Experimental study of the mechanism of grouting colloidal nano-silica in over-broken coal mass

The grouting process is an important subject for stability control of unfavourable geology. A multi-parameter grouting monitoring device was designed to monitor pump pressure, inlet density, outlet density, outlet electrical resistance and osmotic pressure. Taking the multi-source information as the indication, this paper presented an experimental study about the process of grouting colloidal nano-silica into over-broken coal mass, which was verified by percolation mechanics theory and numerical simulation simultaneously. The porosity and permeability of the injected sample were 15.34% and 5.45 mD. The results showed that the variation of pump pressure generally presented an obvious three-stage regularity. The time-dependent curves of outlet density and outlet resistance were consistent with that of pump pressure, while the curve of inlet density was opposite to pump pressure. The law of the dominated penetration mechanism was as follows: miscible displacement mechanism in the 1st stage (0-46th minute), Darcy flow in the 2nd stage (46th∼200th minute) and filtration flow in the 3rd stage. This research can guide grouting theory study and design in the over-broken rock mass.

Experimental Research into the Evolution of Permeability in a Broken Coal Mass under Cyclic Loading and Unloading Conditions

The permeability characteristics of a broken coal mass under repeated loading and unloading conditions exert significance on spontaneous combustion of coal in goaf during the mining of coal seam groups. Considering this, by using the seepage test system for broken coal-rock mass, seepage tests under cyclic loading and unloading conditions, were carried out on broken coal masses. The test results show that the fitting curves between permeability and effective stress, strain and porosity are a logarithmic function, cubic function and power function, respectively. Besides, the permeability of a broken coal sample under cyclic loading and unloading conditions is determined by its porosity, which conforms to the cubic law. With increased cyclic loading and unloading times, the permeability loss, stress sensitivity and the crushing amount of the broken coal sample were gradually reduced, but the particle size gradation of the broken coal sample gradually became better. During one loading and unloading cycle, the stress sensitivity of the permeability of coal samples in the loading stage was far higher than that in the unloading stage. In the loading stage, the re-arrangement, breakage and compressive deformation of coal particles can lead to a reduction in porosity, consequently resulting in a decreased permeability. In the unloading stage, only the permeability reduction of coal samples due to particle deformation can be recovered.