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已有桩基负摩阻力研究多关注于桩周土体为粘土的情况,桩周土为砂土的模型试验相对较少。设计实施了砂土中单桩及双桩负摩阻力模型试验,试验中对桩顶与土表进行了分级加载,测定了模型桩桩身应变、桩顶位移以及土体分层沉降。试验结果表明:由于土表分级荷载作用下的土体沉降增加值大于桩沉降增加值,桩基中性点位置存在下移趋势。受群桩效应的影响,桩顶荷载作用下单桩沉降小于双桩沉降,土表超载作用下3倍桩间距的双桩沉降最小。从桩身轴力最大值分析,5倍桩间距的双桩可忽略群桩效应。随土表超载分级加载,单桩负摩阻力有效应力系数因侧摩阻力发挥所需桩土相对位移的影响而逐渐减少,而双桩的负摩阻力有效应力系数逐渐接近单桩时的数值。
The negative friction resistance of pile foundation has been mostly concerned with the situation that the soil around the pile is clay, and the model test of soil around the pile is relatively small. The model test of negative skin friction of single pile and double pile in sandy soil was designed and implemented. The pile top and soil surface were graded and loaded in the experiment. The strain of pile body, the displacement of pile top and the delamination settlement of soil were measured. The experimental results show that as the added value of soil mass under the graded load of soil surface is larger than the added value of pile settlement, the neutral point position of pile foundation has a downward trend. Under the influence of pile groups, the settlement of single pile under the pile top load is smaller than the settlement of double piles. The settlement of double pile with 3 times of pile spacing under the overload of soil surface is the smallest. Based on the analysis of the maximum axial force of pile, the double piles with 5 times the spacing of piles can neglect the group pile effect. The effective stress coefficient of negative skin friction of single pile gradually decreases with the influence of lateral friction resistance, and the effective stress coefficient of negative skin friction gradually approaches the value of single pile.