震動夯實造成之土壤應力及密度變化

Abstract

本研究計畫探討震動夯實造成砂土內垂直應力、水平應力及密度之變化。傳統上， 大地工程師多係採用Jaky 理論來評估靜止土壓力，在現地土方施工，一般土壤都須經 過機械或壓路機等不同方式的滾壓夯實，夯實過後的回填土組織及行為和疏鬆狀態者大 不相同。Chen and Fang（2008）發現夯實後回填土頂部附近的土壓力接近以Rankine 公 式求出之被動土壓力。 本研究將運用國立交通大學之剛性土槽試驗設備，以氣乾渥太華砂為回填土進行實 驗研究，本研究採用電動平板夯實機在試驗室夯實土壤。在夯實前，砂土是以空中霣降 的方式落入土槽中，形成疏鬆砂土試體。在震動夯實後，土體內各點密度發生改變。本 研究將許多個土壤密度盒(soil density cup)埋在土體不同深度、不同位置，夯實後小心挖 掘出密度盒，量測盒內砂土的單位重，依試驗結果探討夯實造成之土壤密度改變。 為了觀察夯實造成土體內各點的應力改變，本研究將許多個土壓力計(soil pressure transducer)埋置於土體內之不同深度、不同位置，量測土體內各點因夯實造成的土壓力 改變。本研究計畫將分三年進行，第一年探討「單點震動夯實」對土體的影響。在將渥 太華砂落入土槽後，以不同震動時間夯實疏鬆砂土，本研究將探討之主題包含：(a) 夯 實造成之砂土密度改變; (b) 夯實造成之砂土垂直應力改變; 及(c) 夯實造成之砂土水平 應力改變。本研究第二年探討「條型震動夯實」及「全面震動夯實」對土體之影響，討 論內容與第一年之研究內容相同，此外本研究將量測因夯實而造成之土層沉陷。為模擬 現地平滑鼓式振動壓路機的夯實效果，本研究第三年將採用第一年經費所研發的小型振 動鼓(drum)式夯實機，進行土壤夯實試驗，依據實驗結果比較平板式與鼓式振動夯實機 施工對土壤應力及密度影響之異同。

This study investigates the change of vertical and horizontal stresses in a soil mass due to vibratory compaction. Traditionally, the earth pressure at-rest is estimated with the Jaky’s formula. However, in the field, the loose soil is generally compacted with different types of compactors to a desired density. Soil behaviors before and after the vibratory compaction would be quiet different. Chen and Fang (2008) reported that the lateral earth pressure measured near the soil surface after compaction was almost identical to the passive earth pressure estimated with the Rankine theory. In this study, the non-yielding soil-bin model of the National Chiao Tung University is used for all experiments. Air-dry Ottawa silica sand is used as the backfill material. A flat-plate vibratory compactor is used to compact the loose fill. The dry sand was placed into the soil bin with the air-pluviation method to form a loose specimen. Before compaction, many soil-density cups are buried in the soil mass at different elevations and locations. After compaction, the cups are carefully dug out of the backfill and the density of soil in the cup is determined. Based on the test results, the change of soil density due to compaction is reported. To observe the change of stresses due to compaction, many soil pressure transducers are buried in the soil mass at different elevations and locations. In the first year of this project, vibratory compaction is carried out at a point on the soil surface. Subjects to be investigated includes: (a) density change due to compaction; (b) vertical stress change due to compaction; and (c) horizontal stress change due to compaction. In the second year of this study, vibratory compaction is applied to a strip on the surface of the backfill. Ground settlement due to vibratory compaction is also investigated. To simulate the effects of compaction with a smooth-drum vibratory roller in the field, a small drum vibratory roller model is developed. In the third year of this project, the effects of soil compaction with the drum-type roller compactor are compared with that compacted with a plat-type vibratory compactor.