摘要
地震是一种常见的突发性地质现象,地震时引发的地面运动是导致建筑物破坏的直接原因。尤其是分布有多条重大断裂带的攀西地区,受断裂带活动的影响,地震活动频繁而强烈,加之这一地区广泛分布的昔格达具有明显不同于其他第四系地层的物理力学特性,因此研究攀西昔格达场地动力响应特性具有重大的现实意义。
山区建筑往往将基地置于不同标高的多个平台之上,因此,研究场地的地震放大情况主要归结为考察平台标高、斜坡放坡坡度及挖填方对加速度放大的约束效应。本文以昔格达地层的物理力学特性为背景,固体介质中应力波的传播理论为基础,借助于MTS振动台试验和有限元数值模拟手段,对西昌昔格达地层的场地动力特性进行了探讨,具体工作有以下几个方面：
(1)应用固体介质中的应力波传播理论分析了场地加速度响应的空间分布特征及其约束机制。分析得出地震波传播到两种介质交界面处会发生反射叠加,从而导致地表地震动放大。两种介质的泊松比差异越大,其放大效应越显著。具体表现在：入射P波和SV波时,其能量转换只与入射角及场地岩土泊松比有关,垂直入射时引起的自由界面上介质质点位移比介质内部质点的位移大一倍；而入射SH波时,无论以何种入射角入射,引起的地表地面运动均为入射波的两倍。
(2)利用MTS振动台模型试验对不同放坡结构、不同频谱特性及0.25 g和0.5 g两种输入水准强度下的地震差异响应进行了模拟；利用有限元数值分析软件研究了昔格达地基动力响应行为的约束机制。研究结果表明：对于确定高差的两平台,放坡次数越多,顶级平台的加速度放大水平可能越低；输入地震动的强度越大,地表峰值加速度越大,但放大系数越小；同一输入地震动下,平台高差越大,地表地震动响应越强烈,且填方区地震动响应普遍大于挖方区域。
所以,多级平台场地中的边坡应尽量采用分台放坡的结构形式,尽量避免采用直立挡墙,并应遵循“多挖少填”的原则。
Earthquakes are a common burst geological phenomenon. Strong ground motion which earthquake induced, in particular the Xichang region with a number of major active faults, is capable of causing damages to buildings. By the influence of active faults, coupling with the Xigeda stratum in this region has different physical-mechanical characteristics from other quaternary strata, leading to stability of crust is poor in this region, Therefore, it is extremely important to study dynamic response of the Xigeda stratum at region of Xichang.
Researches on site amplification of earthquake mainly includes platform elevation, slope step-slope grade and restraint effect of acceleration amplification by cut-fill, because structures located in mountainous area often put the base on the platforms with variable elevations. This thesis which backing on physical-mechanical characteristics in Xigeda stratum, studied site dynamic characteristics of the Xigeda stratum by stress wave propagation theory in solid medium, MTS shaking table test, and finite element numerical simulation. The tasks done are follows:
(1) Seismic wave reflected and Superposed when propagating to the two medium interface and inducing ground motion amplification, it is obtained based on analyzing spatial distributing characteristics of site acceleration response by stress wave propagation theory in solid medium and restriction mechanism. The bigger difference of Poisson ratio between two medium, the greater amplification effect. Energy conversion was related to incident angle and poisson's ratio while incident wave was P or SV. Particle displacement in free-interface is twice as much as inner at normal incidence. But whatever incident angle is, particle displacement in free-interface is twice the incident wave while it was SH wave.
(2) Using MTS shaking table test, seismic response difference between different step-slope strutures, different spectrum characteristics and two input standard strength (0.25g and 0.5g) was simulated. And restriction mechanism of dynamic response in the Xigeda stratum was studied by finite element numerical simulation. The results show that as two platforms with specific height difference, the more slope units, the decreaser acceleration amplification level in the top platform, the greater input earthquake intensity, the increaser ground peak acceleration, but the smaller the magnification factor, as the same input eathquake intensity and spectrum characteristics, the greater the elevation platform, the more strongly surface ground motion on the top platform, seismic response in embankment section is larger than excavation section.
Therefore, slope with multi-platforms should adopt structure of slope unit form accord- ing to platforms'height and try to avoid using vertical retaining-wall. The rules to be followed is "more excavation and less embankment".
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