Recession of Limestone Clifis in Ryukyu Islands

从动力学的角度研究了琉球石灰岩海岸悬崖的退缩现象。选择冲绳岛、宫古岛和菊海岛的几个岩石海岸作为调查区域。对岩壁高度、缺口深度、落块长度和岩体纵波速度进行了现场测量。崖高2 ~ 23 m，缺口深度2 ~ 9 m，块长4 ~ 13 m。通过室内试验测量了悬崖材料的物理力学性能。岩石强度随试样尺寸的增大而急剧降低，即琉球灰岩具有岩石强度的尺度效应。结合室内尺度效应强度折减结果以及室内和现场试验的纵波速度结果，导出了岩体强度估算方程。采用梁分析模型对悬崖失稳进行了分析。忽略尺度效应的岩体强度稳定性分析不能很好地解释断崖破坏。考虑崖后张裂缝，导出了新的稳定性分析方程。将估算的岩体强度和岩壁材料的单位重量代入方程，得到了岩壁高度和临界缺口深度关系下的临界稳定线。结果表明，目前的悬崖剖面数据绘制在稳定区，而落块的长度和宽度绘制在临界线上。由此得出结论，考虑岩体强度和拉裂的梁破坏类型分析将为预测琉球石灰岩悬崖的临界缺口深度提供一个有用的框架。

Recession of coastal cliffs made of Ryukyu Limestone was studied from the dynamic viewpoint. Several rocky coasts of Okinawa-jima, Miyako-jima and Kikai-jima were selected as the investigated areas. Field measurements were carried out for the cliff height, the depth of notches, the length of fallen blocks and longitudinal wave velocity in rock mass. The cliff height, notch depth and block length range from 2-23 m, 2-9 m and 4-13 m, respectively. Physical and mechanical properties of the cliff material were measured through the laboratory tests. The rock strength drastically decreases with increasing size of specimen, i.e., Ryukyu Limestone has the scale effects in rock strength. Combining the laboratory results of strength reduction due to scale effect and the results of the longitudinal wave velocity obtained from both laboratory and field tests, an equation was derived for estimating of the rock mass strength.Cliff instability was analyzed using a beam analysis model. The stability analysis using the strength of rock mass ignoring scale effects could not well explain cliff failures. Considering tension cracks behind cliff, a new equation for stability analysis was derived. Substituting the estimated rock mass strength and unit weight of cliff material into the equation, we obtain the critical stability lines in cliff height and critical notch depth relationships. Results show that the data from the present cliff profile was plotted in the stable area, while the length and breadth of fallen blocks was plotted on the critical lines. This leads to the conclusion that the analysis of beam failure type, which takes account of the rock mass strength and tension crack, would provide a useful framework for predicting the critical notch depth of cliff made of Ryukyu Limestone.