设计,取1.5的安全系数时所对应的按本文方法计算的抗倾覆安全系数范围。若按库仑理论进行设计,实际的被动土压力合力作用点位置比库仑理论设定的要高,其安全系数达不到1.5的要求,所以,应优化挡墙的结构,控制挡墙的变位模式,使合力作用点降低,提高抗倾覆安全系数。
5 坡面起伏和非均匀超载对被动土压
力的影响
设挡墙高6 m,墙后土重度γ=18 kN/m3,内聚力c=10 kPa,内摩擦角φ=20°,挡土墙倾角α=70°, 坡面形状函数g(x)和非均匀超载q(x)均用三角函数模拟坡面的起伏和非均匀超载。表2为计算结果,可以看出,被动土压力合力和作用点受坡面的起伏以及坡面超载的不均匀性影响不能忽视。
6 结论
基于墙后滑楔体整体极限平衡方程,利用变分法原理推导了被动土压力泛函极值的变分模型,提出了相应的计算方法,当作用点位置已知时,可有效确定被动土压力大小和滑裂面形状位置。算例分析表明,墙后土体的极限平衡状态对应于作用点位置系数范围,随着作用点位置系数的增大,被动土压力呈非线性增长,在作用点位置下限处,被动土压力最小,滑裂面为对数螺旋曲面,但随着作用点位置的上移,滑裂面曲率半径逐渐增大,到上限处变为平面。此时,被动土压力值达到最大,且与按库仑理论计算的被动土压力一致,但作用点位置位于墙背中点以上,与库仑理论所假定的合力作用点位置不一致。
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(编辑 王秀玲)