Earth pressure balance shield machine in the excavation, under what circumstances with bentonite, under what circumstances use foam?
So the bentonite slurry is suitable for coarse and fine material content of gravel, cobble and boulder strata and so on. The main reason lies in the bentonite slurry can be added in the relative lack of gravel soil fine particle content, improve workability and gradation, which can improve the sealing property. (2) high permeability soil bentonite slurry added this soil material suitable for high permeability, this is mainly because of low permeability formation, bentonite mud relatively difficult to penetrate the soil and pore filling, into a short distance and it is difficult to combine a wide range of soil and soil particle inclusions. The low permeability of soil eventually results in the formation of low permeability bentonite mud film around soil particles.
Suitable for the use of bentonite modified strata (1) small amount of fine particles of soil according to the construction experience of many, in the earth pressure balance shield type, in order to make the excavation down waste has liquidity and sealing certain, ensure the normal advance of shield machine, soil shield machine pressure cabin must ensure that the fine particles certain content, content of the fine particles should be above 35%.
Suitable for the use of modified foam formation (1) foam is more suitable for the soil in grading is relatively good for soil particles with good gradation, the particle size distribution of a wide range, and the size of the bubble itself is not uniform, it is easier to fall between soil particles in the pores, closer contact with soil particles. The rate of dissipation of foam in a simple two-phase (water + gas) is much higher than the rate at which it dissipates in a three-phase (soil + water, ten gas). This is because in the soil, the full contact of foam and soil particles can prevent the flow of liquid film from being blocked and reduce the possibility of liquid film dewatering and foam bursting. In the soil gradation is relatively good, because the bubble and soil particles combine more complete and dense, pore water can more fully in the soil replacement and filling the original pore, so easy to form more closed bubbles. It is because of the existence of a large number of enclosed foam that the permeability coefficient of soil is reduced and water retention is enhanced. (2) foam is more suitable for the soil with larger average particle size; the finer the particle of soil is, the closer it is to viscosity; the stronger the hydrophilicity of minerals is, the greater their suction force is. Obviously, the small particles of silty clay will adsorb the free water contained in the liquid film on the surface of the foam, which will lead to the collapse of the foam after the dehydration of the liquid membrane, and reduce the sealing performance of the mixed soil
