Studies on the mechanical behaviour of rocks, including volcanic rocks, usually seek for unaltered and simple material: rocks without macroscopic defects. However, volcanic rocks are often naturally altered due to the circulation of hydrothermal fluids. This alteration may influence mechanical and physical properties. Here, we study the effect of hydrothermal alteration on the elasticity and failure properties of andesite. A homogeneous block of natural andesite was retrieved from a quarry. Three samples were first heat-treated and then artificially altered at different temperatures by soaking them in a brine for one month at a pressure of 20 MPa and temperatures of 80, 180 and 280 • C. Heat-treated unaltered and altered samples were hydrostatically loaded up to 50 MPa and unloaded, while strains and elastic wave velocities were recorded. Samples were also triaxially deformed to failure at a constant strain rate and a confining pressure of 15 MPa. At ambient pressure, increased alteration temperature resulted in increased wave propagation velocity, thus increased dynamic elastic moduli. During hydrostatic loading, volumetric deformation at a given effective pressure decreased with alteration temperature denoting increased static elastic moduli. During triaxial loading, the degree of alteration decreased elastic compaction and peak stress at failure. These observations are interpreted as the result of microcracks in-filling by alteration minerals, and in particular smectite, a swelling-clay mineral with a low friction coefficient. The mechanical behaviour of a volcanic rock subjected to triaxial loading was modelled with a damage model based on crack propagation from pre-existing flaws. A decreasing friction coefficient within the flanks of the cracks leads to a decrease of the peak stress and explains the experimental observations.

中文翻译：

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热液蚀变对瓜德罗普岛热处理安山岩弹性行为和破坏的影响

对包括火山岩在内的岩石力学行为的研究，通常寻求未改变的简单材料：没有宏观缺陷的岩石。然而，由于热液流体的循环，火山岩通常会自然改变。这种改变可能会影响机械和物理特性。在这里，我们研究了热液蚀变对安山岩弹性和破坏特性的影响。从采石场开采出一块均质的天然安山岩。三个样品首先进行热处理，然后通过在 20 MPa 的压力和 80、180 和 280 • C 的温度下将它们在盐水中浸泡 1 个月，在不同温度下进行人工改变。热处理未改变和改变的样品被静压加载高达 50 MPa 和卸载，同时记录应变和弹性波速度。样品也在恒定应变率和 15 MPa 的围压下三轴变形至破坏。在环境压力下，增加的蚀变温度导致波传播速度增加，从而增加动态弹性模量。在静水载荷期间，给定有效压力下的体积变形随温度变化而减小，表明静弹性模量增加。在三轴加载期间，改变程度降低了弹性压实和破坏时的峰值应力。这些观察结果被解释为蚀变矿物填充微裂纹的结果，特别是蒙脱石，一种具有低摩擦系数的膨胀粘土矿物。火山岩在三轴载荷作用下的力学行为采用基于预先存在缺陷的裂纹扩展的损伤模型进行建模。裂纹侧面内摩擦系数的降低导致峰值应力的降低并解释了实验观察结果。