地震誘發遽變式山崩之臨界位移;Critical displacement of earthquake-triggered catastrophic landslides

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Title:	地震誘發遽變式山崩之臨界位移;Critical displacement of earthquake-triggered catastrophic landslides
Authors:	許暢軒;Hsu,Chang-Hsuan
Contributors:	應用地質研究所
Keywords:	遽變式山崩;臨界位移;殘餘強度;Newmark位移法;速度位移相依摩擦律
Date:	2016-08-30
Issue Date:	2016-10-13 14:56:50 (UTC+8)
Publisher:	國立中央大學
Abstract:	前人利用Newmark位移法評估地震誘發山崩時，常用臨界位移作為判斷邊坡是否達破壞之門檻值，其物理意義為滑動面上材料剪力強度達殘餘值所需之位移，此一位移量常透過恆速之土壤強度試驗求得。不同材料具不同臨界位移值，一般為數公分至數十公分。然而，當材料所承受剪切速度不同時，其強度將有明顯變化，因此，文獻中對臨界位移的給定義可能過度簡化。本研究以遽變式山崩為對象，整理近二十年來高速旋剪儀針對各種土壤、岩石材料之試驗結果，並於考慮強度之速度相依性後，重新定義臨界位移為運動不再停止前邊坡的累積位移量，並探討滑動面材料特性、傾角以及地震特性對臨界位移之影響。研究假設剛塊置於一傾角之斜面(臨界摩擦係數=tan 0.268)，輸入東西方向不同頻率(0.5、1.2、2.0Hz)之正弦波地震加速度歷時(PGA=600gal)，並利用Newmark位移法，計算滑動面於不同速度位移相依摩擦律條件下之運動行為。結果顯示：遽變式山崩啟動的關鍵在於材料強度是否易隨剪切速度的上升而降低以及材料處高速剪切下穩態強度是否足夠低；另外，臨界位移隨材料的(1)尖峰強度、(2)高速滑移下的穩態強度、(3)弱化距離、(4)強度低於臨界摩擦係數所需之速度等四項摩擦特性之數值增加而增加，其中尤以弱化距離影響最為顯著。地震特性方面則是臨界位移隨地震頻率增加而增加，但不受尖峰地動加速度之影響，不影響之主因在於所輸入地震加速度為正弦波形。對於滑動面傾角的改變亦影響相異材料間臨界位移值大小之關係。綜合以上分析，本研究考慮材料強度之速度位移相依性後發現遽變式山崩的啟動關鍵以及臨界位移大小明顯受材料影響，同時地震特性以及滑動面傾角亦是影響臨界位移因素之一，因此，評估地震誘發山崩的觸發條件時須同時考慮滑動面強度之速度相依、地震特性以及滑動面傾角之大小。;The critical displacement is referred to as a threshold of a slope from stable to unstable, compared with the calculated permanent displacement under seismic load using Newmark displacement analysis. The critical displacement, usually obtained from laboratory shear tests under constant/low shear rates, is defined as the coseismic displacement beyond which strengths along sliding surface approach residual values. The typical values ranges from several centimeters to several tens of centimeters, which depends on the sheared materials. However, this definition of the critical displacement is oversimplified because the strengths along sliding surface is velocity-dependent. Therefore, we collect a large number of parameters which change with different materials of friction tests in order to take into account velocity dependence. Besides, this study redefines critical displacement as the accumulated displacement before initiation and discusses how the critical displacement changes with different dominant frequencies and materials. A simple two dimensional rigid block model incorporating velocity-displacement dependent friction law on sliding plane is adopted. The inclination of the sliding plane is assumed as 15°. The seismic load is simplified as sinusoidal wave with peak ground acceleration of 600 gal. Different seismic frequencies of 0.5, 1.2, 2.0 Hz are used to evaluate the influence of dominant frequency on the critical displacement. The influence of the parameters in the displacement/velocity dependent friction law on the occurrence of block instability is also assessed. Firstly, this study reveals the leading cause which results in catastrophic failures. Secondly, we discuss how material, dominant frequency, peak ground motion acceleration and dip angle of sliding surface influence the critical displacement. Catastrophic failures will not form if the strengths of sliding surface are either not easy to decrease with increasing velocity or remains high under ultrahigh-speed shearing. In addition, if catastrophic failures occur, we find that the higher the peak friction coefficient, steady-state friction coefficient at infinite velocity, slip weakening distance, and critical velocity of material, the higher the critical displacement. In particular, slip weakening distance have great influence on critical displacement. Moreover, the critical displacement is also affected by dominant frequency but not peak ground motion acceleration. Being not affected by peak ground motion acceleration mainly because of the input sinusoidal wave. This study highlights the initiation of landslides and the critical displacement are extremely complex, both obviously rely on materials. Also, velocity-displacement dependent friction law must be taken into account for landslide triggering assessments.
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