Approach to determining the unsaturated hydraulic conductivity of GMZ bentonite using the NMR technique

Incorporating the nuclear magnetic resonance (NMR) technique, a newly developed infiltration instrument constructed from polytetrafluoroethylene is designed for the rapid determination of unsaturated hydraulic conductivity of bentonite materials. The proposed approach is founded on measuring the variation of the NMR signal in numerous cross sections of the specimen during the infiltration test, enabling the determination of water content distribution. Through integration with the instantaneous profile method, unsaturated hydraulic conductivity can be obtained using Darcy's law. The mechanism underlying the observed U-shaped trend of unsaturated hydraulic coefficient with decreasing suction can be attributed to the relatively high value controlled by water vapor diffusion at higher suction stages, as well as the hydraulic conductivity increasing near saturation, similar to the behavior of most unsaturated soils. The consistency of test results with those acquired from conventional measurements, regarding amplitude level and evolutionary pattern, provides evidence of the reliability of the proposed method. The primary improvement lies in the real-time acquisition of the water content profile using the spatial encoding technique in the NMR in the infiltration test. Simultaneously, the approach substantially reduces the test duration from thousands of hours to dozens of hours while maintaining measurement accuracy.