Influence caused by initial tectonic stress on relative measurement of borehole ground stress in viscoelastic media

In this paper, we discussed the influence of the initial tectonic stress on the relative measurement value of the ground stress when a long cylindrical straingauge is used in the viscoelastic media. We simulate the whole system consisting of the stratum, cement ring and the straingauge cylinder with a three-layer-medium model and obtain the solution of radial displacement on the inner wall of the straingauge under the action of initial tectonic stress by using the correspondence principle. The result showes: 1) The influence of initial stress making of the radial displacement on the inner wall of the straingauge increases gradually with time at a gradually reduced rate and tends to a certain value eventually; 2) The principal stress with a magnitude of C ₀+C ₁ exp (?ω ₁t)+C ₂ exp (?w ₂ t]σ ₁₀ in the opposite direction of initial stress will be superimposed to the measuring result of stress change without considering the initial stress. whereC ₀,C ₁,C ₂ andω ₁,ω ₂ are related only to the mechanical property of the stratum,σ _i0 (i=1, 2) are the two principal components of the initial tectonic stress. It is pointed out that the initial tectonic stress affects obviously on the measurement of the relative change of ground stress in the cases of soft strata and high initial stress.