本研究為單根模型土釘在砂土中拉出行為的探討，研究參數包括土釘牙距、土釘直徑、砂土粒徑等；經試驗結果獲得：(1)當砂土平均粒徑小於土釘牙距時，尖峰、殘餘視摩擦係數隨螺紋牙距增大而增加，且顆粒越大，尖峰、殘餘值也越大；當砂土平均粒徑大於土釘牙距時，視摩擦係數則隨螺紋牙距之增加而增大，而殘餘視摩擦係數皆下降至某一範圍。(2)粗糙土釘隨直徑增大，視摩擦係數呈遞減的趨勢，但遞減的趨勢最後趨於平緩；殘餘視摩擦係數則隨不同粒徑之砂土而有固定的趨勢。(3)各試驗砂之拉出峰後殘餘行為皆不同：編號9矽砂隨位移增加而平滑下降，其拉出力殘餘值為尖峰值的50%~65%；編號315石英砂則有上下跳動之現象，其拉出力殘餘值為尖峰值的70%~80%；在編號313石英砂中的殘餘值趨向一定值(相差6%以內)。本研究透過一系列拉出試驗結果，配合定義之粗糙度與單一長度內螺紋數量，運用Luo之理論模式，歸納單釘視摩擦係數與粗糙度及螺紋數之關係。關係式分為平均砂土粒徑小於螺紋牙距(編號9矽砂、編號315石英砂)與大於螺紋牙距(編號313石英砂)兩種情況。預測值與試驗值相比，編號9矽砂相差13%；編號315石英砂相差6.1%；編號313 石英砂則相差17.2%。 Pullout behavior of model nail in a sand box under various conditions was investigated through a series of laboratory tests. The conclusions from the result of laboratory experiment are summarized: (1) When D50 of the sand is smaller than the screw length, peak and residual apparent friction coefficients at the soil-nail interface are increased with the increase of screw length. When D50 of the sand is larger than the screw length, the peak apparent friction coefficient increases with the increase of screw length, but the residual apparent friction coefficient decreased and reaches a constant value. (2) The increase of sand particle creates greater value of the peak apparent friction coefficient. Apparent friction coefficient at residual varies with the particle size of the tested sand. (3) The residual pullout resistance for nail pulled out from No 9 silica and No. 315 sands decreased to 50-60% and 70-80% of peak values, respectively. The residual resistance of nail in No.313 Quartz sand reaches a constant value for all sizes of nails. According to the results of laboratory experiment, roughness was defined to evaluate the peak apparent friction coefficient from nail and soil properties. Two formulas are used for the D50 of sand is smaller and greater than screw length, respectively. The predicted values of silica sand are 13% greater than the values of laboratory experiment, while the No.315 Quartz sand and No. 313 quartz sand are 6.1% and 17.2% greater than the tested values.