低应变条件下桩土相互作用的阻尼系数

DAMPING COEFFICIENTS OF PILE-SOIL INTERACTION WITH LOW STRAIN

低应变条件下桩土相互作用的阻尼系数是基桩缺陷量化分析的一个关键参数。为通过试验直接确定桩土相互作用的阻尼系数,建立桩土相互作用的定解方程,分析两端自由的摩擦桩在不同阻尼系数时的桩顶速度波时程变化,推导桩顶速度响应幅值比与阻尼系数之间的关系。在此基础上,选用6种桩侧土,针对土层饱和前、后两种状态,进行不同上覆有效应力作用下土层与桩相互作用的模型试验,确定桩侧土阻尼系数,研究阻尼系数随土性、土层上覆有效应力的变化。试验结果表明,6种桩侧土阻尼系数均随土层上覆有效应力增加而呈指数关系增加,不同土类指数关系中的系数不同;与饱和前的砂土与粉土阻尼系数相比,相同土层上覆有效应力下饱和后砂土与粉土的阻尼系数降低25%～60%;当土层上覆有效应力从85 kPa增加到850 kPa时,饱和粉质黏土的阻尼系数从250 s-1增加至890 s-1;当土层上覆有效应力从20 kPa增加到150 kPa时,饱和软黏土的阻尼系数从330 s-1增加至420 s-1。最后,将试验得到的阻尼系数与其他方法确定的阻尼系数进行比较,进一步说明通过试验测定桩土相互作用阻尼系数的必要性。

The pile-soil interaction-damping coefficient with low strain is a key parameter to quantitatively analyze defects of piles. The pile-soil interaction equation was established to determine the damping coefficient using experiments. The time-history velocity at pile top was analyzed corresponding to different damping coefficients and free-end boundary condition. The relationship between the incident and reflected velocity amplitudes ratio at the pile top and the coefficient was developed. Six types of soils were selected to conduct pile-soil interaction model tests and determine the coefficient for different effective overburden pressures, unsaturated and saturated states. Effects of soils and the effective overburden pressure on the coefficient were researched. The experimental results show： （1） the exponential relationship can be used to describe the increment of coefficients with the effective overburden pressure for six soils, but determining coefficients in the relationship are different for different soils; （2） damping coefficients of saturated sands and silts decrease 25% to 60% compared with the coefficients of unsaturated sands and silts. If the effective overburden pressure varied from 85 kPa to 850 kPa, the damping coefficient of the saturated silty clay increased from 250 s^-1 to 890 s^-1. However, if the effective overburden pressure varied from 20 kPa to 150 kPa, the damping coefficient of the saturated soft clay will increase from 330 s^-1 to 420 s^-1. The test results were also compared with damping coefficients determined using other methods to show the necessity of determining damping coefficients using the model test method.