ABSTRACT

Dense networks of dilated fractures occur locally in the upper 5–15 m of bedrock in basement gneisses in eastern Sweden. Near Forsmark, pre-existing sub-horizontal fractures have been jacked open and filled with water-lain sediment, likely during the latest Weichselian glaciation. Despite extensive previous research, it is uncertain whether subglacial hydraulic jacking led to the generation of new fractures, in addition to reactivation of pre-existing ones. Re-analysis of historic photos from excavations near the Forsmark power plant indicates formation of two types of new fracture. Firstly, rock fragments were broken off the main fracture surfaces as existing fractures were jacked open. Secondly, fracture analysis shows that whilst few subvertical fractures occur above tight sub-horizontal fractures, a higher density of vertical fractures occurs above dilated sub-horizontal fractures, suggesting new formation. We apply a model of beam failure theory, borrowed from structural engineering, to constrain potential new fracture generation, using assumptions based on measured water pressure fluctuations from beneath the Greenland Ice Sheet. This modelling shows that beam failure is a plausible mechanism for the generation of new vertical fractures during a subglacial water fluctuation cycle under a range of realistic glaciological conditions. This implies that hydraulic jacking can result in further in situ disruption and brecciation of the shallow rock mass, decreasing the rock mass strength and increasing its hydraulic conductivity. Altogether, hydraulic jacking of existing fractures and the formation of new vertical fractures results in effective subglacial mechanical weathering of the shallow rock mass.

摘要

在瑞典东部基底片麻岩的基岩上部 5-15 m 局部出现密集的扩张裂缝网络。在福斯马克附近，可能是在最近的魏克瑟利亚冰川作用期间，已经存在的亚水平裂缝被顶开并充满了水沉积物。尽管先前进行了广泛的研究，但除了重新激活先前存在的裂缝之外，冰下水力顶进是否导致新裂缝的产生尚不确定。对 Forsmark 发电厂附近发掘的历史照片的重新分析表明，形成了两种类型的新裂缝。首先，随着现有裂缝被顶开，岩石碎片从主要裂缝表面上脱落。其次，断裂分析表明，虽然很少有近垂直断裂发生在紧密的亚水平断裂之上，较高密度的垂直裂缝出现在扩张的亚水平裂缝之上，表明新的地层。我们应用从结构工程中借用的梁破坏理论模型来限制潜在的新裂缝生成，使用基于格陵兰冰盖下方测量的水压波动的假设。该模型表明，梁破坏是在一系列现实冰川条件下冰下水波动循环期间产生新垂直裂缝的合理机制。这意味着液压顶升可以导致进一步 使用基于从格陵兰冰盖下方测量的水压波动的假设。该模型表明，梁破坏是在一系列现实冰川条件下冰下水波动循环期间产生新垂直裂缝的合理机制。这意味着液压顶升可以导致进一步 使用基于从格陵兰冰盖下方测量的水压波动的假设。该模型表明，梁破坏是在一系列现实冰川条件下冰下水波动循环期间产生新垂直裂缝的合理机制。这意味着液压顶升可以导致进一步浅层岩体的原位破碎和角砾化，降低了岩体强度并增加了其导水率。总之，现有裂缝的液压顶进和新垂直裂缝的形成导致浅层岩体有效的冰下机械风化。