Predictive analysis of stress regime and possible squeezing deformation for super-long water conveyance tunnels in Pakistan

The prediction of the stress field of deep-buried tunnels is a fundamental problem for scientists and engineers. In this study, the authors put forward a systematic solution for this problem. Databases from the World Stress Map and the Crustal Stress of China, and previous research findings can offer prediction of stress orientations in an engineering area. At the same time, the Andersonian theory can be used to analyze the possible stress orientation of a region. With limited in-situ stress measurements, the Hock-Brown Criterion can be used to estimate the strength of rock mass in an area of interest by utilizing the geotechnical investigation data, and the modified Sheorey＇s model can subsequently be employed to predict the areas＇ stress profile, without stress data, by taking the existing in-situ stress measurements as input parameters. In this paper, a case study was used to demonstrate the application of this systematic solution. The planned Kohala hydropower plant is located on the western edge of Qinghai-Tibet Plateau. Three hydro-fracturing stress measurement campaigns indicated that the stress state of the area is SH - Sh 〉 Sv or SH 〉Sv 〉 Sh. The measured orientation of Sn is NEE （N70.3°-89°E）, and the regional orientation of SH from WSM is NE, which implies that the stress orientation of shallow crust may be affected by landforms. The modified Sheorey model was utilized to predict the stress profile along the water sewage tunnel for the plant. Prediction results show that the maximum and minimum horizontal principal stres- ses of the points with the greatest burial depth were up to 56.70 and 40.14 MPa, respectively, and the stresses of areas with a burial depth of greater than 500 m were higher. Based on the predicted stress data, large deformations of the rock mass surrounding water conveyance tunnels were analyzed. Results showed that the large deformations will occur when the burial depth exceeds 300 m. When the burial depth is beyond 800 m, serious squeezing deformations will occur in the surrounding rock masses, thus requiring more attention in the design and construction. Based on the application efficiency in this case study, this prediction method proposed in this paper functions accurately.

Applications of FBG-based sensors to ground stability monitoring

The four diversion tunnels at Jinping Ⅱ hydropower station represent the deepest underground project yet conducted in China, with an overburden depth of 1500-2000 m and a maximum depth of 2525 m.The tunnel structure was subjected to a maximum external water pressure of 10.22 MPa and the maximum single-point groundwater inflow of 7.3 m^3/s. The success of the project construction was related to numerous challenging issues such as the stability of the rock mass surrounding the deep tunnels, strong rockburst prevention and control, and the treatment of high-pressure, large-volume groundwater infiltration. During the construction period, a series of new technologies was developed for the purpose of risk control in the deep tunnel project. Nondestructive sampling and in-situ measurement technologies were employed to fully characterize the formation and development of excavation damaged zones（EDZs）, and to evaluate the mechanical behaviors of deep rocks. The time effect of marble fracture propagation, the brittleeductileeplastic transition of marble, and the temporal development of rock mass fracture and damage induced by high geostress were characterized. The safe construction of deep tunnels was achieved under a high risk of strong rockburst using active measures, a support system comprised of lining, grouting, and external water pressure reduction techniques that addressed the coupled effect of high geostress, high external water pressure, and a comprehensive early-warning system. A complete set of technologies for the treatment of high-pressure and large-volume groundwater infiltration was developed. Monitoring results indicated that the Jinping II hydropower station has been generally stable since it was put into operation in 2014.