Niumiangou valley, the epicenter of the 12 May 2008 Wenchuan earthquake (MS 8.0), became an area with frequent and dense debris flow disasters post-earthquake. Based on the in situ investigations after the earthquake on 14 August 2010 and a series of gathered data, characteristics and dynamic analyses of post-earthquake debris flows in Niumiangou valley were conducted, and then their hazard degree was assessed. Some research conclusions are obtained: (1) these post-earthquake debris flows have some typical characteristics, such as rainstorm viscous-type debris flow, happening usually between 11 p.m. and 5 a.m., broken out in the main channel as well as six branch gullies at the same time and also induced in the branches with good vegetation; (2) the dynamic parameters of Niumiangou debris flow (including volumetric weight, velocity, peak discharge, impact force, total amount of debris flow, total amount of solid materials washed out by single debris flow, maximum height of the debris flow rises and super elevation in bend) are relatively significant, and due to which it can be indicated that these debris flow disasters have great destructive power and harmfulness; (3) the hazard degree of debris flow in Niumiangou valley is very high, compared with the debris flows that occurred in the years of 2008 and 2013 in Niumiangou valley post-Wenchuan earthquake, and the comparison result shows that the hazard degree of debris flow in Niumiangou valley is relatively higher, which is consistent with the current situation. Therefore, according to these results, debris flows in Niumiangou valley are in the development phase and large-scale rainfall-induced debris flow disasters, with greater damage and stronger wallop, will easily occur in the rainy seasons of the 20 years after the earthquake.

Penetration grouting technology is an important technical means to improve the mechanical properties of gravel soil layers, and the time-varying characteristics of Newtonian fluid viscosity have an important influence on the morphology and effect of penetration grouting. However, these time-varying properties are not considered in the current research on the mechanism of Newtonian fluid penetration grouting. In this paper, by studying the basic rheological equation of Newtonian fluids and its dynamic viscosity time-varying law, the penetration motion equation of viscosity time-varying Newtonian fluids is discussed, by means of theoretical analysis and experimental research. Based on this, the time-varying viscosity Newtonian fluid columnar penetration grouting diffusion mechanism (TVNCPGDM) equation is derived, the application scope of the equation is analyzed and a grouting experiment is designed to verify it. The results show that the theoretical value of the grouting diffusion radius calculated by the TVNCPGDM equation, is closer to the experimental value than that obtained by the equation of columnar penetration grouting without considering the viscosity time-varying Newtonian fluid, with a 12.9% improvement in accuracy. This shows that the TVNCPGDM equation derived in this paper, can better reflect the diffusion law and diffusion morphology of column penetration grouting of Newtonian fluid, which changes with time in the injected medium; and the diffusion radius obtained for penetration grouting is more in line with the actual grouting engineering demands. The research results can provide some theoretical guidance for the actual grouting of loose gravel soil layers.