An experimental study on the stress field in rocks uniformed under in situ stresses

岩石在单轴压缩载荷下的非弹性变形反映了先前施加在岩石上的应力。这种先前的应力对非弹性应变的影响称为应力记忆的岩石性质。根据岩心应力记忆的特性，提出了用变形速率分析（DRA）方法从钻孔岩心样品中估算原位应力的绝对值。我们把对地应力绝对值的记忆称为地应力记忆。然而，从现有的岩石变形知识来看，原位应力记忆是一种意想不到的特性。我们进行这项研究的目的如下：1)确认将DRA法估算的岩心应力作为取样处的地应力，2)提出地应力记忆的解释假设，3)证明假设的有效性。在842 m深度的钻孔岩心样品上，将DRA估算的垂直应力与计算的覆盖层压力进行了比较。a)对比证实岩石具有原位应力记忆。b)提出了岩石在长时间几乎恒定的地应力下应力场是均匀的假设，并从理论上证明了地应力记忆可以用这一假设来解释。该假设预测，当施加的应力几乎等于长期施加在岩石试样上的恒定应力时，岩石试样的顺应性降低到最小。c)我们通过实验发现，岩石试样在恒定应力作用超过3天后，其顺应性有所下降。

Inelastic deformation of rocks under uniaxial loading of compresion reflects the stresses that have been previously applied to the rocks. This effects of the previous stresses on the inelastic strain is called the rock property of stress memory. The absolute values of in situ stresses are suggested to be estimated from boring core samples by the method of deformation rate analysis(DRA) which has been developed on the basis of the property of the stress memory. We call the memory of the absolute values of in situ stress as in situ stress memory. In situ stress memory is however an unexpected property from the established knowledge on the deformation of rocks. We carried out this study for the purposes as follows ; 1) to confirm that the stresses estimated from core samples by DRA are regarded as the in situ stresses at the places where the samples were retrieved, 2) to propose a hypothesis for the explanation of in situ stress memory and 3) to prove the validity of the hypothesis. We compared the vertical stresses estimated by DRA on boring core samples up to 842 m depth with the calculated overburden pressures. a) We confirmed by the comparisom that rocks have in situ stress memory. b) We proposed the hypothesis that stress field in rocks has been uniformed at the in situ stresses which are almost constant for a long time and theoretically proved that the in situ stress memory is explianed by the hypothesis. The hypothesis predicts that the compliance of rock specimens decreases to the minimum at an applied stress nearly equal to the constant stress that has been being applied to the specimen for a long time. c) We experimentally found the decrease in the compliance for the rock specimens which have been subjected to a constant stress for a time more than 3 days.