High Pressure Permeability And Micro-mechanism Of Chemically Grouted Sands

Quicksand or sand and water inrush in underground coalmines have threatened the production safety, regular service of shafts, tunnels and panels. Chemical grouting technology for seepage prevention and strata enforcement technologies has widely utilized in the treatment of shaft rupture and quicksand hazards prevention. Permeability of grouted sand is one of important geotechnical properties, This dissertation focuses on the permeability and anti-seepage mechanism of chemically grouted sands, Which is a part of the project"Mechanism and process control of chemical grouting treatment for quicksand hazards in coalmines"supported by National Natural Science Foundation of China (NSFC) under Grant No. 40772192. Chemical grouts propagation and permeability partitioning features of grouted sand, high pressure permeability of chemically grouted sands and its micro-mechanism of anti-seepage were studied through scaled model test, permeability test under high cofining pressure and microanalysis. The main understandings are as follows:(1) The engineering geological characteristics of thick unconsolidated formations in coal mining areas, such as structure, layer group characteristics, composition, physical and mechanical properties were analyzed in detail. Permeability for sand in the thick unconsolidated layers changes with depth were investigated from the view of penetration and groutability. And sand granulometric composition in several deep coal mining areas indicates their poor groutability for granular grouts.(2) Semi-cemented sandstone specimen were simulated, and permeability before and after chemical grouting for the specimen has been tested. Results show that the permeability for the semi-cemented sandstone specimen mainly depends on the effective grain size and the content of coarse sand. Hydraulic conductivity and effective grain size and coarse particle content have an approximate relationship of exponential function. Grouting simulation tests indicate that groutability to semi- cemented sandstone for the first time under certain osmotic pressure difference depends largely on the porosity with large opening (the water absorption at atmospheric pressure). After the second grouting, impervious performance of chemically grouted semi- cemented sandstone tends to be equivalent.(3) A simulated grouting experiment was conducted on semi-cemented sandstone and results show that interface of grout penetration in sandstone column is a curved surface with the surrounding slightly below the center. Grouting infiltration over time around the grouting borehole is basically a concentric circle-shaped extension. Grouts diffusion rate in the different permeability media at same pumping pressures have significant differences.(4) Grouts diffusion concentration variation was gained using image processing technology. Results show that the concentration is generally decreasing with the distance of grouting hole. Howeverthere is also local enrichment for grouts. Partition of grouts concentration strongly implies the differences in permeability and anti-seepage behavior.(5) Apatent aerostatic pressure controlled equipment for preparing mechanically grouted sand was developed, Which can be used to simulate grouting into soil mass at different depth with different pumping pressure.(6) Static high pressure triaxial test system was used to conduct permeability test in confining pressure conditions for chemically grouted sands in different grouting conditions and different grouts-sand ratio under different osmotic pressure and different confining pressures. Results suggest that hydraulic conductivity increased with the increase of osmotic pressure. However, the influence of confining pressure on hydraulic conductivity relating to specimen composition and confining pressure exhibit different characteristics. That is, in general, hydraulic conductivity reduction was induced by the confining pressure increase in chemically grouted sand containing a higher proportion of grouts. Under high confining pressure, hydraulic conductivity for chemically grouted sand increased with the increase of confining pressure due to the rupture of particles.(7) A micro-strucure classification for chemically grouted sands and a simplified pore and throat network conceptual model have been proposed based on micro structural characteristics of chemically grouted sand. Micropores in chemically grouted sand mainly consist of large and super-large pores. Retro-mercuometric curve for semi-cemented sandstone show a sudden drop, which indicates the existence of thin-neck-bottle-shaped pores and poor connectivity. The conceptual model of pore throat is a complex network system. It is clearly indicated from the conceptual model that the decisive influencing factors on permeability of chemically grouted sand include the degree of filling, cracks and fissures within particles, grouts and particle-grouts boundaries (interfacial cracks).(8) The relationship between micro-structue and permeability of chemically grouted sands is analyzed and results show that the total pore volume, pore area, porosity increase with increasing chemical grouts content, and medium pore diameter in volume and average pore diameter decrease, which result in a obvious decrease in permeability. Permeability has a directly proportiona to cavity to throat size ration, medium pores diameter in volume and average pore diameter.