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Erdaoya tunnel in the highway connecting Shiyan (Hubei province) and Manchuan, (Shanxi province) was studied in the study. Various problems occurred in the construction of the tunnel, including large deformation of surrounding rocks, cracks of preliminary supports and secondary linings as well as creeping down and collapse of surrounding rock masses along schistosity planes. In which, the most serious problem was large deformation of surrounding rocks, which not only dramatically delayed the normal construction progress of the tunnel, but also brought a huge challenge for the safety of builders. In order to solve this problem, detailed investigations were made on site in the study to reveal the regional geological, engineering geological and hydrogeological conditions of the tunnel. By conducting geo-stress measurement on site, the characteristics of geo-stress field of Erdaoya tunnel were analyzed from four aspects, including magnitudes of stress, coefficient of lateral pressure, orientation of stress and the relationship between magnitudes of geo-stresses and depths. By applying the DIPS software for interactively analyzing geological locator data, structure characteristics of rock masses were analyzed for preferred structural planes and schistosity planes in the tunnel site. Furthermore, the authors tested the physical and hydrological properties of the rock specimens collected on-site. So various physical properties (density, void fraction, and wave velocity) and water absorption hydrological property of schist were obtained, which provide underlying parameters support for the deformation and support of surrounding rocks of the tunnel. [ABSTRACT FROM AUTHOR]

Based on 232 sets of well-test results measured at depths from 135.9 to 1407.19 m in 15 coalbed methane (CBM) development blocks in China, correlations between parameters including reservoir pressure (P o ), shut-in pressure (P c ), breakdown pressure (P f ), in-situ stress, coal permeability, and depth ( D ) were determined. The distribution of in-situ stress in the eastern margin of the Ordos Basin (EMOB), southern Qinshui Basin (SQB), and western Guizhou province (WGP) were analyzed systematically, and its control on the coal reservoir permeability was also addressed. The minimum and maximum envelope curves of the stress ratio (including k (( σ H + σ h )/2 σ v )), K H ( σ H / σ v ) and K h ( σ h /σ v )) variability with depth was obtained using the Brown and Hoek (1978) method, and the average envelope curves were established by mathematical derivation on the condition that horizontal principal stresses increase linearly with depth. The results show that the maximum principal stress ( σ H ), minimum principal stress (σ h ) and vertical stress (σ v ) vary with depth and location. Generally, there are two main stress regimes in major coalfields in China: σ H > σ h > σ v (59.48%) and σ v > σ H > σ h (34.52%). The σ H > σ h > σ v stress regime has the smallest proportion (6%), which appears mainly in relatively shallower coal seams (< 742.84 m). Stress ratios, including k , K H and K h , were plotted between two envelopes, which can be used to estimate the range of σ H and σ h at a certain depth. With increasing depths, three ratios decrease toward a fixed value, e.g., K H → 0.86, k → 0.74, and K h → 0.6. For coal seams with a depth > 1000 m, k is generally < 1, indicating that the σ v gradually becomes the dominant stress. Coal reservoir permeability decreases exponentially with the increase in effective in-situ stress (EIS, ( σ H + σ h + σ v )/3 − P o ). Vertically, four distinct vertical bands of coal permeability can be found in the EMOB, SQB and WGP, defined by the compression and deformation of the coal-matrix under certain stress regimes: in shallow coal seams (EMOB, < 800 m; SQB, < 650 m; WGP, < 500 m), coal reservoirs are within a strike-slip faulting stress regime ( σ H > σ h > σ v ) where σ H and σ h tend to increase, and coal permeability decreases exponentially with an increasing depth; in intermediate coal seams (EMOB, 800–1000 m; SQB, 650–825 m; WGP, 500–750 m), the σ v > σ H > σ h stress regime plays a major role, with relatively high coal permeability. For deep coal seams (EMOB, > 950 m; SQB, > 825 m; WGP, > 750 m), CBM development conditions deteriorate due to a state of “extremely low permeability and extremely high in-situ stress”. [ABSTRACT FROM AUTHOR]

