Large-scale 3D printed model test on seepage distribution in water diversion tunnel and surrounding fractured rock

Abstract

Hydro-geomechanical model test is a prevailing method to study the seepage field in the underground engineering. Most of the existing test models are equivalent continuous medium models, whereas the deeply buried diversion tunnel in rock-matrix with fractures in practical engineering exhibit discontinuities, unevenness and heterogeneousness. To this gap, current study constructs an 1800.0 × 1800.0 × 1800.0 mm³ division tunnel model with fractures in the surrounding rock through 3D additive-subtractive printing, which registers the largest ever 3D printing hydro-geomechanical test model. The current model is based on a practical project of the Jinping II Hydropower Station. The geological strata along the longitudinal direction of the tunnel are surveyed to identify the most critical section in view of instability. Six major cracks in the critical section are identified and measured. According to the scalability principle in the fluid–solid coupling theory, the current model with dimension, density, and permeability scalability coefficients of respective 35, 1, and $\sqrt{35}$ to those of the prototype target section is designed and 3D printed. In this 3D printed model, the dip angle and direction of the six major fractures are reproduction of those of the practical fractures, while the locus spacing is reduced by a scale of 35. Structural matrix is additively printed using cement mortar, while fractures are subtractive printed with a customized cutter. The fracture space is filled with a paste medium with variable permeabilities to approach the potential permeability range in the practical fractures. To reproduce the practical hydraulic-mechanical condition in the test, the 3D model is loaded with initial stresses of 150.0 kPa and seepage condition of 20.0 kPa around the model for 7 days prior to the hydraulic loading. To cover the potential practical seepage extent, the external hydraulic pressure is set gradually at five loading levels of 40.0 kPa, 60.0 kPa, 80.0 kPa, 100.0 kPa and 120.0 kPa. The results show that, as the circulating hydraulic pressure increases, the seepage pressure inside the fracture plane and outside the lining structure gradually increases. Test results are in good agreement with the field monitoring data. There is a significant correlation between the seepage pressure in different inclination planes and the distribution characteristics of fractures in its surrounding rock. With denser fractures in the surrounding rock on the lining outside, the seepage pressure in this area will be higher. The test results reflect the discontinuity characteristics of the seepage field inside the fractured rock mass to provide guidance for the stability evaluation and long-term maintenance of water diversion tunnel engineering.