地震波类型对砂土震陷影响的数值模拟分析

Numerical Simulations of the Impact of Different Types of Seismic Waves on the Seismic Compression of Sands

地震波是一种随机的、不规则作用的动荷载脉冲,可分为振动型和冲击型。不同类型的地震波会对砂土液化和变形等产生重要影响,而传统的砂土震陷计算方法往往忽视这种因素,只考虑最大加速度幅值。通过编写UMAT子程序,在非线性有限元软件ABAQUS中开发亚塑性砂土边界面模型,对不同地震波类型下不同相对密度的砂土进行动单剪试验模拟,得到一系列砂土剪应变及竖向应变的时程曲线,并与室内试验结果进行对比分析。研究表明:在同一工况下,同类型的地震波引起的砂土竖向应变相近,不同类型引起的竖向应变差异明显;振动型地震波比冲击型引起的竖向应变更大。

Earthquakes can result in huge losses of human life and property. According to exten- sive field surveys, a significant number of foundation failure cases are due to sand settlement, as foundation upsetting and cracking can be caused by large deformations or differential settlements. Seismic waves are a type of random and irregular dynamic pulse and have a great influence on the seismic settlement of sand. By setting the boundary value in a time history curve to 60% of the maximum peak acceleration, various seismic waves can be divided into two types： vibration-type waves, which have more than two wave crests within the boundary value, and impact-type waves. Different types of seismic waves have different effects on the liquefaction and compression of sand, possibly due to the problem of energy release. However, traditional methods for calculating the seismic settlement of sand neglect the different types of waves; they merely consider the max- imum acceleration amplitude to simplify this complex problem by using an equivalent sinusoidal quantity. This simplification may cause larger errors during practical applications. In this paper, the nonlinear finite element software ABAQUS, which is a frequently used commercial software for numeric calculations, is used to develop a bounding surface hypoplaticity model for sand that includes a UMAT subroutine. The formulation of this constitutive model is based on the boun-ding surface plasticity theory, which a large number of laboratory experiments have proven to be effective in describing the cyclic behavior of sand. The dynamic simple shear test has a few advan- tages in assessing the seismic behavior of soil. Primarily, since it applies cyclic shear stresses onto horizontal planes and permits the continuous rotation of principal stress axes, this test replicates field loading conditions much more accurately than the cyclic triaxial test. By simulating the dy- namic simple shear test using sand with different relative densities under different types of seis- mic wave loading, this paper studies the influence of wave type on the process of seismic settle- ment and the final settlement value, as well as the deformation of sand under earthquake loading. Finally, a series of time-history curves comparing shear strain versus vertical strain were be ob- tained from this series of numerical experiments, and were compared with laboratory results. The main conclusions are as follows. The wave band prior to the maximum peak acceleration governs the controlling effect on seismic settlement; however, the left wave band also has an obvious in- fluence when the peak value attains 80~ of the maximum value under vibration-type seismic loading. An analogous experiment yielded similar compression results; under the same conditions but under the influence of different types of seismic waves, the compression results were dispa- rate, as vertical strains caused by vibration-type seismic waves were greater than those caused by impact-type waves. By taking into account the wave type, this method is much more practical for the simulation of sand seismic settlement at actual sites. As the model parameters related to the dilatancy effect are constant, there is a certain amount of bias between the simulated and experi- ment results. Further research into these parameters is needed.