岩块波速–应力关系及其卸荷效应

Relation between P-wave velocity and stress of rock samples and their unloading effect

当岩块从原有的地质环境中取出时,它在原位岩体中受到的应力将被解除,其物理力学性质亦会发生变化,从而产生卸荷效应。通过自行设计的岩块波速–应力试验,研究了岩块的波速–应力关系。研究发现:波速–应力关系曲线可以较清楚地反映出岩石受荷过程中体积变形的3个阶段;波速–应力关系曲线基本上以σ=σc=(1/4～1/3)Rc为界,当σ<σc时,岩样波速随应力增大迅速增加,波速–应力关系符合幂函数曲线,当σ>σc时,岩样波速逐渐达到峰值,随后迅速下降,波速–应力关系符合二次函数曲线;波速–应力试验过程中,当应力增加到岩样在原位状态下同试验加荷方向下所受的应力时,波速增加速度急剧变缓。根据岩块的波速–应力关系提出了岩样的卸荷指标,并将其定义为岩块在原位所受应力对应的波速与未受荷时的波速之比,通过试验和计算得到黄岛地下水封石油洞库场区片麻岩和闪长岩的卸荷指标分别为1.13和1.08。

When a rock sample is extracted from the underground rock mass,the interior stress of the rock sample will release and its mechanical properties will change.The unloading effect will happen.According to the experiments on P-wave velocity and stress designed by the authors,it is found that the P-wave velocity vs.stress curves（V-S curves） of rock samples can reflect the three stages of the bulk distortion during the loading process.The total V-S curve can be divided into two parts.When the stress is less than σc,that is,one third to one fourth compression capacities of rock samples,the P-wave velocities of rock samples increase rapidly with the stress and the V-S curves can be fitted with power function.When the stress is larger than σc,the P-wave velocities of rock samples gradually reach the peak value,then they dramatically decrease.The V-S curves can be fitted with conic function.During the V-S experiments,the increasing velocity of the P-wave velocities will dramatically decrease if the stress reaches the stress which is undergone by rock samples in the field.The unloading effect index which is the ratio of the P-wave velocity with the initial geo-stress to that without stress is put forward according to the V-S relation.After the rock samples of gneiss and diorite from Huangdao water sealed underground oil tank are tested,the calculated unloading effect indexes of the gneiss and the diorite are 1.13 and 1.08 respectively.