Tectonic activities are frequent in the Lhasa terrane because of the ongoing collision between the India and Eurasia plates. Knowledge of the stress state is critical to evaluate the crustal stability and the design of underground excavations. Because of the limitations imposed by natural conditions, little research has been performed on the present crustal in situ stress in the Tibetan Plateau, and further study is imperative. In this study, hydraulic fracturing measurements were conducted in Nyching County (LZX) and Lang County (LX), Lhasa terrane to characterize the shallow crustal stress state. The results indicate that the stress state in the LZX borehole is markedly different from that in the LX borehole, in both magnitude and orientation. At the same measurement depths, the magnitudes of horizontal principal stresses in the LX borehole are 1.5-3.0 times larger than those in the LZX borehole. The stress regime in the LX borehole favors reverse faulting characterized by SH> Sh> Sv, where SH, Sh, and Sv are maximum horizontal, minimum horizontal, and vertical principal stresses, respectively. The SH and Sh values are approximately three and two times greater than Sv. Fracture impression results reveal that SH in the LX borehole are predominantly N-S, while in the LZX borehole the maximum horizontal principal stress is mainly in the NNE-direction. The heterogeneity of the regional stress state might be a result of the population and distribution of local structures and seismic activities. The stress state in the LX borehole has exceeded the critical state of failure equilibrium, and there is an optimally orientated pre-existing fault near the borehole. It can be concluded that the optimally orientated fault is likely to be active when the stress has built up sufficiently to destroy the frictional equilibrium; it is suggested that research focus should be placed on this in future. The stress states in boreholes LZX and LX indicate uniformity of the regional stress field and diversity of the local stress fields resulting from the interactions among regional dynamic forces, tectonic stress field, and geological structures. [ABSTRACT FROM AUTHOR]

The Longtan tunnel is one key tunnel along the Hurong (Shanghai-Chengdu) national roadway in the Hubei province of China. This tunnel is a typical long and deep tunnel, with a length of approximately 8700 m and a depth of approximately 500 m. Because of the complicated geological conditions and high initial stress, to compute the stability of the surrounding rock, it is very important to back analyze the material parameters and initial stress from the measured displacements. Based on a novel evolutionary neural network proposed by the author, a new back analysis method is proposed that can be used to simultaneously determine the material parameters and the initial stress. Using this new back analysis method, some typical monitoring sections of the Longtan tunnel are computed. From numerical computations using the back analysis results, it is found that the stability of the tunnel is consistent with engineering practice and that the initial stresses determined from the back analysis coincide very well with the measured values. Therefore, using the proposed back analysis method, the relevant parameters for the entire tunnel can be conveniently obtained. [ABSTRACT FROM AUTHOR]

The crustal stress and seismic hazard estimation along the southwest segment of the Longmenshan thrust belt after the Wenchuan Earthquake was conducted by hydraulic fracturing for in-situ stress measurements in four boreholes at the Ridi, Wasigou, Dahegou, and Baoxing sites in 2003, 2008, and 2010. The data reveals relatively high crustal stresses in the Kangding region (Ridi, Wasigou, and Dahegou sites) before and after the Wenchuan Earthquake, while the stresses were relatively low in the short time after the earthquake. The crustal stress in the southwest of the Longmenshan thrust belt, especially in the Kangding region, may not have been totally released during the earthquake, and has since increased. Furthermore, the Coulomb failure criterion and Byerlee's law are adopted to analyzed in-situ stress data and its implications for fault activity along the southwest segment. The magnitudes of in-situ stresses are still close to or exceed the expected lower bound for fault activity, revealing that the studied region is likely to be active in the future. From the conclusions drawn from our and other methods, the southwest segment of the Longmenshan thrust belt, especially the Baoxing region, may present a future seismic hazard. [ABSTRACT FROM AUTHOR]