Analysis of bank slope stability under strong seismic response for super long span bridges
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摘要:
在高烈度山区设计修建公路桥梁时，耦合多种不利条件的在强震作用下超大跨径桥梁高陡岸坡稳定性最为复杂，易形成滑移、碎屑流等岸坡失稳灾害。实际震害调查结果表明不规则地形对地震动力具有明显的放大作用，对边坡的稳定性和桥梁的安全性构成不利的影响，如何考虑复杂地形的地震动力放大效应具有重要的工程价值。以位于四川省凉山彝族自治州高烈度深切峡谷地段的主跨
1200 m特大悬索桥岸坡为例，对此类超大跨径桥梁岸坡在强地震力作用下的基岩面地震危险性概率和失稳破坏模式机理进行研究，建立了含卸荷裂隙的三维坡体结构模型，采用动力时程分析方法给出了不同失稳破坏模式下岸坡上各特征点的峰值地震加速度并据此获得了修正的放大系数。基于修正的放大系数对坡体地震稳定性的拟静力计算方法进行改进，采用改进后的方法对该桥位的稳定性进行了评估。结果表明：边坡遵循峰值地震水平加速度及放大系数地表最大，随着坡体深度的增大而递减，且递减速度减缓并趋于稳定的规律，且坡度变化率对此影响极大。坡度变化率大且地貌突出部位的地震响应极为强烈。大范围分布的碎块石土覆盖层、变坡率的地貌突出的浅表层、风化卸荷带内的表层风化碎裂岩体极易在地震作用下产生变形，应当加强防护。未考虑修正放大系数的地震工况计算结果偏于不安全，安全系数的计算结果减少了2%～6%。据此提出一整套针对高烈度山区特大跨径桥梁岸坡的地质灾害风险评估方法和与考虑桥梁结构两水准抗震相适应的边坡稳定性计算方法及防护措施建议思路，为相关工程的研究与设计提供参考。Abstract:Designing and constructing highway bridges in high-intensity mountainous areas present significant challenges. The stability of high and steep bank slopes for large span bridges coupled with various unfavorable conditions under strong earthquakes is particularly complex, which is prone to formation of bank slope instability disasters such as sliding and debris flow. Investigations into earthquake damage reveal that irregular terrain has a significant amplification effect on earthquake dynamics, which has an adverse impact on the stability of slopes and the safety of bridges. Assessing the seismic dynamic amplification effect of complex terrain is of important engineering value. This study examines the bank slope of a 1200m-long suspension bridge located in the high-intensity, deep canyon region of the Liangshan Yi Autonomous Prefecture, Sichuan Province. We conduct an in-depth analysis and research on the seismic hazard probability and instability failure mode mechanisms of the bedrock surface under strong seismic forces. A three-dimensional slope structure model with unloading cracks was developed. The peak seismic acceleration of each characteristic point on the bank slope under different instability failure modes was obtained using dynamic time-history analysis method and modified amplification coefficient was derived based on these findings. Improvements were made to the static calculation method for slope seismic stability using this modified coefficient. The improved method was used to evaluate the stability of the construction site. The results indicate that the slope's peak seismic horizontal acceleration and amplification coefficient are highest at the surface and decrease with increasing slope depth, with the rate of decrease slowing and stabilizing. The rate of slope change significantly impacts this response. The seismic response is exceptionally strong in areas with high slope change rates and prominent landforms. Widely distributed fragmented rock and soil cover layers, shallow surfaces with varying slope rates, and surface weathered fragmented rock masses within weathering unloading zones are prone to deformation under seismic action, and protection should be strengthened. The calculation results of seismic conditions without considering the correction of amplification factors are unsafe, with safety factor results decreasing by 2% to 6%. A complete set of geological hazard risk assessment methods, and slope stability calculation methods, and protective measures suitable for considering the two-level seismic resistance of bridge structures are proposed based on this for the bank slopes of ultra large span bridges in high intensity mountainous areas, providing a reference for the research and design of related engineering projects in high-intensity mountainous areas.
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表 1 岩土体的物理力学参数
Table 1. Physical and mechanical parameters of rock and soil
区域 弹模
/MPa泊松比 黏聚力/kPa 内摩擦角/（°） 容重/（kN·m−3） 天然 暴雨 天然 暴雨 天然 暴雨 碎块石土 60 0.32 15 13 27.0 24.0 20.0 21.0 卸荷带 300 0.28 120 108 42.2 38.0 24.0 25.0 中风化岩 800 0.27 507 456 49.9 44.9 26.5 27.0 
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表 2 监测点峰值地震水平加速度和放大系数（ξ）
Table 2. Peak seismic horizontal acceleration and amplification factor of monitoring points
编号 西昌岸 香格里拉岸 峰值地震水平加速度 ξ 峰值地震水平加速度 ξ 1 18.779 2.61 16.249 2.26 2 26.536 3.69 19.068 1.40 3 21.003 2.92 14.210 1.28 4 18.763 2.61 15.963 2.22 5 12.581 1.75 43.467 6.04 6 19.471 2.71 18.124 2.52 7 18.848 2.61 13.097 1.82 8 16.650 2.32 14.919 2.07 9 12.434 1.80 41.169 5.72 10 15.802 2.20 16.488 2.29 11 13.193 1.83 12.499 1.74 12 11.940 1.66 14.474 2.01 13 15.797 2.20 10.257 1.43 14 12.253 1.70 12.484 1.76 15 10.975 1.53 9.688 1.35
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表 3 考虑修正放大系数下不同工况边坡FS及稳定状态
Table 3. FS and stable state of various conditions with considering the correction amplification factor
岸坡 天然工况 暴雨工况 E1地震 E2地震 FS 状态 FS 状态 FS 状态 FS 状态 西昌 1.35 稳定 1.26 稳定 1.13 稳定 0.97 失稳 香格里拉 1.26 稳定 1.12 稳定 1.06 基本稳定 0.98 失稳
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表 4 未考虑修正放大系数下地震工况的FS及稳定状态
Table 4. FS and stable state of seismic conditions without considering the correction amplification factor
岸坡 E1地震 E2地震 FS 状态 FS 状态 西昌 1.15 稳定 0.99 失稳 香格里拉 1.11 稳定 1.04 欠稳定
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