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钻孔偏心对空心包体应力解除法地应力测量的影响分析

Analysis of the Effect of Borehole Eccentricity on In-situ Stress Measurement by the Hollow Inclusion Stress Relief Method

  • 摘要: 基于空心包体应力解除法原理和围压率定试验,对钻孔偏心造成的误差进行了分析。将围压试验中的偏心岩心视为平面上偏心圆环受压模型,利用双极坐标法推导了该模型的应力分布计算公式,并通过实例分析了不同围压和不同偏心距下小孔内壁环向应力的分布规律与特征。在此基础上,结合空心包体中应变花的布设角度及围压率定公式,提出了偏心下岩石弹性模量测量值的修正方法,并利用现场地应力实测数据,得到偏心前后各测点的岩石弹性模量、k系数值、地应力分量及主应力计算结果。结果表明:(1)随偏心距增加,后期围压测定的岩石弹性模量误差逐渐增大,最大可达13.41%;(2)各测点k系数值、主应力方向及倾角受偏心的影响十分微小,可忽略不计;(3)钻孔偏心对地应力分量及主应力大小影响较大,3个测点的平均最大主应力、平均中间主应力、平均最小主应力误差分别为4.23%、4.19%、4.24%,在实际地应力测量工作中不可忽略。

     

    Abstract: Based on the principle of the hollow inclusion stress relief method and the confining pressure calibration test,the error caused by borehole eccentricity was analyzed. The eccentric core in the confining pressure test is regarded as an eccentric circular ring compression model on a plane. The bipolar coordinate method is used to derive the stress distribution calculation formula for this model,and the distribution laws and characteristics of the tangential stress on the inner wall of the small hole under different confining pressures and eccentricities are analyzed through examples. On this basis,combined with the layout angle of the strain rosette in the hollow inclusion and the confining pressure calibration formula,a correction method for the measured rock elastic modulus under eccentricity is proposed. Using the field measured in-situ stress data,the rock elastic modulus,k coefficient,ground stress components,and principal stresses at each measuring point before and after considering eccentricity are obtained. The results show that:(1) With increasing eccentricity,the error in the rock elastic modulus determined by subsequent confining pressure gradually increases,reaching a maximum of 13. 41%;(2) The influence of eccentricity on the k coefficient,principal stress direction,and inclination angle at each measuring point is very small and can be neglected.(3) Borehole eccentricity has a significant influence on the magnitudes of ground stress components and principal stresses;the errors of the average maximum,intermediate,and minimum principal stresses at the three measuring points are 4. 23%,4. 19% and 4. 24%,respectively,which should not be ignored in practical in-situ stress measurement work.

     

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