Evaluating the Mesostructural Changes of Laboratory Created Soil-Rock Mixtures Using a Seepage Test Based on NMR Technology

Abstract

Abstract Soil-rock mixtures (S-RMs) have great potential as a material for use in hydropower engineering. However, seepage can change the porosity and structure of such mixtures, which can in turn threaten hydropower engineering structures. The objective of this study was to understand the mesostructural changes in S-RMs when S-RMs undergo seepage failure. Laboratory seepage tests were performed using a custom-made seepage instrument under different hydraulic gradients, i.e., 0, 20, 24, 27, and 30, and under a constant critical hydraulic gradient (30) for different lengths of time, i.e., 15 min, 30 min, 60 min, 120 min, and 180 min. Nuclear magnetic resonance (NMR) can directly determine the physical properties of S-RMs, such as the pore distribution, porosity, and permeability. The results of the NMR tests reveal that the pores in the S-RMs are well developed. As the hydraulic gradient and time increase, the pores are expanded by the hydraulic pressure, and the connectivity of the pores increases. The structure and cohesion of the S-RM can be damaged by hydraulic pressure. When the hydraulic gradient is equal to the critical hydraulic gradient, the S-RM experiences erosion or seepage failure, and these phenomena become increasingly obvious with time. A relatively strong correlation exists between porosity and permeability. Movable fluid porosity contributes considerably more to permeability than does irreducible porosity.