振动沉桩过程“跳机”现象动力学特性

摘要

为了找出振动桩锤在沉桩作业过程中产生“跳机”现象的根本原因,从而采取有效措施解决“跳机”这一工程实际问题,从非线性振动力学的角度出发,建立二自由度分段非线性振动桩-土动力学模型,基于数值积分方法,采用四阶龙格-库塔法运算程序进行数值仿真,研究激振频率、地基土的刚度、激振器上下层偏心块的相位差对振动沉桩动力学特性的影响.结果表明:引起桩端与土有脱离的因素主要是激振力和地基土的刚度,激振力和地基土的刚度越大,则桩端与土脱离的程度越严重;当桩端与土没有脱离时,桩和机架的振动频率与激振频率相等,而在桩端与土有脱离时,机架的振动频率将小于激振频率,桩的振动频率也略有降低,当机架频率降低至一定范围时就会出现“跳机”现象.消除“跳机”现象的措施主要是减小激振力,可以通过适当降低激振频率、减小偏心力矩、增大激振器上下层偏心块相位差的方法实现.%In order to explore the reason that causes vibratory hammer to "dance" in piling and solve the "dancing" problem, a two-degrees-of-freedom pile-soil mechanical model of piecewise-nonlinearity was established in view of nonlinear vibration mechanics. By numerical simulation based on four order Runge-Kutta method, the impact of soil stiffness, vibration frequency and eccentric block phase difference on the dynamic characteristics of vibratory hammer piling was studied. The results show that the separation of the pile and the soil widens with the increase of pile exciting force and soil stiffness. No separation occurs when the vibration frequency of the pile and the rack is equal to their exciting frequency; when separation occurs, pile vibration frequency declines a little and rack vibration frequency drops sharply lower than rack exciting frequency. "Dancing" occurs when rack vibration frequency drops to a certain degree. The vibratory hammer "dancing" can be eliminated by reducing pile exciting force, which can be realized by reducing either eccentric torque or exciting frequency, or increasing eccentric block phase difference.