Recession of Limestone Clifis in Ryukyu Islands

从动力学的观点研究了琉球石灰岩海岸峭壁的后退。冲绳岛、宫古岛和纪海岛的几个岩石海岸被选为调查区域。现场测量了悬崖高度、切槽深度、落石长度和岩体纵波速度。悬崖高度、缺口深度和块体长度分别为2-23 m、2-9 m和4-13 m。通过室内试验测定了崖体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，琉球石灰岩在岩石强度方面具有尺度效应。结合室内尺度效应引起的岩体强度折减结果和室内及现场纵波波速测试结果，推导了岩体强度的估算公式，并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崖体破坏。考虑崖后拉裂缝的影响，推导了新的稳定性分析方程。将估算的岩体强度和崖体材料容重代入方程，得到了崖体高度与临界切槽深度关系的临界稳定线。结果表明，目前的悬崖剖面数据绘制在稳定区，而下降块的长度和宽度绘制在临界线。因此，考虑岩体强度和张裂的梁破坏类型分析将为预测琉球石灰岩悬崖的临界切槽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.