黄土斜坡动力响应特征分析

Dynamic Response Characteristics of Loess Slopes

斜坡动力响应特征与斜坡形态密切相关,若入射地震波主频接近斜坡卓越频率就会放大斜坡动力响应,甚至造成斜坡失稳。汶川地震对远离震中的黄土地区造成了较为严重的破坏,局部场地震害和地震动放大效应显著。选取汶川地震典型黄土斜坡场地,利用地形台阵流动观测和数值模拟计算相结合的方法,系统开展强震动作用下黄土斜坡场地动力响应特征研究。结果表明:坡顶卓越频率最小,其PGA放大系数甚至达到坡底的1.98,这种现象可能与斜坡高差和入射波波长之比密切相关,比值0.2时坡顶放大效应达到最大。随斜坡坡度增加,放大效应增强,坡顶反应谱卓越周期放大系数可达5,说明斜坡地形对强震地面运动有显著影响。数值计算结果与实际强震观测基本吻合,其结果对黄土地区建设工程抗震设防具有重要的科学与实际意义。

The Loess Plateau is seated on the upper and middle stream of the Yellow River in northern China, covering an area of 440 000 km 2, with loess deposit thickness ranging from several meters to more than 500 meters. The Loess Plateau is one of the most tectonically active areas of the world and one of the most seismically active regions. More than 1.4 million people have been killed by the earthquakes in the region. The Wenchuan M S8.0 earthquake in 2008 collapsed or seriously damaged enormous buildings, houses, and infrastructure. The field investigations, observations, and analyses indicate that a large number of casualties and tremendous economic losses were caused not only by the collapse and damage of houses with poor seismic performance, but also by the amplification effects of site conditions, topography, and the thickness of loess deposits on ground motion. The morphological characteristics of slopes determine the predominant frequency, which may amplify the incident seismic wave with the same or similar frequency range, and thus, increase the slope dynamic response and even trigger landslides. The field investigations of the Wenchuan earthquake indicate that the amplification effects of site conditions and topography on ground motion were very obvious in loess regions. In this paper, we chose a typical loess site for temporary strong motion array and numerical analysis, and aim to explore the dynamic response characteristics of the loess slope. The results reveal the following:(1) The minimum predominant frequency occurred at the slope crest. The highest peak ground acceleration ( PGA ) amplification coefficient reached 1.98 at the slope crest. The phenomenon of the low predominant frequency corresponding to the high PGA amplification at the slope top may be related to the ratio of slope height to the wavelength of incident wave. The PGA amplification was maximum when the ratio was 0.2.(2) The amplification effects are more predominant with increase in slope gradient. The predominant period amplification coefficient of response spectrum at the slope top may reach 5. The numerical results are basically consistent with the ground motion observations, and thus, they have high scientific and practical significances for engineering seismic fortification in loess regions.