不同坡角斜坡动力响应频谱特征研究

SPECTRAL CHARACTERISTICS OF DYNAMIC RESPONSE OF SLOPE WITH DIFFERENT ANGLES OF INCLINATION

斜坡频谱特征曲线与斜坡形态密切相关,若入射地震波主频接近频谱曲线中极大谱比值所对应的卓越频率,就会放大斜坡动力响应,甚至造成斜坡失稳。以汶川极震区青川狮子梁斜坡为典型实例,根据青川3次余震作用下斜坡不同高程实测地震记录及地脉动测试结果,通过单点谱比法,获得斜坡地震动和地脉动的频谱特征曲线,通过对比分析曲线发现:坡顶卓越频率最小;同时,实测水平地震加速度峰值放大系数随高程增加表现出先减小后增加的"凹型"特征,坡顶的放大系数甚至达到坡底的1.25。如果从斜坡形态因素进行分析,坡顶存在低频但较高放大系数的现象可能与斜坡高差与入射波波长之比密切相关,在比值为0.2时,坡顶放大效应达到最大。为进一步确定斜坡形态对频谱特征影响,利用实测汶川地震卧龙波拟合成多频率地脉动作为动力输入,在FLAC有限差分软件中建立3种不同坡角均质斜坡模型,经动力计算后发现,3种模型坡顶、坡中及坡脚的卓越频率基本一致,该结果与输入单一频率Ricker子波时斜坡动力响应一致,说明斜坡角度改变不会影响斜坡的卓越频率。研究结论有助于增强斜坡自身频谱与动力响应之间关系的认识。

Morphological characteristics of slopes determine the predominant frequency,which may amplify the incident seismic wave of the same or similar frequency range,thereby increase the slope dynamic response,and even trigger landslides. The microtremors and seismic observations were carried out in the adits along the Shiziliang slope surface in the meizoseismal area of Wenchuan earthquake(Ms 8.0). The spectral ratios of horizontal to vertical component from the microtremor and the seismic records of 3 aftershocks were analyzed and the spectral characteristic curves were obtained. It was found that the minimum predominant frequency occurred at the slope crest. The PGA amplification decreased initially and then increased with the elevation,exhibiting a concave shape. The biggest PGA amplification reached 1.25 at the slope crest. The phenomenon of the low predominant frequency corresponding to the high PGA amplification at the top of slope may be related to the ratio of slope height to the wavelength of incident wave. The ratio was 0.2 when the PGA amplification reached the maximum value. To further investigate the morphological amplification effect on spectral characteristics,dynamic analysis was performed using the finite difference software FLAC by inputting the waves of multiple frequencies from the seismic acceleration records at Wolong station during Wenchuan earthquake(Ms 8.0). 3 different angles of inclination were assumed for the slope being simulated. The calculated predominant frequencies of the slopes with different angles of inclination were found to be the same,which were verified by the results from inputting Ricker wavelets of different mono-frequencies. This indicated that the slope angle had no effect on the predominant frequency.