反復扭剪夯實試驗之扭剪角度對砂土密度及應力之影響

Abstract

本論文以實驗方法探討反復扭轉剪力夯實造成砂質填土之地表沉陷量,土體內相對密度, 垂直與水平應力及CPT試驗錐尖阻抗的改變。本研究使用自行設計建造之反復扭轉剪力夯實儀，施加靜態垂直應力及反復剪應力在土層表面。本研究以氣乾之渥太華砂為填土，填入4層高150 mm之疏鬆砂土。填土初始相對密度為35.5 %，試體採用直徑為300 mm的鋼質圓盤施加垂直載重9.2 kPa，然後施加反復剪應力，扭剪轉角分別為+1°、+3°、+5°、+7° 及 +10°，反復扭剪作用為20次。本研究採用雷射測距儀量測土體表面沉陷量，埋設密度控制盒埋置於試體內部以量測土壤各點之相對密度，置於土壓力計試體內部以量測垂直及水平應力變化，以圓錐貫入儀測量土體各深度之錐尖阻抗值。根據實驗結果，本研究可獲得以下幾項結論: 1. 加載靜態垂直荷重於四層 150 mm 厚的試體表面，造成土體表面15至22.3 mm 的沉陷量；平均表面沉陷量為19 mm，沉陷量約為試體厚度600 mm的3.2%。很明顯地，靜態垂直載重是夯實鬆砂有效的方法 2. 靜態垂直加載，砂土的相對密度從35.5%增加大約至62.0%，比緊砂要求的標準(Dr =70%-85%)略低。 3. 與未夯實的鬆砂內的應力相比，靜態載重對於砂土中的垂直土壓力和側向土壓力的影響不顯著 4. 在垂直載重的作用後，上層之錐尖阻抗qc有明顯的增加。受壓後正常化錐尖阻抗qc / qc,loose由1.0增大至4.6。 5. 在四層且每層為150 mm 厚之土層的表面，經過以扭轉角度為±10° 扭轉20次後，平均表面沉陷量為38.2 mm；由於反覆扭剪夯實造成額外的表面沉陷量約為19.2 mm。很明顯地，反覆扭轉剪力是有效的夯實方法 6. 若靜態載重為9.2 kPa，且試體厚度為150 mm，以扭轉角度為±5° 扭剪20次後，土壤的相對密度可達72%至84%。達成之土壤相對密度隨扭剪角度之加大而增加 7. 經過反覆扭剪夯實後，土層中的垂直土壓力沒有受到影響；然而，經過反覆扭剪夯實後，土層中側向土壓力增加從27%至88%。 8. 在經過反覆扭剪夯實後，正常化的錐尖阻抗 qc / qc,loose 從4.6增加至9.0。實驗結果說明了反覆扭剪夯實明顯的增加土層中的錐尖阻抗。

This paper presents experimental data on the surface settlement change of relative density, vertical and horizontal stresses, and cone resistance in a cohesionless soil mass due to static vertical load and cyclic torsional shearing compaction. A new cyclic torsional shearing compactor was with a 300 mm-diameter circular shearing disc designed and constructed at National Chiao Tung University. Air-dry Ottawa sand was used as fill material. The initial relative density of the fill was 35.5 %. The static vertical load and cyclic torsional shearing were applied on the surface of the four 150 mm-thick lifts. Then cyclic shearing was applied with rotation angles of +1°, +3°, +5°, +7° and +10° for 20 cycles. Surface settlement of the fill was measured with a laser distance meter. Soil density cups were buried in the cohesionless specimen to monitor the relative density of soil. Soil pressure transducers were buried in the compacted fill to measure the change of the stress. A cone penetrometer used to measure cone resistance due to compaction. Based on the test results, the following conclusions were drawn. 1. With the vertical loading q = 9.2 kPa on the surface of the four 150 mm-thick soil lift, the induced surface settlements varied from 15.0 to 22.3 mm. The average surface settlement was 19.0 mm, which was about 3.2% of the soil thickness. Static vertical loading is an effective method to compact the loose fill. 2. Static vertical loading represents the dead load of the cyclic torsional shear compactor. After the application of q = 9.2 kPa, on the average, the relative density of soil increased from 35.5% to about 62%, which was less than the target value Dr = 70-85% for dense sand. 3. As compared with that for uncompacted loose sand, the effects of static vertical loading on the vertical and horizontal earth pressure, in the compressed soil mass were not significantly. 4. The application of static vertical loading q = 9.2 kPa significantly increased the cone resistance of the compressed fill. The normalize cone resistance qc / qc,loose increased from 1.0 to 4.6 due to static compression. 5. After 20 cycles of torsional shearing with the rotation angle of □ = ±10° on the surface of the four 150 mm-thick lifts, the average surface settlement was 38.2 mm (volumetric strain = 6.4%). The extra surface settlement due to the torsional shearing compaction was about 19.2 mm. Cyclic torsional shearing compaction (static plus cyclic loads) is an effective method to densify loose soil. 6. With static load q = 9.2 kPa and the lift thickness of 150 mm, after 20 cycles of torsional shearing with angle □ of ±5°, the relative density achieved was 72 to 84%. The compacted relative density increased with increasing □ angle. 7. The vertical earth pressure in the fill was not influenced by the cyclic shearing compaction. However, after cyclic shearing compaction, the horizontal earth pressure in the compacted fill increased from 27% to 88% 8. After cyclic torsional shear compaction, the normalized cone resistance qc / qc,loose increased from 4.6 to about 9.0. Test results showed the cyclic shearing compaction effects on the cone resistance in the fill was quite obviously.