单向冻结条件下模拟螺旋桩的冻拔响应（英文）

Modeling frost jacking behaviors of screw piles subjected to one-dimensional freezing condition

季节性冻土区活动层土体在寒季的冻结会引起与冻胀相关的工程病害,如埋设在冻土中的地基会随冻胀的发展发生冻拔等。针对该问题,寒区工程一般选用螺旋桩来减轻影响。采用COMSOL Multiphysics软件建立有限元模型,模拟单向冻结条件下螺旋桩-地基土系统的响应。考虑水分、温度、应力3种物理场的顺序耦合,即自定义水分-温度耦合计算模块,在理查德方程中考虑冰水相变、冰对水分迁移的阻抗作用等,得到某时刻的全局体积含冰量分布,根据半经验模型建立冻胀量与体积含冰量、温度之间的关系,用以计算应力场中冻胀、冻拔变形,得到应力与位移分布,最终实现水分、温度、应力3个物理场的顺序耦合。根据几何对称性,建立二维轴对称计算模型。比选了3类螺旋桩(多叶片螺旋桩、宽叶片双螺旋桩、窄叶片双螺旋桩),并设置1组直线形桩作对照。计算结果表明:边界温度设为-15℃,经过7d冻结,土体冻深超过了桩的埋设深度,3种螺旋桩的冻拔位移小于直线形桩,证明了其减轻冻拔的作用;此时,螺旋桩底层叶片将承担最多的应力,桩身可以传递更多的冷量到下部土层,促使桩周土体冻结;通过对比不同桩形,总结出叶片布设的主要几何参数,即螺距、叶片宽度与数量及顶层叶片位置等对螺旋桩-地基土系统的冻拔响应会产生较大影响,并初步探索了寒区工程中一种基础的抗冻拔措施。

Soil in active layer undergoes volume expansion during winter,and foundations will be ejected from the ground due to the frost-heaving force acting on them.Screw piles are usually applied to foundations of projects in cold regions to minimize such problems.FE model was established by using commercial software COMSOL Multiphysics,to simulate screw pile-soil system under one-dimensional freezing condition.Hydro-thermal calculation was sequentially coupled with stress analysis,i.e.,within self-defined module,the ice-water phase change and resistance of ice were taken into consideration for Richard＇s equation.A semi-empirical model was proposed to connect frost heave with volumetric ice content and temperature,and stress analysis was conducted by using data derived from previous hydro-thermal calculation.The stress and displacement were obtained on the basis of this.Due to geometric symmetry,the computational approach was performed under axisymmetric condition.Three kinds of screw piles,including small double-helix pile,large double-helix pile and multi-helix pile,were compared with a no-helix pile as a control.The results show that when freezing depth exceeds the full embedment length of pile after seven days＇freezing（boundary temperature-15℃）,screw piles generally perform better in mitigating frost jacking than no-helix pile,and bottom helix bears more loads than the rest helices.Cooling energy transferred through pile shaft gives rise to freezing of soil adjacent to the pile.By comparisons among different piles,main geometric parameters are summarized,and it is found that screw pitch,blade width and number,position of top blade are crucial to frost jacking performance of screw pile.This work is a preliminary step towards optimized designing of screw piles in cold regions engineering applications.