗以及動態均向壓密不排水試驗，並且所有試體皆以濕夯法進行製作，以軸向應變等於15%時的軸差應力作為土壤強度，並且更進一步探討相對密度及孔隙比對二元混和物之適用性。 根據本研究實驗結果顯示，在控制孔隙比為0.65且細顆粒含量等於35%時，土壤強度並非一直隨著超額孔隙水壓比上升而下降，在孔隙水壓比低於0.5時，土壤強度會隨著孔隙水壓比上升而下降；在孔隙水壓比超過大約0.5時，試體強度則會隨著孔隙水壓比上升而有上升的趨勢，但相同孔隙比於細顆粒含量等於15%時，則會出現不同之結果，在孔隙水壓比小於0.45時，試體強度並不會產生太大之變化，而大於0.45後則會有強度折減之情形發生。本研究也發現在動態荷載期間試體之軸向應變為一可能產生此結果之原因，並且透過Thevanayagam在2007年提出的修正孔隙比公式計算出之孔隙比可以更有效的描述二元混和物實際之顆粒接觸情形。

Taiwan is located on the circum-pacific seismic belt, where more than 16 thousand earthquakes along with liquefaction occurs each year. Liquefaction may make the buildings tilted and induce ground settlement. In-situ soils are composed by various sizes and different types of particles. The arrangements may change the behavior of the mixtures. The previous experimental result shows fines content equals to 35% is a limit value (the densest condition). Based on the framework of binary packing, those two proportions both locate on the left side To observe the post-cyclic behavior of the granular mixtures, this research uses different proportions of fine sands by weight to conduct three types of triaxial tests, which include CID, CIU, and CIUcyc tests. All the specimens are prepared by wet tamping method. The result shows that, the reduction of the soil strength is not a decrement trend when ru value increases at the fines content equals to 35% (ru = 0.5 is a critical point). While the ru value is approximately less than 0.5, the deviatoric stress at monotonic loading becomes lower with the more excess pore water pressure is generated in cyclic loading. But, the ru value exceeds the 0.5 approximately, the soil strength of specimen becomes greater when the excess pore water pressure becomes higher. In addition, there is a different performance for the specimens with fines content equals to 15%. The result shows a huge decreased phenomenon of the soil strength when the ru goes beyond 0.45. Furthermore, the soil strength exhibits the similar value when the ru value goes below 0.45. This research also points out the diversification of the axial strain during the cyclic loading is a main reason to occur this kind of result. The intergranular void ratio is considered a better parameter to describe the actual contact condition of the coarse and fine particle in granular mixture.