具節理強度岩體因落石作用撞擊碎解與堆積行為

Fragmentation and Deposition of Rock Mass with Joint Strength Due to Rock Fall

連俊凱(Tsun-Kai Lien)

指導教授 ： 林銘郎

國立臺灣大學/工學院/土木工程學研究所/碩士(2010年)

https://doi.org/10.6342/NTU.2010.01103

摘要

前人研究落石運動主要以單顆落石運動或落石群運動為主，有關岩體撞擊後破碎的研究甚少，故本研究欲從基本的室內落石試驗，探討岩體的節理強度和撞擊破碎之關係，然後以分離元素法之PFC3D軟體分析之，描述具節理強度岩體撞擊破碎其能量之損耗情形及岩體破裂後塊體運動行為，彌補實驗上觀察之不足，並建立具節理強度岩體撞擊破碎之運動行為。接著再將經校核之數值分析方法應用至現地案例-早安斷崖落石區，先以航照判釋推估落石區之崩落岩體之大小，配合現地調查之實測，整合上述之方法引用至現地案例數值分析，做逆推反算分析。
由航照判釋發現早安斷崖在2004年敏督利颱風過後，其上邊坡距中橫公路約140公尺處有一大塊落石崩落，量體約35000m3，道路線形被岩屑覆蓋，很難看清，但明隧道並無全面破壞，且在崖錐堆積上，並無看到特別大的岩體堆積，初步推估此落石在崩落過程中有因撞擊而破裂成更多的小岩塊。
現地調查結果顯示，發現早安斷崖上邊坡有馬崙山複背斜通過，且有數條小型的剪裂帶通過，為地質構造主控。證明本區長期處於不穩定狀態、岩體較為破碎，易有落石現象發生。且在早安斷崖上邊坡及下邊坡觀察崖錐堆積上的岩塊的大小，均無發現岩塊大到足以一次砸毀明隧道的岩體，推論有可能是受到當地節理(至少3組)的影響，也和航照判釋的推論結果相符。
由室內落石試驗、數值分析結果顯示，具節理強度岩體撞擊破碎試驗，除了節理強度的大小和塊體滾動距離有負相關趨勢，還有隨著量體的增加，具節理強度岩體撞擊後破碎其顆粒滾動距離會愈來愈遠，而坡趾處的塊體也會愈向坡趾處靠近，這和Okura等人(2000)作無膠結性岩體之量體與其滾動距離試驗有相符趨勢。與前人不同之處為，岩體撞擊後破裂所形成的堆積體之組成為有大岩塊與小岩塊。且岩體撞擊45度斜坡破碎其散佈範圍會比90度垂直運動來的廣，且顆粒滾動也較遠，塊體也較破碎，可能是由於在塊體撞擊45度坡面，塊體和坡面一開始接觸，可能有較大的應力集中現象，且運動路徑較長，增加塊體間的碰撞頻率。爾後塊體在沿著斜坡往堆積區運動時，由於顆粒滾動撞擊，又會產生再次破裂情形。
由現地數值節理強度敏感度分析發現，岩體在運動過程中逐漸破裂之總動能為最大，推測最大動能以岩體在運動過程中逐漸破裂為最大其可能原因為在岩體部分破碎時，因為非均勻破裂，故破裂後的岩塊體較大，而大塊的岩塊運動時，塊體和塊體間交互碰撞作用不大，因此總動能最大。其觀察結果和室內數值模擬45度斜坡塊體撞擊部分破裂、部份不破裂有相同的趨勢。

關鍵字

落石 ； 具節理強度岩體 ； 撞擊 ； 落石試驗 ； 分離元素法 ； PFC3D

並列摘要

The pre-research mainly discuss the single rock fall movement or cluster of rockfall movement, the research of fragmentation of rock mass was less. This research starts from the small scale rock fall tests to discuss the relationship of joint strength of rock mass and fragmentation. Then use the distinct element method PFC3D to analyze and to discuss the energy disappearance and the movement of the fragmentation. Then use the adopted numerical analysis to analyze the full scale model, the Tzau An Cliff rock fall area. And this research would use aerial-photo interpretation to estimate the volume of rockfall of the source area and combines the field investigation to calibrate.
Through the aerial-photo interpretation, we found that there is a rockfall above the rock-shed 140m whose volume is about 35000m3 has fallen after year 2001, and the rock-shed has not been totally destroyed. Also the talus is composed of small rocks. So we figure out that the rockfall must have crush through its movement.
The field investigation shows that there are Malaishan Synclinorium and a lot of small shear zones through the slope. It results that the geological structure here is very broken and it may result rockfall. Observaing the talus, there are not founded that the rock is as big as possible to destroy the rock-shed. It is supposed that it is controled by the joint (at least 3 sets), and the results is the same with the aerial-photo interpretation’s.
Through the small scale rock fall tests and PFC3D simulation, we found that the joint strength has the negative trend with the run out distance, in addition, with the volume increasing, the run out distance of the fragmentation will run farther and the fragmentation which is near the toe will be more . This result has the same trend with Okura et al. (2000). In addition, it is found that the run out distance of the fragmentation which impact on 45 degrees slope is farther than the one which impact on 90 degrees, and it also founded that the rockfall which impact on 45 degrees slope, its fragmentation is more broken.
Through the sensitive of joint strength analysis of the full scale simulation, the rockfall which fragmentize gradually along its movement has the maxium kinetic energy. It is because that the fragmentation was big enough, so that the interaction between fragmentations was not much fierce. It is observed that the results have the same trend with the rockfall which fragmentizes partly on 45 degrees slope in the small scale rockfall test simulation.

並列關鍵字

rockfall ； rock mass with joint strength ； impact ； rockfall test ； distinct element method ； PFC3D

參考文獻

李偉竹(2007)，結合岩體評分法與網格不穩定力平衡模式應用於廣域山崩行為之模擬-以溪頭為例，碩士論文，國立台灣大學土木工程研究所。

羅佳明(2009)，落石區崖線崩退與崖錐堆積形態之研究，博士論文，國立台灣大學土木工程研究所。

Goodman, R.E (1976), Method of geological engineering in discontinuous rock, ASTM Publ. St. Paul West.

Giacomini, A, Buzzi O., Renard, B., Giani, G.P. (2009) Experimental studies on fragmentation of rock falls on impact with rock surfaces, International Journal of Rock Mechanics and Mining Sciences, Volume 46, Issue 4, June 2009, Pages 708-715.

Okura, Y., Kitahara, H., Sammori, T., Kawanami, A. (2000), The effect of rockfall volume on runout distance, J. Eng. Geo., 58, 109-124.