土釘工法為近年來所發展之現場土壤加勁技術，大都應用於支撐開挖面或不穩定邊坡之防護。加勁方式乃利用鋼棒在開挖坡面進行鑽孔後插入鋼棒，再以水泥砂漿等固結材料填充於孔洞中，與現地地層結合成一體，以抵抗牆後土壓力或邊坡下滑力，防止開挖面或邊坡的變形。 土釘-土壤間之界面摩擦阻抗為土釘擋土結構之主要加勁機制，界面剪力阻抗與土釘驅動相鄰砂顆粒之範圍息息相關。過去常以巨觀模式探討土釘拉出行為，對於拉出過程中，土釘周圍砂顆粒之運動過程與剪力帶範圍，僅有限的資訊可供參考。 有鑑於此，本研究以二維微觀方式於土釘拉出試驗，觀測顆粒實際運動情形。試驗控制因子，包含土釘牙距(Sp)、顆粒直徑(d)及相對密度(Dr)等。選擇4種土釘表面粗糙度，分別為光滑面，牙距1 mm、2.05 mm及4.10 mm等；而顆粒直徑分別採用0.84 mm與1.90 mm。試驗結果顯示土釘牙距與砂顆粒之幾何關係影響剪力帶厚度最鉅；當Sp/d接近1時，剪力帶厚度與尖峰視摩擦係數越大。由粒徑凸出比Ｅ值與剪力帶厚度值、尖峰視摩擦係數之關係得知，當Ｅ值越接近1時，剪力帶厚度值與尖峰視摩擦係數也越大。 Soil nailing is an in-situ reinforcement technique in recent years, to retain excavations and stabilize slopes. The nails are installed at predetermined locations using a drilling and grouting method appropriate for the soil to retain excavations and stabilize slopes. Nails may be driven as well. The frictional characteristic of the interface between the nail and soil is the most important reinforcing mechanism for nailed retaining structures. The interfacial shear resistance depends on the effect range of neighbor particles subject to the nail pullout. During the pullout of a nail, the nail tends to drag the surrounding soil particles to move together. The friction on the soil-nail interface would cause adjacent particles to move through the friction between their contacts. This propagates further away from the soil nail resulting in shear band formation and development. In this study, the microscope movement of soil particles around a soil nail is observed through a two-dimensional soil nail pullout test in a model sandbox. The test parameters included variations in the screw pitch of nail (Sp), the particle diameter (d), and the relative density of particle (Dr). The surface screw pitches of the nails were 1 mm, 2.05 mm, and 4.10 mm, respectively. A smooth surface was used to represent a nail with zero roughness. The diameters of sand particles were 0.84 mm and 1.90 mm, respectively. The experimental results showed that the thickness of shear band depends on the geometric relationship between the surface roughness and the particle size of sand. The shear band thickness and the peak apparent friction coefficient were the maximum when the ratio of the screw pitch to particle diameter (Sp/d) or the diameter protrusion ratio (E) were approached one.