In situ stress measurement and analysis of the stress accumulation levels in coal mines in the northern Ordos Basin,China

For non-directional drilling cores,selection of samples and the test methods for in situ stress measurements to evaluate the Kaiser effect(KE)were proposed,and the magnitude and direction of the principal stresses were derived from first principles.Based on this approach,the KE for 423 samples in the Burtai and Baode coal mines in the northern Ordos Basin(NOB),China,have been investigated.The results show that the maximum horizontal principal stress(σ_(H)),the minimum horizontal principal stress(σ_(h))and the vertical stress(σ_(v))varied with depth and location,and the values increase with increasing depth.Generally,the horizontal stresses play a leading role.For the main stress regimes in the NOB,σ_(H)>σ_(h)>σ_(v)(Burtai Mine,<172 m;Baode Mine,<170 m)andσ_(H)>σ_(v)>σ_(h)(Burtai Mine,170-800 m;Baode Mine,170-400 m),and theσ_(v)>σ_(H)>σ_(h)stress regimes are mainly distributed in moderately deep to deep coal mines.For rock masses with a depth of 350 m,k((σ_(H)+σ_(h))/2σ_(v))tends to 1,indicating that a deep critical state will gradually emerge.The test results are compared with those for the overcoring(OC)method,the anelastic strain recovery(ASR)method and micro-hydraulic fracturing(HF).The relative errors forσ_(H),σ_(h)andσ_(v)were 14.90%,19.67%,15.47%(Burtai Mine)and 10.74%,22.76%,19.97%(Baode Mine),respectively,and the errors are all within an acceptable range,thus verifying the reliability of the KE method.The dominant orientation for theσ_(H)(Burtai mine,NE-NNE;Baode Mine,NEE)is obtained via paleomagnetic technology,and the data are consistent with those(NE-NEE)of the earthquake focal mechanism solutions for the area.Based on the Byerlee-Anderson theory,a discussion is given on the levels of stress accumulation in the rock mass of the mines.For dry rocks or hydrostatic pressure rocks,the friction coefficients of the faults are low for both locations,and the values are less than the lower limit(0.6)of the strike-slip faults slip,indicating that the stress fractures at a low level around the study areas are lower than the friction limit stress.The stress accumulation levels in the Baode Mine are slightly larger than those in the Burtai Mine.