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以前施加在岩石上的应力。这种先前应力对非弹性应变的影响称为岩石的应力记忆特性。建议用基于应力记忆特性的变形速率分析(DRA)方法从钻孔岩心样品中估算地应力的绝对值。地应力绝对值的记忆称为地应力记忆。然而，从已有的关于岩石变形的知识来看，地应力记忆是一个意想不到的特性。我们开展这项研究的目的如下：1)确认DRA从岩心样品中估计的应力被认为是取样点的地应力，2)提出一个解释地应力记忆的假说，3)证明该假说的有效性。我们将DRA对深达842m的钻孔岩心样品的垂直应力估计值与计算的覆岩压力进行了比较。A)通过对比证实了岩石具有地应力记忆。2)提出了岩石中的应力场在长期几乎恒定的地应力下是均匀分布的假设，并从理论上证明了该假设解释了地应力记忆。该假设预测，当施加的应力几乎等于长期施加于岩石试件的恒定应力时，岩石试件的柔度减小到最小。C)实验发现，恒定应力作用3天以上的岩石试件柔度降低。

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.