壓入式沉箱施工以NF膜降低土層摩擦力之研究

Abstract

本論文以噴氣直剪試驗探討壓入沉箱施工，噴氣壓力大小對降低箱體周面摩擦力之效應。本研究先至施工瞭解現場紀錄壓入沉箱之施工，並採用交通大學直剪試驗儀模擬壓入式沉箱下沉時，箱壁與土壤間的介面摩擦行為。本研究之試驗土樣採取渥太華砂，並以實驗室自拌混凝土模擬箱體材料。透過直剪試驗儀與自行建置之噴流設備，模擬壓入沉箱下沉時採用的兩種輔助工法：NF工法與噴氣工法。依據沉箱施工現場取回之NF膜，配合直剪試驗可以了解NF膜對降低沉箱壁體與土壤間介面摩擦力之成效。實驗研究改變空氣壓力σa與直剪試驗正應力σn的壓力比APR(air pressure ratio)，了解施加氣壓大小與引致介面摩擦角δ的關係。直剪試驗之結果顯示，使用NF膜可以降低混凝土試體與渥太華砂之間的摩擦力。實驗測得混凝土試體與中等緊密砂土的介面摩擦角為22o，將平滑鋼片製成之NF膜包覆於直剪試驗上盒底部後，得到的混凝土試體與NF膜界面摩擦角為16.7o，NF膜的介入使摩擦角下降了5.3o。噴氣直剪試驗結果顯示，隨著噴氣壓力的增大(壓力比自0增大到3)，混凝土試體與NF膜間的界面摩擦角逐漸減小。界面摩擦角自16.7o(當APR=0)減小到δ=13.0o(當APR=1)，摩擦角δ=9.6o(當APR=2)，摩擦角δ=8.8o(當APR=3)。噴氣直剪試驗結果顯示，當壓力比大於3.0(氣壓σa為正應力σn的三倍)，界面摩擦角保持下限值8.8o，δ不再隨著壓力比之增大而下降。壓入式沉箱施工現場案例顯示，噴氣工法可有效降低箱體的周面摩擦力。但是於地盤中灌注大量壓縮空氣，可能對周圍地盤造成擾動，須謹慎使用。

This thesis investigated the reduction of interface friction between the pressed-in caisson and the soil by the air-jet direct shear tests. A special air jet direct shear experiment was designed and constructed at the National Chiao Tung University soil mechanics laboratory to simulate no-friction method(NF) method and air jet method for the construction of a pressed-in caisson. Direct shear tests were used to study the interface friction between concrete sample and Ottawa sand, and the reduction of interface friction angle by the thin steal of NF sheet. The relation between the applied air pressure and the interface friction angle was studied by varying the air pressure ratio(APR). The APR is defined as the ratio of applied air pressure (σa) and the interface normal stress (σn), during the air-jet direct shear test. Test results indicated that the residual internal friction of the medium dense Ottawa sand(Dr=55%) was 31.3°. By using the NF sheet, the interface fiction angle between the concrete sample and Ottawa sand was reduced to 22°. By place of the NF sheet between the sand and the concrete sample, the interface angle was reduced to 16.7°. Test results of the air-jet direct shear test indicated that the interface friction angle between NF sheet and the concrete sample was reduced to 12.7°, 9.6°, and 8.8°, corresponding to an APR ratio of 1, 2, and 3 respectively. However, the interface friction angle remain a constant after the ratio exceeded 3. In other words, an increase of APR beyond 3 has no effect on the friction angle between the concrete sample and the NF plate. The ultimate interface angle was 8.8°. Test results of press-in caisson indicated that the air- jet method could effectively reduce the friction between caisson and soil up to 7.9°. However, pumping compressed air into the ground might have adverse effects to the deposits surrounding the caisson.