Mauri McSaveney教授讲座通知
报告人：Mauri McSaveney教授
单位：GNS Science, New Zealand
题目：From creep to catastrophic collapse – an evolution mechanism for the 2009 Jiweishan landslide
时间：2017年9月29日（周五）9:00-10:30
地点：新实验楼211教室（珙桐园对面）
讲座简介：
Shortly before 3:00 P.M. on June 5, 2009, after years of monitored creep deformation, a mass of about 13.3 million tonnes of limestone, dolomite and shale unexpectedly accelerated and slid rapidly along a weak shale interlayer on Jiweishan Mountain, Wulong County. Although nearly a thousand townsfolk from the immediate area below the mountainside had long been removed to safety though relocation of Tiekuang Town, some 79 people were killed in the unanticipated giant rock avalanche which in less than two minutes, devastated the wider valley floor of Tiejiang Creek, destroying farms and an iron mine.
When WH showed MM a hand specimen of a beautiful striated and polished erosion pavement he had collected from the landslide source area on Jiweishan Mountain, we knew that we had to visit the location and learn more about the landslide that had made this pavement. Exposures of the base of very large rapid landslides are extremely rare internationally, which has left scientists free to speculate wildly about the basal conditions. In this hand specimen, we saw an opportunity to end much of the speculation. The landslide was already well documented, so we were able to focus on a few square metres of the exposed base with reasonable knowledge of the greater landslide deposit around us. Here we discuss some of what we have learned to date in this ongoing study.
Mining of ore-grade hematite since the 1920s below Jiweishan Mountain had left a legacy of collapsed mined-out areas. Deformation of the overlying rock mass began possibly as early as 1958. A crack had opened to the top of the cliff above Tiekuang Town by 1996. The historical mine collapse was previously reasoned insufficient to have triggered the large rock-slope failure at Jiweishan Mountain on its own. Our new information suggests otherwise: that the mine collapse may have indirectly led to the fatal landslide.
The new information also provides an explanation of how competent professionals could get their prediction of the outcome so wrong. In the lead-up to catastrophic failure, the larger rock mass was not creeping towards the cliff face: it was creeping obliquely away from the cliff face, in a highly improbable direction — directly down dip. Movement in that direction would be expected to have been blocked by the in-situ mass of Jiweishan Mountain. Scratches on the pavement indicate that the landslide maintained that down-dip movement direction even as it suddenly accelerated to send the torrent of fractured debris cascading at high speed from the top of the cliff. The dramatic acceleration may have been triggered by collapse of an unseen cavern system. The shale previously had been an aquiclude, with a karst aquifer perched above it. Mine collapse had cracked the shale and allowed the aquifer to drain through it. Runoff that had previously created the cavern system above the shale was rerouted and may have started to slowly cut through the remaining limestone at the base of the cave weakening the resistance of the mass to sliding. We estimate from the current composition of spring water and previously measured mine drainage that solution could have enlarged the karst system by at least 1000 m3 over the 50-or-so years.
By June 2009, those monitoring the site had identified the front of a faster moving fractured mass that was causing rockfalls to spill from the clifftop, but not that this was allowing motion of a much larger mass. If they had recognised the immense scale and direction of movement of the more slowly moving mass, however, it is very likely that they would have concluded that its failure was highly improbable. It would have appeared kinematically impractical for the block to fail in the true-dip direction. We now know differently.
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地质灾害防治与地质环境保护国家重点实验室
2017年9月25日
