ESTIMATION AND RECHECK OF STRESS FIELD AT TAOZIYA DEEP-BURIED SUPER-LONG TUNNEL

Stress determination of deep-buried super-long tunnel has attracted more and more attention from scientists and engineers. How to determine the stress state especially for deep and complex geological conditions with high efficiency and accuracy has become a major concern in tunnel engineering and underground excavation. We developed an intergrated stress determination method for the stress determination of deep-buried super-long tunnel during engineering design and excavation. The method includes in-situ stress orientation and magnitude estimation based on multi-source data and stress recheck after excavation. For in-situ stress determination,we utilized the hydraulic fracturing in-situ stress measurements to obtain the stress magnitude and orientation of Taoziya tunnel in one deep borehole ZK3 before excavation. We analyzed the regional stress state using multi-source data by combining geomechanical trace analysis,Anderson's faulting theory,focal mechanics and stress inversion based on focal mechanism and stress database of China Mainland. We are based on the limited number of in-situ stress measurements in survey and design stage and regional multi-source data and used the modified Sheorey model combined with Hoek-Brown criterion to predict the initial stress along the designed tunnel axis. Finally,we rechecked the predicted stresses by hydraulic fracturing in-situ stress measurements of 4 boreholes in Taoziya tunnel during the tunnel construction. Results show that the preferred principal stress orientation is around N15°W~N40°W which is consistent with the regional stress orientation obtained from multi-source data except for the focal mechanics results. The stress orientation in shallow depth may be influence by topography and deviates from deep crustal stress. The relationship between three principal stresses is S_H≥S_v>S_h,which indicates that the stress regime is in favor of reverse faulting and strike-slip faulting. The predicted stress magnitudes are in the range of field measurements and database data near the borehole. The predicted magnitudes of horizontal maximum and minimum principal stresses are around 9.8 MPa and 6.6 MPa at the buried depth of 300 m,respectively,and 16 MPa and 10 MPa at the buried depth of 600 m,respectively. In the deepest buried depth,the predicted maximum and minimum horizontal principal stresses are around 24 and 16 MPa,respectively. The stress recheck in 4 boreholes along the tunnel axis indicates that the predicted stresses using modified Sheorey model are consistent with these stress measurement results in general. Local influence of lithology changes and fault fracture zone may lead to some excepts such as local bias and deviation of stress magnitude. The integrated stress prediction and recheck approach developed in this paper can ensure the efficiency and reliability of stress prediction results and reduce the cost for in-situ stress measurement in survey and design stage. And this method can also provide powerful evidence and data support for timely change of tunnel excavation scheme and budget adjustment,which is of great importance for successful completion of the project cost control.