集集地震造成擋土牆破壞之案例探討

Abstract

本研究蒐集921集集大地震造成各種型式擋土結構物之破壞案例，以個案方式逐一探討，希望能從中找尋出破壞的機制和原因。本研究自各破壞案例現地採取回填土樣，於交通大學土壤力學實驗室進行土壤物性實驗、強度試驗及夯實試驗，並進行擋土結構物之靜態及動態的分析。依據擋土牆破壞案例研究，獲得以下各項結論。（1）涵碧樓停車場擋土牆破壞案例顯示，座落於斜坡上的擋土結構物，可能會因為往復作用之水平地震力作用，整體安全性不足，產生邊坡滑動式破壞。（2）台中港案例顯示，地震力可能造成濱水擋土結構物向海水側外移，回填土之側向伸張可能造成牆後主動土楔之滑移，並在回填土造成與牆體平行之地表裂縫。（3）台影文化城之破壞案例顯示，若擋土牆正巧座落於地震斷層上方，斷層上盤垂直方向的上舉將牆踵頂起可能造成牆身傾覆，上盤水平方向的側推可能造成牆身向外滑移及牆趾前方土壤之隆起。（4）地震來襲時，擋土牆可能因牆趾部分支承力不足而產生沈陷。根據Tatsuoka（1998）的研究，此現象會引致傾覆彎矩逐漸加大，導致傾覆式破壞。

In this study, cases of retaining wall failure due to the 1999 Chi-Chi earthquake has been collected. Each case has been investigated to explore the mechanism of wall failure under seismic excitation. Backfill soils behind the damaged gravity walls had been sampled from the site, and tested in the soil mechanics laboratory of National Chiao Tung University. Physical properties, compaction characteristics, and shear strength of the backfill were determined. Static and dynamic stability analyses had been conducted for the damaged gravity walls. Based on the adequacy check of the retaining structures, the following conclusions can be drawn. (1) The failure of retaining wall at the Han-Bi-Lou parking lot indicates that, as a result of cyclic ground shaking, the retaining wall located on a slope might lost its stability and moved downhill with the sliding block. (2) The Tai-Chung harbor case indicates that, cyclic earthquake shaking caused the waterfront retaining structure to move toward the seaward side. Lateral expansion of the backfill caused the active wedge to slide along failure planes. Surface cracks parallel to the wall can be observed at ground surface. (3) For the wall failed at the Taiwan Cinema Culture Town, the retaining structure is located right above the Cher-Lung-Pu fault. The uplift of the fault caused the heel of the wall to move upward, and the body of the wall to overturn. The lateral thrust of the fault caused the wall to slide outward, and the soil in front the toe to heave. (4) As a result of earthquake vibration, the bearing capacity under the wall may not be sufficient to resist the maximum normal stress at the toe. Based on the study of Tatsuoka (1998), the settlement at the toe will decrease the resisting moment of the wall, and cause overturning failure for the wall.